Journal of Conference Abstracts

Volume 4 Number 1


Symposium O11
Structural Geology and Tectonics



Session O11:4A

O11 : 4A/09 : F4

How to Make Stretching Lineations ­ A Recipe

Sandra Piazolo (piazolo@mail.uni-mainz.de) &

Cees W. Passchier (cpasschi@mail.uni.mainz.de)

Universität Mainz, Tektonophysik, Becherweg 21, 55099 Mainz, Germany

For decades field geologists have been using stretching lineations to determine the presence and direction of tectonic transport and the position of the principal elongation axis assuming that the geometric axis of a lineation represents the maximum principal strain axis. Also, strength of a lineation is commonly assumed to be associated with strain intensity. Our investigations of lineation development in discrete greenschist to amphibolite facies shear zones of the Variscan Pyrenees show that this issue is not as simple as commonly assumed. Field work in the Cap de Creus area near Cadaques, Spain, reveals that in the same discrete shear zone stretching lineations are strongly developed in certain lithologies while in other lithologies no apparent lineation is detected in handspecimen and hardly, if at all, in thin section. In our case, medium to coarse grained granitic gneisses develop a lineation while very fine grained metapelites rarely develop a lineation, except where sillimanite is present. Coarse grained pegmatitic dykes and quartzo-feldspatic gneisses exhibit a profound lineation of quartz ribbons and recrystallized feldspar aggregates, while pure quarzites are devoid of apparent lineations. Thin section studies of these lithologies reveal that the interplay of size and shape of grains, rate and mode of intercrystalline and intracrystalline deformation, rate of nucleation, recrystallisation and grain growth plays an important role in the development and preservation of both mineral and aggregate stretching lineations. Especially important is the relative original grain size of different mineral species and their different rates of intercrystalline and intracrystalline deformation. Initial small grain sizes commonly hinder the development of strong stretching lineations and additionally enhance fluid flow which may result in higher rates of dissolution. A consequence of dissolution may be that the main constituent of a possible lineation, eg.quartz, may be lost to the system and therefore no apparent lineation can develop.

Overall, field geologist should take care in interpreting the absence of stretching lineations in terms of deformation intensity. The characteristics of stretching lineations are as much or more dependent on the original fabric of a rock before deformation, as on strain intensity.

O11 : 4A/10 : F4

The Geometry of Folds and Mineral Lineations: Examples from the Cascades Crystalline Core (Washington, USA)

Hermann Lebit (hlebit@earth.usc.edu)1,

Catalina Lüneburg1 &

Martin Casey2

1 Department of Earth Sciences, University of Southern California, Los Angeles, USA
2 Department of Earth Sciences, Leeds University, Leeds, Great Britain

Folds and mineral lineations are widely used to infer regional kinematics in ductile deformed rocks. However, models of regional displacements commonly consider kinematic approaches, implying isotropic rheological properties and thus homogeneous material behaviour. These models are not suited to address the role of mechanical instabilities such as folding, although folds are common structure at all scales in all orogenic belts. Therefore, it is significant to determine if local strain features reflect regional deformation and kinematics, or local mechanical instabilities. This problem is illustrated by discussions on the tectonic interpretation of mid-crustal rocks exposed in the Cascades crystalline core of Washington and British Columbia. There subhorizontal, NW-SE trending mineral lineations, apparently parallel to fold hinges, have been used in support of strike-slip dominated transpressional models. Other models suggest SW-directed thrust tectonics with the mineral lineations orthogonal to thrust displacement. Detailed structural analysis demonstrates that the geometrical relationship of mineral lineations and folds is not as simple as transpressional models assume. Although mineral lineations are often subparallel to fold axes close to the hinge, they tend to form higher angles in the limbs. At least two other, partly isoclinal fold sets predate these folds and suggest that the finite strain pattern was developed in already existing strongly anisotropic rocks. The fold superposition is almost coaxial forming a complex type III interference pattern. In addition, kinematic indicators and the schistosity in fold profile sections show the typical morphology of mechanical folds, although these sections are normal to the lineation. Similar geometries in finite strain patterns were obtained by numerical experiments where two sets of folds were arranged with their fold axes either coaxial or forming acute angles. According to our field observations, the first tight folds are of similar type, while the second fold set represents buckle folds taken from a finite element model. Finite strain was calculated from the cumulated deformation of both fold events. In a perfectly coaxial arrangement the calculated stretching lineation parallels the fold axes if flattening deformation is considered, whereas non-coaxial fold superposition leads to stretching lineations sigmoidally curving across the later folds.These results and the fact that the lineation form geometries not continuously parallel to the fold hinges indicate that mineral lineation is related to local fold development. We suggest that the mineral lineations reflect the cumulative heterogeneous strain resulting from the superposition of folds onto pre-existing isoclinal folds. However, in this case the orientation of the mineral lineation is not indicative for regional kinematics.

O11 : 4A/11 : F4

Mica Fish and Other Fish-Shaped Shear Sense Indicators

Saskia M. ten Grotenhuis

(tengrote@mail.uni-mainz.de) &

Cees W. Passchier (jsg@mail.uni-mainz.de)

Tectonophysics, Department of Geosciences, University of Mainz, 55099 Mainz, Germany

The most well known fish-shaped shear sense indicators are mica fish, but several other minerals, such as hypersthene, tourmaline and plagioclase show the same type of structure. The fish commonly have a lozenge shape and tails of small fragments extended into the matrix from the tips of the fish. The occurrence of fish-shaped structures in many mylonite zones (Lister and Snoke 1984), and the ability to determine the shear sense, using their asymmetrical form and the stair-stepping of the tails, has made the importance of these structures widely recognised. Some shear sense indicators, such as sigma and delta clasts have received considerable attention in the last few years. The genesis and kinematic significance of mica and other mineral fish, however, have remained relatively unexplored.

This study is based on two different methods. The first method is microstructural investigation of samples from different mylonite zones. The studied structures are mainly muscovite and biotite fish in mylonites derived from micaceous quartzites. The studied metaquartzites consist, besides the mica fish, completely of quartz. The internal structures of the objects are in some cases very complicated, including folds and cut-off structures. The studied mica fish seem not to rotate, this in contrary to sigma and delta clasts. The fish-shaped structures have a stable position with respect to the mylonitic foliation. This position is with the long axis of the fish tilted back against the sense of shear, and an angle of 5-40° to the mylonitic foliation. The second method used is experiments with rock analogues and rigid fish-shaped objects. With these experiments the behaviour of a fish-shaped object in a homogeneous matrix, and the effects of the fish-shaped objects on the flow pattern of the matrix around the object are investigated.

Lister and Snoke (1984) have suggested the idea that mica fish can be considered as a special form of SC-fabric. However, this idea is unlikely because the fish-shaped structures are very well developed in otherwise homogeneous rocks. The first results have lead to the following two conclusions. One is that the role of pressure solution is probably very important for the genesis of the cut-off structures in the mica fish and of their general shape. The other conclusion is that the role of inhomogeneous deformation in shear zones containing these structures has to be considered. This could probably be an explanation for the absence of rotation of the fish-shaped structures.

Lister GS & Snoke AW, J Struct Geol, 6, 617-638, (1984).

O11 : 4A/12 : F4

Timing and Kinematics of Paleoproterozoic Shear Zones in Central Sweden

Karin Högdahl (karin.hogdahl@nrm.se)1 &

Håkan Sjöström (hakan.sjostrom@geo.uu.se)2

1 Museum of Natural History-LIG, Dept. of Geology and Geochemistry, Stockholm University, Sweden
2 Department of Earth Sciences, Uppsala University, Sweden

Several, steep ductile shear zones, active during the time range 1.85-1.70 Ga, exist in the Svecofennian part of central Sweden (Wijbrans et al. 1995; Bergman & Sjöström 1994; Stephens & Wahlgren 1996). The two most prominent of these, the Storsjön-Edsbyn Deformation Zone (SEDZ) and the Hassela Zone (HZ) envelope the western and northern margins of a c. 130x180 km calc-alkaline intrusion (Ljusdal Batholith). Kinematic data show that both were dominated by dextral shearing (Bergman & Sjöström 1994). The WNW- to NW-striking HZ was formed under wrench conditions (Bergman & Sjöström 1994). The relative timing of activity is bracketed by its imprint on the 1.84-1.85 Ga Ljusdal Batholith (Delin 1993; Welin et al. 1993), and its relation to the regional low-pressure metamorphism (LPM) at c. 1.82 Ga (Claesson & Lundqvist 1995). Conjugate sinistral shear zones were formed at a late stage of the bulk dextral shearing. U-Pb titanite data from one of these zones yield a fabric age of 1796±3 Ma.

The NNW-SSE striking SEDZ exhibit mylonites ranging from ductile to brittle reflecting repeated activity (Bergman & Sjöström 1994). During ductile deformation, the regional pattern was characterised by a change from shallow- to steep plunge of stretching lineations outside and within the SEDZ, respectively, indicating transpressive conditions. Large variations in the plunge of lineations locally, support this interpretation.

The junction of the SEDZ and HZ is defined by a c. 50 km wide pattern of anastomosing shear zones, having a more pervasively deformed marginal, north-eastern part. Titanites in a c. 1 km wide C/S-mylonite from this area have been dated at 1816±2 Ma; another deformation zone, developed in a limb of a large scale shear fold, has been dated at 1799±7 Ma. Similar ages have been recorded from shear zones south and south-east of SEDZ and HZ; 1810±2 Ma (the Hagsta Gneiss Zone) and 1798±2 Ma (Ljusne Shear Zone). Altogether these results define a period (c. 1800-1815 Ma) of development of regional scale, ductile shear zones at a late stage of the Svecokarelian orogeny. The overlap in time with the emplacement of various kinds of granitoids derived from deep as well as more shallow crustal levels, suggests that shear zone development and magma emplacement were closely related.

Claesson S & Lundqvist T, Lithos, 36, 115-140, (1995).

Bergman S & Sjöström H, Research report SGU, pp46, (1994).

Delin H, SGU ser C, 823, 13-16, (1993).

Stephens MB & Wahlgren C-H, Abs Nordic geological winter meeting, 22, 203, (1996).

Welin E, Christiansson K & Kähr A-M, GFF, 115, 285-296, (1993).

Wijbrans JR, Beunk FF, Pitka S, van Lil R, Boeken WB, Wahlgren C-H, Stephens MB & Verdurmen EAT, Terra Abs, 7, 44, (1995).

O11 : 4A/13 : F4

Structural Development of the Central Part of the Krusné Hory (Erzgebirge) in the Czech Republic ­ An Evidence for Changing Stress Regime During Variscan Compression

Jiøí Konopásek (kony@prfdec.natur.cuni.cz) &

Karel Schulmann (schulman@prfdec.natur.cuni.cz)

Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Albertov 6, 128 43, Praha 2, Czech Republic

In the central part of the Krusné hory (Erzgebirge) are exposed two main tectonic units - para-autochthonous metasedimentary basement has been overthrust by a crustal nappe of fine- and coarse-grained orthogneisses. The thrust boundary is defined according to the presence of mafic eclogites associated with layer of garnetiferous micaschists as there is significant inconsistency in peak pressure conditions between eclogites and all surrounding lithologies. Four stages of deformation can be recognized in the are studied. The D1 is present exclusively in eclogites and cannot be tepmporarily correlated with the deformation in surrounding rock types. It represents subduction-related eclogitic foliation and lineation with strong omphacite fabric. The D2 is associated with thrusting of an allochthonous nappe over the autochthonous metasedimentary basement. The PT conditions of this event are close to amphibolite-eclogite facies transition and in eclogites it leads mainly to formation of an intrafoliation boudinage. In all other rock-types, the D2 deformation leads to formation of main metamorphic S2 foliation and L2 lineation. Locally, the development of F2 folds is associated with this event. Numerous kinematic criteria show top-to-the-west sense of movement during D2. The D3 deformation leads to large-scale folding of both autochthonous and allochthonous units. In basement rocks can be recognized the development of numerous F3 folds and kink bands, sometimes with strong axial S3 clevage. This D3 event is responsible for refolding of an originally flat lying allochthonous slab, for steepening of the S2 planar fabric in all lithologies and its transformation into E-W trending S3 foliation. Last stage of deformation (D4) is characterized by development of the kink band-like F4 folds. These folds develop almost exclusively in those parts of the structure, in which the D3 folding has produced steep planar fabric. The F4 folds develop only in metasediments and often concentrate on their boundary with orthogneisses. In those localities, where the D4 has produced conjugated set of kink bands, their geometry suggests sub-vertical direction of principal compression. Concentration of the F4 folds in zones of steep S2-S3 metamorphic foliation suggests that these structures are associated with growth of the F3 folds. Our observations suggest two main deformation events. An early westward compression is responsible for the emplacement of allochthonous orthogneiss body with eclogites on its base. After this event, the principal compression has switched into N-S trending compressional event producing large-scale D3 folding of both allochthonous and autochthonous units. During D3, growing F3 folds produce local stres field with sub-vertical principal compression leading to formation of F4 folds in metasedimentary lithologies.

O11 : 4A/14 : F4

Characterisation of the Pressure Solution Transfer System in the Cretaceous Chalk from East England and the North Sea Central Graben

Markus Safaricz (m.safaricz@gl.rhbnc.ac.uk) &

Ian Davison (davison@gl.rhbnc.ac.uk)

Department of Geology, Royal Holloway - University of London, Egham, Surrey TW20 0EX, England

Volume loss, mass transfer and cementation as consequences of 3D pressure solution (PS) are described on the basis of detailed studies of chalk and more than 2000 pressure solution surfaces from the southern cliff coast of Flamborough Head, Yorkshire, UK, and from drill cores from the Machar Oilfield, UK North Sea. PS in a porous rock in a non-tectonic setting is driven by compressive effective stresses in all directions and the resulting strain ellipsoid shows three-dimensional shortening. Progressive dissolution causes a continuous evolution from micro-stylolites to massive PS residue seams. PS residue seams in the study areas reach lengths greater than 800 m and thicknesses up to 0.15 m and have been described formerly as sedimentary beds. Geometrical and chemical evidence indicates that they are PS residues. The bulk rock volume loss due to PS was determined with a geometrical method (based on measuring stylolite amplitudes) and a chemical method (based on quantification of immobile element enrichment in the residue seams) and is approximately 50% for both study areas. This suggests that a significant part of the subsidence in the North Sea central grabens of several hundred metres may be due to PS.Volume balance calculations of the amount of dissolved carbonate and the porosity reduction in the preserved sequence show that at both localities, a considerable amount of material must have left the studied chalk, in particular at Flamborough Head where the chalk shows a general lack of veins. The pressure solution transfer system in the Chalk must, therefore, have been open. This is supported by stable isotope data and by the occurrence of celestite bodies in the chalk. Considering the low solubility of calcite, the large amount of dissolution implies that for each cubic decimetre of rock, several hundred litres of an initially carbonate-free fluid are needed to remove the dissolving material. The source of the fluid flux must be a major convection system, and it is suggested that this probably extends up to the seawater. In cases of substantial open-system pressure solution transfer, like those of the studied areas, the fluid flux is assumed to be the rate-controlling factor for pressure solution.

Session O11:4B

O11 : 4B/25 : F4

Three-Dimensional Geometry and Permeability of Gold-Mineralized Faults in the Sigma and Lamaque Mines, Quebec, Canada

Stephan K Matthai (matthai@erdw.ethz.ch) &

Paolo Garofalo

Institute for Isotope Geology and Mineral Resources, Department of Earth Sciences, Swiss Federal Institute of Technology (ETH), Zürich, Switzerland

The Sigma-Lamaque mines have produced more than 250 tonnes of gold from quartz-tourmaline-carbonate veins (QTC) within subvertical oblique-reverse faults with foliated margins and subhorizontal, mechanically associated mineralized joints. The joints occur at or near fault tips, or where fault slip varied along fault planes. The faults also contain deformed and partially mineralized tonalitic dikes. Some dikes terminate at depth within porphyritic diorite which intruded the steeply-dipping host rock sequence of basaltic to andesitic lavas and pyroclastics. QTC mineralization is continuous in the subsurface to a depth of more than 2 km. To this depth and over an E-W strike length of 1 km, the mine geology is constrained by level maps with a 40-m-spacing and additional cross-sectional maps of raises. Our three dimensional interpretation of these data identifies the mineralized fault zones as curvilinear splays originating from anastomosing km-scale vertical strike-slip faults. Individual splays are continuous for up to six-hundred meters, but continuously QTC-vein-mineralized splay segments obey a Gaussian size distribution with a maximum near 30 meters.

While the disc-shaped QTC-mineralized joints have vein textures indicative of episodic dilation over their entire diameter (¾100 m), only certain segments of the splays were dilated, containing QTC. These segments were dilated by wall separation or fault-rock and dike brecciation during fault slip. In contrast to mineralized splay segments, QTC-free segments and the km-scale master faults are barren and marked by smooth discrete slip planes enveloped by strongly foliated and sheared host rock.

We infer from these characteristics that faulting enhanced host-rock permeability strongly only in higher-order extensional structures, creating high-permeability patches on the 30-300 meter-scale. However, the patch-normal spacing and the number of patch intersections permitted clustering and hydrologic interactions among patches. This is verified by means of fluid-flow simulations.

O11 : 4B/26 : F4

Fault Sealing by Clay Smear Based on Ring-Shear Experiments

Jill Angelique Clausen (jill.clausen@geol.uib.no) &

Roy H. Gabrielsen (roy.gabrielsen@geol.uib.no)

Geologisk Institutt, Universitetet i Bergen, Allegt. 41, N-5007 Bergen, Norway

Fluid seals are characterised by reduction in the flow potential across barriers due to the fluid transmission capacity on mediums being juxtaposed against another, either stratigraphically or by faulting. Clay smear involves entrapment of clay or shale within the fault zone, thereby giving the fault a high entry pressure. The aim of the present work is to evaluate the effect of clay smear on the sealing potential of faults, and the work is based on experiments with a ring-shear apparatus. The coherence of the clay smear is analysed in relation to parameters which naturally varies in fault zones, such as normal stress (<sigma> n), number of clay layers (n), deformation rate (<epsilon> ), fault throw (d) and the water content in the clay (PH2O). The samples consisted of stratified sand (Baskarp sand No.15) and clay (Drammen Clay) in a net/gross relationship of 95.8. Baskarp sand No.15 is a industrial sand and was deposited a few miles north of Jönköping (Sweden) under melting of the icecap ca. 10.000 years ago. The sand consists of more than 90% quartz. It is sorted by the contractor and the grains are classified as angular to sub-angular. During the experiments the sand had a porosity of 44.25% and a relative density of about 28.57 g/cm3. Hence, it is characterised as a loose sand. The Drammen Clay is post-glacial (ca. 3000 years) and has been sampled (drilled) at 7.3-7.6 m depth. The clay consists of 60-70% clay minerals, where illite dominates over kaolinite and smectite. The original water content of the clay varies from 41.55% to 44.44% with a mean value of 42.04%. Its liquid limit (WL) is 52.25, its plastic limit (WP) is 24.54, its plasticity index (IP) is 27.71 and its liquid index (IL) is 0.36. Accordingly, the clay is characterised as very plastic. The clay has a undrained shear strength of about 60 kPa. The stress-displacement deformation path in all the experiments was characterised by a post-peak stadium, peak, strain-hardening and strain-softening and eventually stick-slip sliding. The vertical deformation was characterised by dilation and compression. When a normal stress <sigma> n ~6 kPa was utilised, no clay smearing was seen, but some very small (mean height of 0.21 cm, mean length of 0.63 cm, mean width of 0.19 cm) circular and oblong clay segments with an orientation normal to the displacement/rotation were developed along the fault plane, mainly in the area of 0-180° rotation. When normal stress was set to <sigma> n~25 kPa, the clay was smeared along almost the entire fault plane, but several "holes" devoid of clay developed along it. When stresses of <sigma> n~50 kPa, <sigma> n~100 kPa, <sigma> n~150 kPa and <sigma> n~200 kPa were utilised, a partly continuous clay smear developed. When a normal stress of <sigma> n~400 kPa was utilised, the clay smear was continuous, but some "holes" were observed. The deformation rate seems to have little effect on the continuations of the clay smear in these experiments, whereas the water content of the clay seems to have influenced the coherence of the clay smear to some degree.

