0954
The geometrical profiles and surface velocity distributions in active accretionary wedges are functions of the bulk rheology and the tractions on the basal and rear surfaces
. Analytical and numerical solutions of the force-balance and corner flow circulation equations indicate that the mechanical parameters should in principle all be directly determinable from the observational variables. With sufficiently precise geodetic determinations of the strike-normal and strike-parallel velocity distributions, together with the wedge geometry, it should therefore be possible to distinguish different bulk rheologies and determine all the main rheological variables.
The angle of taper and surface slope of Coulomb and plastic wedges depend on the yield strength of the wedge and the basal shear stress. The surface profile of a viscous wedge is a function of the bulk viscosity, the viscous coupling at the base, and the scale. Wedges with a bulk viscous rheology are unique in that they show corner-flow circulation in the absence of accretion. The strike-normal velocity away from the backstop produced by this circulation is a function of the bulk viscosity and the viscous coupling at the base, and can therefore be used together with the surface profile to determine both variables. Current constraints on the rates of corner flow suggest a bulk viscosity > 3x1019 Pa.sec, and a viscous coupling at the base < 0.3 (i.e, the shear stress at the base of the wedge is 0.3 of the value required if the shear strain were distributed throughout the wedge).
Obliquely converging non-accreting plastic and Coulomb wedges show no distributed strike-parallel shear. If the obliquity is below a critical angle, the wedge moves with the upper plate, and relative motion at the wedge front has the same obliquity as the plate motion vector. For higher angles of obliquity, the wedge is separated from the upper plate by a strike-slip fault, defining a forearc sliver. Relative motion at the wedge front in this case has a lower obliquity than, and is independent of, the plate motion vector. The critical angle of obliquity and the velocity of the fore-arc sliver are functions of the boundary tractions and the wedge geometry. In obliquely convergent viscous wedges, the strike-parallel velocity is distributed throughout the wedge, decaying exponentially from the rear to the front. Its value at the rear is a function of the viscosity and the viscous coupling on both the rear and the base, and the length scale for the decay is depends on the viscosity and the coupling at the base.
3642
New structural and kinematic data are presented on the high pressure, low-temperature (HP/LT) Middle Penninic units from the Western Alps that clarify the important role of syn-collisional normal faulting. These HP units are constructed of a W-dipping arcuate wedge inserted between the external Briançonnais zone in the west and the internal crystalline massifs (ICM) in the east that is continuous from the Graie Alps to the Ligurian Alps. A major west dipping ductile normal fault assisted the exhumation of the HP units. The hanging wall of this ductile fault is represented by the external Briançonnais zone at shallow crustal levels (¾ 4-5 kbar) in which post-Eocene E verging folds represent the first tectonic event. The footwall is represented by rocks of the outer HP/LT zone characterised by fresh carpholite (ca 8 kbar, T¾350° C) indicative of their rapid exhumation along a cooling path. Jadeite-lawsonite W-dipping blueschists below pass progressively downward to garnet-zoisite assemblages. They form the middle part of the wedge that encompasses Mesozoic oceanic units and reworked basement below. Monviso-type eclogites (ca 20 kbar) form the deep part of this edifice that overlies the edge of the MCI massifs and related cover. The MCI crust records a very complex HP to UHP evolution ending with Tretiary thrusting onto greenschists, doming and tectonic unroofing.
We consider that the W-dipping Tertiary structures of the wedge formed within a cold and low-angle, W-dipping subduction zone representing the northward prolongation of the well constrained early Oligocene Sardinian-Corsican subduction. HP fabrics in this arcuate wedge display a regular mean NE trend of mineral and stretching lineations (glaucophane, carpholite, etc...) throughout the southern part of the Alpine arc. The partly retrogressed HP fabrics of most rocks displaying ambiguous shear-sense indicators record the combined effects of plate convergence (top to NE) and syn- to late-convergence exhumation (top to W). Differential and diachronic low-angle (top to NE) uplift of slices of subducted material of both oceanic and continental origin may explain the complex pattern of metamorphic zones and facies. The 3D geometries of the outcropping pre-Alpine inner Briançonnais basement is that of large sheath folds formed during the evolution of low-angle, forced extrusions. This pre-Oligocene exhumation tectonics is at variance with the Swiss Alps nappes that essentially record NW-directed thrusting.
1864
The occurrence of high-grade rocks in orogens indicates considerable unroofing and possible links to tectonic extension as well as erosion, which in turn must link to external zone dynamics. In the Alps we have shown in a six-year program of structural, metamorphic and geochronological work that there was a period of extension on a 1-2 km thick SE-directed shear zone (the Gressoney Shear Zone, GSZ), which unroofed the HP and UHP eclogites in its footwall to greenschist facies depths. This extension lasted from 45 to 36 Ma. It is non-trivial to confirm that thrusts operated at this time. Rather than direct examination of external zone thrusts, the foreland basin provides key evidence for shortening. In this time period the edge of the foreland basin advanced NW by ca. 50 km, which is a proxy for movement of the thrust load. Geometric arguments show that there was ca. 100 km of extension. Therefore this was a major component of Alpine evolution. Even if the extension is overestimated and thrusting underestimated, they are of comparable magnitudes. Thrusting linked to internal zone extension can be explained by buoyant uprise of pips or wedges. Other dynamic models are possible, but must accord with our demonstration that extension is the same order of magnitude as thrusting in the Alps in the late Eocene.
0402
One of the challenging questions in metamorphic petrology, the preservation of HP- and UHP-LT rocks, imposes that rocks form and evolve along persistent cold geotherms characteristic of subduction-related environments (Ernst, 1971). Despite the existence of several distinct models having reconciled many geological observations and increasing numerical modelling, the exhumation of alpine UHP rocks is still a matter of debate (for a review : Platt 1993, Duchêne et al., 1997). There is nevertheless increasing recognition that (1) alpine UHP rocks were exhumed following a two-step evolution characterized by a rapid early ascent; (2) though not negligible, erosion alone cannot account for rapid exhumation; (3) exhumation probably proceeds during active convergence, as stressed by plate kinematics reconstructions; (4) UHP metamorphism could be mostly Tertiairy in age as indicated by a wealth of recent geochronological data.
In view of combining new structural and metamorphic data bearing on exhumation in the Alps, we focussed our study on the Schistes Lustrés from the Cottian Alps, because (1) this unit links the external domains and the UHP Dora Maira massif, (2) patterns of exhumation in the Schistes lustrés unit were not given much attention to date, (3) the lateral evolution of the tectonometamorphic patterns can be studied almost continously, due to good outcrop conditions along the transect.
Metamorphic conditions are shown to increase from west to east, from 11 kbar 350°C to 20 kbar 550°C close to the Dora Maira massif, on the basis of a detailed study of carpholite and high-pressure phengite occurrences (Agard et al., in prep.). Two distinct exhumation stages are recognized: (1) a pervasive east-vergent, relatively coaxial, ductile D2 event which took place under low blueschist-facies to greenschist-facies conditions. This event is responsible for most of the exhumation of the Schistes lustrés unit as well as for the preservation of carpholite occurrences at the front of the unit, and was probably active during the period 50-40 Ma; (2) a west-vergent ductile-to-brittle non-coaxial D3 event, marked by a deformation intensity decreasing from east to west. This event takes place at subgreenschist-facies conditions and is thought to be coeval with the west-vergent greenschist deformation taking place in the Dora Maira massif. It is suggested that this event reflects the major exhumation movements taking place in the more internal domains by ca. 38-30 Ma.
Duchêne S, Lardeaux JM & Albarède F, Tectonophysics, 280, 125-140, (1997).
Ernst WG, Journal of Petrology, 12, 413-437, (1971).
Platt JP, Terra Nova, 5, 119-133, (1993).
1876
The Western Alps offer an excellent opportunity to study significant volumes of well preserved high to very high pressure and low temperature metamorphic rocks. Especially a section from the Pelvoux massif (External French Alps) to the Dora Maira massif (Internal Italian Alps), through the Monviso ophiolitic complex, shows important metamorphic gaps between the different tectonic units. This crustal-scale cross-section was investigated in the framework of "Geo-France 3D Alpes" program of geological and geophysical imagery.We present and discuss the results of an integrated study combining structural and metamorphic analyses, seismic tomography and geochronological systematics. These different investigations point out the existence of important partitionning of syn-convergence deformation between high and low pressure metamorphic units and allow three types of tectonic contacts to be distinguished:- low angle ductile to brittle normal faults (Viso / Queyras contact),- thrusts, sometimes reactivated as normal faults (contacts within Dora Maira massif separating eclogitic and blueschist domains)- combinated normal and inverse faults indicating tangential extrusion process (Acceglio / Queyras contact). Each of these structures participate actively to the total exhumation process and the salient results are:- exhumation rates decrease through time, - extensional tectonics corresponds to the accomodation of vertical extrusion (displacement) of the higher pressure units,- vertical extrusion of crustal units is related to mantle indentation. The mantle indenter acts like a rigid piston pushing crustal units toward the earth surface during convergence, inducing the progressive development of both inverse and normal faults.
0918
The Central Unit is a NE-dipping band of mylonitic rocks which constitutes the suture between the Ossa-Morena Zone (OMZ) and the Central Iberian Zone (CIZ) in the SW Iberian Massif. In the Central Unit, high-pressure assemblages have been locally preserved; these assemblages were retrogressed first to amphibolite and then to greenschist facies conditions during an intense left-lateral extensional shearing that affects the whole unit. The CIZ constitutes the hanging wall of this suture, being the tectonic evolution of its southernmost part directly related to the one of the suture. The first Variscan structures in the southernmost CIZ were produced in a transpressional ductile regime that gave rise to NW-SE trending km-scale recumbent folds verging to the NE, coeval with a top-to-SE shearing parallel to the axial surfaces. The metamorphic conditions during the recumbent folding and shearing were of low- to medium-grade and ~4-5 kbar. The age of these folds is Middle to Late Devonian since they affect Lower Devonian rocks, but Lower Carboniferous sediments unconformably overlay them. At the same time in the OMZ, the footwall of the suture, SW-vergent recumbent folds and thrusts were being formed. All these structures resulted from the compressional evolution of the OMZ-CIZ suture, being interpreted those of the southern border of the CIZ as conjugate structures with respect to the main SW-vergent thrust that superposed the CIZ over the OMZ. Subsequently to this compressional stage, the rear part of the thickened crustal zone collapsed by means of the NE-dipping Matachel Oblique-Extensional System. This system is responsible for the intense mylonitic retrogression affecting the Central Unit, and has a throw of ~30 km according to the metamorphic gap between the eclogites included in the suture zone and the southern CIZ rocks. This extensional collapse provoked the exhumation of the previously eclogitized rocks included in the suture zone, and produced subsidence and magmatism in the southernmost CIZ. Thus, a large NW-SE trending ~90 km wide basin (the Guadiato-Pedroches Basin), filled in with up to ~7 km of "Culm" facies synorogenic sediments, was formed during the Early Carboniferous. Inversion of the Lower Carboniferous basin began with the development of a top-to-the-NE thrust in Early Namurian times. This shortening episode continued with the formation of NW-SE trending upright folds in the Late Westphalian and, finally, with left-lateral NE-directed reverse faults. The strike-slip component of the late brittle faults is the final expression of the important lateral movements during the whole evolution of this Variscan collisional system.