O11 : 4B/27 : F4

Application of Artificial Neural Networks to Fracture Analysis

Manhal Sirat (Manhal.Sirat@geo.uu.se) &

Christopher J. Talbot

Dept. Earth Sciences, Villavägen 16, 752 36 Uppsala, Sweden

AbstractThis study investigates the potential of artificial neural networks (ANNs) to recognize, classify and predict patterns of different fracture sets with depths down to 450 m in crystalline rocks at the Äspö Hard Rock Laboratory (HRL), south-eastern Sweden. ANNs are computer systems composed of a number of processing elements that are interconnected in a particular topology, which is problem dependent. ANNs have the ability to learn from examples using different learning algorithms. These learning algorithms involve incremental adjustment of a set of parameters to minimize the error between the desired output and the actual network output. Six fracture sets with particular ranges of strike and dip have been distinguished with poor linear correlations between fracture dips and depth. A series of trials were carried out using back-propagation (BP) neural networks for supervised classification. The BP networks recognized different fracture sets and approximated the noise among them to the closest set. Self-Organizing Neural Networks have been used for data clustering analysis with supervised learning algorithms; (competitive learning and learning vector quantization), and un-supervised learning algorithms; (self-organizing maps). The self-organizing networks adapted successfully to different fracture clusters (sets). A set of trials has been carried out to investigate the effect of changing the network's topologies and transfer functions on the performance of the BP networks. Using 2 hidden layers, with tan-sigmoid and linear transfer functions is beneficial for the performance of BP classification. Other combinations of input data of dips and depth values were introduced to the BP network to examine and predict correlations between fracture curvature with depth. The BP network recognized and predicted these correlations with high accuracy. These findings confirm earlier results indicating that fractures are curved on scales of hundreds of meters. This adds a significant factor to the conceptual model of the Äspö HRL, and should help in finding the appropriate site for isolation of Sweden's high-grade radioactive waste in crystalline bedrock.

O11 : 4B/28 : F4

Deformation Mechanisms Along a Brittle-Ductile Thrust (Clark Mountains, CA)

Catalina Lüneburg (catalina@earth.usc.edu),

Hermann Lebit &

Jean Morrison

Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089, USA

The Mesozoic Clark Mountains Thrust Complex (E-California) forms part of the southern extension of the Cordilleran fold and thrust belt. In this complex, the oldest thrust, the Winters Pass thrust, exposes brittle to ductile deformation at various crustal levels. In order to investigate deformation mechanisms and involvement of fluids we performed microstructural studies, texture goniometry measurements, and whole rock oxygen isotope measurements in three selected areas. The structurally uppermost part of the thrust (area I) is represented by a 1-2 m wide brittle fault with breccias showing no preferred grain shape or crystallographic orientations. The oxygen isotope values (15.9 to 30.1‰) likely reflect original sedimentary compositions. In area II the thrust is a 50 m wide discrete ductile shear zone characterized by almost completely recrystallized quartz mylonites. C-axes distributions of quartz show asymmetric maxima to point maxima. At the contact, a thin zone of ultramylonites exhibits no preferred grain or crystallographic preferred orientations. Isotope values are homogenized in the vicinity of the contact. The structurally lowest part of the Winters Pass Thrust (area III) is a 700 m wide ductile shear zone with partially recrystallized quartz mylonites. Detrital quartz grains are strongly flattened and quartz textures show c-axes distributions of simple girdles and cross girdles. Isotope values (8.5 to 13.7%o) are reduced in a zone which is characterized by brittle overprinting possibly opening pathways for external fluids. Along the Winters Pass thrust deformation regimes change from higher to lower crustal levels but not as predicted for increasing depth or temperature. Deformation processes change from cataclastic (area I) to strongly crystalloplastic deformation with almost complete recrystallization and grain boundary sliding at the fault (area II) to crystalloplastic deformation with only partial recrystallization (III). Therefore, local (metamorphic) fluids and concentration of strain may be critical parameters controlling deformation mechanisms in the brittle-ductile transition.

O11 : 4B/29 : F4

Deformation Mechanisms and Ar Isotope Systematics in Micas: Implications for Absolute Deformation Ages

Steven Reddy (sreddy@lithos.curtin.edu.au)1 &

Graham Potts (gpotts@liv.ac.uk)2

1 Tectonics Special Research Centre, Curtin University, Perth WA 6102, Australia
2 Dept. of Earth Sciences, University of Liverpool, Liverpool L69 3BX, U.K.

The extent of diffusive equilibration in a mineral is strongly temperature dependent but is also a function of grain size. In deformed rocks, deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape and therefore affecting isotope systematics. The range of possible deformation mechanisms in micas has significant implication for grain size modification and the resetting of isotope systems during deformation. Different deformation mechanisms may lead to increasing or decreasing grain size and may or may not lead to bulk syn-deformational isotopic resetting.

Relationships between diffusion, temperature and grain size in geological materials may be considered in terms of the mathematical approximation known as the "closure temperature" (Tc). We consider three different thermal scenarios with deformation taking place during a) the prograde heating path, b) at the closure temperature of the deformed mineral and c) at temperatures significantly below the closure temperature. We have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages. This modelling illustrates that in deforming rocks in which there is no change in grain size, it is difficult to establish deformation ages. In the cases where deformation causes a change in grain size, it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation. Without this information, distinction between cooling and deformation ages is equivocal. The development of grains with Tc greater than the deformation temperature may record a deformation age. This situation will arise in the following cases; i) neocrystallisation, ii) when grain size reduction occurs at temperatures below Tc of the reduced grain size and the deformation mechanism has reset the grains; or iii) when deformation-induced grain coarsening has occurred.

O11 : 4B/30 : F4

Kinematic Analysis of Strain Fringes

Daniel Koehn (koehn@mail.uni-mainz.de),

Cees W. Passchier &

Paul D. Bons

Inst. fuer Geowissenschaften, Becherweg 21, 55099 Mainz, Germany, Germany

Fibrous crystals in strain fringes are commonly used in modern kinematic analysis to determine the incremental strain history of deformed rocks. Displacement-controlled fibres are thought to track the opening trajectory of these strain fringes, representing the incremental extension direction or Z-axis path. Computer modelling shows that the roughness of the central object in the fringes is the critical parameter that controls the tracking ability of fibres. Face-controlled non-tracking fibres develop around smooth objects and displacement-controlled fibres around objects with a rough surface. Whether or not the displacement-controlled fibres are tracking the opening trajectories of the fringe depends on the rotation of the central object. In pure shear progressive deformation, the computer modelling predicts that displacement-controlled fibres are indeed tracking the opening trajectories of the strain fringe, and represent the Z-axis path if the central object is not rotating. Non-coaxial progressive deformation leads to more complex fibre geometries. For example, displacement controlled strain fringes that formed during non-coaxial progressive deformation around rough round pyrites from the Yilgarn Craton, Australia, show crystal fibres that have a pronounced stronger curvature than the fringe. Computer modelling of such fringes confirms that the observed fibre geometries can only be produced if the central object is rotating. This has important implications for the interpretation of displacement-controlled crystal fibres in strain fringes. Fibres in fringes are NOT necessarily tracking the incremental strain history of rocks: they are reflecting the movements of the central object. Only if the central object is not rotating, the displacement-controlled fibres are tracking the opening vector of the fringe that is representing the Z-axis path. If the object is rotating, the fibres are tracking both the opening of the fringe and the rotation of the central object. In this case, the effect of the rotation has to be determined and fringe geometry can only be translated in terms of a Z-axis path by advanced kinematic analysis.

O11 : 4B/33 : F4

Calcite Single Crystal Deformation in Torsion

Luigi Burlini (luigi@erdw.ethz.ch),

Karsten Kunze &

Marco Pieri

ETH Zentrum, Sonnegstrasse, 5, Zurich, Switzerland

Torsion deformation experiments were performed on single crystals of calcite with the {10-14} plane normal to the cylinder axis at 1000 K and constant shear rate (3 x 10-4 s-1) to variably large shear <gamma> ranging between 0.1 and 3. The gradient in shear rate and the varying shear direction throughout the cylinders caused the single crystal to deform inhomogenously. At <gamma> ~ 1 the samples showed clear macroscopic evidence of shear localization and the shape of the specimen was not anymore cylindrical but irregular. Within section planes normal to the cylinder axis (therefore parallel to the {10­14} plane), at <gamma> = 0.1 we observed a sector in which several twins with lensoid shape were present. The twins were thick near the periphery of the cylinder and very thin near the centre. At larger <gamma> , in a section cut parallel to the cylinder axis through the centre of the cylinder, we observed very few twins in the centre (where the shear deformation is minimum) and many lensoid twins of different generations, often with both boundaries curved, towards the periphery of the cylinder. Some bands of newly recrystallized small grains (grainsize of 10 to 50 µ m) were observed near the periphery of the cylinder, where shear localized. Some twins were displaced by the {10­14} plane. These microstructure are compatible with deformation dominated by twinning and slip occurring along the {10­14} plane. The stress strain curves were compared to that of the Carrara marble deformed in torsion at the same temperature and strain rate conditions. From these, we observed:

- For any finite shear, the single crystal in this special orientation is always softer than the Carrara marble. The yield stress of the single crystals was consistently at about 32 MPa, which is about half the yield stress of the polycrystalline aggregate (around 65 MPa). The yielding point was below <gamma> <0.01 for the single crystal and <gamma> <0.05 in the polycrystal. Strain hardening varied between the different single crystal tests. Peak stresses between 58 and 75 MPa were reached at a shear near <gamma> =1, which is at a similar finite shear, but between 5 and 25% below the peak stress of the polycrystal.

- The strain softening following the peak stress was more pronounced for the single crystals than for the aggregate. A steady state flow stress of about 55 MPa was approached at <gamma> =3, which falls about 20% below the flow stress of the marble.

O11 : 4B/34 : F4

Rheological and Microstructural Development in Calcite During Deformation and Recrystallization. Numerical Simulation of High Strain Torsion Experiment

Marco Pieri (pieri@erdw.ethz.ch)1,

Karsten Kunze (kunze@erdw.ethz.ch)1,

Luigi Burlini (luigi@erdw.ethz.ch)1 &

Hans-Rudolf Wenk (wenk@seismo.berkeley.edu)2

1 Sonneggstr. 5, Zürich 8092, Switzerland
2 301 McCone Hall, Department of Geology, University of California, Berkeley, CA, 94720-4767, USA

The evolution of lattice preferred orientation (LPO) is studied during high shear strain deformation and dynamic recrystallization of Carrara marble. Of particular interest is the development of a LPO that aligns easy slip systems in grains with the bulk shear plane and direction as one of the possible mechanisms by which strain softening is achieved. Carrara marble has been deformed in torsion to shear strains up to <gamma> = 11 within the dislocation creep regime, at constant temperature of 1000 and 1200 K and strain rate of 3x10-4s-1. At both temperatures, a peak stress (80 MPa at 1000K and 30 MPa at 1200 K) was reached followed by strain weakening (5-7%). Crystallographic fabrics were analyzed by Orientation Imaging (automated EBSD). The starting material had an initially isotropic LPO. The deformation texture was achieved at <gamma> = 1. For larger shear strains the LPO changed completely into sharp recrystallization texture, which had stabilized at <gamma> = 5 (1000 K) and <gamma> > 6 (1200 K) and remained stationary for higher shear strains with a preferred orientation aligned for easy slip on (r) <a>. At low strains the microstructural evolution differed for both temperatures but it seemed to converge for high shear strains. At 1000 K, grains started to deform homogeneously and produced a shape fabric consistent with the macroscopic shearing. At higher strains <gamma> >= 5 grains recrystallized by subgrain rotation forming stretched clusters of 5 mm grains with similar lattice orientation and an oblique shape preferred orientation (SPO). At 1200 K, grain shape was modified by grain boundary migration that was competitive with the development of a SPO. At <gamma> = 5 pervasive grain size reduction by dynamic recrystallization occurred down to 15 mm. At both 1000 and 1200 K rheology and microfabric did not show further evolution for 5 ¾ <gamma> ¾ 11. It is concluded that a steady state behaviour is established at significantly higher strain than previously reported. Numerical simulation using the self-consistent plasticity model has been applied with considerations of the effects of dynamic recrystallization. Two different recrystallization mechanisms have been evaluated: growth and nucleation. In both cases modelled textures resemble very well the experimental ones. However, grain growth is not consistent the significant grain size reduction.

O11 : 4B/35 : F4

High Resolution Orientation Imaging Using Computer-Integrated Polarization Microscopy (CIP) and Ultra-Thin Sections of Quartzite

Renée Heilbronner (heilbronner@ubaclu.unibas.ch) &

Lukas Rosenthaler (rosenth.foto.chemie.unibas.ch)

Geological Institute, Bernoullistr. 32, Basel, Switzerland

Orientation imaging (OIM) is a method by which crystallographic orientations and misorientations are visualized. In general, a scanning electron microscope (SEM) is used, and by electron back scatter diffraction techniques (EBSD), the full orientation of the minerals, i.e. the orientation of all crystallographic axes, is obtained at every pixel of the image. An easier, but much less comprehensive way of OIM is offered by computer-integrated polarization microscopy (CIP); this method works only for uniaxial minerals, and is only capable of deriving the c-axis orientations and none of the other crystallographic directions (Panozzo Heilbronner & Pauli, 1983 & 1984). It has been demonstrated, however, that c-axis orientation images (COI) are sufficient to explain quite a number of microstructural or textural phenomena (Pauli et al., 1996, Herwegh et al., in press) and that they represent a method of analysis which is a good complementary to the SEM-based method (VanDaalen et al., in press).

One of the important questions when comparing EBSD versus CIP-derived orientation images, concerns the spatial and orientational resolution of the methods. The resolution of the previously published method for the calculation of c-axis orientation images of uniaxial minerals (CIP) can be improved significantly if ultra-thin sections are used. Infact, the spatial resolution which can now be achieved is as good as the optical resolution of the microscope objective.

In this contribution, a new version of the CIP sofware is presented. What used to be a series of separate programs is now combined into one platform-independent application. High resolution COIs of experimentally deformed, dynamically recrystallized quartzite will be presented.

Herwegh, M, Handy, M & Heilbronner, R, Tectonophysics, (in press).

Panozzo Heilbronner, R & Pauli, C, J. Structural Geology, 15(3-5), 369-382, (1993).

Panozzo Heilbronner, R & Pauli, C, Textures of geological materials (edited by Bunge, H. J., Siegesmund, S., Skrotzki, W. & Weber, K.) DGM Informations gesellschaft Verlag, Oberursel, 147-164, (1994).

Pauli, C, Schmid, SM & Panozzo Heilbronner, R, J. Structural Geology, 18, 1183-1203, (1996).

VanDaalen, M, Heilbronner, R & Kunze, K, Tectonophysics, (in press).

O11 : 4B/36 : F4

Localized Versus Distributed Deformation in Heterogeneous Viscous Rocks

Dani Schmid (dan@erdw.ethz.ch) &

Yuri Podladchikov (yura@erdw.ethz.ch)

Geologisches Institut, ETH Zentrum, 8092 Zürich, Switzerland

Structural softening is an important mechanism during the deformation of heterogeneous, viscous rocks. It causes strain to be localized without altering material properties or grain size reduction and is purely due to the reorganization of heterogeneities. Weaker parts are aligned along planes which focus strain. These shear zones may maintain an absolute position or may passively follow the material flow.We studied the deformation of heterogeneous, viscous rocks with a numerical simulation. Based on a finite difference / spectral method it provides the required resolution of the heterogeneities and reaches high strains, for both simple and pure shear. The model assumes a constant strain rate boundary condition for pure shear, but constant stress level for simple shear and allows arbitrary shear combinations. We used the model to investigate the deformation of heterogeneous, viscous rocks and how heterogeneities influence distributed deformation mechanisms such as folding. We run our experiments with increasing amounts of heterogeneity under pure, simple and combined shears. From our results we can:

- construct mode diagrams for localized versus distributed deformation- predict limitations of classical folding theories if heterogeneities are introduced

- illustrate the structural softening mechanism and predict the behavior of the corresponding shear zones

We show that even strongly heterogeneous rocks accommodate strain in a predictable manner. Considering the heterogeneous nature of most rocks, any structural interpretation must include structural softening.

O11 : 4B/37 : F4

Viscoelastic Folding: Importance of Elasticity for Slow Deformation of Rocks

Stefan Markus Schmalholz (stefan@erdw.ethz.ch) &

Iouri Podladchikov (yura@erdw.ethz.ch)

Geologisches Institut, ETH Zentrum, 8032 Zürich, Switzerland

Viscoelastic folding is examined using analytical solutions and numerical simulations based on spectral/finite difference methods. The material behaviour is described with a linear viscoelastic (Maxwell) rheology. The analytical solution provides several dimensionless parameters which describe (i) the onset of active folding and (ii) the change from a quasi-viscous to a quasi-elastic behaviour of the folding process.

The analytical solution and the numerical simulations show a variety of elastic effects as e.g.:

1.) Elasticity allows the development of relatively small wavelength to thickness ratios for large viscosity contrasts. This resolves the discrepancy between small viscosity contrasts inferred from the application of pure viscous folding theory to field observations and the large variability of effectiv viscosities of natural rocks from laboratory measurements.

2.) Increasing elastic behaviour leads to the development of single layers with increasing chevron shape. Stronger chevron shapes can be produced in multilayers where also viscous rheology provides chevron type shapes.

3.) Shortening of single layers with a random initial perturbation (red noise) produces the analytically predicted wavelength to thickness ratios for quasi-viscous behaviour. For increasing elastic behaviour a regular wavelength to thickness ratio cannot be observed anymore and deformation is localized in irregular distributed folds.

4.) For increasing elastic behaviour the maximum pressure at the bottom of the fold hinge decays stronger during progressive folding.

Elastic effects are often ingnored since (i) elastic strains are small and (ii) the tectonic time scale is much larger than the Maxwell relaxation time. However, elasticity may cause ultrafast stress changes and hence the characteristic process time (e.g. folding) decreases and approaches the Maxwell relaxation time.

In contrast to the differences between viscous and viscoelastic folding there are several universal constants which are unaffected by elastic effects like the amplitude to thickness ratio at the stage of maximum folding velocity. These universals are used to develop methods which can be applied by geologists to estimate e.g. strain and rheology from fold shapes.

O11 : 4B/38 : F4

Lithospheric Scale Folding and Rock Exhumation: Numerical Modelling and Application to the Himalayan Syntaxes

Yu. Podladchikov (yura@erdw.ethz.ch) &

J. -P. Burg (jpb@erdw.ethz.ch)

Geologisches Institut, ETHZ, Sonneggstrasse 5, CH 8092 Zurich, Switzerland

We first describe the eastern and western Himalayan syntaxes, which are large-scale antiforms situated at geodynamically similar locations and whose metamorphic evolution is coeval in the India-Asia collisional history. In order to understand the mechanical plausibility of the structural interpretation, we present two-dimensional finite element modelling of lithospheric folding. Our 2D FEM modelling couples plane strain mechanical and thermal calculations with non-linear lithospheric rheologies. The models reveal the coeval development of adjacent synformal basins, which are recognised as the Peshawar and Kashmir basins on both sides of the western syntaxis. Similarities between geological data and calculated models indicate that lithospheric buckling is a basic response to large-scale continental shortening and an efficient mountain building process. Our 2D-FEM simulations suggest that certain behaviours are characteristic:

- Models initially undergo homogeneous shortening and coeval thickening before they become unstable and buckle. Hot lithospheres undergo more distributed shortening than cold ones. Regardless of thermal regime, buckling is a basic response of lithospheres to applied, far-field compression.

- At the next stage lithospheric folding is mechanically preferable to homogeneous thickening and can cause/drive mountain building and exhumation of deep seated rocks.

- Buckle amplification is limited by a saturation. A cold (strong) lithosphere tends to exhibit higher amplitude folding with a longer wavelength of ca. 200 km than a hot lithosphere. Km-scale amplification is achieved in the strain range of 10-25%.

- With shortening beyond the saturation condition, buckling propagates laterally and adjacent crustal folds develop. Propagation is less pronounced in hot than in cold lithospheres.

- Folding of both crustal and sub-crustal levels indicates coupling of all lithospheric layers during this deformation mode.

- In all cases, asymmetry and crust decoupling grow gradually and becomes dominant after ca 25% shortening.

- Synformal, small amplitude basins develop on both sides of the growing anticlines.

The Himalayan syntaxis areas are preferential sites of large-scale folding. Both syntaxes have grown within the last 4 Myr, and were accompanied by the formation of fast subsiding synformal basins. Geological and modelling information indicate that these structures have reached their locking stage. Vertical movements should decelerate and buckling of the Indian lithosphere is expected to propagate laterally.

Session O11:5B

O11 : 5B/21 : F4

The Moyenne Durance Active Fault (Southeastern France) is Presently Dextral and Moving in Response to the Alpine Push

Jean-Claude Hippolyte

(Jean-Claude.Hippolyte@univ-savoie.fr)

Université de Savoie, LGCA Campus Scientifique, 73376 Le Bourget du Lac, France

The Moyenne Durance Fault, near Manosque in northern Provence, is one of the most famous French active fault. It is known to have generated about one damaging earthquake a century. The authors consider that this 'foreland' fault is an Hercynian NNE-trending basement fault, reactivated during the Oligocene extension, and presently moving sinistral in consistency with a N-S compressional stress field that results from the Africa-Europe convergence. Several palaeostress determinations had confirmed this N-S compression. However the most recent sediments concerned by the fault analyses were those of the Miocene-Pliocene Valensole Basin, which is in fact mostly pre-Messinian in age.

To better constrain the Quaternary stress field, we look for deformation in the most recent deposits: the alluvial terraces of the Durance Valley and its Alpine tributary, the Bléone River. Several outcrops in the lower quaternary terraces (Mindel, Riss and Würm) reveal the presence of an active out-of-sequence thrusting in the Digne Nappe, moving in consistency with a N70E compression. One site along the Durance fault reveals deformation in a Riss terrace. This deformation allows to characterize a similar ENE-WSW direction of compression. It results that the NNE-trending Moyenne Durance active Fault is in fact moving dextral reverse. This discovery has important consequences for the understanding of the alpine deformation : (1) the Durance Fault is an old foreland fault which is now involved in the foreland progressing alpine deformation; (2) the Valensole molasse Basin is located in the alpine compressionnal domain; as it undergoes vertical movements and folding, we conclude that it is presently, like the Swiss molasse Basin, partially or totally lying above an incipient alpine décollement.

O11 : 5B/22 : F4

Tectonic Inversion and the Role of Pre-Existing Structures, an Example from the Baronnies, SE France

Jan Kees Blom (j.c.blom@ta.tudelft.nl)

Subfaculty of Applied Earth Sciences, Delft University of Technology, Mijnbouwstraat 120, Delft, The Netherlands

Tectonic inversion is a process which is being recognised in many parts of the world. One of the most important factors controlling the process is the lay out of the structures that are present before the onset of inversion. For instance, thrust faults in an inverted terrain may not exhibit the low-angle ramp-flat geometries that can be observed in many foreland basins. Here, we examine an example of an inverted terrain, situated on the southern margin of the inverted extensional Vocontian Basin of SE France. Extension started in the Triassic, with NE-SW striking normal faults, as well as minor NW-SE striking faults. Continued extension during the Jurassic and Early Cretaceous caused a series of EW trending normal faults, separating tilted blocks. During the Late Cretaceous, the onset of the Pyrenean phase of Alpine tectonics resulted in NS directed compression in the Baronnies area. The Alpine phase, resulting in EW compression in de Vocontian Domain, had very little effect in the Baronnies Area. This inversion resulted in the reactivation of the EW trending extensional faults as steep reverse faults, generally blocking the formation of low angle thrust faults. In some (adjacent) areas, the hanging wall of these faults moved toward the S, in others toward the N. Separating these domains are the NE-SW and NW-SE trending faults, which generally show very little vertical movement, but act as transfer or compartimental faults.

O11 : 5B/23 : F4

Red Sea Extension Influenced by Pan-African Fabrics in Eastern Eritrea

Woldai Ghebreab &

Christopher J. Talbot

Uppsala University, Dept. Earth Sciences, Villavagen 17, S-752 36, Uppsala, Sweden

Middle to lower crustal rocks with dominant flat-lying fabrics developed at amphibolite metamorphic facies in Pan-African time are exposed along the actively extending Red Sea coast of Eritrea. These rocks are structurally overlain by upper crust of greenschist facies metamorphic rocks with steep fabrics dominant. Three Pan-African PAD1-3 phases of deformation were superposed during the Cenozoic by two major Red Sea RSE1-2 phases of lateral extension in eastern Eritrea. PAD1 deformation is characterised by steep penetrative foliation S1 which is axial planar to upright F1 folds. During PAD2 deformation, subhorizontal shear zones and F2 recumbent folds refolded F1. PAD3 deformation resulted in strike-slip shear zones and a steep S3 foliation axial planar to open upright F3 folds.

NE-SW lateral extension of the Red Sea that occurred in two main phases, RSE1 and RSE2, was influenced by steep PAD1 and PAD3 and flat-lying PAD2 fabrics in different parts of the Pan-African basement of eastern Eritrea. The first phase of extension RSE1 was dominantly semi-brittle and characterised by a stack of top-to-basin low-angle normal faults that sole out to a subhorizontal basal detachment. This suite of low-angle detachments involved minor landward block tilting and exploited embrittled pre-existing Pan-African low-angle ductile shear zones of PAD2. Reverse faults locally accompanied RSE1 detachments. The second phase of extension RSE2 involved seaward block tilting on a new system of moderately to steeply (~400) domino-style normal faults and dykes with NW-SE strikes and mainly landward dips. These younger coast-parallel faults and dykes rotated ~30-400 landward by still younger detachment shear. Extensional structures of the second phase concentrate in zones of maximum crustal flexure seaward across the escarpment. The RSE2 structures truncate the PAD2 subhorizontal shear zones, but exploited steeper structures along the escarpment and on the plateau. Normal faults and dykes that flank the Damas, Hergigo and Zula half-grabens consistently dip landward. These half-grabens are separated from the Danakil depression half-graben by the Alid-Senafe transfer zone which is about 60 km long. These fault-dyke geometric relationships suggest that the Red Sea escarpment in Eritrea is a monoclinal flexure.

O11 : 5B/24 : F4

Strike-Slip Fault-Propagation Cleavage in Carbonate Rocks of the Southern Apennines, Italy

Andrea Billi (billi@uniroma3.it) &

Francesco Salvini (salvini@uniroma3.it)

Universita Roma Tre, L.go S.L. Murialdo 1, 00146 Rome, Italy

Fault-related cleavages are carriers for hydrocarbons or groundwater, yet they may be difficult to be detected in the subsurface. Consequently, understanding the nature and the spatial arrangement of fault-related cleavages is a first-order problem for inferring major fault geometry and kinematics and for modelling fluid flow through them.

Field investigations along and within some regional strike-slip faults in thick carbonate successions of the Southern Apennines revealed that the fault deformation zones are typically 2-300 m wide subvertical fractured bands, where a fault core encompassed by a damage zones is recognised.

Within fault cores the dominant lithologies are gouges and cataclasites developing along the master plane. Carbonates affected by subsidiary slip planes and patterned, mm- to cm-spaced solution cleavages prevail within damage zones. At the exposure scale, these cleavages are penetrative, aligned, planar to curviplanar surfaces whose spacing tends to slightly reduce in approaching the master slip plane. The cleavage surfaces are decorated with stylolites testifying for pressure solution mechanisms. In some cases, slickenlines along the cleavage surfaces as well as dilational fractures and/or solution seams emanating from the end of the cleavage planes suggest some small shear displacement occurred along the cleavage planes. Cleavage is restricted to the fault-related deformation zones and occurs at the both sides of the master and subsidiary slip planes. A number of rather uniform geometrical relationships was recognised as existing between fault and associated cleavage planes: 1) a parallelism between the cleavage-fault intersection lines and the fault rotational axes; 2) a rather constant cleavage-fault angle. These relationships seem to apply to the entire length of the studied faults.

The geometry, spatial distribution and displacement field associated with these cleavages suggest their development to occur as secondary, fault-propagation structures in front of the fault tip in response to the stress concentration in that region. Three distinct sequential processes may be inferred as having operated. (1) Typical mm- to cm-spaced pressure solution cleavage formed with slight stylolite and insoluble residue selvages in the distal tip zone of the advancing fault. (2) As the fault tip advanced, minor shear displacements integrated these nascent cleavages into an interconnecting network of centimetre to decimetre spaced lithons. Minor fibrous slickolites and small unfilled dilational fractures indicate a continuing left-lateral shear. (3) Eventually, the fault (zone) propagated across and through these structures.

According to this model, as "undeformed" carbonate rocks are progressively involved in the advancing region of influence of stress and elastic strain concentration, fault-related cleavages initiate as solution seams ahead of the propagating fault tip. As the fault propagates disrupting the former tip region, a new stress configuration induces part of the original solution planes to slip synthetically as regards the kinematics of the related fault.

O11 : 5B/25 : F4

The Role of Relay Zones Along Border Faults in Changes in Architecture of the Baikal Basin (Eastern Siberia)

C. Matton (cmatton@africamuseum.be)1,

A. Fronhoffs (afronhof@vub.ac.be)2,

J. Klerkx1,

S. Ceramicola (silvia.ceramicola@rug.ac.be)3 &

M. De Batist3

1 Department of Geology, Royal Museum for Central Africa, Tervuren, Belgium
2 Department of Geology, Free University of Brussels, Brussel, Belgium
3 Renard Centre of Marine Geology, University of Gent, Gent, Belgium

The structural development of the Baikal basin is mainly controlled along its western side by the main border faults. Embryonic sedimentary basins develop along relay zones between overlapping faults segments. Two examples are discussed: the Zavorotny area offshore and the Zama area on land.

The Zavorotny area corresponds to an offshore relay zone between two overlapping parallel-trending synthetic fault segments of the Baikalsky border fault. Detailed lake floor morphology and high resolution seismic profiling show that the normal downthrow is transferred from one fault segment to another. The area in between is broken up by approximately N-S tending faults, transverse to the major NNE-SSW oriented faults. Narrow basement blocks are tilted and create elongate partially infilled sedimentary basins of half-graben type with different polarity.

The Zama area corresponds to an onshore graben-like depression comprised between a fault splay constituted by the Zunduk fault in the south and the Primorsky fault in the north. Dominantly normal faults are clearly expressed in the present-day morphology and seem to have been active during the Late Cenozoic. The faults have a slightly different trend and diverge toward the northeast. The rectilinear Primorsky fault continues as border fault toward the north while the Zunduk fault slightly bends toward the east and plunges into the lake basin.

Active transtensive movements in sinistral sense along the transverse Academician Ridge are invoked to explain the changes in basin architecture. Main orientations of recently tilted basement blocks within the ridge and the fault patterns observed in the sedimentary cover on the southern part of the ridge confirm that lateral movements are currently affecting this major structure. This displacement results in clockwise rotation of the central basin relative to the northern basin and consequently in sinistral or left-stepping relay zones between the border faults. The major consequence of this movement is the present development of new basins in the relay zones between parallel-trending segments of the border faults, and structurally more complex basins when the fault segments diverge.

O11 : 5B/26 : F4

The Role of Arcuate Listric Versus Planar Normal Faults in the Kinematics of Deep Rift Basins (East Africa and South Siberia)

Jean Klerkx (jklerkx@africamuseum)1,

Boris Dehandschutter (bodehand@vub.ac.be),

Damien Delvaux (ddelvaux@africamuseum.be)1,

Michael M. Buslov (misha@uiggm.nsc.ru)2 &

Eugene M. Vysotsky

1 Africa Museum, B-3080 Tervuren, Belgium
2 United Institute of Geology, Geophysics and Mineralogy, SB-RAS, 630090 Novosibirsk-90, Russia

The architecture of continental rift basins is generally formed by the combination of individual segments with asymmetric "half-graben" to more symmetric "full-graben" morphology. These are combined in different ways to produce the characteristic geometry of the rift basin. There has been also speculations on the possible listric nature of the major border fault (break-away) of these individual graben units. Seismic reflection data show that the border faults of the Rukwa basin tend to be listric at depth but linear in map view, while intrabasinal faults are more curved in map view. By contrast, the border faults of the Tanganyika, Malawi, and Baikal rift basins are more planar.

The presence of listric faults has been recently recognized in the Lake Teletskoye basin, a very immature graben in the Altai belt in Gorny-Altai, South Siberia. This graben is 40 km long, 4-5 km wide and has a vertical throw of the top of basement of 2-3000 m. It lies in a particular extensional setting in the Altai transpressional belt, linking two strike-slip faults of opposing sense. It is occupied by a 300 m deep lake, but most of the major controlling faults are observable along the margins of the depression. Nice curved faults in plan view are observable in the morphology. The biggest one (Baskon fault) has 15 km long and the hanging wall was rotated towards the fault, suggesting its listric character. This structure is similar as for a very large landslide with a spoon-shaped rupture surface.

We want here to investigate the possible causes of these different morphologies of faulting: the controlling effect of pre-existing structures, the kinematic history, and their consequences for the architecture of extensional basins.

O11 : 5B/29 : F4

Tectonically Controlled Switching in a Giant Delta: The Palaeo-Amur, Neogene of Sakhalin, Russian Far East

David Macdonald (david.macdonald@casp.co.uk) &

Rachel Flecker (rf211@esc.cam.ac.uk)

CASP, Department of Earth Sciences, Univeristy of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

The Amur (the tenth largest river catchment on Earth) has been producing significant quantities of coarse sediment since at least the start of the Neogene. This paper describes the structural controls on sedimentation in its delta, which covered the island of Sakhalin, Russian Far East. Around the Oligocene-Miocene boundary, the north-south plate margin on the west side of the Sea of Okhotsk switched from westerly subduction to dextral strike-slip, which persists to the present day.

At the same time a large delta (radius of up 400 km), fed by the ancestral Amur, began to prograde from the west across the island, perpendicular to the strike of the fault system; this too, has persisted throughout the Neogene. Subsidence of the Mesozoic-Paleogene basement (forearc) was controlled, in part, by transtension, which appears to have created pathways through pre-existing topography. Later transpressional deformation created rising highs which deflected and funnelled coarse clastic sediments into several different sedimentary basins. These were then uplifted and exposed, giving an unrivalled opportunity to study the internal architecture of a large delta.

Despite the scale of the delta and the rapidity of sand sedimentation, the structurally controlled pathways funnelled all or part of the delta into counter-intuitive patterns, including causing it to prograde back towards the continent. Palaeo-highs also mean that in middle Miocene time there was close juxtaposition of coarse deltaic clastics and diatomite produced by high oceanic productivity. These deltaic sedimentation patterns differ markedly from classic delta models.

O11 : 5B/30 : F4

Structural Geology of the Kokchetav UHP Metamorphic Belt, North Kazakhstan

Kaneko Yoshiyuki (kaneko@geo.titech.ac.jp)1,

Ryo Anma (anma@arsia.geo.tsukuba.ac.jp)2,

Shigenori Maruyama (smaruyam@jupiter.geo.titech.ac.jp)1,

Hiroshi Yamamoto

(hyam@sci.kagoshima-u.ac.jp)3,

Masahiro Ishikawa (ishikawa@ed.ynu.ac.jp)4,

Masaru Terabayashi (tera@eng.kagawa-u.ac.jp)5,

Christopher Parkinson (chris@geo.titech.ac.jp)1,

Yoichi Nakajima (698g5067@mn.waseda.ac.jp)6,

Ikuo Katayama (katayama@geo.titech.ac.jp)1 &

Tsutomu Ohta (tohta@geo.titech.ac.jp)1

1 Dept.Earth & Planet. Sci., Tokyo Institute of Technology, Ookayama 2-12-1, Meguro, Tokyo, Japan
2 Institute of Geoscience, Tsukuba University, Ten-no dai 1-1-1, Tsukuba, Ibaraki, Japan
3 Dept. Geology, Kagoshima University, Kagoshima, Japan
4 Yokohama National University, Yokohama, Japan
5 Institute of Technology, Kagawa University, Saiwai-cho 1-1, Takamatsu, Japan
6 Institute of Earth Science, Waseda University, Tokyo, Japan

The Kokchetav ultra-high pressure (UHP) metamorphic rocks contain metamorphic microdiamond and coesite inclusions inside the rigid capsules like garnet and zircon crystals in the white-schist and carbonate rocks that encase eclogite bodies. The UHP rocks and associated HP rocks have been dated by SHRIMP and other methods to be 530 Ma (Jagoutz et al., 1989; Claoue-Long et al., 1991). The highest grade part may have reached 70 kbar at ca. 1000 degree C (Okamoto, 1997). Such UHP assemblage maintained in the rigid capsule suggests that the crustal rocks were once dragged downward with subducting slab to the depths of around 200 km, although primary UHP minerals are not well preserved due to late Barrovian overprint during exhumation and later collision events in the Caledonian time. We use structural data to argue the intrusion and exhumation mechanisms of the UHP rocks.

The Kokchetav HP and UHP rocks were subdivided into three units from the bottom to the top: Unit I consists of leptite and amphibolite, Unit II of pelitic and psammitic gneiss, with minor white schist and eclogite, and Unit III of orthogneiss, migmatite, amphibolite and eclogite. Metamorphic diamond and coesite are found only in Unit II where the metamorphic grade is the highest. The eclogite bodies in the Units II and III occur as isolated lenticular blocks or layers and could be interpreted as melange. Structures inside the HP units are sub-vertical trending roughly EW and dipping toward both direction. However, detailed mapping revealed that the same amphibolite sequences are exposed repeatedly and the sub-vertical structures in these units were formed due to later folding. The primary structure appears to be sub-horizontal and the total thickness for the Units I, II, and III is estimated at Å 2 km. These HP rocks are bounded by sub-horizontal faults along the top and bottom boundaries to thefeebly metamorphosed unit and low-P and high-T unit, respectively. Kinematic indicator shows top to the south sense of shear along the top, and top to the north displacement along the bottom boundaries. These shear senses, together with the metamorphic gradients suggest that thin sheets of UHP rocks were intruded obliquely toward the north into the feebly to unmetamorphosed rocks. Wedge extrusion must have been the most effective mechanism for the exhumation of the UHP rocks.

Jagoutz E, Shatsky VS & Pokhilenko NP, Workshop on Diamonds, 28th IGC Extended Abstracts, 32-35, (1989).

Claoue-Long JC, Sobolev NV, Shatsky VS & Sobolev AV, Geology, 19, 710-713, (1991).

Okamoto K, Tokyo Workshop on Kokchetav Deep-Drilling Project, 36-40, (1997).