2884
The 300 x 300 km Rhodope metamorphic system formed in the Cretaceous to Eocene times at the southern active margin of Europe against the Tethys ocean. It includes thrust nappe units of both continental and oceanic origins. P,T paths went through eclogitic conditions before pervasive recrystallisation under amphibolite conditions. Exhumation went at a fast rate and retrogressive metamorphism is restricted to ductile fault zones. The overall structure displays both top to SW shear (lower-medium part) and top to NE shear (upper part) related to the same foliation pattern and to similar amphibolite conditions. We interpret the metamorphic belt through upward-forelandward expulsion of continental terranes that were driven at depth along the subduction plane and returned to the surface due to their high buoyancy. A typical clue for SWward expulsion with respect to both the lower and upper plates is given by the long scar left between the lower (top to SW) and upper (top to NE) parts. The older post-metamorphic deposits are Late Cretaceous in age. The Cretaceous Vardar subduction trench, established immediately SW upon an Early Mesozoic oceanic crust, received decakilometric olistoliths that derive from the metamorphic wedge. Both the metamorphic ages and the geological history in the lower and upper plates point to a continuous underthrust - continuous exhumation process that lasted some 50 Myears. The post-Eocene tectonic-magmatic activity and renewed uplift is likely related to final detachment of the subduction slab.
Ricou L-E, Burg J-P, Godfriaux, I & Ivanov, Z, Geodinamica Acta, 6/6, (1998).
1289
In the Rhodope domain to the north of the Recent Aegean back arc basin, from the early Tertiary to the Miocene, during and after continental collision, a variety of crustal metamorphic rocks was exhumed. HP-metamorphic rocks developed at various episodes in the interval between the Cretaceous and Tertiary. The structural, metamorphic and geochronological record of this domain mostly reflect the retrograde, i.e. the exhumation histories of individual superimposed tectonic metamorphic complexes. The kinematics of tectonic movements associated with exhumation of HP- rocks involving early Oligocene and Miocene extensional tectonics and custal stacking and is depicted by: (i) penetrative mylonitic fabrics that overprinted HP- rocks during decompression and (ii) sharp tectonic contacts between tectonic units that substantially differ in exhumation/cooling histories. Such contacts are interpreted as extensional detachment faults.
An important extensional detachment fault that caused substantial crustal thinning and that significantly contributed exhumation of a Tertiary HP-metamorphic complex can be can traced over more than 100 km from the Xanthi area (Central Rhodope) to the Bulgarian boundary in the eastern Rhodope. The lower plate consist of strongly mylonitized gneisses locally containing ambhibolitized eclogites. Present geochronological data constrain a Tertiary age of HP-metamorphism and exhumation of this unit between 37 and 35 Ma. The detachment surface locally truncates at high angle the mylontic foliation and the lithological succession of the lower plate and is interpreted as an extensional ramp. Mineral ages of the lower plate and structural relationships of dated granites constrain formation of this detachment surface between 37 and 30 Ma following formation of the mylonites. The upper plate is also composed of a high grade HP metamorphic complex, however characterized by early Cretaceous HP-metamorphism and early Teriary exhumation (Kimi-complex). This complex, which reaches a thickness of only a few 100 meters is superimposed by weakly metamorphosed Mesoszoic sequences that both were overlain by a transgressive contact by unmetamorphosed Late Eocene to Oligocene sediments. We will discuss the kinematics and magnitudes of tectonic movements on the mylonites and this detachment surface and the relationships between compressional and extensional tectonics during Alpine collision in the Eocene and Oligocene time in this region based on new structural and petrological data.
0951
If the post-orogenic Miocene exhumation of metamorphic rocks of the Aegean Sea is clearly governed by extension the earlier syn-orogenic Eocene exhumation is still poorly understood. Previous authors have argued in terms of extension (Avigad et al, 1989) or compression (Lister and Raouzaios, 1996). Our observations on Sifnos and Syros show that exhumation of HP-LT rocks involves crustal scale detachments during the Eocene. Top-to-the-NE asymetric ductile shear begins in blueschist facies at 42 Ma, and carries on until greenschist facies conditions until 18 Ma. The deformation history and the P-T-t paths suggests a continuum of extension from the syn-orogenic to the post-orogenic stage responsible for exhumation and thinning of units. P-T paths are calculated using the variations of composition of chlorites and/or phengites. Applying this method to invariant parageneses, we obtain PT points instead of PT fields. Several P-T paths are evidenced with this method for the different structural levels of a single unit. The preservation of eclogitic parageneses at the top of the structural pile is correlated with a cold P-T path. In contrast, the progressive retrogression observed in underlying rocks is correlated with a nearly isothermal decompression. In Syros island, the main tectonic contact separates the upper Vari Unit that remained cold since the Cretaceous from the underlying eclogites of Ermoupoli Unit of Eocene age. We conclude that this contact has probably worked as a ductile detachment until the Early Miocene (Burdigalien). Similar conclusions may be reached for Sifnos although no major detachment is observable. The history of deformation and metamorphism can account for a simple shear extensional model. Comparison of these results to available data on Crete island allows us to reconsider a tectonic model of evolution of the Aegean domain during the Cenozoic.
Avigad D, Garfunkel Z, Terra Nova, 1, 182-187, (1989).
Lister G, Raouzaios A, Journal of structural geology, 18, 1417-1435, (1996).
0140
The mechanics and kinematics of mountain building, including the onset of extension during convergence, is - at least for simple geometries - largely understood. Much of the earlier advances in this field were summarised in the review paper of Molnar and Lyon-Caen (1988) and Earth scientists have since expanded the simple one-dimensional models to explore more complicated and geologically more realistic scenarios (England and Houseman, 1989; Beaumont et al., 1996). These studies include some fully three-dimensional models (Braun, 1992; Platt, 1993). However, in many integrated studies of surface uplift, exhumation and extension, the reference frames are confused. In order to clarify this confusion, Molnar and England (1990) provided a clear definition of reference frames for vertical motions in the crust. This study was expanded by Stüwe and Barr (1998) who explored the depth dependence of the vertical velocity field during convergence with a simple one-dimensional model.
We admit that the simplicity of the model does not allow direct application to any orogen, but emphasize its use for visualising the possibility of the simultaneous occurrence of different vertical motions in the crust. This contribution aims to clarify the reference frames during vertical motion using this model. The model assumes homogeneous thickening of the lithosphere.
It is shown that - despite the restrictive boundary conditions - rocks may move upwards or downwards in the crust only depending on (a) the relative rates of denudation and thickening and (b) on the initial depth of rocks in the crust. The deepest initial depth from which rocks can be exhumed depends on the density contrasts of crust and mantle lithosphere, but is independent of the denudation and thickening rates. For a range of reasonable parameters, this maximum depth is around 30 km or less. Thus, there is no conflict in the observation of thickening deformation phases at the time of decompression of metamorphic rocks, if the peak pressures are less than about 10 kbar.
The model also predicts a characteristic association between surface uplift (defined as the vertical motion of the surface with respect to sea level) and exhumation (defined as the vertical motion of rocks with respect to the surface). Thus, detailed documentation of the relative timing of uplift and exhumation histories may be a useful tool to constrain the exhumation process. Finally, the model can be used to explore the possible relationships between up- or downward vertical motion of rocks in the crust and extension versus compression as a function of depth.
Molnar P & Lyon-Caen H, Geol. Soc. Am. Spec. Pap., 218, 179-207, (1988).
England PC & Houseman G, J. Geophys. Res., 94, 17561-17579, (1989).
England PC & Molnar P, Geology, 18, 1173-1177, (1990).
Braun J, EOS, 73, 292, (1992).
Beaumont C, Ellis S, Hamilton J & Fullsack P, Geology, 24, 675-678, (1996).
Stuewe K & Barr T, Tectonics, 17, 80-88, (1998).
0764
The Neogene to Present evolution of the Eastern Alps east of the Tauern window show a close relationship of present topographic data as revealed by a digital elevation model, apatite fission-track (FT) data, fault activity and sedimentary basins related to these faults. Published (Hejl, 1997) and new apatite FT data lead to the following results: (1) In the Niedere Tauern block, 10 FT ages with a mean of 18 Ma display no vertical gradient over 1.5 km, which means a fast exhumation pulse in late Early Miocene times. (2) Detrital apatite in the Tamsweg basin, adjacent to the Niedere Tauern block and the Tauern window, reveals similar, nearly synsedimentary FT ages. (3) 7 FT-ages from the Gurktal block to the south of the Niedere Tauern are around 32 Ma (Early Oligocene). The Oligocene apatite ages characterize an area where remnants of a formerly extended paleosurface are preserved. In contrast, the Miocene apatite ages in the Niedere Tauern characterize a tectonic block with steep slopes and deeply incised valleys, testifying young uplift. The Tamsweg basin, which does not contain Penninic pebble material from the Tauern window, received its detrital material from an area with fast exhumation; the logic candidate for its source area are fastly eroding deep levels of the Austroalpine crystalline basement above the still buried Penninic rocks of the later Tauern window. A kinematic analysis of faults, along which the intramontane sedimentary basins like the Tamsweg basin were formed in late Early Miocene times, reveals a complex history with changing paleostress directions.In conclusion, in late Early Miocene times an important tectonic event in the area east of the Tauern window caused a fundamental change in the uplift, erosion, and sedimentation pattern. Differential uplift is reflected by contrasting thermochronologic and geomorphologic signatures.
Hejl E, Tectonophysics, 272, 159-173, (1997).
2352
Along the southwestern edge of the Tauern Window, Penninic, Austro-Alpine and South Alpine units are separated by non-parallel shear zones defining a west-east-trending wrench corridor. Tectonics were mainly active in Oligocene times and accounts for exhumation of the Tauern Window. Differential vertical displacement within this broadening wrench corridor is evident from:(1) Granitoid intrusions with decreasing depth of emplacement from west to east. (2) Variable horizontal (north-south) shortening corresponding with differential vertical stretch as estimated from profile balancing and strain analysis. (3) Spatial variation of thermal isogrades. 3D-kinematic models hab been used to quantify vertical displacement. The model has to explain observed structural data including horizontal stretching lineation and vertical shear planes and to fit data on variable vertical stretch. Input data to constrain flow geometry include estimates on mean kinematic vorticity using microstructural techniques and published data on Tertiary plate motion. We conclude that: (1) Deformation can be described by a transpressive regime with divergent flow or differently convergent flow in an oblique convergent wrench corridor becoming broader from west to east. (2) Transpression is combined with lateral extrusion to stabilize horizontal lineation in a pure shear dominated kinematical frame. (3) Boundary effects include east-west-coaxial stretch along the rheologically weak Tauern Window boundary and discrete shear along the stiff Periadriatic Lineament.
1139
The eastern Lepontine Alps in Switzerland are both a structural and a metamorphic high. Within the gently northeastward dipping nappes metamorphic grade increases gradually towards the South where it reaches upper amphibolite facies to granulite facies conditions. The Insubric Line represents the abrupt southern border of Alpine high-grade metamorphism. Immediately north of it a series of backfolds brings the nappes from their shallow orientation into an upright to overturned orientation. This "Southern Steep Belt" is generally regarded as a ductile high strain zone along which the Lepontine Alps were exhumed by backthrusting and erosion (Schmid et al., 1989).
The field area in the lower Valle Mesolcina is located in the vicinity of the backfolds, nearly entirely within the Adula-nappe. Three major deformation-phases can be distinguished. The main foliation S1 in the northern part is the axial-plane foliation of isoclinal D1-folds. The stretching-lineation L1 and the D1-fold axes both dip gently northward. Shear-sense indicators show a top-to-the-north transport direction. Towards the South a second generation of open to isoclinal folds D2 overprint the D1 structures. These folds are consistently southwest vergent and locally produce a new axial-plane foliation S2. The southeastward dipping stretching lineation L2 is parallel to the D2 fold axes and related to top-to-the-Southeast directed shear senses. Large backfolds of the third phase subsequently reorient the whole nappe pile into the southern-steep-belt. Axial-planes of major D3-folds strike NNW-SSE in the northern part of the area and turn into an E-W-orientation in the southern part. As small-scale folds in the southern steep belt could best be regarded as D2 folds we interpret the foliation there as an S2 structure. Metamorphic assemblages in metapelites record decompression on the order of 5 kbar between D1 and D3 and probably heating at rather low pressure during D3.
Retrodeformation of D3-folds brings the southern steep belt into a south-dipping orientation. It would then represent a D2 high-strain zone with a sinistral-transtensive motion. This interpretation is in line with the observed decompression between D1 and D3. The D2 deformation in the Southern Steep Belt is interpreted to represent the continuation of the Turba-Mylonite-Zone (Nievergelt et al., 1996) at the base of the Eastern Alps in Graubünden which is of mid Oligocene age. The structural and metamorphic data suggest that the Lepontine dome forms a metamorphic core complex exhumed in mid Oligocene time in an extensional setting (Bradbury and Nolen-Hoeksema, 1985).
Bradbury HJ & Nolen-Hoeksema RC, Tectonics, 4/2, 187-211, (1985).
Nievergelt P, Liniger M, Froitzheim N, Ferreiro Mählmann R, Tectonics, 15/2, 329-340, (1996).
Schmid SM, Aebli HR, Heller F, Zingg A, Alpine Tectonics, Geological Society, 153-171, (1989).
1644
A kinematic model on the Oligocene-Present tectonic exhumation of fault-bounded blocks within the Austroalpine-Penninic nappe stack in the North-Western Alps is inferred from an integrated study combining remote sensing, structural geology, geochronology and seismology. Two brittle tectonic patterns have been recognised by crosscutting relationships along the geological transect between the Mont Blanc Massif and the Canavese Line. In the Oligocene, a NW-SE extension (perpendicular to the belt axis) took place, consistently indicated by the displacements along four conjugate fault sets and by extension directions on gold-bearing quartz veins and andesitic dikes, both yielding radiometric ages around 31 Ma (Diamond 1990, Dal Piaz et al. 1979).This extensional phase lead to the differential exhumation of large fault-bounded blocks of the nappe stack, well represented by cooling rate contour maps based on published cooling ages (Ap FT, Zr FT, Rb/Sr Bt ages; Hunziker et al. 1992, Seward and Mancktelow 1994). A T-related rheological transition took place at that time in the ophiolite-bearing Piemont calcschists (underlying the Austroalpine system), since diffuse low-angle detachments evolved into sharp high-angle fault planes, always showing the same NW extension direction. Conversely, in harder rocks like Austroalpine and Penninic gneisses, only high-angle brittle faults, and no low-angle detachments, have been found. Hydrothermal activity preferentially developed along large normal fault zones, as evidenced by gold-bearing quartz veins transecting the nappe pile and by carbonatic metasomathism on serpentinites (listvenites). The second tectonic pulse is related to the well known lateral escape coeval with the tectonic unroofing of the Lepontine dome along the SW-dipping Simplon line (Hubbard and Mancktelow 1992). The large SW-escaping block is bounded to the SE by the sinistral Ospizio Sottile fault (extending from Val Sesia to Champorcher) and shows a strong internal deformation due to a complex network of faults first detected on satellite images (LandsatTM and ERS-1). Most of these faults reactivated Oligocene structures.This displacement pattern lasted from the Miocene to the Present, as is proven by cooling ages and microseismicity. Ductile structures have been recognised at any scale to be older than brittle features, always showing direct to transcurrent displacements. Hence, a pre-Oligocene age may be envisaged for the ductile backthrusting-backfolding processes across the North-Western Alps, previously referred to the Oligo-Miocene by a comparison with the Central Alps.
Dal Piaz GV, Venturelli G & Scolari A, Mem. Sc. Geol. Padova, 32, 16, (1979).
Diamond LW, American Journal of Science, 290, 912-958, (1990).
Hubbard M & Mancktelow NS, Geology, 20, 943-946, (1992).
Hunziker JC, Desmons J & Hurford AJ, Mémoires de Géologie (Lausanne), 13, 59, (1992).
Seward D & Mancktelow NS, Geology, 22, 803-806, (1994).
0666
The fine grained siliciclastic Lech Formation of the "Kreideschiefer basin" within the northwestern portion of the Northern Calcareous Alps (NCA) of western Austria and southern Germany reflects the Austroalpine Aptian to Cenomanian (?) transition from a passively subsiding carbonate-dominated platform- and -basin enviroment to an active submarine deformation front. The margins of the marine Kreideschiefer basin were partly reactivated low-angle extensional faults of probable latest Triassic (?) Jurassic ancestry and partly frontal thrusts. Thus basinal lows received both distally derived fine grained siliciclastic detritus and locally detached carbonate olistoliths ranging in size from metres to hundred of metres. The contemporary northwesterly advance and emplacement of the strongly segmented Inntal thrust sheet and continuing deformation of footwall strata in the Lechtal thrust sheet reflects a dextrally-convergent setting that also produced a first cleavage. The Kreideschiefer basin thus reflects locally divergent settings within a northwesterly prograding thrust wedge during Aptian to Cenomanian (?) times. In other parts of the western NCA the ascent of basaltic dikes at about 100 MA (Trommsdorff et al. 1990) was probably also related to this transitional setting in which local extension alternated with regional incipient contraction. Triassic-Jurassic klippen of the Inntal thrust sheet and other thrust masses now preserved as isolated klippen on top of the Lech Formation indicate that the first deformation preceded the regional emergence of the western NCA above sea-level. This emergence is illustrated by the unconformity below coarse grained clastics of the basal Gosau Group of Turonian to Santonian age. Subsequent north- to northeast-directed contraction during the latest Cretaceous- Paleogene long distance movement of the Austroalpine complex superimposed fold-thrust structures corresponding in age to a second cleavage in the Kreideschiefer basin and a first cleavage in the Gosau Group. This second deformation was probably related to the northerly emplacement of the entire Austroalpine thrust wedge onto the distal European margin.
Trommsdorff V, Dietrich V, Flisch M, Stille P & Ulmer P, Geologische Rundschau, 79, 85-97, (1990).
0258
In the internal part of modern orogenic belts, the late deformation field post-dating nappe emplacement, during the last stages of continental collision, under brittle-ductile to brittle conditions, is strongly neglected while it has a great impact on the actual nappe geometry. At large scale, it is partitioned between late extension and strike-slip faulting, i.e. between vertical and lateral extrusion (e.g. Tapponnier et al., 1977). In order to better understand the late deformation field in the internal domain of the eastern Central Alps, we used a multi-scale approach running from the regional scale to the meso-scale. The internal deformation of a deformable block, deduced from the fault-slip analysis of minor fault populations, is compared with the deformation at its boundaries, defined by major discontinuities. To avoid problems due to previous deformations and strong mechanical anisotropy, studies are focused on Oligo-Miocene intrusions in the Bergell and Insubric areas.The Bergell triangular block is delimited by 4 major discontinuities with kinematics compatible with its early Miocene northeast-directed lateral escape. The NW-SE oriented Forcola and Muretto normal faults, at the western and eastern block boundary respectively, are bounded in the north and in the south by two major strike-slip faults, the NE-SW oriented, sinistral-reverse Engadine Line and the dextral E-W oriented Insubric Line. Complex brittle faulting at the Muretto fault suggests a continuous transition from normal faulting to dextral strike-slip tectonic regime, the extension axis remaining sub-horizontal and consistently NE-SW to ENE-WSW oriented. In the Bergell block, the overall small scale fault pattern includes strike-slip, oblique normal and normal faults. The kinematic analysis of linked extensional and compressional structures, revealed local-regional permutations between the short and intermediate axes of the strain ellipsoid, while its long axis remained sub-horizontal and NE-SW to ENE-WSW oriented. The internal deformation of the block is quite homogeneous and points out the compatibility of deformation fields at the meso- and regional scales. This quasi-systematic low-angle clockwise misorientation of the long axis of the strain ellipsoid is interpreted as the recording of the quantity of deformation by minor fault populations, inside the Bergell block. This overall fault pattern at all scales is the result of the early Miocene NW oriented dextral transpression along the Canavese-Insubric indenter, which caused an horizontal stretch parallel to the orogenic strike. The major discontinuities bounding the system accomodated most of the displacements and kinematically allowed this lateral escape. This lateral escape doesn't imply a strong component of gravity spreading during the indendation.
Tapponnier P & Molnar P, JGR, 82 (20), 2905-2930, (1977).
3041
The Southern Carpathians, are, from bottom to top and from external to internal, made up by Danubian nappes (exposed in the Danubian window), the oceanic Severin unit, and Getic and Supragetic nappes. Structural and kinematic analysis of fault rocks from this area revealed a two-stage Alpine tectonic evolution for the major tectonic units. Kinematic indicators associated with Upper Cretaceous top-SSE directed nappe stacking are only preserved in the Lower Danubian Schela nappe. The scarcity of these earlier shear senses is mainly due to the strong overprinting effect of a second event, namely WSW-ENE orogen-parallel extension. Top-ENE directed shearing in lower greenschist facies mylonites from the eastern part of the Danubian window is associated with an east-dipping low-angle detachment at the base of the brittely deformed Getic nappe (the Getic detachment). Below the dome-shaped Getic detachment normal faulting led to tectonic omission of most of the Severin unit while the low-grade metamorphic Danubian units were rapidly exhumed. Rather rapid cooling (ca. 25°/Ma) and inferred exhumation took place in the Eocene for the western part and in Oligocene time for the easternmost part of the Danubian units. We assume that the Danubian units, during their exhumation, passed from ENE to WSW through a "rolling hinge" located close to the eastern end of the Danubian window, which explains westward increasing cooling ages. Reheating of the cold hangingwall is evidenced by bimodal track lengths distributions in apatites from the Getic unit.