O11 : 5B/31 : F4

Tectonic Subsidence Analysis of the Cretaceous-Tertiary Post-Rift Interval in the Northern North Sea Basin; Importance of Palaeobathymetric Control

Rune Kyrkjebø (Rune.Kyrkjebo@geol.uib.no)1,

Jan Inge Faleide (j.i.faleide@geologi.uio.no)2,

Roy Helge Gabrielsen (Roy.Gabrielsen@geol.uib.no)1,

Tomas Kjennerud (Tomas.Kjennerud@iku.sintef.no)2,

Gavin Gillmore (gavin.Gillmore@NENE.AC.UK)3 &

Willy Fjeldskaar (Willy.Fjeldskaar@rf.no)4

1 Geologisk Institutt, Universitet i Bergen, Allegt. 41, N-5007 Bergen, Norway
2 Institutt for Geologi, Universitetet i Oslo, N-0257 OSLO, Norway
3 IKU Petroleum Research, N-7034 Trondheim, Norway
4 Nene University College, Northampton NN2 7AL, UK
5 Rogaland Research, Postboks 2503, Ullandhaug, N-4004 Stavanger, Norway

The Cretaceous-Tertiary post-rift sequence in northern North Sea basin between 58°N - 62°N has been studied, using deep seismic reflection data, conventional seismic reflection data, and data from selected wells in different structural positions. The post-rift interval was divided into 16 seismic sequences, 6 in the Cretaceous age and 10 in the Tertiary. The seismic sequence framework was utilized in subsidence analysis along 4 regional deep seismic lines.

The Cretaceous-Tertiary post-rift development, which followed the late Jurassic-early Cretaceous NW-SE extension in the northern North Sea, is associated with thermal subsidence, passive infilling of the basin and hence burial of the rift topography. In addition analysis of reflection seismic data suggests that the post-rift sequence was affected by faulting associated with basin flank uplift and intraformational fault activity at different stratigraphic levels, probably driven by instabilities due to basin gradients. The subsidence analysis is used to detect events which are not easily observable on seismic data, and to study the interplay between basin flanks and the interior basin.

Palaeobathymetric data are important in subsidence analysis to constrain over/under estimations of the quantitative subsidence/uplift expiration and for general understanding of the depositional history. Previous studies have however, demonstrated that estimating "correct" palaeowaterdepth is difficult.

The present study, which is based on interpretation of micropalaeontological data gave robust quantitave estimates of palaeowaterdepths, and a qualitive consistent framework of shallowing/deepening trends by combining micropaleontological data with results from structural restoration. A "most likely" and a "maximum" scenario of waterdepth expiration is suggested for key-wells located on or close to the regional transects , and profiles of the waterdepth throughout the Cretaceous/Tertiary development was reconstructed at the regional transects. Subsidence analysis was performed, integrating waterdepth development, the seismic framework, lithological parametres, tectonic/isostatic parametres in the modeling tool BMT (Basin Modelling Toolbox, Rogaland Research).

Deepening/shallowing trends reflected in the waterdepth profiles which are believed to directly reflect tectonic events are observed in the Campanian (subsidence), Maastrichtian (uplift), Palaeocene (subsidence), Eocen (subsidence), late Oligocene-Miocene (uplift) and Pliocene (subsidence). Causal mechanisms to explain these events may be long-wave thermal basin scale processes, perhaps combined with short-wave deformation caused by for instance intra-plate stresses.

O11 : 5B/32 : F4

Evidence for Extension Perpendicular to Plate Convergence Along the Lesser Antilles Volcanic Arc

Nathalie Feuillet (feuillet@ipgp.jussieu.fr),

Paul Tapponnier (Tappon@ipgp.jussieu.fr),

Isabelle Manighetti (manig@ipgp.jussieu.fr) &

Jean-Claude Lépine (lepine@ipgp.jussieu.fr)

IPGP, Laboratoire de Tectonique, 4, place Jussieu, 75252 Paris, France

The lesser Antilles volcanic arc results from subduction of Atlantic seafloor under the Caribbean plate. During the historical period, several large earthquakes (M > 7, 1690, 1843, 1974) as well as smaller, more frequent events (M ¾ 5, 1851, 1897...) have brought significant damage to many of the islands. Shallow earthquake depths imply that the overriding Caribbean plate is deforming. Many events show strike-slip or normal fault-plane solutions, with roughly E-W striking nodal planes. We investigate the mechanisms of such deformation at several scales, and its relationship with the subduction process, to better assess seismic hazard.In the field, using both aerial and SPOT imagery, and topographic maps (1/25000), we have mapped active normal faults and open cracks that cut uplifted coral platforms on the limestone islands of the Guadeloupe archipelago. Using available marine geophysical data, we show that the faults mapped onland extend offshore and bound an E-W striking rift, the Marie-Galante rift, which follows the V-shaped trough between Marie-Galante and Grande-Terre. The progressive width and depth increase of this trough towards the east suggests that the rift propagated westwards. Relocation of M >= 2 earthquakes recorded by the regional seismic network between 1992 and 1996 and computation of several focal mechanisms confirms that the shallow seismicity is related to the normal faulting. The Marie-Galante rift is typical of other troughs tranverse to the northeastern edge of the Caribbean plate. We interpret such troughs, orthogonal to the arc, to result from extension perpendicular to plate convergence. Such extensional structures are progressively less developped southwards, implying southward propagation of the deformation.

O11 : 5B/33 : F4

Relationships between Cenozoic Volcanism and Tectonics in Central Andes

Nicolas Boudesseul (nicoboud@yahoo.com),

Jean de Breond d'Ars

(bremond@univ-rennes1.fr),

Peter R. Cobbold (cobbold@univ-rennes1.fr),

Denis Gapais (gapais@univ-rennes1.fr) &

Erwan Hallot (hallot@univ-rennes1.fr)

Géosciences Rennes UPR 4661 CNRS, Université de Rennes 1, 35042 Rennes Cedex France.

Magmatism provides an efficient heat transfer mechanism which may weaken the continental crust and induce a localisation of the deformation. Conversely, pre-existing tectonic structures like faults can facilitate the ascent of magmas. This work consists in determining the spatial and temporal relationships between recent volcanism and tectonics at the scale of Central Andes and at more local scales.

To do this, we have established a structural map of Central Andes by digitizing structural data and reporting all known volcanic landforms. Statistic orientations of faults were compared to directions of volcanic alignments calculated from density maps, Fry method and azimuth analysis.

The density maps show that volcanism is localised at the western edge of the volcanic arc during Cenozoic times. This can be explained either by an increasing dip of the subducted slab, or by a localisation of the deformation on this edge of the Cordillera. At the scale of the Central Andes, the volcanic arc can be divided into three linear segments which are correlated to the geometry of the subducted slab and to the main directions of the fault system: these are major westward-verging and eastward-verging thrust faults which limit the volcanic arc. On the southernmost part of Central Andes, directions of volcanic alignments and of tectonic structures are well correlated. A N40 trend corresponds to dextral strike-slip faults and to the bending of the volcanic arc at 23°S. A N130 trend of volcanoes in NW Argentina reflects a sinistral strike-slip fault system (see figure).

Volcanic and Structural map of the Southern Part of Central Andes.

Heavy Lines Represent N130 Trending Fault Systems; Grey Bands Underlined N40 Trending Fault Systems.

O11 : 5B/34 : F4

Active Tectonics in Central Sulawesi (Eastern Indonesia)

Olivier Bellier (bellier@geol.u-psud.fr)1,

Thierry Beaudouin1,

Michel Sébrier1,

Marc Massault1,

Didier Bourles2,

Diane Seward3 &

Michel Villeneuve4

1 CNRS ORSAYTERRE, Bat. 509, Univ. Paris-Sud, 91405 Orsay, France
2 CSNSM, Univ. Parsi-Sud, 91405 Orsay, France
3 Dept. Erdwissenschaften, ETH-Zurich, CH-8092, Switzerland
4 URA-CNRS 1208, Univ. de Provence, 13331 Marseille, France

In the framework of the GEODYSSEA program (an EU-ASEAN program of SE Asia GPS survey) a subprogram was set up to study the active tectonics of the Central Sulawesi fault system, in eastern Indonesia. This system consists of complex distributed left-lateral strike-slip fault zones located within a triple junction area between three tectonic plates: the Pacific, Indo-Australian, and Eurasian plates. Seismicity in Central Sulawesi documents shallow earthquakes located on and around the Central Sulawesi faults. To characterise the active fault trace geometry and to localise the high seismic potentiality zones, we analyse SPOT images covering Sulawesi. This analysis shows, from west to east, the occurrence of two major active fault zones: The NNW-trending Palu-Koro (PKF) and the WNW-trending Matano (MF) fault zones. The northernmost segments of the Palu-Koro zone bounds the western part of Palu basin. Study of the Palu-Koro segments evidences late Quaternary left-lateral geomorphologic feature offsets ranged between 50 and 3000 m (streams, alluvial fans, etc.), as well as faceted spurs, shutter ridges and 300-400 m high triangular and trapezoidal facets. The western edge of the Palu basin is controlled by a N-trending 2000-2500 m high escarpments. The significant dip-slip component of the Present-day faulting is confirmed by Quaternary uplifted marine and alluvial terraces that reach up to 200 m high. Thus, these geomorphologic characters evidence combined strike-slip and normal faulting for the Palu-Koro fault Present-day activity. This transtensional tectonic regime has been confirmed by the fault slip-vector analyses. We perform an along strike survey of the Sulawesi faults analysing and dating fault offsets to subsequently determine fault long-term slip-rate. Recent streams correlated to fan deposits of about 11,000 yr. (dating from 10Be measurements undertaken at the Tandetron AMS facility at Gif Sur Yvette, Fr.) are left-lateraly displaced. These streams offsets permited to calculate an about 30 mm/yr horizontal slip-rate for the PKF. Same order of results are obtained by datings performed using the 14C method (Tandetron AMS facility at Gif Sur Yvette) on the offset marine terraces. In addition, fission track analysis applied on granodiorites which form the escarpment constrained the vertical slip rate and shown an high vertical slip rate between 4 and 6 Ma. To define precisely the Late Quaternary seismic history, we excavated seven trenches across the North Palu-Koro fault zone which exposed paleosoils and colluvial wedges in fault contact with a poorly weathered alluvial bedrock. These provide evidence for 3 major seismic event (maximum Mw of about 7) during the last 2000 yr.

Session O11:5P

O11 : 5P/01 : PO

Phyllosilicate Preferred Orientation, X-Ray Diffraction Intensity Ratios, and Cleavage/Bedding Fissility Ratios as Criteria of Cleavage Development in Low-Grade Metamorphic Rocks

Gerd Jacob (jacob@geologie.uni-halle.de)

Institut für Geologische Wissenschaften, Domstraße 5, 06108 Halle, Germany

Phyllosilicate preferred orientation, X-ray diffraction intensity ratios, and c/b cleavage/bedding fissility ratios were determined on a series of metasediments from various regions. The purpose of these different investigations was to compare the results, and to verify the parameters of relative cleavage intensities, such as X-ray intensity ratios, and c/b cleavage/bedding fissility ratios, by means of the intensities of phyllosilicate preferred orientations. The relationship between the parameters of phyllosilicate orientation depends strongly on the lithology and the cleavage morphology of the measured samples. There are clear relationships between phyllosilicate preferred orientations and both X-ray diffraction intensity ratios and c/b fissility ratios. The degree of phyllosilicate preferred orientation increases with increasing c/b fissility ratios for samples with cleavage-parallel oriented phyllosilicates, while the degree of preferred orientation remains essentially unchanged with increasing fissility ratios for samples with bedding-parallel phyllosilicate orientations. This relationship is not unexpected, because samples with cleavage-parallel oriented phyllosilicates show stronger cleavage and, therefore, a better cleavage fissility. In contrast the bedding fissility is higher in samples with bedding-parallel preferred phyllosilicate orientations. The relationship between phyllosilicate preferred orientations and X-ray intensity ratios (I 10A(0.5°) C/B) is similar: the higher the maximum intensity ratios, so too, the higher the X-ray intensity ratios for samples with cleavage- and/or bedding-parallel preferred phyllosilicate orientations. In spite of the lower intensity values for samples with bedding-parallel phyllosilicates, the regression line is parallel to that for samples with cleavage-parallel or between cleavage and bedding oriented phyllosilicates. However, both regressions show considerable scatter. With additional investigations on the influence of lithology and microtextural effects (cleavage morphology) it should be possible to quantify the cleavage intensity by determination of the X-ray intensity ratios. On the other hand, the field-based measurements of fissility ratios (Durney & Kisch, 1994) would seem to be an excellent method to get a reasonable estimate of cleavage intensity.

Durney DW & Kisch HJ, AGSO J. Austral. Geol Geophys, 15, 257-295, (1994).

O11 : 5P/02 : PO

Multiple States of Stress Determined from Calcite Twins in Forelands of the Taiwanese and Pyrenean Belts: Stress Perturbations or Polyphase Tectonism?

Muriel Rocher (rocher@lgs.jussieu.fr),

Olivier Lacombe (lacombe@lgs.jussieu.fr) &

Jacques Angelier (ja@lgs.jussieu.fr)

Univ P. & M. Curie, Lab. Tectoniqu Quantitative, Tr 26-25, E1, 4 pl. Jussieu, France

In forelands of collision belts, faults are few and tectonic structures are smooth or buried under post-orogenic sediments. In this context, calcite twins appear to be a powerful indicator and recorder of paleostresses responsible for the deformation during the collision. We first studied the foreland of a still active collision belt, and then applied the same method to the north pyrenean foreland, buried by post-Oligocene sediments. Taiwan is undergoing a N105 compression since the Pliocene. In the SW foreland, we analysed 13 samples, extracted from pleistocene reefal limestones situated on the top of NNE-trending anticlines. We reconstituted a syn-folding NW-SE compression and a NW-SE extension, then an ENE compression, and local events.In the north pyrenean foreland, we studied 11 samples extracted from paleocene to miocene limestones, affected by N110 to N170 trending anticlines. We identified a syn-folding NNE compression sychronous with a NNE extension, followed by a perpendicular extension, then an earlier NNW compression followed by a perpendicular extension. Locally, where folds parallelise to NNW major faults, we observed an early ENE compression. These numerous states of stress are also identified using fault slip data where available. Relative dating of these events was performed by local field observations. Despite the apparent complex tectonic history of these forelands, these numerous states of stress can be explained by few events. In SW Taiwan, the NW-SE and ENE compressions are interpreted as deviations of the regional N105 compression along NNE trending anticlines. In the north pyrenean foreland, the NNE and NNW compressions are both consistent with the nearly N-S convergence between Europe and Africa. The ENE compression is interpreted as a local deviation of the NNE compression along NNW major faults.The NW-SE extension in Taiwan and the NNE extension in Pyrenees, perpendicular to fold axes, probably correspond to stretching at anticline hinges. The extensions perpendicular to compressions are interpreted as due to diapirism or post-collisional stress relaxation. Thus, the multiple states of stress usually reconstituted are not an artefact due to the method of analysis, but represent the variation through space and time of a single compressional event, undergoing deviations due to structural inheritance, or change in relative stress magnitude inducing compression-related extension.

O11 : 5P/03 : PO

Microstructures Related to Dynamic Overprint in Statically Recrystallized Carrara Marble (Alpi Apuane, NW Italy)

Nils Oesterling (oesterling@ubaclu.unibas.ch)1,

Renee Heilbronner &

Giancarlo Molli2

1 Bernoullistr. 32, Geologisch-Palaeontologisches Institutl, Universität Basel, Switzerland
2 Dipartimento Scienze della Terra, Università di Pisa, I - 56126 Pisa, Italy

In this study the deformational behaviour of calcite under different natural conditions (P, T, strain rate) is investigated. Carrara marble is choosen because its experimental behaviour is well known (e.g. Schmid et al. (1980)); the aim of this study is a comparision of natural with experimental microstructures.

Carrara marble belongs to a series of mesozoic carbonate platform sediments deposited on the former Italy-Adriatic continental margin (Carmignani & Kligfield, 1990). The Alpi Apuane are located in the north-western Apennines and represent a large tectonic window (Carmignani et al., 1994). During the main deformational phase (late Oligocene - early Miocene), Triassic to Cretaceous sediments and the underlying paleozoic basement were folded isoclinally under greenschist facies conditions. After the deformation the whole region was heated statically, which caused the annealing of the marble. Along an E-W cross-section the annealing temperature increases from 380°C in the eastern part of the Alpi Apuane to 430°C in the West (Molli & Giorgetti unpublished).

In gerneral Carrara marble is believed to be completely annealed. However, there are several post-thermal-peak shear zones, which were active after the main deformation phase (Molli & Meccheri, 1997 and Molli unpublished). In contrast to the typical "foam structures" of the main body of Carrara marble, these shear zones show microstructures indicative of dynamic recrystallization that is clearly associated with the late overprinting deformation.

As a start we compare the microstructures of the annealed marble, which we consider to be the relative "starting material", with the microstructures of the late shear zones. We focus on shear zones located in the western part of the Alpi Apuane. Orientation imaging, grain size and shape analysis (CIP, Panozzo Heilbronner & Pauli (1993), StripStar Heilbronner & Bruhn (1998), etc) are employed to analyse the microstructures. The transition from staically annealed to dynamically overprinted marble can be observed within a single sample. Apart from the grain size reduction towards the shear zones (150 µm to 50 - 20 µm) a shape preferred orientation (SPO) oblique to the main stretching direction is recognizable in the more intensively deformed regions. A first set of CIP orientation images reveals a strong preferred orientation of c-axis (CPO). It can be demonstrated that given deformation and recrystallisation mechanisms produce characteristic SPOs and CPOs. The shear sense of the analysed shear zones as determined by SPO and CPO is top to the northeast with complex kinematic pattern.

Carmignani L, Decandia FA, Fantozzi PL, Lazzarotto A, Liotta D, Meccheri M, Tectonophysics, 138, 295 - 315, (1994).

Carmignani L & Kligfield R, Tectonics, 9, 1275 - 1303, (1990).

Heilbronner R & Bruhn D, J. Structural Geology, 20, 695 - 705, (1998).

Panozzo Heilbronner R & Pauli C, J. Structural Geology, 15, 369 - 382, (1993).

Molli G & Meccheri M, Alpine Meeting Oropa 29, Vol Abst, Quaderni di Geodinamica Alpina e Quaternaria, 4, 202, (1997).

Schmid SM, Paterson MS, Boland JN, Tectonophysics, 65, 245 - 280, (1980).

O11 : 5P/04 : PO

The Significance of Pre-Deformational Crystallographic Preferred Orientations for the Deformation of Dolomite Rocks

Bernd Leiss (bleiss1@gwdg.de) &

Klaus Weber (kweber@gwdg.de)

IGDL-Univ. of Göttingen, Goldschmidtstr. 3, 37077 Göttingen, Germany

In addition to other deformation related fabric elements, the type, the symmetry and the intensity of crystallographic preferred orientations (textures) can reveal information on deformation mechanisms, deformation history and rheological behaviour of rocks. For a proper interpretation of deformation fabrics, the knowledge of textures of the starting material is essential. No systematic investigations on the existence, the development, the types and the significance of textures in undeformed carbonate rocks have been carried out yet.

The dolomitic carbonate platform of the Southern Margin Zone of the Damara Orogen in Namiba has been deformed under greenschist facies metamorphic conditions. Generally, the dolomite rocks exhibit mylonitic fabrics and clearly developed deformation textures. In the southern areas very weakly to undeformed starting material can be found. Of special interest for this study are dolomite rocks with alternating coarse- and fine-grained layers which show no macroscopically visible deformation.

The coarse-grained layers are about 1 to 15 mm in height and consist of an upper and lower row of palisade-like arranged crystals. Cathodoluminescence microscopy (CL) reveals a growth sector zoning of the crystals parallel to the layer. The grains of the matrix are 0.01 to 0.1 mm in size and show straight to curved grain boundaries. CL reveals grain cores with a round to oval shape exhibiting concentric structures. A very weak grain shape preferred orientation indicates a weak deformation. Quantitative x-ray texture determinations clearly show different textures for the coarse- and the fine-grained layers. In the fine-grained layers a distinct c-axis maximum is developed and oriented close to the normal of the layering. The corresponding a-axis maximum is oriented nearly parallel to the general stretching lineation. These textural features are typical for deformation fabrics of dolomite (Leiss & Helming 1996). In contrast, the coarse-grained layers show a strong regular c-axis girdle oriented parallel to the layering and an a-axis maximum perpendicular to the layering. The rhombohedral planes are distributed on small circles. A texture with such an orientation in correlated with the observed grain fabric can be best explained by an oriented crystal growth. Crystals growth occured perpendicular to the prism planes in the direction of the c-axes.