Hence the eastern part of the Danubian window represents a greenschist facies core complex, the grade of metamorphism within the footwall decreasing to diagenetic illite crystallinity values westwards, i.e. away from the E-dipping detachment. Yet, apart from this predominant E-W gradient clearly caused by orogen-parallel extension, an increase in grade of metamorphism is also observed from S to N. This N-S gradient implies exhumation of relatively warmer and deeper parts of the Cretaceous nappe stack in the north during core complex formation.
It is proposed that this orogen-parallel stretch was originally N-S oriented and that it formed when the Rhodopian fragment (which includes the Getic and Supragetic nappes) moved northward into an oceanic embayment, past the western margin of Moesia. This Eocene extension was part of a process of oroclinal bending in the area of the Southern Carpathians and their continuation into the Balkan mountains. Extension was followed by about 50° clockwise rotation of the Southern Carpathians, associated with dextral wrenching along the northern boundary of Moesia and compression in the Moldavides and the Eastern Carpathians.
0932
Well constrained concordant ion-microprobe U-Pb ages of 21.4 ± 0.5 Ma and 19.3 ± 0.4 Ma have been obtained from complex polyphase zircons separated from a high-P garnet granulite and a sillimanite K-feldspar gneiss, respectively, from two different locations in the Betic Cordillera of southern Spain. These ages are supported by four discordant 206Pb/238U ages in the range 19.8 - 22.9 Ma from the outermost growth zones of zircons from several other locations covering a 240 km strike length along the Cordillera: these discordant ages can confidently be interpreted as maxima. Taken together with published early Miocene Ar-Ar ages on metamorphic micas from the Betic Cordillera and from ODP Site 976 in the basement of the adjacent Alboran Sea, the new data provide definitive evidence of regional early Miocene high-grade metamorphism and rapid exhumation in the Alboran Domain. After Alpine collision and crustal thickening, the Alboran Domain underwent late orogenic extension in the late Tertiary, and subsided in part below sea-level in late Aquitanian to early Burdigalian time (about 20 Ma), forming the Alboran Sea. High-T metamorphism was therefore contemporaneous with and directly associated with the formation of a marine extensional basin.
Thermal modelling of the cooling path of the garnet granulite indicates that this rock was exhumed at between 5.5 and 7 km/m.y. This suggests that extension started at approximately 27 Ma, when the rock was at about 52 km depth; cooling below 800°C and closure of the U-Pb system in zircon did not take place until the rock reached about 16 km depth.
2159
U-Pb ages of magmatism and high-grade metamorphism have been obtained in the uppermost allochthonous units in the eastern part of the Ordenes Complex (NW Iberian Massif). The Corredoiras orthogneiss, a huge granodioritic pluton, has been transformed into a coarse-grained metagranite, or an augengneiss when heavily deformed. One fraction of euhedral, clear zircon prisms from the orthogneiss, gave a slightly discordant analysis, with a 207Pb/206Pb age of 500±2 Ma. The nearly concordant zircon age suggests that the possible thermal resetting caused by subsequent metamorphism and the participation of inherited components were limited. This is considered the age of intrusion.
A tectonic event induced a prograde metamorphism, which reached the amphibolite-granulite facies transition. The orthogneiss developed a granoblastic fabric with Bt+Grt aggregates and recrystallized feldspars, without muscovite. Within the orthogneiss, hornfelses and migmatites outcrop as kilometric xenoliths. Hornfelsic in origin, as indicated by preserved relics, the xenoliths were migmatized during the high-grade metamorphic event. The mineral association Bt+Grt+Pl+Kfs+Sil+Qtz, without stable muscovite, characterizes the partial migmatization of the hornfelses. This metamorphism has been dated in two migmatite samples, using monazites included in biotite from the high-grade association. One of the samples yielded a concordant 207Pb/206Pb age of 493±3 Ma, whereas the other seems slightly younger, with a age of 207Pb/206Pb 484±2 Ma. The temperature necessary for migmatization is above closing temperaure of U-Pb system in monazite, ensuring that the ages date the high-grade metamorphic fabric and that monazites are not contact metamorphism relics.
The Barrovian character of the high-grade metamorphism points to an Early Ordovician compressional event. Following crustal thickening, an extensional detachment brought into contact metasediments of the garnet zone, above, with the high-grade orthogneisses, below. Subsequently, both the orthogneisses and the detachment were folded and overprinted by compressional shear zones, developed under a new Barrovian metamorphic gradient, in conditions of the staurolite zone. Later on, a new decompressive event gave rise to the Corredoiras extensional detachment (Díaz García et al., in press), at the base of the orthogneiss and separating it from lower allochthonous units of the Ordenes Complex. Amphibolite-facies mylonitization in the detachment gave an age of 375 Ma (40Ar/39Ar, Dallmeyer et al. 1997). Structural analysis combined with isotopic ages allows to define two complete cycles of burial followed by exhumation, revealing a polyorogenic history for the uppermost units of the Ordenes Complex. The first is Early Ordovician, while second is probably early Variscan, as suggested by the Upper-Middle Devonian age of the Corredoiras detachment.
Díaz García F, Martínez Catalán JR, Arenas R & González Cuadra P, Geologische Rundschau, (in press).
Dallmeyer RD, Martínez Catalán JR, Arenas R, Gil Ibarguchi JI, Gutiérrez Alonso G, Farias P, Bastida F & Aller J, Tectonophysics, 277, 307-337, (1997).
0974
In the study of the timing and kinematics of late orogenic extensional basins the analysis of the syn-depositional relationship between the basin-fill and the rifting of the basement is crucial. In the Sorbas-Tabernas basin, SE Spain, a later stage compressional event has exhumed the basin floor to reveal some aspects of this relationship.
The Sorbas-Tabernas basin is an east-west trending, elongate (45 km long, 15 km wide), Neogene basin, one of a series of fault-bounded basins that are located on the metamorphic rocks of the Internal Zone of the Betic Cordillera. The basin lies between two ridges cored by metamorphic rocks, the Sierra de los Filabres to the north, and the Sierra Cabrera and Sierra Alhamilla to the south, which were source areas and/or formed palaeoslopes, of varying importance, throughout the basin history.
The kinematics and syn-sedimentary deformation of two sedimentary units near the base of the sequence, at either end of the basin, were analysed. To the east, are the red continental conglomerates of the Gafarillos member, and to the west, shallow marine conglomerates deposited on the western flank of the Sierra Alhamilla. Both sedimentary units show a coarse clastic input at the initiation of the sedimentary system. These sediments were syn-depositionally deformed by extensional faulting during the rapid subsidence at the early stages of basin formation. The basin-floor topography is controlled by (low-angle) normal faults, where, at both locations, the main spreading direction is northeast-southwest. Where there are faults with large displacements this syn-sedimentary relationship results in northwest-southeast trending extensional troughs or half-grabens acting as depocentres bounded by horsts. This local constraining of the syn-tectonic deposition controls unit thickness variations, lithological concentrations and palaeocurrent directions.
The termination of the (pure) extension episode appears to have been quite abrupt, as overlying sediments (marls with graded sand beds), are not affected by the same style of faulting. Dating of the units, by micropalaeontological and strontium isotope work, indicates that this extensional period had stopped sometime in the early - middle Tortonian.
0294
In the southern Menderes Massif to the south of Büyük Menderes Graben (western Turkey), the boundary between the so-called core granitic augen gneisses and overlying cover schists is marked by a major, south-dipping Alpine extensional shear zone along which the granitic protoliths of the core rocks in the footwall were metamorphosed and converted into mylonitic augen gnesisses during a top-to-the S-SW shearing under lower amphibolite-upper greenschist facies conditions. Shear zone also contained syn-shearing muscovite-quartz pegmatite from which undeformed or slightly deformed two large muscovite books yielded early Eocene Rb-Sr age of ~58-52 Ma, interpreted as indicating the time of movement in the shear zone. Previously reported 43-37 Ma Ar-Ar mica cooling ages from the host rock augen gneisses suggest that subsequent cooling occurred during the middle Eocene. The rocks of the southern submassif is unconformably overlain by the unmetamorphosed Lattofian-Rupelian mollase sediments of Kale-Tavas basin which in turn suggests that southern submassif was at the surface by the early Oligocene while the central and northern submassifs were still at greater depths.
Avaliable structural and geochronologic data from different parts of the Menderes Massif confirms that exhumation of the metamorphic rocks occurred through successive events and that while extensional collapse occur at one part of the massif, in adjacent areas convergence continued. These data also suggests that plate convergence along the Yzmir-Ankara Neotethyan Suture continued until late Miocene, whereas locally collapse began earlier in Eocene in the southern submassif, and in early Miocene in central and northern Menderes submassifs. This can be explained by the convergence of irregular margins during which incomplete collision let to extensional collapse in small areas, whereas in adjacent areas convergence continued.
It is also suggested that Menderes Massif as a whole is not a single core-complex, rather it is a more complex metamorphic culmination that comprises three (southern, central and northern) or may be more small core-complexes that differ from one another on age of exhumation, style of deformation, kinematics, and hanging wall rocks to the extensional shear zones that are responsible for their exhumation.
1348
Five major stages may be distinguished in the geological evolution of the western Anatolia; These are, 1-The pre-graben stage, 2- The east-west extension stage 3- the earier stage of the north-south extension 4-the cessation of the north-south extension 5-the later stage of the north-south extension
The pre-graben stage corresponds to the late Cretaceous-Pre Miocene period. During this stage the region underwent the collision of the Pontides with the Taurides. During the post collisional convergence thrusting propogated across the Pontides and the Taurides. Within the Western Taurides the Lycian nappe package travelled southward, and finally emplaced onto the Lower Miocene basin fill of the Antalya basin before the Late Miocene.
The east-west extension stage produced approximatelly north-south trending grabens during the early Miocene. Within the north-south grabens fluviale and lacustrine sediments were deposited. Along the fault zones the calc-alkaline, intermediate volcanic rocks of hybrid origin were extruded.
The north-south extension began during the late Miocene. The east-west trending Bozdag horst, located in the Middle of the Menderes Massif was elevated during this period. A detachment fault flanking the horst in the north and the south began to form. The initial stage of development of the east-west trending normal faults also occurred during this stage.
The contiunity of the north-south extension appear to have been ceased to the end of late Miocene or at the beginning of Pliocene as evidenced by the regionwide development of a flat-lying erosional surface. The present east-west trending grabens formed after the development of the erosional surface as the north-south extension has been rejuvanated during the Plio-Quaternary.
1376
The Gediz-Alasehir graben is one of good represantatives of the western Anatolian active graben system. The graben trends aproximately E-W, and developed under an ongoing N-S extensional regime. Age of inception of the graben is widely debated. According to one group of views the Gediz-Alasehir graben began to form during the Late Oligocene-Early Miocene and developing eversince (Cohen et al., 1995; Seyitoglu et al., 1996). According to the other group of views the graben is quite young; it began to form after the Late Miocene-Early Pliocene (Sengör and Yilmaz, 1981; Yilmaz et al., 1997). In order to solve the controversy, we stuided the region extensively and produced geological mappes in critical areas. According to the data, derived from this research, in the southern side of the graben area, the sequence, is formed from three different tectono/stratigraphic groups of rocks. These are; the lower unit, the middle unit and the upper unit. The lower unit begins on the high grade metamorphic rocks of the Menderes massif with a pebble-cobble conglomerate, formed as debris flow, fault scree and fan deposits. The source, according to the paleocurrent directions, is a fault-induced elevation of the Evrenli fault trending N-S. Away from the source westward the grain size are reduced significantly. The coarse clastic rocks give way to sandstones, marls and bitumuneous shales, which dominate the succession in the west. They were developed in a low energy lacustrine environment.
The seismic data display that the Evrenli fault may be traced under the present fill of the Gediz graben, where it delimits also a N-S lying graben, trapped within the E-W Gediz graben. Therefore the sedimentary rocks observed in the southern side of the present graben are infact the products of grabens of different orientations, formed under different tectonic regime during different periods. The older graben is Early Miocene and the present graben is post Early Pliocene in age. In this paper the successions and the geology maps of the grabens will be documented to detail these conclusions
Cohen HA, Dart H, Akyuz S & Barka A, J. Geol. Soc. London, 152, 629-638
Seyitoglu G & Scott B, Geol. J, 31, 1-11
1357
The Eastern Mediterranean region is ideal to study processes of continental collision and related extension and normal faulting, in view of excellent access, manageable scale and continuing tectonic activity. Specifically, western Turkey provides an entire exposed section through an orogen, beginning with relatively authochthonous Panafrican basement, overlain by a Palaeozoic-Mesozoic cover, including a Tethyan carbonate platform. The basement is overlain by the Lycian Allochthon that restores as a proximal-distal north-facing passive margin (Collins and Robertson, l997, l998). The structurally overlying Lycian Ophiolite was generated by supra-subduction zone spreading in the Late Cretaceous. Subduction culminated in melange accretion and trench-margin collision, driving initial southward emplacement of the Lycian Allochton onto the Tethyan continental margin (Menderes Massif). This was followed by restoration of a passive margin and construction of an Early Tertiary carbonate platform. Convergence resumed from the Early Eocene onwards, related to suturing of Neotethys to the north. The Lycian Allochton was reactivated and episodically thrust further further southeast, resulting in development of Mid Eocene and latest Oligocene-Early Miocene piggy-back basins (e.g. Tavas basin), formation and subsequent consumption of foreland basins (e.g. Faralya basin), followed by a final foredeep in the Miocene (Kas-Aksu basin system). Convergence ended in the Late Miocene with thrusting of the Lycian Allochton over the western edge of the Miocene foreland basin (Isparta-Elmali area). Simultaneous with outward thrusting the inner (northwesterly) zones of the orogen began to exhibit normal faulting and basin formation in latest Oligocene-Early Miocene. Extension was dominantly down to the south in the south (Buyuk Menderes graben), but down to the N in the north (Alasehir, Gordes and Selendi grabens; Hetzel et al l995). In the north, extension was achieved by regional-scale simple shear detachment along presently low-angle normal faults (Verge, l993). On a regional scale the detachment fault surface is deformed parallel to the extension direction into giant corrugations with a ca. 30 km wavelength-ca. 1.5 km amplitude, to produce NE/SW trending basins (e.g. Demirci basin; Purvis, l998). The detachment system locally exhibits strongly brecciated and sheared metamorphic basement (e.g. Alasehir graben) and is segmented along strike, but less corrugated than the northerly basins. After initial localised lacustrine deposition, coarse alluvial breccias were shed from the hanging walls of the graben and overlain by finer grained fluvial sands and lacustrine deposits of Early-Middle Miocene age (Purvis, l998). During continuing extension the fills of the Alasehir and Buyuk Menderes graben were faulted, back-rotated and reactivated by a renewed phase of extension in latest Miocene time, with continuing extension as part of the active east Aegean graben system. The regional diving force in convergence and collision was the closure of a Neotethyan ocean basin along the Izmir-Ankara-Ercincan suture. The probable main factor in extension was "orogenic collapse" and "roll-back" along a subduction in the south Aegean. Westward tectonic escape of Anatolia from the Africa-Eurasia collision zone in the east probably played a role from latest Miocene time onwards.
Collins A & Robertson AHF, Geology, 25, 255-258, (l997).
Collins A & Robertson AHF, J. Geol. Soc. (London), 155, 759-772, (l998).
Hertzel R, Ring, U, Akal C, & Troesch M l995, J. Geol. Soc. (London), 152, 639-654, (l995).
Verge N, Terra Abstracts, 5, 249, (1993).
Purvis M, Univ Edinburgh Unpubl PhD thesis, (l998).
1095
The Cyclades blueschist belt is tectonically overlain by a dismembered LP-MT metamorphic unit and relics of molasse basins formed since the Early Miocene in relation to extension and low-angle normal faulting in the Aegean region. The Miocene clastic sequences are a unique source of information on the part of the orogen that has been eroded or obliterated by extension, and on the style of the extensional tectonics. We studied the mineralogy, petrology and stratigraphy of molasse sequences on the islands of Paros and Mykonos, and performed K/Ar and 40Ar/39Ar analyses of metamorphic and igneous pebbles.From bottom to top, the lithology of the sedimentary sequences ranges from turbiditic marl and sandstone, to coarse conglomerate whose matrix and pebble size grow upwards. Pebbles having an exotic origin, often well rounded, are predominant in the lower part of the section and are still found at intermediate stratigraphic levels. They are mostly quartzite-gneisses containing abundant MP phengites (Si=3.2). Radiometric dating of 6 phengite concentrate from Mykonos and Paros yields ages of 82-93 Ma. These ages correspond to a metamorphic stage previously not detected in the Cycladic region. Their ages, composition, and metamorphic grade suggest that the investigated pebbles were derived from a Pelagonian hangingwall (of the type that overlies the Olympos region) now completely removed. Mica schists containing Si-rich phengite (Si>3.4) first appear in the molasse at the bottom of the intermediate levels. Mica composition suggests that these pebbles were derived from the underlying blueschist unit, and radiometric dating required to confirm this are underway. Sedimentary and low-grade metamorphic pebbles that may have been derived from the Upper Unit that overly the Cycladic blueschist belt at present, occur at intermediate levels too. Three whole-rock analyses yielded unexpectedly-young ages of 9-13 Ma. Similar ages were obtained by us on low-grade phyllites that comprise the Upper Unit of Paros (Dryos) and we speculate that they reflect resetting due to nearby intrusions or late ductile shear. Granite boulders yielding K-Ar ages of 10 Ma were derived from the now-exposed footwall below the molasse and are abundant in the upper levels of the sequences together with volcanic rocks. We suggest that during the Paleogene the structure of the Cyclades resembled that of the Olympos region with a Pelagonian block overriding the Cycladic blueschists belt. Subsequently to the lower Miocene extension gradually exposed the orogenic structure. Sediments were shaded into the extensional basins from progressively deeper parts of the footwall until the blueschist unit has been exposed in the middle Miocene. Subsequent extension, coeval with 5 Ma volcanism, dragged to the surface 10-Ma granites that were exposed in relatively steep fault scarps possibly defining breakaways zones.
0440
The 110 km long, 5 to 30 km wide, and up to 900 m deep asymetric Corinth-Patras rift shows a strong seismicity indicating the activity of a shallow, low angle (=20°) northward dipping detachment zone beneath the Gulf. But in northern Peloponnesus, Pleistocene and active normal faults striking N90°E to N130°E show steeper dip angles, around 40°. The major question concerning the rifting process is to understand the geometrical and mechanical relationships between the low angle detachment beneath the Gulf and the steeper normal faults outcropping in northern Peloponnesus.
The geometrical disposition and sedimentological characters of the thick syn-rift Pleistocene detritic series which widely cover parts of northern Peloponnesus show that their deposition was controlled by the activity of the normal faults. They also evidence that these faults have jumped from the South to the North to their present location.
The southernmost of these faults is quite different: it is a more than 70 km long, flat or low angle north dipping contact, with a mean N115°E strike. It is located along a major lithological and mechanical limit in the Hellenic nappe pile. Offsets show that this regional structure is a detachment fault. It corresponds to the inactive southern emergence of the seismic detachement zone active beneath the Gulf. Due to uplift and bulging resulting from extension, backtilting of the southern part of the detachement progressively locked its tail, and successive high-angle normal faults formed from the South to the North, linking the end of the active detachment to the surface.
These data show that the main structure which formed the Pleistocene Gulf of Corinth-Patras is a regional northward low dipping detachement zone, reactivating at least one major lithological discontinuity in the Hellenic nappe pile. This mechanism is in agreement with seismological data, and also explains:- The northward displacement of the Gulf and the regressive polarity of its sedimentation.- The general uplift and bulge of Northern Peloponnesus, and its northwards migration.- The successive norhward jumps of normal faults above the end of the active detachement. Although impressive on field, and until now considered to explain the regional extension, these normal faults are only steeper structures branching off the regional detachement.
Stratigraphic and structural investigation combined with ra-diometric age de-termi-nation show that the Alpine evolution of western Bulgaria includes Cretaceous nappe stacking followed by late Eocene to Oligocene extension, which was responsible for the exhumation of the crystalline Serbo-Macedonian Massif (SMM).
The nappe system, including an upper nappe (the Supragetic Morava nappe) and a lower (Struma) nappe, overlies metamorphic rocks of the so-called Serbo-Mace-donian Massif (SMM). Paleostress analysis of striations, combined with shear-sense criteria and fold vergence in this system, indicates two com-pressional stages during the Cretaceous. The related movement directions are generally top to the E. Nappe emplacement occurred between sedimentation of Lower Cretaceous flysch and Upper Eocene clastic sediments.
From late Eocene to Oligocene (possibly even to Miocene), post-orogenic relaxation led to pronounced extension. NE-SW oriented extension pro-duced a series of SW-dipping and NW-SE-striking normal faults. The faults are listric and cut down into the basement where they presumably merge into one or more low-angle detachments. Microstructures indicate general top to the SW normal faulting. Resulting crustal thinning led to the exhumation of the am-phibolite-facies (Hbl, Grt, Ms) basement rocks of the SMM exposed in high-altitude culminations between low-altitude basins. Retrogressive metamorphism of greenschist facies (Chl, Ep, Ilm, Czo) and formation of cata-cla-sites is associated with the shallow dipping detachment fault zones along which Pb-Zn-ores occur in tectonic breccias.