The results show different texture types and different orientations of the textures with respect to the layering. For the coarse-grained layer, the pre-deformational texture leads to a rheological hardening. For the fine-grained layer, the syn-deformational texture development leads to a rheological softening. This leads to an increased rheological contrast which has to be considered for the interpretation of a more developed deformation fabric.

Leiss B & Helming K, Proceed. Eleventh Int. Conf. Textures of Materials, 1275-1280, (1996).

O11 : 5P/05 : PO

The Evolution of Crystallographic Preferred Orientations and Microstructures within Naturally Deformed Eclogite Facies Rocks

Walter Kurz (walter.kurz@kfunigraz.ac.at)1,

Wolfgang Unzog (unzog@kfunigraz.ac.at)1 &

Franz Neubauer (neubauer@sbg.ac.at)2

1 Institut für Geologie & Paläontologie, Karl-Franzens-Universität Graz, Heinrichstr. 26, A- 8010 Graz
2 Institut für Geologie & Paläontologie, Paris-Lodron-Universität Salzburg, Hellbrunnerstr. 34, A- 5020 Salzburg

Eclogites of different structural and tectonic settings within the Eastern and Western Alps have been investigated in order to get information on the evolution of Crystallographic Preferred Orientations (CPOs) of omphacite and garnet. In some cases, the textures and microfabrics of eclogites allow the establishment of the eclogitic and post-eclogitic deformation histories from burial by subduction to subsequent exhumation. Several types of eclogites have been affected by different tectonometamorphic conditions.

Within several settings the eclogite fabrics show continuous transitions from coarse-grained eclogites with relics of gabbroic fabrics to fine grained eclogitic mylonites. Within coarse grained low-temperature eclogites the omphacites show the development of subgrains and CPO patterns that are interpreted to be related to a shape preferred orientation subparallel to the penetrative foliation. At advanced stages of deformation, omphacite shows fabrics that are chararacteristic for subgrain rotation recrystallization. Very often core and mantle fabrics can be recognized. Within fine grained eclogite mylonites the c- axes of omphacite show cluster distributions near the X-axis of the finite strain ellipsoid.

Generally, omphacite shows fabrics that are characteristic for dislocation glide within LT eclogites, and fabrics that are typical for dislocations creep within HT eclogites. The development of CPOs within LT eclogites (at temperatures below 550°C) seems to be primarily related to the shape preferred orientation and rigid body rotation. Within HT eclogites (at temperatures of ca. 600° C) the CPO evolution is primarily related to crystal plasticity.

O11 : 5P/06 : PO

Quartz Texture Analysis in Granulite Rocks Using Neutron Time-of-Flight Diffraction: Example from Pan-African Intrusions in Southern Madagascar

Sanna Mrkwiczka

(mrkwiczk@mail.uni-freiburg.de)1,

Djordje Grujic (grujic@sun2.ruf.uni-freiburg.de)1 &

Klaus Ullemeyer (kulleme@gwdg.de)2

1 Geologisches Institut, Albert-Ludwigs-Universität Freiburg, Germany
2 Institut für Geologie und Dynamik der Lithosphäre, Universität Göttingen, and Joint Institute of Nuclear Research - Frank Laboratory for Nuclear Research, Dubna, Germany

The aim of this study is to unravel the tectonic setting of "stratoid" granitoid intrusions in the southern Madagascar (Nédélec et al., 1995). The controversy exist whether the intrusions are sin- or post-tectonic; whether they were emplaced during collision (presumed top-to-the-east thrusting) or during a post-orogenic collapse (i.e. E-W extension and normal faulting). Unfortunately, the regional kinematic history has not been ascertained yet owing to the scarcity of reliable shear sense indicators. In order to obtain kinematic information we carried out microfabric analyses of quartzites and sheared granitoids in the roof of three intrusions. Because of coarse-grained, mostly heterogeneous and polyphase rocks the textures could not be analysed by classical methods like optical measurements and X-ray diffraction. To obtain representative textures bulk sample analyses were carried out on the basis of neutron diffraction. The Time-of-Flight (TOF) technique was applied, where the d-patterns are measured. We performed the texture measurements at the reactor IBR-2 (Dubna, Russia), a research facility with a pulsed neutron source. The new multidetector diffractometer (SKAT) and the GEOTOF computer software allow simultaneous measurements of several lattice patterns and the calculation of pole figures, respectively (Ullemeyer et al., 1998). Estimated from the c- and a-axes distribution patterns two types of deformation geometry were identified: (a) non-coaxial vertical flattening with roughly top-to-the-west sense of shear. In the corresponding outcrops S-type fabrics was; (b) approximately constrictional strain with uncertain non-coaxial component. In the field L-type fabrics was found. Field observations suggest that these two types of microfabrics might be the result of strain partitioning. Most quartz textures are indicative of deformation at medium-grade metamorphic conditions. In one sample, however, the texture is typical for low temperature deformation which is consistent with the field observations. Since the quartz-texture kinematic indicators are compatible with other medium- to high-grade microfabric kinematic indicators, we suggest that the interpreted deformation geometry and kinematics are representative for the tectonic setting during emplacement of the granitoids. An important vertical shortening and roughly E-W extension suggest intrusions during orogen collapse. However, still under high temperature conditions, the stratoid granitoids of Madagascar are folded with axes striking approximately N-S. This makes debatable the interpretation of intrusions as post-tectonic.

Nédélec A, Stephens WE & Fallick AE, J. Petrol, 36, 1367-1391, (1995).

Ullemeyer K et al, Nuclear Instr. Meth, A 412, 80-88, (1998).

O11 : 5P/07 : PO

Non-Rotated Porphyroblasts in a Folded Metapelite, Eastern Finland

Paul Evins (pevins@babel.oulu.fi) &

Kauko Laajoki (laajoki@babel.oulu.fi)

University of Oulu, Department of Geology, Linnanmaa, 90570, Oulu, Finland

The notion of porphyroblast non-rotation has recently resurfaced (Kraus et al., 1997). One of the most convincing arguments put forth by non-rotationalists has been that of consistently parallel inclusion trails within porphyroblasts across a later folded structure (Fyson, 1980), (Steinhardt, 1989) and (Johnson, 1990). Previous evidence for this has been criticized for its 1) general lack of orientation data, 2) almost complete lack of three-dimensional data, 3) confinement of measurements to a single fold limb and 4) high variablity in Si orientations (Passchier et al., 1992). The study presented here addresses all four of the above criticisms.

The orientations of 750 garnet porphyroblast early internal foliations (Si) were measured from a Precambrian metapelite outcrop in Posio, Finland. They varied only slightly with respect to geographical coordinates and their enveloping foliation (Se) which is axial planar to asymmetrically folded bedding (S0) in the metapelite. Furthermore, this variation is not systematic with respect to the later (Se) axial planar folding, but instead represents warping and S4 crenulation of Si before the growth of garnet.

At this location, garnet porphyroblasts and their pressure shadows did not rotate during simple-shear dominated folding. Classical theories of rotation of high viscosity materials in lower viscosity matrices do not apply to the Posio outcrop. Instead, deformation partitioning has preserved the orientation of older foliations in higher viscosity material during rotation and development of a new foliation in a lower viscosity matrix. Finally, rigid porphyroblasts may not rotate with respect to geographic coordinates within rheologically similar layers within certain structural domains. The authors recognize the validity of previous studies which aptly defend porphyroblast rotation based on systematic variability of Si across a fold, chaotic Si, and millipede and spiral Si. However, we suggest that porphyroblast non-rotation can and does occur during deformation involving significant components of simple shear.

Fyson WK, Can. J. Ear. Sci., 17, 325-332, (1980).

Johnson SE, J. Struc. Geol., 12, 727-746, (1990).

Kraus J, Williams PF, J. Struc. Geol., 20, 61-77, (1997).

Passchier CW, Trouw RAJ, Zwart HJ & Vissers, RLM, J. Met. Geol., 10, 283-294, (1992).

Steinhardt CK, Tectonophysics, 158, 127-140, (1989).

O11 : 5P/08 : PO

Can the Effects of Shear Heating be Identified by Fission Track Analyses? ­ Projected Fieldwork and Experiments

Maurice Brunel (brunel@dstu.univ-montp2.fr)1 &

Anke S. Wendt (a.wendt@ucl.ac.uk)2

1 Universite Montpellier 2, Laboratoire GGP UMR 5567, Place Eugene Bataillon, 34095, Montpellier, France
2 University College London, Geological Sciences, Rock & Ice Physics, Gower Street, London WC1E 6BT, England

Fission track lengths and densities depend mainly on temperature and potentially on the deformation history of a rock. The influence of temperature changes on fission tracks is well established while the influence of deformation on fission tracks is almost unknown. For studying the thermodynamic behaviour of fission tracks in a non-hydrostatical environment, we propose the analyse of fission tracks lengths and densities through natural low grade shear systems and to calibrate the natural observation by deformation experiments. This will contribute detailed information on shear heating and on fission track dating.

The natural case study will be concentrated on the low grade (lower than 300 degree C) granitic shear zones in the Aar Massif in the Swiss Alps. The comparison of fission track lengths and densities in zircons and apatites between the undeformed and the deformed material at several crustal levels (300 m to nearly 4000 m) will allow to construct maps of fission track lengths and densities across the shear systems.

The calibration of these maps to the influence of temperature, pressure and differential stress will be provided by deforming large, irradiated single crystals of zircons and apatites at temperatures from 50 degree to 550° and constant high pressure (400 MPa). The experiments will be performed in a gas confining medium using a triaxial deformation apparatus under constant load (creep experiments). During the creep experiments, the differential stress will be applied with an angle of 45 degree to the main slip plane of zircon and of apatite. Irradiation should be strong enough to strain soften both minerals before deformation.

O11 : 5P/09 : PO

North-South Trending Fracture Zones in the Sunnfjord Region, Western Norway

Silje Stören Berg (Silje.Berg@ngu.no)1,

Alvar Braathen (Alvar.Braathen@ngu.no)2 &

Agust Gudmundsson (Agust.Gudmundsson@geol.uib.no)1

1 Geological Institute, University of Bergen, Allegt.41, N-5007 Bergen, Noway
2 Geological Survey of Norway, pb 3006 Lade, N-7002 Trondheim, Norway

Fracture permeability, which requires opening along fracture planes, is strongly affected by the existence of secondary mineralization and fault breccia. As a consequence, development of fractures is crucial in evaluating fracture permeability. Also, connectivity must be considered in a permeability analysis, in order to understand the importance of dense networks of smaller fractures.

Detailed field observations of 5 fracture zones, appearing N-S trending lineaments in the Sunnfjord region of Western Norway have been carried out. The 6-16 km long lineaments are partly situated in the Western Gneiss Region, the Precambrian basement, and partly in Caledonian allochtonous basement and cover units. Fracture measurements were made along 48 profiles, using two methods: (1) the fixed circle method and (2) the traverse method. The fixed circle method involves registration of the orientation and number of fractures intersecting an 80 cm wide circle every 5 meters along 200 m long profiles. In the traverse method all fractures along 0.15-40 m long profiles are measured, providing registration of fracture clustering, orientation, length, width, geometry, connectivity, termination, displacement and associated secondary mineralization.

The analysis shows systematic patterns of fracture appearance, distribution, and orientation within and outside the lineaments. Based on their orientation, two populations of fractures are defined within the lineaments. The dominant group of lineament-related fractures is joints (mode I cracks) striking NNW-SSE. These constitute population 1. Population 2 consists of shear fractures, dominated by NNE-SSW striking normal faults. Some of the population 2 fractures in the central zone of the lineaments contain fault breccia.

The two populations are believed to represent two deformation systems of different intensities and orientations of strain axes. There are indications that the first system led to formation of tension gashes (population 1) constituting the initial fracture set associated with master joint zones. The second system generated shear fractures and breccias, thereby changing the status of the observed lineaments into immature fault zones.

The strain history for the lineaments has implications for the fracture permeability, both because of the formation of breccia and enhanced fracturing. There is a remarkable increase in fracture intensities towards the central zones of the lineaments. This change in intensity, together with changes in other fracture parameters, strongly affects the overall permability of the lineaments. The results also suggest that there may be a zonal variation in the hydromechanical properties of the lineaments.

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Analysis of Large-Scale Lineaments in the Dale Region, Western Norway

Helena Amaral (agust.gudmundsson@geol.uib.no)

Geological Institute, University of Bergen, N-5007 Bergen, Norway

One of the most striking geological features of the western part of Norway is the regular system of lineaments, easily observed on aerial photographs and Landsat images (Ramberg et al., 1977; Gabrielsen and Ramberg, 1979). While the large-scale fracture pattern is relatively well known, few detailed field studies have been made of these lineaments. This abstract reports the preliminary results of a study of exceptionally well-exposed fractures in the Dale area, just east of the city of Bergen. This area shows a regular pattern of fractures dissecting mainly Caledonian gneisses, highly deformed supracrustal rocks (Precambrian to Caledonian in age) and strongly altered sedimentary rocks (Cambrian to Silurian in age). The study area includes the boundary between the Major Bergen Arc, a ductile shear belt containing Ordovician ophiolitic rocks, in the SW, and the Bergsdalen Nappes in the NE, and shows the variation in fracture style and intensity between these two structural elements. There are two main fracture sets in the Dale region: one set has mainly NNW-SSE trending fractures, the other mainly NE-SW trending fractures. Large-scale and small-scale fjords and valleys in the area commonly follow these fracture trends, suggesting that many fjords and valleys are, at least partly, tectonic in origin. While most of the lineaments in both sets appear to be very old faults and fractures, there is evidence that some of these lineaments have been reactivated in Holocene due to postglacial crustal doming. It can be shown that the vertical uplift associated with postglacial rebound and crustal doming in this part of Norway generated shear stresses that were sufficiently large to reactivate old, but favourably orientated, faults within these two major sets (Gudmundsson, 1998). Data from other parts of Norway indicate that sets similar to these occur over wide areas of western Norway, which has implications not only for the tectonic evolution of the area but also for the hydromechanical properties of the crust.

Gabrielsen, RH & Ramberg, IB, Proc. Norw. Sea Symp. Tromsø, NSS/23, 1-28, (1979).

Ramberg, IB, Gabrielsen, RH, Larsen, BT & Solli, A, Geol. Mijnbouw, 56, 295-310, (1977).

Gudmundsson, A, Tectonophysics, submitted, (1998).

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Fault Pattern Analysis of the Northeastern Part of the Cretaceous Kyongsang Basin, Korea

Byung-Joo Lee (bjlee@rock25t.kigam.re.kr) &

Jae-Ha Hwang (jhhwnag@rock25t.kigam.re.kr)

Dept. of Geology, KIGAM, Yusong p.o.box 111, S. Korea

The study area, part of the Euisong block of the Cretaceous Kyongsang Basin, consists of the Sagog and Chunsan Formations, the Yuchon Group with volcanic rocks in ascending order, and late Cretaceous granitoids. The sedimentary and volcanic strata are partly covered by Tertiary strata. The faults of the study area are categorized into two groups; the WNW-ESE trending, sinistral strike-slip faults represented by the Kaum fault and the dextral strike-slip faults of the NNE-SSW direction. The latter group is very important in both academic and engineering aspects because is included therein a set of major faults including the Yangsan fault. The wide open folds with axes of E-W direction are common and are cut by the WNW-ESE sinistral strike-slip fault. The paleostress field related to the Kaum fault, indicates that <sigma> 1 direction is approximately E-W. Another wide open fold in the eastern part of the study area, with the axis of N-S direction, was probably formed also by the E-W compression by which the Kaum fault was developed. The dextral strike-slip fault of NNE-SSW direction was made by NE-SW compression direction(<sigma> 1) and cut the Kaum fault. Deformation sequence of the area is summarized as the following; (1) During the late Cretaceous time, N-S compression made the wide open folds which have E-W axis. (2) Compression direction was changed, being nearly E-W direction, which formed the WNW-ESE trending sinistral strike-slip fault as well as the wide open fold with a N-S axis. (3) These deformation events were followed by the activation of a dextral strike-slip fault of the NNE-SSW direction, caused by the predominance of NE-SW compression towards the Tertiary.

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SW-NE Seismic Trends on the Unstable Flank of Mt. Etna Related to Dyke-Associated Volcanism

Alex Ciocanel (Alex.Ciocanel@Brunel.ac.uk) &

Derek Rust (Derek.Rust@Brunel.ac.uk)

Brunel University, Department of Geology and Earth Science, Uxbridge, Middlesex, UB8 3PH, UK

A linked system of active faults and associated cinder cones (the Ragalna fault system), extending down slope in a general SSW direction on the southern flank of Mt. Etna, has been suggested as one boundary to a proposed large scale collapse instability sector affecting the eastern and southern flanks of the volcano. However, this system has received only preliminary study and is situated outside the established geodetic monitoring network on Mt. Etna.

Analysis of published epicentral data for the volcano shows a correlation between eruptions associated with dykeing, principally flank eruptions, and seismicity patterns of the Ragalna sector. For example, a clustering of seismic events occurred before, during and after the 1974 Mt. De Fiore eruption associated with a WSW-ENE magma body intrusion beneath the volcano, in the vicinity of the town of Ragalna. Similarly, the 1981 fissure eruption, although occurring on the northern flank of the volcano was related with seismicity on the SW-NE fault trend, and this structural zone was again highlighted by events associated with the 1983 and 1989 eruptions. The most recent flank eruption (1991, 1993), which was largely a result of magma transfer from depth to the volcanic pile along the SE Rift fault zone, has lead to the conjugate SW-NE Ragalna fault system to be activated in a right-lateral mode.

These relationships highlight the importance of the system in accommodating stress within the volcanic edifice itself, rather than operating as a regional tectonic feature exhibiting a more random pattern of activity, and therefore reinforcing the interpretation of the system as a boundary to a large collapsing sector made unstable by a combination of magma pressure and gravitational stress.

By comparing the parameters of those eruptive events, it can be hypothesised that particular features of flank or lateral eruptions of Mt. Etna depend on the involved structural trends, in which nonetheless the Ragalna fault system plays an important part. Further analysis of these relationships coupled with geodetic monitoring of the Ragalna fault system may provide new insights of the geodynamic behaviour of the volcano.

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New Trigonometrical Method for Calculating Total Slip of Fault

Jae ha Hwang (hajh@rock25t.kigam.re.kr) &

Dae-ha Lee (renee@rock25t.kigam.re.kr)

Korea Institute of Geology, Mining and Materials

We propose a new trigonometrical method for calculating total slip (T) of faulting. In general figure of the fault, the angle between the strike slip vector and the dip slip vector shows orthogonal, the magnitude of the strike and dip slip vectors L and S, respectively, are defined as

T = L x (cos r + sin r) / cos r or T = S x (cos r + sin r) / sin r Where r and f are rake of the fault striation, dip angle of the fault respectively.

As presenting a set of three equations, the strike slip can be obtained from recognized strike separation of a fault.

L = K x tan sf x sin a / (tan r x sin g + tan sf x sin a)

L = K x tan sf x sin a / (tan sf x sin a - tan r x sin g)

L = K x tan sf x sin a / (tan r x sin g - tan sf x sin a)

Another set of three equations is derived for calculation of the dip slip when we can determine the dip separation of a fault.