The high-grade metamorphic rocks of the SMM include gabbroic to granitic rocks from high-K calc-alkaline island arc magmas, derived from an enriched mantel and intruded during the Cambrian (556-545 Ma). The high-grade rocks are intruded by undeformed K-feldspar granites and partly covered by rhyolites, both composed of typical conti-nental rift magmas, contaminated by amphibolite facies crustal rocks. The crystallization age (U-Pb on zircons) for the granite is 32±2 Ma. This age is consistent with an Eocene-Oligocene exhumation of the SMM amphibolite grade rocks related to exten-sional tectonics and accompanied by anatexis.
0775
A new tectonic map of the Cévennes area in the para-autochtonous domain of the Hercynian belt of the French Massif Central is proposed on the basis of the identification of three lithological series, namely: i) the Cézarenque gneiss series, ii) the black micaschists series and iii) the sandstone-pelite unit. These three series form an imbrication of five tectonic units which overlie the unmetamorphozed Paleozoic unit of the Viganais to the south. To the north, the micaschist stack of nappes tectonically overlies a gneissic unit distinct from the "lower gneiss" nappe of the Massif Central. This "infra-micaschist" gneiss unit which outcrops in a tectonic window corresponds to the deepest domain outcroping in the Cévennes area. It is also an allochtonous thrust stack which ends in a wedge-like style below the Mt-Lozere pluton in agreement with gravimetric modelling.
The tectonic, metamorphism and magmatic evolution of the Cévennes area is divided into three stages.:
1) S-SW shearing. Several deformation phases with a general southward vergence are responsible for nappe emplacement coeval to a MP/MT metamorphism dated by 40Ar/39Ar around 340 Ma.
2) Early anatexis. The infra-micaschist gneiss unit is reworked by a melting event under T<750°C and P>5 kb conditions older than the Velay migmatite. A radiometric date of 315±4 Ma might suggests relationships between plutonism and migmatization.
3) Namurian plutonism and extensional tectonics. During the postcollisional crustal thinning, around 315 Ma, granodiorite emplacement is controlled by E-W stretching. The plutons are the driving power of the hydrothermal convective flows responsible for the early formation of diffuse arsenopyrite grains in the thermal aureole. Gold bearing sulfides are afterward concentrated in quartz veins along brittle normal and strike-slip faults related to the last stage of pluton emplacement. Lastly, ore bearing quartz pebbles are eroded and sedimented in the Stephanian coal basin.
2064
Mountain belts often show large metamorphic domains affected by flat-lying foliations attesting to large strains due to crustal ductile flow often related to post-thickening extensional collapse. These domains contain mylonitic décollement zones in which deformation-metamorphism relationships suggest that deformation processes were fluid-assisted. In these zones, questions concern the nature and origin of fluids, the scale of their transfer, and the coupling between strain localization and fluid availability.
We present preliminary results from a major décollement zone located within the Variscan Belt of southern Brittany (Gapais et al., 1993). Rocks studied are orogenic volcanoclastic deposits of dacitic to rhyolitic compositions and of upper Silurian to lower Devonian age. The rocks, mostly composed of quartz, alkali feldspar and phengite are affected by pervasive large strains (often more than 400% stretching), with only local remnants of low-strain domains. In peak metamorphic conditions (ca. 300-350°C), deformation is accommodated by extensive dissolution-crystallization processes involving quartz and feldspar and combining continuous (grain dissolution and crystallization in shadow zones) and discontinuous mechanisms of crack-seal type. The progressive deformation is marked by (i) an increase of the scale at which crack-seal and solution-transfer processes occur (from grain scale up to outcrop scale), and (ii) a change from distributed deformation to strain localization within ductile shear zones.
Consistently, whole-rock geochemistry indicates that channelling of fluids within shear zones (with albitization) has followed more pervasive solution transfers involving quartz and feldspar. O isotope analyses made on successive generations of quartz veins reveal a strong lithological control of O isotopic variations, which suggests that both the amount of fluids and the scale of fluid transfers were limited.
In overall, the available data suggest that the décollement zone acted as a trap for fluids. Rheological implications are discussed.
Gapais, D & al, C. R. Acad. Sci., Paris, 316, 1123-1129, (1993).
1259
Thermobarometric data from plutonic rocks of the Kdyne complex (westernBohemian Massif) yield crystallisation temperatures of 1102 to 1137°C, (olivine-clinopyroxene thermometer) and 1028 to 1244°C (clinopyroxene thermometer) for gabbros and gabbronorites, 927 to 1151°C for diorites (clinopyroxene thermometer) and 886 to 1003°C (olivine-clinopyroxene thermometer) for ferrodiorites. Decreasing crystallisation temperature from gabbros to diorites and ferrodiorites supports the idea that olivine gabbros crystallised from a hot initial magma and ferrodiorites represent relatively cold residual magma from the roof of the chamber. Disequilibrium in Mg-Fe between coexisting ortho- and clinopyroxenes yields anomalously high temperatures. Low equilibration temperatures of mineral pairs in diorites (688-901°C, two-pyroxene thermometry) and gabbronorites (696-846°C, amphibole-plagioclase thermometry) suggest slow cooling of the rocks following their magmatic crystallisation.
Pressure estimates come from phase transitions in the contact aureole of the plutonic complex and the mineral stability in coronas present in the olivine gabbros. Transition from phylites through biotite-muscovite schists, sillimanite schists to cordierite hornfelses in the aureole suggests pressures between 3.8 and 4.8 kbar. Absence of sillimanite zone, presence of andalusite pseudomorphs and lower AlT content of amphibole in the northern part of the complex indicates an increase in the depth of the intrusion towards the south. Phase transitions in the contact aureole point to pressures below 3.1 kbar for the northern part of the complex. Estimated pressure corresponds to the intrusion depth of 11 to 14 km in the south and less then 9 km in the north.
The estimated intrusion depth of the Kdyne complex is higher then the presumed depth of other Cambrian intrusions in the area. This suggests that the Variscan uplift of the Kdyne complex was faster than the uplift in the rest of this part of the Tepla-Barrandian terrane. Dipping of the intrusion to the south may also be an indication of the tilting of the adjacent part of the Tepla-Barrandian (Domazlice crystalline complex). This partly contradicts previous observations that the Domazlice crystalline complex did not rotate considerably (Zulauf 1994). During the Variscan uplift the hot Moldanubian crust had moved faster than the adjacent Tepla-Barrandian (Zulauf 1994). The Moldanubian crust could therefore pull the adjacent rocks of the Tepla-Barrandian and cause their tilting. This movement was more pronounced in the south-east of the Tepla-Barrandian due to the influence of two major Variscan structures - West Bohemian Shear Zone and Central Bohemian Shear Zone.
Zulauf G, Geol Rundsch, 83, 276-292, (1994).
0417
U-Pb zircon analyses by the ID-TIMS and SIMS methods were acquired on five monzonite samples of the anorthosite complex of the Lindås Nappe, islands of Holsnøy and Radøy, Bergen Arcs, Caledonides of W Norway. This Proterozoic magmatic complex was affected by penetrative Sveconorwegian granulite-facies metamorphism and by eclogite- and amphibolite-facies Caledonian overprinting. The Caledonian overprinting is spatially restricted along fractures and normal shear zones, and was triggered by fluid infiltration. Deeply abraded prismatic zircon of a two-pyroxene granulite yields an intrusion age of 951 ± 2 Ma. Rounded zircon of a garnet-bearing granulite yields the age of granulite-facies metamorphism at 929 ± 1 Ma. Two samples affected by Caledonian amphibolite-facies overprinting, one undeformed and the other one sheared, yield similar ages of 951 +10/-4 and 933 ± 2 Ma for both events. Zircon growth during granulite-facies metamorphism is evidenced by CL images and SIMS analyses showing luminescent overgrowths with Sveconorwegian ages (Th/U = 0.52) on magmatic zoned cores (Th/U = 1.13) and also by the occurrence of coronas of fine zircon (10 µm) at ilmenite grain boundaries. SIMS analyses of overgrowths and surfaces of grains failed to provide any evidence for Caledonian zircon growth. One sample showing static eclogite-facies overprinting, display a specific zircon population with prismatic to formless overgrowths. Prismatic overgrowths on small zircon in coronas around rutile were observed. SIMS analyses of luminescent to oscillatory zoned prismatic overgrowths display Caledonian ages (Th/U = 0.03). ID-TIMS analyses of prismatic to rounded zircon define a six point discordia line with intercepts at 419 ± 4 Ma and 917 ± 7 Ma. Flat and amoebic zircon define a four point discordia with intercepts at 423 +16/-17 Ma and 895 +19/-18 Ma. Relative position of the analyses along the discordia lines correlates with Th/U ratio, indicating the discordia is a mixing line between low Th/U overgrowths and higher Th/U cores. Crystallization of the overgrowths at 419 ± 4 Ma is attributed to the formation of the HP assemblage, garnet + omphacite + rutile. It records fluid infiltration and eclogitization in the crystalline margin of Baltica, forced to mantle depth during early Scandian continental collision. It corresponds to normal shearing, drop of coherence of the crust, and probably decoupling of the Lindås Nappe relative to the subducting slab. It thus shortly precedes its exhumation and thrusting to an upper allochthon level, together with the low grade Hardangerfjord Nappe of Iapetan affinity, containing ophiolites, granites (430 ± 6 Ma) and Ashgill-Llandovery sediments. Thrusting of the allochtons in S Norway is estimated at ca. 410 Ma (published Ms 40Ar/39Ar ages). Fluid induced eclogitization in the para-autochthonous Western Gneiss Region is possibly coeval but probably younger than in the Lindås Nappe.
2810
The Caledonian mountain belt formed in the Silurian by collision of the rapidly westward-moving Baltic plate, into the larger, steady Laurentian plate. Today this deeply eroded Paleozoic orogen compares remarkably with the interior of the active Himalayan/Tibetan orogen, with the Scandinavia and East Greenland Caledonides as analogues to the Himalayas and Tibet, respectively. During the late Silurian Baltic continental crust was subducted to extreme (> 100 km) depths, below a thick stack of Baltic, oceanic/Iapetus and Laurentian (?) nappes, today exposed in west-central Scandinavia. During the Devonian time the subducted Baltic crust educted, along an major W-dipping detachment system, above which collapse basins formed in rotated half-grabens. In contrast the East Greenland Caledonides represent an area far (ca. 500 km) into the overriding Laurentian plate, where Caledonian, upper-crustal shortening is far less pronounced. Nevertheless do studies of eclogites indicate that also parts of Laurentian crust were overthickend (>85 km) from Early Silurian to late-Early Devonian times. Thickening of the Laurentian "overriding plate" above the partly subducted Baltic crust, is enigmatic in an area with little evidence of crustal scale thrust-stacking, however data from the former middle crust of central East Greenland, may explain this tectonic scenario. Migmatites and ductily deformed footwall rocks separated by an E-dipping extensional detachment from a thick sequence of mostly low-grade, Neoproterozoic-to-Ordovician sediments in the hanging wall. Structures and metamorphic mineral assemblages in this metamorphic core complex, have been regarded as Precambrian, with only a mild Caledonian overprint. Our preliminary isotopic data (U-Pb and 40Ar/39Ar), however, indicate that these structures formed in Caledonian time, synchronous with extensional collapse of the upper crust. Based on a combination of structural and isotopic data we furthermore argue that this mid-crustal top-to-the-NW ultra-ductile or fluid flow thickening the crust overlapped in time (430-420 Ma.) with upper crustal thinning and top-to-the-E movement of the hanging wall of the detachment zone. Despite extreme upper crustal thinning did the orogen remain subaerial until the Late Permian. This may suggest that light upper crustal rocks removed by extension were not replaced from below by updoming of heavy mantle rocks. We, therefore, propose that mid-crustal flow of partially molten rocks into extended regions kept the overthickend crust in gravitational balance. The tectonic setting of the exhumed, deep-crustal rocks of East Greenland may, accordingly, represent an exposed analogue to the deep Tibetan crust, where fluid structures in partially molten rocks is imaged on recent deep seismic sections.