S = (AM x sin b - i / cos i) x tan r x sin g / (tan r x sin g + tan sf x sin a)

S = (AM x sin (b + i) x tan r x sin g / sin (90 + i) x (tan sf x sin a - tan r x sin g)

S = (AM x sin (b + i) x tan r x sin g / sin (90 + i) x (tan r x sin g - tan sf x sin a)

Where K is strike separation of the fault and a is apparent dip angle of the bedding plane on the orthogonal plane to the fault. g is angle between fault trace and bedding trace on the vertical plane included dip direction of the fault. i is angle between the fault trace on the horizontal surface and the bedding trace on the fault plane. sf is angle between the fault trace and the bedding trace on the horizontal surface. AM is apparent dip separation. b is angle between the fault trace on the horizontal surface and a trace of arbitrary vertical cutting plane on the fault plane.

The equations proposed can be adapted independently for the corresponding relationship between an index planar structure and a direction of the fault striation but hardly fit to the polyphase faulting.

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The Energy Balance and the "Choice" of Folding Mode

Eric Mercier

(Eric.Mercier@chimie.univ-nantes.fr) &

Fatima Outtani

1Université de Nantes, Planétologie et Géodynamique, France

In external zones of the mountains belts, several kinds of fault-related folds have been described (mainly: Fault-bend fold and Fault-propagation fold: Mercier et al, 1997). We suppose that the mode of deformation that is favoured in the nature is this one that consumes less energy (Goguel, 1948; Elliot, 1976; Mitra & Boyer, 1986).

But, when a fold begins to grow, the "nature" does not "know", in advance, if a lot of energy will be necessary to end it's growth. In fact the "choice" between different mode of folding is posed at each instant (at each increment of deformation). We have therefore calculated the total energy involved in emplacing a fault-related fold for each increment of deformation. Consequently, we have need to know the movement of each element of the fold ("particle") during an increment of deformation. For this purpose, we have generalised the analysis of Hardy & Poblet (1995).

For each increment of deformation, energy consumed by a ramp-related fold is equal to the sum (1) of the work involved in propagating the ramp, (2) of the work done against gravity, (3) of the work of the basal sliding, and (4) of the work of the internal deformation.

These different terms depend notably on two mechanical parameters: the basal shear stress and the yield or flow stress for deforming rock. We show that, in order that a ramp related fold could grow, it is necessary that these two parameters decrease during the deformation. This allows us to suppose that before the deformation, rocks undergo a "hardening" (probably related to the Layer Parallel Shortening), then they undergo a "softening" during the folding.

Furthermore, we show that, a very rapid rock "softening" and a basal shear stress clearly higher than the flow stress for deforming rock, promote the growth of Fault-propagation fold. In conclusion, the geological credibility of these hypotheses is discussed.

Elliot D, Phil. Trans. R. Soc. London, A283, 289-312, (1976).

Goguel J, Mem. Carte Geol. France, 1-530, (1948).

Hardy S & Poblet J, Marine and Petroleun Geology, 12, 165-176, (1995).

Mercier Eet al, J. Struc. Geol, 19, 185-193, (1997).

Mitra G & Boyer SE, J. Struc. Geol, 8, 291-304, (1986).

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Transformation of the Porous Network in Chalk Close to the Normal Fault Planes: Clues Provided

M. Coulon (michel.coulon@univ-reims.fr)1,

C. Schroeder (cschroed@lgih.ulg.ac.be)2,

P. Gaviglio (patrick.gaviglio@univ-fcomte.fr)3,

F. Bergerat (francoise.bergerat@lgs.jussieu.fr)4,

C. Dubois (claude.dubois@univ-fcomte.fr)3 &

S. Vandycke (vandycke@fpms. ac.be)5

1 Sciences de la Terre, Univ. Reims, 51100 Reims, France
2 LGIH, Université de Liege, 4000 Liege, Belgium
3 Geosciences, 5 Microanalyses Nucléaires, Univ. Franche-Comté, Besançon, France
4 Tectonique quantitative, Univ. P & M. Curie, Paris, France
5 Geologie Fondamentale et Appliqueé, Faculté Polytechnique, Mons, Belgium

Matrix deformations are closely associated with normal faulting in the Campanian chalk of the Mons basin (Belgium). Pressure solution and crystallization are responsible for deep modifications of the porous network (porosity ranging from 41 to 31%). The experimental capillary rise provides a good insight of the decrease in porosity close to the fault plane. The height and the volume of absorbed water are in a linear function of the square root of time : the slopes are lower against the fault plane, where the porosity is low. The variation of the kh product clearly points out the decrease in hydraulic conductivity despite the increase in capillary succion. The analysis of the propagation of elastic waves in the material reveals the existence of cracks associated with the porous network along the fault plane where the block forms the footwall and a cementation of the intergranular joints where the block forms the hanging wall (figure).

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Study of Pico del Grado Fold: A Finite Element Modelling

Antonio Rivas (rosatej@eucmax.sim.ucm.es),

Rosa Tejero,

Monica Fernandez &

Alejandro Laurrieta

Departamento de Geodinamica, Fac. CC. Geologicas, Universidad Complutense, 28040, Madrid, Spain

Alpine Orogeny (Upper Cretaceous-Tertiary) deformation in central Iberian Peninsula results of new generated structures and the previous structures reactivation, latest present in the Variscan basement (Upper Devonian - Middle Carboniferous). Thus the Mesozoic-Paleogene cover deformation is controlled by the reactivation of inherited structures and by the new ones.

A period of intensive faulting took place at the ending of Hercynian Orogeny (Upper Carboniferous) event known as Late Hercynian (Parga, 1969; Arthaud, and Matte, 1975) controlling the Mesozoic sedimentary record. Alpine tectonic evolution can be divided into two main episodes: 1.- Eocene to Early Miocene. The mean shortening direction was N10ºE; 2.- Middle Miocene-Quaternary. The mean shortening direction was N165ºE (De Vicente et al., 1994; De Vicente et al., 1996). One of this structures is the Pico del Grado monoclinal fold. The fold trends NE - SW and involved a sequence of carbonatic sediments. This package of layers keeps constant thickness along its limbs. This structure is located in the link zone of two Alpine chain: the Spanish Central System and the Iberian Range. The basement of this zone is constituted by igneous an metamorphic rocks, whose ages range from Cambrian to Middle Devonian, and the cover includes formations ranging from Permian to Upper Cretaceous in age.

Finite element method allow us, through rocks rheological properties, to model the mechanical behaviour of basement and cover involved in this fold. For thus a profile and a floor view of this structure has been considered. The model consists of three different blocks: two represent a basement of quartz-feldespatic nature (Cambrian to Middle Devonian formations), separated by a discontinuity related to a basement inferred fault. The third corresponds to the cover (Triassic to Campanian formations), and in it several discontinuity planes parallel to bedding have been established. Several stress distributions are recorded in the models, ones shown high stress levels concentrated mainly around hinge zones in the cover whereas, others show that high stress levels are associated with flank zones. Evidence of the main role played by the fault shown in both solutions. The field data constrain the results, that high strains represented by intense brittle deformation are associated with hinge zones.

Parga JR, Geol. Rundschau, 59(1), 323-336, (1969).

Arthaud F & Matte P, Tectonophysics, 25, 139-171, (1975).

De Vicente G, et al, Cuadernos Lab. Xeolóxico de Laxe, 19, 175-190, (1994).

De Vicente G, et al, Tectonophysics, 266(1-4), 405-442, (1996).

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Finite Element Modelling of Convergent Margin Wedge Dynamics

David Voelker (voelker@uni-tuebingen.de) &

Martin Meschede (meschede@uni-tuebingen.de)

Institute of Geology, Sigwartstr. 10, D-72076 Tuebingen, Germany

Convergent continental margin wedges form at the collision zone of oceanic and continental crust. Sediments and/or continental crustal material are being transferred either from the subducting oceanic plate to the overriding continental plate or from the overriding to the subducting plate. The geometry of the wedge as well as the tectonic stress regime and uplift and/or subsidence history of the entire forarc region result from the equilibrum between a number of processes each of which permits material transfer between the plates: frontal accretion, accretion by underplating, frontal and basal tectonic erosion, and fluid motion. The factors which govern the steady state represented by an actual convergent margin are supposed to include the subduction velocity, the basal friction of the subducting plate, the amount and variety of sediments and the rheological properties of the rocks.

Some of the mentioned dependencies have been tested with the help of analogue (sand-box) models. We aim to get a deeper insight into the question by employing a numerical model using the finite element method (ANSYS). The aim of the 2-D model is to simulate the dynamic system of a convergent margin wedge as an entity. It is particularly interesting which factors are controlling basal erosion, e.g. the upward jump of the decollement which is believed to preceed the subduction of basal sheets of material of the overriding plate. Principally, stresses are being calculated from the rheological properties of the materials and the displacements resulting from subduction up to a point where localized stress requires yielding. After faulting has ocurred, the buildup of stress continues to the next step of faulting. Ideally the model has to account for the interrelationship between the temperature field and rheology (in particular the brittle-ductile transition) and the changes in rheology due to fluid migration and mixing of materials of oceanic and continental origin.

The region of reference which is used to test some of the modelling results is the Pacific Continental Margin of Middle America (Costa Rica), where net tectonic erosion of the continental plate is supposed to have occured for at least the past 15 Ma (Meschede & Zweigel, this volume). The poster shows some of the modelling results and relates them to the data observed off Costa Rica, e.g., during ODP Leg 170.

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Where Have all the Mountains Gone..?

Per-Gunnar Andréasson

(per-gunnar.andreasson@geol.lu.se)

Geologiska Institutionen, Sölvegatan 13, Lund, Sweden

The thrust sheets of the Scandinavian Caledonides preserve an almost complete assembly of tectonic elements of a mountian belt, formed during opening and closing of the Iapetus Ocean. However, in the region supposed to represent the core of the orogen, the evidence for such an orogenic evolution is almost lacking. This paradox is not only a first rate tectonic problem still waiting for its solution, it has important bearings upon reconstructions of ancient mountain belts.

Following a classical excursion route (Östersund - Trondheim) with detours to other sections across the Scandes when needed, the poster demonstrates the various components and structures of the mountain belt. Excursion stops include undeformed cratonic rocks and the sedimentary cover of the foreland; the imbricated continental margin including smashed granites of the craton, rift basins Mts. in the north. However, in western Norway, the peaks of Sylarna, Snasahögarna and Åreskutan are represented by banded amphibolites in their exact tectonostratigraphic position, but often only some tens of metres thick. Why, how and when did such extreme excision of almost the entire Swedish Caledonides except a few tens of metres of the basal nappes occur? In the same (?) way, the thick nappe succession of the central Trondheim Region, including ophiolites, suprasubduction magmatism and turbidites is reduced in thickness to a few hundred metres in coastal exposures of the western Trondheim Region. The poster describes some interpretations of the excision.

Finally, the poster takes us X Ma into the future, when the Scandinavian Caledonides have been eroded away and all that remains are, at best, a narrow (even <100 m) belt of augen gneiss, amphibolite, and greenschist in tightly folded and thoroughly reworked Proterozoic bedrock. Ages cluster at c. 400 Ma, c. 900-1000 Ma and c. 1500-1600 Ma. This scenario should ring a bell: if a hundred metres of banded amphibolites and gneisses can hide the whole history of a continental margin, including subduction and supra-subduction magmatism, how much of the orogenic evolution do and dolerites, eclogites and intercalated mantle fragments; obducted ophiolites, sulphide ores and turbiditic cover; suprasubduction magmatism including the trondhjemite type locality, boninites and island arc volcanics; melange, and rocks of the inferred Laurentian continental margin. In the Swedish Caledonides, thrust sheets derived from the continental margin and the continent-ocean transition build up high and rugged mountains from Mt. Sylarna in the south to the Sarek-Kebnekaise we actually observe and fully understand when we walk across similar exposures in more ancient mountain belts?

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Structure of a Duplex at the Caledonian Front, Northern Sweden: The Flakaträsk Duplex, Lower Allochthon

Stefano Febbroni

(sfebbron@ix.urz.uni-heidelberg.de) &

Reinhard Greiling

(er8@ix.urz.uni-heidelberg.de)

Geologisch-Paläontologisches Institut, INF 234, D-69120 Heidelberg, Germany

The Flakaträsk Duplex (F.D.) structure represents the lowermost allochthonous level (Lower Allochthon) at the margin of the Scandinavian Caledonides in northern Sweden (Storuman, Sorsele). Its horses are composed of an unmetamorphic sedimentary sequence of Neoproterozoic to Middle Cambrian age, the Jämtland Supergroup. The basal Risbäck Group is composed of a thick sequence of polymictic conglomerates and fine- to coarse-grained feldspathic sandstones with irregular interlayers of siltstones. The overlying Sjoutälven Group comprises Varangian tillite and shales at the base and the overlying Gärdsjön Formation, which is composed of a thick sequence of coarse- (mostly at the base) to fine-grained quartz rich sandstones and irregular interlayers of siltstones. The top of the sequence is composed of black shales of the Fjällbränna Formation.

These black shales and the tillite of the Långmarkberg Formation are the most important overthrust horizons. During the overthrusting of the Lower Allochthon the tillite horizon and the black shales acted, respectively, as basal and roof thrusts for the horses. Irregular siltstone interlayers of the Risbäck Group and of the Gärdsjön Formation occassionally constitute secondary overthrust horizons. Consequently, the thickness of the horses (approx. 500 m) is determined by the stratigraphy. Horses are, in general, 15 km in length (approx. 2 km in width) and terminate both towards NE and SW beneath the overlying units of the Middle Allochthon and, consequently, the F.D. "pinches out" along strike. There, the Middle Allochthon sole thrust is merging with the basal Caledonian detachment surface. Due to stacking of the sedimentary sequence into at least 15 horses, the rock units of the F.D. dip generally W-WNW. The number of horses determines the width of the duplex of approx. 30 km. Towards the foreland and the hinterland, the F.D. roof and floor thrusts merge and, locally, the Lower Allochthon is no longer represented beyond the duplex. Therefore the F.D. forms an isolated antiformal structure, which is surrounded by the higher Caledonian units. According to lineation data and kinematic indicators, the transport direction during the thrusting was approximately 110°, towards the foreland. Due to the generally brittle deformation, joints and fractures are extensively developed throughout the horses.

The basal Risbäck Group occurs mostly in the west and decreases in thickness towards the east and eventually disappears. There, the Gardsjön Formation directly overlies the cristalline rocks of the Baltic Shield. A clear geometric relationship between stratigraphic thickness of sedimentary units and structural horses has been demonstrated. Therefore, the F.D. as a whole is apparently derived from a basin of relatively thick clastic sequences. The lateral termination of the duplex is broadly coincident with the thinning or disappearance of the pre-existing sedimentary layers.

Febbroni S, AZ Marmi: "Rilevamento geologico e dei valori della suscettività magnetica nelle Caledonidi della Lapponia svedese", 131, 55 pp, (1997).

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The HP Granulites of the Vacariza Formation (Cabo Ortegal, NW Spain): Lower Crust Structural Evolution in a Subduction Realm

Pablo Puelles (gobpuolp@lg.ehu.es)1,

Abalos Benito (gppabvib@lg.ehu.es)1 &

Jose Ignacio Gil Ibarguchi (nppgiibi@lg.ehu.es)2

1 Dpto. Geodinamica, Univ. Pais Vasco, P.O. Box 644, Spain
2 Dpto. Mineralogia-Petrologia, Univ. Pais Vasco, P.O. Box 644, Spain

The Cabo Ortegal Complex is a composite nappe pile that forms part of the so-called Allochthonous Complexes of the northwestern Iberian Peninsula. This allochthonous sheet, made of different units separated by thrust contacts, includes ultramafic rocks, metabasites and quartz-feldspathic gneisses metamorphosed under different conditions. High-pressure deformation and metamorphism occurred in this complex.

The HP granulite Vacariza Formation is included within the Cabo Ortegal Complex. Regarding its geochemical composition and outcrop distribution, this formation is considered to be quite heterogeneous. The main granulite types are the following: (i) ultramafic granulites made up of Grt+Cpx±Pl±Zo±Rt, (ii) mafic granulites with an increasing amount of modal Pl composed of Grt+Cpx+Pl±Hbl±Zo±Qtz±Rt, (iii) Mg-rich mafic granulites, (iv) intermediate granulites with an original mineral assemblage made up of Grt+Cpx+Pl±Hbl±Qtz±Zo±Ky± Rt±opq, (v) granulite gneisses composed of Grt+Cpx+ Pl+KFd+Zo+Phg+Qtz±Bt, and (vi) other rock types, such as calc-silicate rocks, Grt-Bt gneisses and gabbros.

Three main deformation phases have been recognized. The first phase F1 took place under HP granulite facies conditions (up to 810°C ,12 kbar), giving rise to the development of a S1 foliation and L1 mineral lineation, defined by the preferred orientation of Cpx. Eye and anvil structures related to the existence of sheath folds have been found in structural YZ sections perpendicular to the lineation. A NNE-directed sense of movement of hangingwall blocks is recognized. The second phase F2 is defined by a LT amphibolite facies mineral assemblage. It led to the syntectonic retrogression of the original mineral assemblage, probably assisted by fluid-flow. An Amp-defined N030E-trending L2, along with a subhorizontal amphibolite facies S2 can be observed. L2 is homoaxial with L1. A latter deformation phase F3, possibly related to a subsimple shear deformation process, is inferred from the existence of spectacular late C-S shear zones. The shear-sense markers indicate a SSW-directed sense of movement.

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Siliceous Fluid Migration Associated with the Evolution of the SW-Iberia Branch of the Variscan Fold and Thrust Belt

L. Sá &

P. Terrinha (pedroterrinha@mail.teleweb.pt)

Dep. Geologia FCUL / LATTEX, Campo Grande, Lisboa, Portugal

This paper describes structures associated with fluid injection in a slate and graywacke fold and thrust belt of Carboniferous age, probably an acretionary prism of the Variscan Belt in SW-Iberia, the South Portuguese Zone. The silica-rich fluids precipitated quartz into bedding surfaces, fold hinges, thrusts, normal faults and tension gashes. Micro-fabrics of these veins are presented.

A diversity of cross-cutting relationships between these structures indicates that the process of injection repeated itself throughout the deformation process. However, it appears that injection of fluids into impermeable layers (or base of these layers) reduced friction allowing layer parallel sliding, which in association with ramps started to accommodate duplication of stratigraphy. These are banded veins alternating quartz and remains of host-rock, mostly shale. This type of deformation preceeded folding and large scale thrusting. Tension gashes, generally perpendicular to bedding in competent graywackes formed during thrusting, due to overload of sediments on footwall of important thrusts. T-geometries of tension gashes and layer parallel veins, strongly suggest formation of channels for fluid migration early in the tectono-sedimentary history of the flysch fomation.

Typical polygonal hydro-fractures formed in the shales. These structures are thought to be associated with dewatering of the sediments; generally they do not show deposition of quartz and their geometry is highly variable. Polygons can be regular or elongated, suggesting a large and continuous spectrum of differential stresses in the shales. Growth of polygonal fractures is sometimes controlled by earlier sets of parallel jointing. Mapping of mesoscale folds shows that distribution of patterns of the hydro-fractures in the shales materialises strain variation in cylindric and monoclinic flexural folds. In some cases simple shear induced by left-lateral transpression also influenced distribution of hydro-fracture patterns.

Tension gashes in graywackes are generally sub-perpendicular to bedding and quartz crystal growth indicate important pure shear mechanism of opening of these veins. Folding of thin tips of these veins caused by transpression, vertical compaction and heterogeneous folding is described.

En échelon tension-gashes (1 cm thick) linked by swarms of very thin (less than 1 mm) quartz veins are described and compared with numerical and photo-elastic models of stress trajectories in jogs between strike-slip faults.

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The Lusitanian and Algarve Basins, West and South Portugal. Comparison of Tectono-Stratigraphic Models: I) Basin Formation

J. C. Kullberg (jck@mail.fct.unl.pt)1,

P. Terrinha (pedroterrinha@mail.teleweb.pt)2,

M. C. Kullberg (carla.kullberg@fc.ul.pt)2,

A. Ribeiro (aribeiro@fc.ul.pt)2 &

R. B. Rocha (rbr@mail.fct.unl.pt)1

1 CEPUNL, Monte de Caparica, Portugal
2 GeoFCUL / LATTEX, Campo Grande, Lisboa, Portugal

The stratigraphic record of the Lusitanian and Algarve Basins shows that the west and south Portuguese continental margins experienced tectonic extension simultaneously, from Triassic to Early Cretaceous times.The N-S and E-W trends of the Lusitanian and Algarve Basins result from a complex interaction of three extensional fault sets in each one of these basins that were later inverted at different times and under different compressive directions.

This paper aims to synthesise the information about the Lusitanian and Algarve Basins and point out their differences and similarities.

Three rifting events have been traditionally assigned to the LB.The LB presents four rifting events, which originated three rift sequences, as follows:

R1-Mid Triassic-Hettangian ENE-WSW to NE-SW oriented extension. First siliciclastic deposits were followed by deposition of evaporites interbedded with pelites and sporadic dolomites.The Monte Real-S. Mamede graben stands out as the most important known structure in which a sequence of evaporites over 2 km thick was deposited.

R2-Pliensbachian-Mid Callovian E-W oriented extension is indicated by dip-slip striae on N-S trending faults. NE-SW trending faults were also activated as shown by variations of facies and paleo-bathimetry and of thickness of sedimentary packages.However, due to strong inversion during NW-SE oriented compression during Miocene inversion, NE-SW faults do not show good extensional kinematic indicators.Halokinesis is also important during this rifting phase.

R3-Mid Oxfordian-Tithonian E-W oriented extension caused important rift shoulder uplift as shown clastic sedimentation along west and east margins of the basin and formation of the Berlengas horst, the western boundary of the basin.

However, a fourth mild rifting event (R4) can be proposed as normal faults in the Lower Cretaceous have been also mapped.Furthermore, R3 and R4 are separated by a mild volcanic and compressive event.

The Algarve Basin presents some differences with respect to the LB:

R1-Mid Triassic-Hettangian extensional event, with a sedimentary sequence similar to the LB, is accompanied by a widespread volcanic event that extends into the Betic Margin.Paleocurrent data from the Triassic fluviatile clastics indicate a depocentre located towards the SW of the Iberian block, which could indicate a NE-SW oriented extension.

R2-This rifting event spans from the Carixian to Early Toarcian and is interrupted by a short lived compressive event on the Carixian-Domerian transition.

R3-Bajocian (Aalenian has not been yet described) to Late Callovian.

R4-Oxfordian-Tithonian

R5-Lower Cretaceous (Berriasian (?)-Valanginian(?) to Albian) extensional event.

Only R1 appears to have had a possible NE-SW extension direction.The remainder show a dominant activity of NE-SW to E-W fault set alternating with extension on N-S to NNE-SSW faults, which are more common in the west.Kinematic indicators suggest that NE-SW faults rotated anti-clockwise, varying from dip-slip to oblique-slip with a dextral component. Transient compressive episodes occurred on the transitions R3/R4 and R4/R5.

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The Lusitanian and Algarve Basins, West and South Portugal. Comparison of Tectono-Stratigraphic Models: II) Basin Inversion

J. C. Kullberg (carla.kullberg@mail.fc.ul.pt)1,

P. Terrinha (pedroterrinha@mail.teleweb.pt)2,

M. C. Kullberg (carla.kullberg@fc.ul.pt)2,

A. Ribeiro (aribeiro@fc.ul.pt)2 &

R. B. Rocha (rbr@mail.fct.unl.pt)1

1 CEPUNL, Monte de Caparica, Portugal
2 GeoFCUL / LATTEX, Campo Grande, Lisboa, Portugal

Comparison of Extensional and Inversion structural maps of the Lusitanian Basin shows:

i) inversion tectonics reworks pre-existent extensional structures; ii) a series of indentors formed due to the fact that the main compression direction was an approximate bissector of the angle made by the 3D basin listric faults.

Near top of the basement map and profiles across the Lusitanian Basin and Arrábida chain show:

i) that some basement faults were partially inverted during inversion; ii) formation of collapse basins due to salt withdrawal during a Paleogene inversion event; iii) reactivation of Paleogene collapse synclines during Miocene inversion.

Re-assessement of the Nazaré Fault kinematics enhanced several aspects:

i) basement overthrusts Cretaceous sediments in the Meseta mainland; ii) Nazaré extensional Fault in the Basin is probably off-set laterally across the Hettangian salt horizon (top to the SE); iii) it is not known wether the inversion of the Nazaré Fault involves a short cut thrust through the previous existent extensional fault or only backrotation of the previous extensional fault.

Main tectonic inversion of the LB occurred during Mid-Late Miocene times.

Comparison of Extensional and Inversion structural maps of the Algarve Basin shows:

i) inversion tectonics also reworks pre-existent extensional faults; ii) main thrusts are south directed, thus cross cutting through normal faults; iii) main transport direction deduced from fault slip data are approximately N-S; iv) age of main inversion episode is poorly constrained onshore because Mid Miocene lies undeformed on top of folded and thrust Lower Cretaceous; v) offshore both Lower Cretaceous and paleogene are deformed but separated by erosion unconformities, suggesting two pulses of tectonic inversion possibly during Late Cretaceous-Lower Paleogene and during Eocene times; vi) Miocene tectonic inversion is much milder and compressive structures are very rare. Small scale kinematic data showing NW-SE transport direction and large scale NE-SW fold axes on the foot-wall of main basin thrust could be a result of a Miocene compression as well as the consistent lack of Mioene on the hanging-walls of main basin thrusts; vii) Neogene and Quaternary evolution of the basin shows evidences of a constritive stress field onshore. On the other hand on the southern flank of the Neogene basin depocentre only extensional faults are seen on seismic profiles.

The striking different evolution of the two basins during Neogene times is possibly associated with i) lateral expulsion of the Betic orogen; ii) the WNW-ESE plate movement vector of Africa with respect to Europe during Pliocene and Quaternary times and, iii) the nucleation of a possible Quaternary subduction zone along the Portuguese western margin.

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New Paleostress Field Patterns in the Central Algarve Basin

Inga Moeck (Schandelmeier@bg.tu-berlin.de)1 &

Heinz Schandelmeier

(Schandelmeier@bg.tu-berlin.de)2

1 Technische Universität Berlin, Institut für Angewandte Geowissenschaften II/BH 2, Ernst-Reuter-Platz 1/10587 Berlin, Germany
2 Technische Universität Berlin, Geowissenschaften II/BH 2, Ernst-Reuter-Platz 1/10587 Berlin, Germany

The Algarve Basin is located close to the Azores-Gibraltar plate boundary, that influenced the tectonic evolution of the basin since Triassic, when the basin opened as a halfgraben. New structural analyses and kinematic analyses of fault-slip data indicate additional stress fields from Triassic to Quaternary. The recent fault grain of the central basin is characterized by NW-SE to NNW-SSE and NE-SW to NNE-SSW strike-slip faults, that are cut by E-W to ENE-WSW oblique reverse faults. Following tectonic phases with related stress fields have been recognized: 1.) Extensional stress regime with an orientation of <sigma> 3 in 7° from Triassic to lower Jurassic. Joint pattern analyses suggest that both <sigma> 3 and <sigma> 2 were tensile. A 90° flip of <sigma> 3 and <sigma> 2 around the vertical <sigma> 1-axis occurred periodically. Pre-existing paleozoic NW-SE striking fold planes and NE-SW striking transfer faults were reactivated as oblique normal faults, whereas E-W trending normal faults developed during the rifting phase. 2.) Strike-slip stress regime with a direction of <sigma> 1 in 20° and <sigma> 3 in 109° during lower Tertiary. This stress field is also known from the Lusitanian Basin, situated to the north along the portuguese westcoast, and shows the influence of the Pyrenean phase in the Iberian peninsula. 3.) Strike-slip regime with an orientation of <sigma> 1 in 189° and <sigma> 3 in 97° during Miocene indicates the collision of Iberia with Africa. The main inversion of the basin fill occurred under influence of this stress field with reverse faulting and flexuring on the E-W striking faults. The pre-existing NW-SE and NE-SW faults were reactivated as dextral and sinistral strike slip faults, respectively. 4.) Transpressional stress regime with a direction of <sigma> 1 in 150° and <sigma> 3 in 62° during the Quaternary. However, the recent stress field is inhomogeneous. Several subareas have been recognized in the central basin part: North of the E-W trending Algibre reverse fault <sigma> 1 is oriented in 154°, south of this reverse fault <sigma> 1 is directed in 129°, along the northern basin margin <sigma> 1 is oriented in 122° and <sigma> 1 is directed in 114° along the sinistral NE-SW trending Silves strike-slip fault. Latter fault forms the western margin of the central basin part.

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Structural Study of Deformed Pliocene Sands Related to Diapiric Activity (west Iberia Margin)

Paulo Fernando Ribeiro (pribeiro@fc.ul.pt) &

João Manuel Cabral (jmlccabral@mail.teleweb.pt)

Departamento de Geologia, Faculdade de Ciências, Bloco C2, 5º Piso, Campo Grande, 1700-Lisboa, Portugal

At the western Meso-Cenozoic sedimentary basin of the Portuguese mainland (West Iberia continental margin), occur several salt structures associated with the lateral migration and rising of the lowermost sedimentary unit - the Dagorda saline marls of Upper Triassic to Lower Liassic age. The studied outcrops, exposed at coastal cliffs North of the town of Nazaré, are located at the southern side of one of those structures (the S. Pedro de Muel diapiric salt anticline), trending NNE-SSW.

For a distance of 1.5 km along the studied coastal cliffs outcrop well stratified micaceous Upper Pliocene sediments which unconformably overlie northerly dipping, faulted and brecciated Liassic limestones, at the north, and directly contact with Upper Triassic to Lower Liassic black gypsiferous marls that compose the core of the regional diapiric anticline, at the south. The Pliocene sediments design a broad asymmetric syncline trending NE-SW, partially related with diapiric instability of the underlying marls.

Besides being folded the Pliocene deposits are strongly fractured at the northern limb. Several microfaults have been measured showing a large dispersion in trend, although with some prevalence of E-W to ENE-WSW directed, N or S dipping surfaces. The observed fracture pattern, showing apparently opposing fault styles, with subparalel extensional and contractional structures, suggests the action of two different processes: forceful indentation by a rising diapir and tectonic lateral shortening. Bedding was kinematically activated, particularly along highly micaceous horizons. Slip was promoted along bedding lying close to the mica angle of friccion suggesting tilting prior to activation. The micas frequentely are crenulated and deformed into small folds whose vergence indicates reverse movement to the NW.

The NW verging syncline affecting the Pliocene sands probably resulted from the combination of a bending process by diapiric indentation and a buckling mechanism suggested by local horizontal shortening.

Strain in the Pliocene sands was accommodated by independent granular flow, microfaults and crenulation in micaceous horizons. Based on reverse microfaults and crenulation a NW-SE maximum shortening trend was found, consistent with the regionally accepted orientation of the present SHmax in the crust (Ribeiro et al., 1996). The local deformation is probably linked to a major NE-SW offshore structure, the Nazaré fault zone (Ribeiro, 1998).

Ribeiro A, Cabral J, Baptista R & Matias L, Tectonics, 15, 641-659, (1996).

Ribeiro P, Estudo de deformações tectónicas plio-quaternárias no bordo meridional da estrutura diapírica de S. Pedro de Muel (Vale de Paredes - Marinha Grande), MSc Thesis, Lisbon University, 107, (1998).

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Tectonometamorphic Evolution of the Eastern Pennine Alps During Tertiary Continental Collision: Structural and Petrological Relationships between Suretta-, Tambo-, Chiavenna and Gruf Units (Switzerland/Italy)

Rachel K. Huber (rachel.huber@geol.unine.ch),

Didier Marquer &

Françis Persoz

Institut de Géologie, E-Argand 11, Neuchatel, SUISSE

The heterogeneously deformed Tambo- and Suretta nappes are polycyclic, polymetamorphic nappes. They are crosscut by early Permian intrusives, overlain by a autochthonous Permo-Mesozoic cover and underlain by the Chiavenna and Gruf units. The Chiavenna unit consists of ultramafic/mafic rocks with a subcontinental to oceanic mantle origin. This units belong to the Briançonnais zone. The Gruf unit consists of granulites and migmatites of presumably pre-Alpine age. Deformation, kinematics and metamorphic assemblages were defined, which allows to propose an tectonometamorphic evolution during Tertiary collision.

The first phase (D1) shows a ductile heterogeneous deformation with top to the NW-moving shear zones and a NW-dipping schistosity and lineation. The mineral recrystallized under HP-conditions (e.g.. Tambo: ~13-10 kb, ~500°C). It is interpreted as nappe stacking during the Eocene subduction. The second phase (D2) creates the main NNE-dipping schistosity, a sub-horizontal E-W trending lineation and shear zones with a top to the E-movement. Decompression with T-max. at the beginning of D2 (e.g.. Tambo: ~11-6 kb, ~610°C) is different to the almost isothermal decompression in the North. The deformation style changes from early HT- to late LT-structures. PT-increase through the nappe pile from top to bottom (Suretta: ~10-5 kb, ~550°C and Gruf: ~10-4 kb, ~730°C). It is related to Eocene-Oligocene syn-orogenic E-W extension due to buoyancy disequilibrium caused by the anomalous thickness of the crust. The T-maximum during the D2, the melt creation of the future intrusions and the induction of the D3-uplift are eventually related to slab detachment.The third phase's (D3) schistosity and lineation are steeply S-dipping and associated to E-W striking folds. They are coeval with steeply inclined S-dipping N-thrusting shear zones and their conjugated minor set. They are responsible for the steepening of the S2 and the uplift of the Bergell area. In the North, lower greenschist facies close to the brittle-ductile transition and in the South conditions of ~4 kb, ~550°C occurs. The Bergell pluton intrudes, provoking contact metamorphism at its top and local melt at its base. Oligocene differential uplift with major elevation in the South provokes erosional exhumation during late continental collision and induces rapid cooling. The fourth phase (D4) involves late orogenic movements along major crustal accidents, the Engadine, Insubric and the Forcola lines, due to E-W escapement of crustal blocs.

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The Marne A Fucoidi Formation: A Relatively Minor Disharmony/Detachment Level in the Umbro-Marchean Apennines (Italy)

Mario Tramontana (igs@geo.uniurb.it) &

Stefano Tosti

Institute of Geodynamics and Sedimentology, Campus Scientifico, Loc. Crocicchia, Urbino, Italy

The Umbro-Marchean Apennines represents a thrust-and-fold belt constituted by a deformed sedimentary multilayer (i.e. the Umbro-Marchean Succession) mainly made up of different carbonate sedimentary units. The succession is characterized by high inner competence contrasts; the marly Marne a Fucoidi Formation (Lower Cretaceous) represents one of the main mechanically weak intervals.

Across the forelimb of the main anticlines, terrains belonging to the Maiolica-Marne a Fucoidi-Scaglia Group Fms outcrop. Lithologically these units show the well-known characters described in detail in the geological literature; the Marne a Fucoidi Formation is constituted by two intervals: an upper mainly calcareous interval and a lower mainly marly one. The latter is characterized by marls and marly clays and, subordinately, by limestones and marly limestones; intercalations of black shales are frequent. Therefore the lower marly interval of the Marne a Fucoidi Fm. represents a marked lithologically inhomogeneous level in comparison with the stiffer overlying and underlying limestones.

The more calcareous upper part of the Marne a Fucoidi Fm. and the lower part of the uppermost Scaglia Bianca Fm. are frequently deformed through many kink and chevron mesoscopic folds, mainly SW verging. Moreover, the marly interval of the Marne a Fucoidi Fm. is affected by several NE dipping reverse shear surfaces. Such features are interpreted as related to back-thrusting across a detachment within the lower interval of the Marne a Fucoidi Fm. All these features are well observable on the eastern limb of M. Nerone anticline (e.g. M. Serrone). The forward propagation towards NE of the anticlines should cause the back-thrusting of the sedimentary multilayer overhanging the less competent unit.

The presence of back-thrusts in this structural position indicates the propagation towards more external zones of buried thrusts along flats also located in correspondence with the Marne a Fucoidi Fm. In fact, the stratigraphic position of a flat can be inferred from the position of a related back-thrust in an anticlinal forelimb.

This indicates that the Marne a Fucoidi Fm. constitutes a disharmony/detachment level whose presence affects the geometry of the Umbro-Marchean Apennine anticlines and their development.

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Geological Setting of the External Part of the Central Apenninic Thrust Belt (Abruzzi, Italy)

Gian Paolo Cavinato1,

Enrico Miccadei2,

Maurizio Parotto3,

Tommaso Piacentini (t.piacentini@uniroma3.it)3 &

Antonio Praturlon3

1 Cento Studi per il Quaternario e l'Evoluzione Ambientale, Dipartimento di Scienze della Terra, Università La Sapienzadi Roma, Piazzale Aldo Moro, 5, Roma, Italy
2 Dipartimento di Scienze della Terra, Università G. d'Annunzio di Chieti, Via dei Vestini, Chieti scalo, Italy
3 Dipartimento di Scienze Geologiche, Università di Roma Tre, Largo San Leonardo Murialdo, 1, Roma, Italy

The central Apenninic thrust belt, as a result of the Europe-Africa incomplete collision, was built up during the Neogene time by juxtaposition of different Meso-Cenozoic palaeogeographical domains, carbonate platforms and pelagic basins. The present chain topography is made up of a series of carbonate ridges formed by Neogene thrust sheets, with synorogenic deposits in between of them, and depressions related to Quaternary extensional tectonics. Three main domains converge in the external part of the central Apenninic thrust belt; they are, from west to east, the Latium-Abruzzi carbonate platform, the Gran Sasso-M. Genzana pelagic basin and the Morrone-Pizzalto-Rotella carbonate platform. The irregular boundaries of these domains, the different reology of their stratigraphical successions, as well as the developing of a strong polyphasic tectonics, have determined a non cylindrical deformation and a complex geological setting ruled by interference and/or interaction of differently trending structures, both in a regional and in a local scale. This work is intended be a new contribution to the definition of the geological setting of this part of the Apennines (between Fucino Plane and Maiella Mountain), using field surveys, structural and stratigraphical surface data, integrated by geophysical data (commercial seismic lines). Field surveys and structural analysis allow us to define geometry and kinematics of the main tectonic line and to identify the polyphasic tectonics; the main events are thrust tectonics, strike-slip tectonics, extensional tectonics. The reconstruction of the geometry and the structural setting in key areas and the distribution of synorogenic foredeep deposits allow us to correlate main thrusts and tectonic lines and to understand the relationship between the different tectonic units. The deep interpretation of surface data has been guided by the analysis of seismic lines.The main tectonic line trend from N-S to NW-SE; some of these were already well known (Gran Sasso thrust, N-S; Morrone thrust, NW-SE; Alto Sangro-Giovenco line, NNW-SSE; Profluo-Tasso-Sagittario line, NNW-SSE); others were not so well investigated (Porrara line, NNW-SSE to NW-SE; Aterno Valley line, NW-SE). The presence of intramontane depressions filled by lacustrine and fluvial deposits is fundamental in the geological reconstruction of this area; Fucino basin and Sulmona basin are the biggest ones and their stratigraphy and tectonics show the importance of extensional tectonics in the evolution of the present structural setting. Extensional tectonics masks thrust tectonics evidence and reactivates strike-slip faults.The dating of the Neogene and Quaternary syntectonic deposits define the timing of the tectonic events.