0788
The Lushan Massif is a topographic high of the Yangtze Block south of the Qinling-Dabie belt. It consists of two main litho-tectonic units separated by a major tectonic contact; an upper Sinian- Paleozoic unit made of nearly unmetamorphosed sandstones, except along it basal contact, and a lower Proterozoic unit mainly made of low-pressure high-temperature gneisses and micaschists. Both units are cut by granitic intrusions. From the younger to the older, several tectonic-metamorphic-magmatic events are recognized. The eastern part of the Lushan Massif is cut by a NE-SW trending ductile normal fault (D3 deformation) coeval to emplacement of 100 -110 Ma old leucogranite dated by 40Ar/39Ar laser-probe on biotite and muscovite. The D2 deformation is responsible for a decakilometre scale NE-SW anticline with NE-SW stretching and NW-SE shortening. The age of this folding is provided by the 127±1 Ma U/Pb sphene date of a syntectonic granodiorite and 40Ar/39Ar ages of amphibole. This Cretaceous age also corresponds to the 40Ar/39Ar ages found on syntectonic muscovites at the base of the Sinian unit which is affected by a D1 deformation related to a top-to-the-NW extensional decollement of the Sinian-Paleozoic series above Proterozoic metamorphics. Since cataclastic kyanite is observed in the decollement, a Late Paleozoic-Early Mesozoic tectono-metamorphic compressional event (Dx) is inferred. We argue that Dx is related to a blind thrust in the continental crust of the Yangtse Block in the S. foreland of the Dabieshan. The relationships of Dx and D1 events are discussed in the frame of the Qinling-Dabie orogenic evolution.
1924
The South Tibetan Detachment System (STDS) of Himalayan chain is a Lower Miocene north-dipping extensional shear zone that juxtaposes sedimentary rocks of Tibetan Sedimentary Sequence onto high grade metamorphic rocks of Greater Himalayan Sequence (Burchfiel et al., 1992). In the Rongbuk valley (north of Mt. Everest) region, it is formed of two major fault zones (Lombardo et al., 1993): (i) a lower extensional ductile shear zone (up to 500 mt thick) separating the low grade rocks of the North Col Formation and the underlying high grade rocks of Rongbuk Formation, and (ii) an upper low-angle normal fault between the North Col Formation and the overlying Ordovician limestone of Tibetan Sedimentary Sequence. The ductile shear zone mainly affect the footwall rocks of Rongbuk Fm. and synextensional leucogranitic sheets. Northeastward gently dipping mylonitic foliations, bear NNE-SSW plunging mineral and stretching lineations and a top to the northeast sense of shear is recognised by S-C fabric, C' extensional shear bands, north-facing mylonitc folds and asymmetric porphyroclasts. Strain features in metamorphic and intrusive rocks underline that ductile shear developed under amphibolitic facies condition (T= 450 - 550°C, P= 2 kbar) The upper low-angle fault, affecting the Ordovician limestones and associated slates, gently dips towards NE with down-dip slickenside lineations. It consists of a metric wide fault zone with cohesive crush breccias, cataclasites and foliated cataclasites, nearly parallel to the mylonitic foliation in the ductile shear zone. Microstructural features in fault rocks indicate the cataclasis as the main deformational mechanisms. Different stages of fracturing and veining suggest a cyclic development of fault-rocks in very low grade metamorphic conditions (lower than 300°C). Moreover in calcite grains mechanical twinnings together shape and crystallografic preferred orientation suggest pressure solution and crystal plasticity as deformational mechanism competing with cataclasis. Based on the microstructural features we can suggest for the fault zone develpment a cyclic alternance of seismic-aseismic deformation, this latter triggerd by lower strain rates and/or low pore fluid pressure. Such a behaviour of the fault rock in the Quomolangma detachment should be taken into account for evaluation of the fault slip rate and tectonic unroofing rate of the Himalayan chain in the Everest area.
Burchfiel BC, Chen Z, Hodges KV, Liu Y, Royden LH, Deng C & Xu J, Geological Society of America, Special Papers, 269, 15-26, (1992).
Lombardo B, Pertusati PC & Borghi S, Geological Society Special Publications, 74, 341-355, (1993).
2778
The Brasília belt (central Brazil) results from E-W collision of the Amazonian and São Francisco cratons during the Neoproterozoic assembly of Gondwanaland. This work focuses the southern segment of the belt, along the 21°S parallel, in which a nappe displaying low to high-grade Brasiliano metamorphism (Passos nappe) and basement slices were thrusted at least 200 km onto a relatively cold foreland zone, the eastern border of the São Francisco craton, with a thin-skinned tectonic style.The uppermost structural unit (Passos Nappe - PN) comprises highly deformed metasedimentary sucession interpreted as the Neoproterozoic (ca. 1.0-0.9 Ga.) passive margin of western São Francisco craton. An inverted (intermediate to high-P) metamorphic gradient ranging from greenschist to upper amphibolite facies is characterised. K-Ar mineral ages from the PN constrain the timing of the thrust-driven uplift/cooling: 674-640 Ma. (hornblende) and 673-566 Ma (white mica). The External Domain is a low metamorphic grade thrust-system with the complex imbrication of basement rocks (Archean Piumhi greenstone, a turbiditic graywacke succession and a calc-alkaline granitoid suite) with thrust-sheets of undated siliciclastic successions (Serra da Boa Esperança sequence). K-Ar dating on white mica from five samples of the siliciclastic sucessions yield a range of 595 to 550 Ma apparent ages, an indication of cooling also related to the Brasiliano tectonic imbrication. The Cratonic Domain comprises basement rocks covered by anchimetamorphic carbonatic shallow marine platform deposits of the Bambuí group (Neoproterozoic). The carbonatic lithology of the footwall rocks under the allochthonous front may have been an important factor favouring the large advancement of shallow thrust sheets. The crystalline basement rocks, covered by the Bambuí group, extend continuously from the SW border of the São Francisco craton, along the southern limb of the Passos nappe. The development of thrust stacking and mountain build up generated a coarse clastic influx of molassic character on the foreland zone, coeval with the exhumation of the External Domain thrust sheets.The cooling (K-Ar) pattern of the basement rocks from the Cratonic Domain of the southern SFC area reveals a clear uplift and cooling event at the end of the Transamazonian orogeny, given by widespread apparent ages around 1.8 Ga. In the gneisses south of the Passos nappe, Mesoproterozoic isotopic Rb-Sr rehomogenization (1.40 Ga.) and K-Ar hornblende dates related to the Transamazonian event (2.25 and 2.00 Ga), point to a complex thermal history for the area. However, the Meso-Paleoproterozoic K-Ar datings indicate that, during the Brasiliano thrust-stacking, the Cratonic Domain basement rocks were not heated enough to have their K-Ar systems reset, yielding pre-Brasiliano ages. Allochthony of the Passos nappe and of basement thrust-sheets is therefore interpreted to have taken place onto a "cold" São Francisco craton foreland, in a thin-skinned thrust setting.
3028
East vergent "thin-skinned" fold and thrust belts (FTB's) are developed as discontinuous discrete morphotectonic elements along the forelandward side of the Andes in Peru, Bolivia and Argentina. In central western Argentina an east vergent FTB of Middle Miocene age, the Argentine Precordillera (AP), is presently being translated and reoriented in a complex triangle zone at its leading edge by westward verging basement uplifts of the Sierras Pampeanas (SP). Our new apatite fission-track constraints from the SP demonstrate that basement ranges of the province commenced exhumation during the Upper Miocene to Pliocene subsequent to the development of the AP. When combined with structural mapping, the cooling ages suggest that this exhumation is accompanied by a bulk westward convergence of the SP with the AP. Further combined structural mapping and apatite fission-track thermochronology to the immediate north of the AP and SP provinces at the southwestern margin of the extensive Altiplano-Puna has identified a similar pattern of events. Here, fault and fold geometries that are indicative of an early episode of east-vergent "thin-skinned" folding and thrusting in Triassic sedimentary rocks have been subsequently incorporated and reoriented by "thick-skinned" contraction and uplift of Palaeozoic rocks on opposed vergence reverse and wrench faults. Apatite fission-track thermochronology suggests that the "thin-skinned" folding and thrusting commenced during the Eocene whereas the basement-involved contraction and uplift commenced subsequent to this during the Oligocene-Miocene. Both cases demonstrate a mode of lateral thickening in the Andes that involves the exhumation and possible westward convergence of basement ranges to the east of an existing FTB followed by a progressive incorporation of the FTB into the thickened orogen. Thermochronological data suggests that this process has been ongoing but diachronous along strike in the Andes.
2609
The Sierra Nevada batholith contains numerous granitic plutons that formed during Cretaceous subduction. In general, the ages of these plutons young to the east. Al-in-hornblende thermobarometry data suggest that crystallization pressures abate across the range, decreasing from 3-5 kbar (11-19 km) in the west to <1 kbar (<4 km) in the east, with elevation differences only able to account for ~0.8 kbar of the observed pressure gradient (Ague and Brimhall, 1988; Ague, 1997). Just how the felsic intrusives attained their final relative positions is unknown. For example, it is enigmatic why coeval plutons that crystallized at 16, 12 and 9 km depth reside at the same level today. Of the Cretaceous granite bodies in the Sierra Nevada, the Mount Givens pluton (MGP) is one of the largest and best exposed. Both this pluton and the surrounding wall rocks are well studied in terms of age dating, cooling history, magmatic and tectonic fabric and geochemistry. Isotopic dates indicate very rapid cooling of the MGP- 400°C from 90 to 88 Ma. To help constrain how the MGP attained its relative position with respect to its surroundings, we demagnetized 277 paleomagnetic samples collected from widespread stations. Linear magnetic components were isolated in 265 samples. Remanent magnetization directions are held in titanium-free, multi-domain magnetite. Together they define an overall mean of D=1.9°, I=63.7° (a95=3.0°). The corresponding paleomagnetic pole is discordant with respect to poles from neighboring coeval plutons or cratonal North America. The discordance is best resolved via a tectonic model of 10° down-to-the-west tilting of the MGP along a horizontal axis that parallels the dominant regional fabric. Previous work on surrounding structures supports the tilt model in terms of timing, kinematics and amount of offset. Together with the paleomagnetic data, they suggest the MGP popped up and tilted along the Bench Canyon reverse fault to the east and the Courtright normal fault to the west during arc-normal compression. Active uplift via faulting could facilitate magma ascent and emplacement and account for the observed age and thermobarometry trend in the batholith.
0409
Exhumation of high to ultra-high pressure metamorphic rocks was approached in most cases in orogenic belts (Caledonides, Alpes, Himalya...). In this collisional zone, early structures linked with subduction are partially obliterate. In contrast, outcrops of HP rocks in active subduction zone suggests a significant role of the subduction in the exhumation processes. The northern Carribean plate and especially the Samana peninsula (Dominican Republic) is an example of transpressive area (southward dipping subduction of the north America plate with a left-lateral strike slip tectonics) where blueschists and eclogites are exhumed in subduction context. Paragenetic analysis and thermobarometric estimations confirm the presence of two different metamorphic units in the Samana peninsula. Three complementary methods are used to evaluate P-T conditions: experimental stability fields of metamorphic assemblages, conventional thermobarometry and the Thermocalc computer program. A first low grade unit is characterised by the albite + lawsonite association and P-T conditions are about 7.5 ± 1 kbar and 320 ± 20°C. The second unit overthrust the low grade unit to the north and blocks of eclogites which are commonly surrounded by blueschist rims are observed in the calcshists. The metamorphic evolution show an isothermal decompression at about 450°C, from 14 kbar (LT eclogite facies) to 9 kbar into the epidote blueschist facies. The late evolution into the greenschist facies is contemporaneous with a syn-convergence extensive tectonic. It is indicated by conjugate normal faults perpendicular to the direction of convergence.The Rb-Sr phengite - whole rock isochron from a high pressure unit eclogite give an age at 32 ± 2 My. This age is compatible with the current subduction of the north american plate, active since about 85 to 90 My. Petrological analysis indicate that this age corresponds to the retrogression into the blueschist facies during the exhumation. The Samana metamorphic complex probably represents an accretionary wedge fragment carried on the active continental plate margin. This transpressive context, where minor collision occurs, show the importance of oblique subduction on exhumation processes. Moreover, the location of the eclogitic unit just north of the the left lateral strike-slip fault (Septentrional Fault Zone) suggests an important role of strike-slip fault during the exhumation. Extension and strike-slip tectonics is a feature of the late orogenic stages (extensional collapse) and is also contemporaneous with the convergence in recent belts (Alpes, Himalaya), it appears they are present in the early orogenic stage, during subduction, before continental collision.
2357
Burial of the continental material to great depths (ca. 100 km) is obviously part of the continental subduction process. This material can be then rapidly returned to shallow depths and be finally exhumed on the surface later on. The exhumed rocks form high-pressure/low-temperature terrain within the mountain belts.
A still unresolved paradox is why these rocks subducted to 100 km depth or more were not heated to temperatures more than 600-700°C. A possible explanation could be a thermal shielding of the deeply subducted continental crust by a relatively cold lithospheric block overlying this crust and subducted with it. This block could correspond to the fore-arc lithosphere detached from the overriding plate during initial stages of continental subduction (subduction of the continental margin) corresponding to the arc-continent collision (Chemenda et al. 1997).
In this paper we present the results from numerical simulation of the thermal regime of such a subduction system, using simplified, purely conductive heat-transfer model set up. Advective heat transfer within the mantle corner between the overriding and subducting plates is taking into account by assuming an effective conductivity coefficient. The thermal structure of the subducting continental crust is estimated with and without the thermal shield, at different subduction velocities and heat production within the crust.
Chemenda A, Matte P & Sokolov V, Tectonophysics, 276, 217-227, (1997).
2531
The majority of the regional metamorphic belts of Japan developed from accretionary complexes that formed as a result of the subduction of oceanic lithosphere below East Asia. We concentrated on very-low-grade metamorphic rocks from the following stack of units, from top to bottom: 1) N-Chichibu belt, 2) Mikabu greenstone belt, and 3) Sambagawa belt. Laser probe 40Ar/39Ar spectra of slates from the N-Chichibu belt show progressively increasing apparent ages, implying that metamorphic temperatures were too low to reset the Ar-system of older, detrital feldspar in the rocks. Metapelites from the Mikabu belt yielded plateau ages from 100 to 115 Ma over substantial parts of the age spectra. Sambagawa metapelites yielded age plateaux in about 50% of the cases, with ages confined to the 83 to 90 Ma range. Age plateaux are less well developed compared to those of the Mikabu belt. This seems to be due to the lower grade metamorphism of the Sambagawa metapelites, that has not fully reset the Ar-system of detrital feldspar. A maximum temperature of around 300°C of the very-low-grade metamorphic rocks of the Mikabu belt implies that the plateau ages indicate the time of the main tectono-metamorphic phase. Consequently, the age of metamorphism and subduction seems to be significantly different for the Mikabu and Sambagawa belts. The generally ENE-WSW striking schistosity of the Mikabu and Sambagawa belts contains a westward plunging mineral and stretching lineation. By contrast rocks of the N-Chichibu belt generally do not contain a stretching lineation. Only quartz-rich rocks of its lowest part in the contact zone with the underlying Mikabu belt are locally mylonitic, with stretching lineations that are parallel to linear structure of the Mikabu belt. The relationship between the Mikabu and Sambagawa belts, with higher metamorphic rocks on top of lower grade rocks, implies that the contact is an overthrust. The N-Chichibu belt is correlated an accretionary complex that formed part of the hanging wall below which the rocks of the Mikabu and Sambagawa belts subducted. As the rocks of the N-Chichibu belt have a substantially lower metamorphic grade, the present-day contact with the underlying Mikabu belt must, however, be a low-angle normal fault. During low-angle normal faulting the HP/LT metamorphic rocks of the Mikabu and Sambagawa belts were partially exhumed, and brought into contact with much less metamorphic rocks. Stretching lineations and the movement on the low-angle normal fault are parallel to the trend of the subduction zone. This implies an important trench-parallel movement during extension, which takes place during continuing oblique subduction. A major left-lateral trench-parallel wrench fault to the North of the Sambagawa belt and the associated Late Cretaceous pull-apart basins fits in this kinematic framework.
1812
In the Bohemian Massif are exposed rocks equlibrated under physical conditions correponding to the middle and lower part of thickenned orogenic root. These rocks are actually forming individual units with different PT evolutions separated by both compressional and normal kilometric scale shear zones. The margins of Variscan orogenic root system in the Bohemian Massif are characterized by rigid Cadomian buttress zones. These zones are composed of imbricated Barrovian metamorphic sequences derived from the Cadomian basement and its Palaeozoic cover. In the east of the Bohemian Massif is the buttress block overthrust by the external zone (ERZ) of the orogenic root system. The overthrusting zone is marked by boudins of MP granulites (14 kb, 750-800°C) and eclogites at the base followed by sillimanite schists and metagranitoids in the hanginwall. The ERZ is again overthrust by an internal zone of the thickened orogenic root (IRZ) composed of HP granulite (18 kb, 850 - 900°C), eclogites and garnet peridotites surrounded by high grade gneisses (10-12 kb, 700 - 750°C). The IRZ and ERZ are separated by several kilometre wide normal shear zone developed in micaschists and metagranitoids (10 kb, 650-700°C). The deformation redeformation regime, in the shear zone, is manifested by extreme ductility of feldspars and quartz forming a banded ultramylonitic orthogneisses. Extreme grain-size reduction of recrystallized feldspars (up to 20 microns) suggest grain size sensitive flow regime associated with weakening of middle crustal rocks under elevate thermal gradient. This structural template allow us to discuss the spatial relationships between the lower crust and the middle crust during extrusion of laterally compressed orogenic root system bracketted between 350 - 330 Ma. We use an integrated microstructural, petrological work and thermal and rheological 2D modelling to constrain the mechanical role of quartzofeldspathic rocks in this high thermal regime. Thermomechanical evolution of overthickened orogenic root system is interpreted in term of sucessive extrusion of first hottest IRZ over ERZ followed by migration of thrust zone towards the buttress boundary. The overthrusting of the base of ERZ over the buttress is accompanied with synconvergent collapse of overthickened and hot root exploiting extreme ductility of metagranitoids in mid-crustal levels at the boundary between ERZ and IRZ.
2018
Mineral ages from metamorphic terranes are usually interpreted as cooling after the peak of metamorphism and, in high-grade terranes, this age information is also often related to exhumation (i.e. decompression). However, cooling and exhumation may not be synchronous. Here we report Ar-Ar (mica and hornblende), Sm-Nd (garnet) and U-Pb (zircon) data from granulite grade gneisses and syn-tectonic granites in the southern part of the Bohemian Massif which demonstrate that decompression and cooling can be dated separately.
IR laser probe Ar-Ar dating of hornblende, biotite and muscovite revealed that all minerals contain variable proportions of excess Ar, the amount of which decreases from centres towards the rims of the grains. Accordingly, the rim ages are interpreted as representing maximum cooling ages for hornblende in amphibolite (331 ± 1 Ma), biotite in granulite gneiss (319 ± 1 Ma) and muscovite and biotite in syn-tectonic granite (316 ± 1 and 310 ± 1 Ma, respectively). A concordant zircon fraction for this granite yields a U-Pb age of 320 ± 1 Ma. The data suggest a cooling rate of 9 to 22 oC/Ma following MP-HT metamorphism reported from this part of the Bohemian Massif at ca 345-338 Ma.
The studied garnets (Alm 0.53, Prp 0.27, Grs 0.19, Sps 0.01, up to 2 mm in diameter) are zoned in major cations with Fe/(Fe+Mg) decreasing from core to rim. The outer rim (ca 200 microns) shows retrograde zoning with increasing Fe/(Fe+Mg), decreasing Ca and a large increase of Mn content. While the major element zoning in the core relates to prograde growth, the formation of the rim can be attributed to garnet resorption during the retrograde decompression path. LA ICP-MS analyses show that REE zoning is on the same scale to that of the major elements, with Sm and Nd concentrations of 10 and 6 ppm in the core and 5 and 1 ppm in the rim, respectively. The Sm/Nd ratio increases from 1.5 in the core to 5.5 in the 200 microns rim. Whole-rock - bulk garnet and whole-rock - abraded garnet core Sm-Nd ages are 351 ± 6 and 354 ± 5 Ma, respectively. The age of the garnet rim calculated from the core and bulk garnet age data, the proportions of the core and rim and their respective Sm/Nd ratios and Nd concentrations is ca 351 Ma, i.e. within the error of the bulk garnet. This effectively dates the final stages of garnet resorption which followed the HP-HT metamorphic event at ca 354 Ma and which is related to decompression