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Post-Carboniferous Structural Evolution of the Istanbul Fragment in the Zonguldak-Cide Area, Western Pontides, Northern Turkey

Timur Ustaomer (timur@istanbul.edu.tr)1,

Sedat Inan2 &

Hakan Hosgormez1

1 Istanbul Universitesi, Jeoloji Bolumu 34850, Avcilar-Istanbul, Turkey
2 TUBITAK-MAM, Marmara Arastirma Merkezi, Gebze-Kocaeli, Turkey

New structural mapping and geotraverses carried out in the Zonguldak-Cide area have revealed important early Cretaceous oblique-slip extensional deformations, followed by fold and thrust belt development. The lower Carboniferous coal-bearing continental clastics crop out in the core of high-amplitude anticlines in the Zonguldak area. Jurassic-lower Cretaceous limestones are preserved as extensional detached slice above the Carboniferous clastics at the fold limbs. The regional fold axial trace both in the Carboniferous and the Cretaceous sediments coincides. No structure in the area studied can be attributed to the Hercynian deformations by confidence. All the structures observed in the Carboniferous sequence are also observed in the Cretaceous units. The faults observed in the Cretaceous (Barremian-Senomanian) sediments are of listric, oblique normal faults, along which individual blocks were rotated. Layer parallel extension, shear fractures, axial planar cleavage development are other mesoscopic structures observed within the shallow marine Cretaceous limestones. This deformation has resulted in irregular horst and graben topography. Overlying the Cretaceous limestones, a Cretaceous flysch (terrigenous and volcaniclastic turbidites) was deposited in lows while the Palaeozoyc basement was uplifted locally (i.e. Goldag) at ridges and re-worked into the lows.Local thrusting and associated folding post-date the extensional deformation in the Zonguldak area. A well-developed fold and thrust belt crop out in the Amasra-Cide area, to the north of the Ulus Basin. Upper Cretaceous sediments are the youngest sediments involved in this deformation. An interesting aspect of this belt is existence of the Permo-Triassic red clastics in the core of the anticlines within this belt. Both the red clastics and the deformed belt are absent to the west of Amasra. We observed that the fold and thrust belt are terminated by transfer faults just to the E of Amasra. We interpret these transfer faults to be re-activated zones of crustal weakness, which were active during deposition of the Permo-Triassic clastics. Main conclusions of this study are that no intense, collisional Hercynian deformations were found in the area. The early Cretaceous extensional deformation is related to the opening of the Black Sea, which has been well-documented by stratigraphic and sedimentological studies (Gorur, 1988, 1991) over a decade. The fold and thrust belt to the E of Amasra is interpreted here as the result of closure of the Intra-Pontide ocean by the Late Cretaceous. Eocene deformations generated high-amplitude folds and caused reactivation of the earlier structures.

Gorur, N, Tectonophysics, 147, 247-262

Gorur, N, Palaeo, Palaeo, Palaeo, 87, 267-288

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Volcanism Associated with Extension at a Consuming Margin, W Pontides, N Turkey

Mehmet Keskin (keskin@istanbul.edu.tr),

Timur Ustaomer (timur@istanbul.edu.tr) &

Mefail Yeniyol (yeniyolm@istanbul.edu.tr)

Istanbul Universitesi, Muhendislik Fakultesi, Jeoloji Bolumu, 34850 Istanbul/Turkey

A thick sequence of north-dipping, Cretaceous volcaniclastic sediments and minor lava flows, termed the Kavaklar Group, crop out to the N of Istanbul, along the south coast of the Black Sea. A Palaeozoic sequence, known as the Palaeozoic of Istanbul, is exposed to the S of the volconogenic sequence. The contact between the two is a major thrust, along which the Palaeozoic sequence tectonically overlies the Kavaklar Group. The Kavaklar Group is represented by a flysch sequence at the base. There are occasional lava and volcanic debris flows within this part of the succession. Soft sediment deformation structures (i.e. slumps and slide blocks) are abundant within the flysch. Above comes a thick sequence of volcaniclastic sediments, ca 1000 m. The volcaniclastic sediments include volcanic breccias, lahar breccias, hyaloclastites, volcaniclastic boulderstones and sandstones. The clasts range in composition from basaltic andesites to rhyolites and in texture from aphyric to plagioclase- and pyroxene- phyric lavas. The lava clasts range in size from a millimetre to a few metres. Thickness of the individual beds range from a few centimetres to ca. 30 m. Dykes and sills of basic to intermediate composition intrude into the volcanicalstic sediments. Associated with the dykes are occasional columnar jointed lava flows. Geochemical data of the lava flows and individual clasts imply a volcanic arc-type eruptive tectonic setting. This is evident from the LIL and LREE enrichment relative to HFSE.Our working hypothesis is that the Kavaklar Group represents extensional magmatism, related to the opening of the Black Sea. We think that initial crustal extension resulted in an irregular topography with structural lows and highs, with associated marginal volcanism. Terrigenous turbidites and volcanic debris flows were accumulated into structural lows while the ridges were eroded. Thick pile of volcanics was accumulated during the initial stages of magmatism. Mass wasting of the volcanic provenance took place, associated with continuous extension. Products were deposited in proximal basins. We believe that the arc signature is inherited from an earlier subduction event that metasomatised the sub-continental lithosphere. Arc signature can be seen in any tectonic setting, provided that an earlier subduction event existed. Basin and Range Province of the W USA, the Aegean Graben System are good examples of extensional setting, containing arc component. Such arc signatures are also found in some of the recent collisonal settings (i.e. NE Anatolia).

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Active Deformation in the Issyk-Kul Basin (Kyrghyz Tien-Shan): From Pull-Apart to Transpressional Ramp Basin

J. Klerkx (jklerkx@africamuseum.be)1,

K. Abdrachmatov (kis@imfiko.bishkek.su)2,

M. Buslov3,

M. De Batist (marc.debatist@rug.ac.be)4,

P. Vermeesch (pieter.vermeesch@rug.ac.be)4,

Y. Imbo (yannick.imbo@rug.ac.be)4 &

R. Hus5

1 Department of Geology, Royal Museum for Central Africa, Tervuren, Belgium
2 Institute of Seismology, Kyrghyz Academy of Sciences, Bishkek, Kyrghyzstan
3 United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
4 Renard Centre of Marine Geology, University of Gent, Gent, Belgium
5 Department of Geology, Vrije Universiteit Brussel, Brussel, Belgium

The Issyk-Kul basin is one of the major basins in the basins-and-ranges configuration of the Tien-Shan. The basin, which is delimited by almost E-W trending border faults, originated in the Miocene. During its evolution the basin-controlling faults shifted progressively towards the inner part, and changed from strike-slip to reverse faults.

Satellite imagery analysis shows a network of lineaments along ENE-WNW and NW-SE directions, supporting the hypothesis of the origin of the basin as a dextral pull-apart.

Active movements are evidenced in the southern part of the basin by a high resolution seismic survey which shows compressional deformation along a ENE trend, resulting in upwarping of the sedimentary deposits and faulting along the same trend.

Structural investigations on land evidence folding of Neogene sediments along the ENE trend and associated faulting. Radon measurements in soils along transverse profiles across the fault zones evidence the present activity of the faults.

It is postulated that the present compressional northwards-directed stress acting on the basin, results in a reactivation of the original basin-controlling faults. At least along the southern boundary of the basin, the ENE-trending faults are reactivated in sinistral transpression, resulting in deformation of the sediments on land and in upwarping of the sedimentary fill in the basin.

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Uplift and Inversion Tectonics in the Easternmost Part of the Great Caucasus (Azerbaijan)

Damien Dhont (damien@lgs.jussieu.fr),

Jean Chorowicz (choro@lgs.jussieu.fr) &

Marie-Francoise Brunet (brunet@lgs.jussieu.fr)

Depatement de Geotectonique, Case 129 - T26-16 E1, UPMC - 4, place Jussieu, 75252 Paris cedex 05 France

Using stereoscopic views from Landsat and SPOT images, we have drawn a new tectonic map of part of the Eastern Caucasus belt (Azerbaijan) at 1:60000 scale. This is complemented by a North-South geological cross-section throughout the belt permitting to analyze the style of the tectonics. We show that the deformations of the Eastern Caucasus are mainly governed by inversion tectonics. Present day reverse faults are former extensional Mesozoic-Paleogene faults inverted during the late Neogene. Anticlines are generally narrow, with sharp hinges associated to divergent steeply dipping limbs separating wide flat synclines. This pattern was named the 'ejective style' by Stille (1917). In plan view, the folds have curved axes or can be circular as is the case for the Baku syncline. More peculiar geometries can be observed because inversion of former normal faults which were branching in different directions has given rise to complex folds forming several periclinal terminations. Most ofthe reactivated faults are presently southward verging. Their syndepositional history is demonstrated by the occurrence of conglomerates, olistoliths and unconformities in the hanging-wall compartment. The Eastern Caucasus is a former rift developed during the Mesozoic-Paleogene time which underwent compression and subsequent inversion tectonics during the late Neogene. It presents similarities with the High Atlas in Morocco. This peculiar tectonic style must be considered for oil exploration.

Stille H, Geologische Rundschau, 8, 89-142, (1917).

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Paleoseismic Investigations Along the Pambak-Sevan Active Fault (Armenia)

Ara Avagyan (avagn@dstu.univ-montp2.fr)1,

Hervé Philip,

Arkadi Karakhanian (georisk@sci.am)2,

Jean-François Ritz1 &

Samira Rebaï

1 CNRS - Université Montpellier 2, CC 060 - 4, place E. Bataillon, Montpellier, France
2 Institute of Geology, Academy of Sciences of Armenia, Erevan, Armenia

The Pambak-Sevan active fault (about 300 km length) is situated in the collision zone between the Arabian block and the Eurasian plate. It corresponds to a NW-SE right-lateral fault system which belongs to the eastern border of the armenian triangular central block. This strike-slip fault system is associated to east-west compressional structures (Spitak earthquake, M= 7.0, 1988) and north-south extensional structures associated to recent volcanism. The kinematics of these structures is compatible with the north-south convergence between Arabia and Eurasia..The Pambak-Sevan fault shows many morphologic evidences of recent activity. Three main segments separated by pull-apart structures can be distinguished on this fault. No major earthquake (M>7) has been generated by this fault during the past 2000 years.On the Central segment, a volcanic cone of 1.42±0.3 My in age, is shifted off right laterally by about 750 m. These results indicate a long-term slip-rate of 0.5 mm/year. A megalithic wall of the Bronze age was shifted by about 1.8 m. This displacement may correspond to the last seismic event with a magnitude close to 7.A trench performed on this segment indicates one seismic event posterior to 6640±90B.P. (C-14). This result is in agreement with late Bronze age ceramic fragments discovered in the same level. On the Northern segment, three trenches were performed. Results of these investigations indicate one seismic event and may be a second one relatively close to the former in time (posterior to 5930±330 B.P. (C-14)). The ceramic stratigraphy confirm that these accured about 4000 years ago.Thus the North and Central segments have been activated during the same period. Pambak-Sevan active fault is a good exemple of a low slip-rate fault that can generate strong earthquakes (Mw>7) with a long recurrence interval (4000-5000 years).

O11 : 5P/34 : PO

The Present Strainfield in Lesser Caucasus: Setting of a GPS Network in America

R. Bayer,

F. Masson (fmasson@dstu.univ-montp2.fr),

E. Doerflinger,

A. Avagyan,

H. Philip,

J. -F. Ritz,

M. Daignieres,

M. Gerbault,

M. Peyret &

P. Collard

CNRS-ISTEEM, Université Montpellier II, France

The mountain ranges of the Caucasus is related to the N-S convergence between the Arabian and Eurasian plates. That results in lateral ejection of the Anatolian block westward and the Iranian block eastward, with the squezzing of the region between the northern edge of the Arabian block and the southern edge of the Eurasian shelf. From the accumulated sum of seismic moments of earthquakes the average velocity of shortening in the Caucasus is about 0.13 cm/yr. Compared with the Arabian-Eurasian convergence velocity, which is estimated at about 2.5-3.0 cm/yr, the average shortening suggests that 80-90% of the deformation is aseismic.

Armenia belongs to the central part of the squezzed region where two main faults are observed: the Pambak-Sevan and the Garni faults. In September 1998, a high-precision GPS network of 22 sites was installed in Armenia to measure crustal deformation within a mountain belt and to quantify the respective parts of seismic and aseismic deformations. This network will be measured every two years. Some GPS stations have been deployed close to the two main faults to measure the seismic and aseismic deformation along the main tectonic structures. Other stations, farther from the faults, will indicate the regional distribution of the aseismic deformation.

The result of the GPS experiment will be interpretated in term of seismic hazard relatively to the paleoseismicity of the Pambak Sevan and Garni faults.

O11 : 5P/35 : PO

New Constraints for the Stratigraphic and Structural Interpretation of the Metaconglomerate Belt from Eastern Lanterman Range (Northern Victoria Land, Antarctica)

Laura Crispini (crispini@dister.unige.it)1,

Giovanni Capponi (capponi@dister.unige.it)2 &

Marco Meccheri3

1 Dipartimento di Scienze della Terra di Genova, Corso Europa 26, 16132 Genova, Italy
2 Dipartimento di Scie, Corso Europa 26, 16132 Genova, Italy
3 Dipartimento di Scienze della Terra di Siena, via Laterina, 8, 53100 Siena, Italy

The Lanterman Range is a key area for understanding the geology of northern Victoria Land (Antarctica), particularly for the study of the Wilson Terrane-Bowers Terrane boundary. This boundary is highly significant because it has the only well-preserved eclogites, found at the Pacific end of the Transantarctic Mountains (Capponi et al., 1994; Di Vincenzo et al., 1997). Another notable feature of this boundary is the occurrence of highly deformed metaconglomerates, felsic to mafic in composition, making a 25 km long-belt, between the high grade metamorphic rocks of the Wilson Terrane to the southwest and the low grade metasandstone of the Molar Formation (Bowers Terrane) to the northeast.

In the literature the mafic and felsic metaconglomerates are known as Husky Conglomerate (HC) and Lanterman Conglomerate (LC) respectively, but new field observations, carried out during three ItaliAntartide expeditions, show that there is a gradual transition from mafic to felsic compositions which supports a stratigraphical continuum from HC to LC and indicate that HC and LC suffered the same structural evolution, therefore there is no reason to distinguish the two types of metaconglomerates, apart from the diversity in the lithological composition. The lithological features of the conglomerates suggest an affiliation with the Bowers Terrane and a generic middle Cambrian age for their deposition.

New data from a detailed structural analysis give further constraints on the Paleozoic tectonic evolution of the boundary between terranes in the Lanterman Range area. In the recrystallized matrix of the metaconglomerates it is possible to recognize three superposed foliations linked to D1, D2, D3 mesoscopic structures; S2 is the most penetrative foliation and commonly completely reorganizes and transposes S1. The comparison between deformation and metamorphism in different structural sites from the matrix of the metaconglomerates (mineral chemistry of some phases obtained by ARL-SEMQ microprobe analyses) shows that the bulk of the metaconglomerates suffered greenschist facies metamorphic conditions (slightly higher than the rest of the Bowers Terrane), but the oldest mineral associations in samples collected close to the boundary between Bowers and Wilson terranes, indicates upper greenschist-amphibolite facies metamorphic conditions. This observation suggests that during the development of S1 there was a progressive increase in metamorphic grade approaching the contact with the Wilson Terrane. As the Wilson Terrane is thrust onto the Bowers Terrane at the regional scale, such distribution of the metamorphism can be explained by a combined action of cover-effect of the Wilson Terrane on the colder Bowers Terrane (see also Buggisch & Kleinschmidt, 1989) and possibly of shear-heating. The entire tectonic pile (Wilson Terrane-metaconglomerates-Molar Formation) and the mylonitic contacts in-between are affected by later folds related to the D3 deformational event that postdates the juxtaposition of the two terranes.

Buggisch W & Kleinschmidt G, Geological Evolution Antarctica-Cambridge University Press, 155-159, (1989).

Capponi G, Meccheri M, Musumeci G, Pertusati PC, Ricci CA & Talarico F, PNRA-IX It Ant Exp, FDR, 16-19, (1994).

DiVincenzo G, Palmeri R, Talarico F, Andriessen PAM & Ricci CA, J Petrol, 38, 1391-1417, (1997).

O11 : 5P/36 : PO

Neogene Tectonics and Rift System within the Magellan Basin (Patagonia)

Marc Diraison (diraison@crpg.cnrs-nancy.fr)1,

Peter R. Cobbold (cobbold@univ-rennes1.fr)2 &

Denis Gapais (gapais@univ-rennes1.fr)2

1 CRPG-CNRS, 15, rue Notre Dame des Pauvres, Vandoneuvre Lès Nancy Cedex, France
2 Géosciences-Rennes (UPR4661-CNRS), Campus de Beaulieu, 35042 Rennes Cedex, France

At the southern tip of South America, the Magellan foreland basin abutts the orocline formed by the southern Andes. Since the Mesozoic, both Cordillera and Magellan basin have developed in a complex tectonic setting. From the Triassic to the Early Cretaceous, extensional deformation led to the development of the Rocas Verdes marginal basin. During the Late Cretaceous and Tertiary, the Andes were uplifted and the Magellan foreland basin developed. Along the arc, compressional structures (thrusts and folds) are subparallel to the Cordillera. Fault-slip data have been collected along the fold and thrust belt of the Magellan basin. Throughout the area, principal directions of shortening are sub-horizontal and sub-perpendicular to the cordillera. Stretching directions are also sub-horizontal but tangential to the orocline. Using subsurface and field data from Argentinian Tierra del Fuego, this study shows that the compressional deformation within the frontal fold and thrust belt is mainly Neogene.

In this area, the Magellan Straits cut both the Andean Cordillera and the Magellan basin. Others arms of the sea intrude the axial zone of the basin. These depressions have long been interpreted as glacial valleys. On the basis of Landsat images, digital topography and field data, we interpret the depressions as rifts or half rifts. These extensional structures are contemporaneous with the Neogene shortening.

The principal shortening and stretching directions are consistent with the pattern of major structures: thrusts, sub-parallel to the cordillera, and rifts, radially distributed around the Magellan basin. Analogue models have shown that simple corner conditions, marking a transition between subduction along the western margin of the continent and strike-slip motion along its southern margin, are sufficient to account for many of the structures observed.

In general, active rifts developed within foreland basins are unusual, but in Patagonia they are consistent with regional deformation and its plate tectonic setting during the Neogene. Furthermore, extensional structures are not restricted to the axial zone of the Magellan basin, but are distributed more widely across the basin.

Tectonic-Structural and Petrological-Metallogenic Individuality of the Eastern Albanides' Ophiolite - The Consequence of the Upper Mantle Heterogenity, from which they have been Generated

Irakli Premti

Geological Survey of Albani

Geological Map 1:200 000 scale of the Albanides' Ophiolite

Lithological petrological section of the ultrabasic massifs and volcanics of both ophiolite belts

Diagram of the chemistry and petrography of the Albanides' Ophiolite, based in the major elements, microelements and rare earths

Elements of the tectonic setting and structures of the ultrabasis massifs and the relations with the surrounding rocks. Structural aspects of the chromium, copper sulphide, platinum etc. deposits.

Diagram showing the metallogenic character and spatial setting of the main mineralizations related to the Albanides' Ophiolite.

Comparison of the above mentioned features of the Eastern belt ultrabasic massifs of Albania and with the other analogous of the alpine belt (Hellenide, Tauride, Oman etc.)

Scheme of the geodynamic mechanism of the ophiolite formation.

The opinions about the formation way and genesis of the main mineralizations related to the eastern belt ophiolites of Albanides are provided here.

The following main conclusion can be drawn:

Although the eastern belt ophiolites of Albanides are characterized of the similar features to those of the island-arc type each massif shows individual features, indicating that their formation and development are particular and the upper mantle tectonic-magmatic conditions haven't been the same.



EUG 10
28th March - 1st April, 1999
Strasbourg, France

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