Orogens developed within the Alpine-Mediterranean region display two fundamentally different styles, which are best exemplified by the Western to Eastern Alps on one hand and by the Apennines, (outer) Carpathians and (outer) Hellenides on the other. The former displays topographically high mountains, large magnitudes of erosion and denudation, exposure of high-grade metamorphic rocks at the surface, extensive deformation of crystalline basement, extensive postcollisional convergence and a protracted history of molasse deposition within the adjacent foredeep basin, and lacks regional extensional features. The latter display topographically low mountains, little erosion or denudation, low-grade to no metamorphism, little involvement of crystalline basement in shortening, little postcollisional convergence, a protracted history of flysch deposition within the adjacent foredeep basin, and regional extension within the upper plate region. These tectonic styles are the result of fundamental differences in the geodynamic processes that affect each orogen. In particular, orogenic style results from the balance between the rate of subduction zone retreat and the rate of local plate convergence (the latter governed by the position of the orogen within the Europe/Africa convergent zone). The importance of slab buoyancy in driving subduction is shown by observations of foredeep basin geometry, gravity anomalies and a general correlation between slab bouyancy and subduction rate. Quantitative modeling of subducted lithosphere within a viscous asthenosphere shows that slab density and imposed convergence rate control the rate and geometry of subduction, the stresses applied to the over-riding and lithospheres, the rate of thrusting and the presence or absence of upper-plate extension. In particular, the introduction of dense lithosphere into the subduction zone results in a rapid increase in the rate of subduction zone retreat (and thrusting) on time-scales as short as a few million years, while the introduction of buoyant lithosphere into the subduction zone results in the slowing or stopping of subduction on similar timescales. Anomalously buoyant terranes as narrow as 100 km (perpendicular to the subduction boundary) result in dramatic changes in thrusting rate and can induce or suppress regional extension within the upper plate. Thus it may be possible to make detailed correlations between density (topography) of subducting foreland regions and changes in thrust belt activity throughout the Alpine-Mediterranean region.
Integrating a wide range of multidisciplinary data, i.a., seismic tomography features, regional tectonic transport directions and ages of subduction/collision, lithospheric extension and slab detachment, suggests a new model for the development of the Alpine belt in the western Mediterranean. The model implies that the backbone of the Alpine orogeny was formed by a composite SW-NE striking subduction system, active until some time before 22 Ma. The system which consumed Mesozoic Tethyan lithosphere was W-ward dipping under the leading edge of Iberia which was drifting E-ward under the influence of the opening of the North Atlantic. Subduction activity was terminated by the formation of the Alpine orogenic belt through collision of Iberian and Betic-Ligurian continental lithospheres. A series of late stage extensional regimes, with local formation of Neogene oceanic lithosphere, in the Valencia Gulf, Provençal-Algerian basin and southern Tyrrhenian basin, and inherent slab roll-back, subsequently have fragmented the original collision belt. This has produced the seemingly eratic distribution of Alpine metamorphic core complexes in the western Mediterranean -- Betic-Rif, Kabylies and the Sicily-Apennines(-Corsica) belt. It follows that in the westernmost Mediterranean N-ward drift of Africa against Iberia/Europe, though influential, e.g., in creating the E-W grain of the Betic Cordilleras, has not been the controlling factor. In contrast, in the eastern Mediterranean the influence of N-S convergence has been much more pronounced due to the hinged sinistral movement of the African plate. A major part of the Europe-Africa N-S convergence was accomodated in a coherent N-dipping subduction zone running from N Africa over Sicily-Calabria to Crete, its importance increasing E-wards.
Betic and Rif Cordilleras developed during the Cenozoic as a response of the collision between African and Eurasian plates. Between the two plates, the Internal Zones act as an independent deformed element. P and S waves seismic tomographic images of the central Betic Cordillera shows a moderate (-3%, -4%) anomaly in the upper mantle below the Internal Zones from 40 to 100 km depths. This anomaly has a slab-like geometry and dips towards the S-SE. The intermediate seismicity (depth<110 km) detected in this region can be easily related with this low velocity anomaly because most of the earthquakes locates inside the slab or very near it. Focal mechanisms of the earthquakes of the central and upper part of the slab (up to 80 km) correspond to reverse fault mechanisms indicating that the slab is undergoing current compression. The stress determination from these focal mechanisms agrees with this interpretation as indicate a nearly subhorizontal NNW-SSE <sigma>1 acting in the upper mantle and inside the low velocity anomaly, in agreement with the convergence direction between Africa and Eurasia. For greater depths, the focal mechanisms of the earthquakes show active normal faults.
The seismic attenuation deduced from the intermediate depth earthquakes (Ibañez, 1990) suggests a continental nature for the low-velocity zone. A modelling of the Bouguer gravity anomaly of the region supports this interpretation due to the good fit between measured and calculated anomalies. We propose that an active continental subduction zone exists below the Betic Cordillera and can accommodate around 150 km of shortening between the Iberian Peninsula and the Internal Zones of the Cordillera. This subduction indicates that the edge of the Eurasian plate locates at the front of the mountain range. The compression of the slab up to 80 Km suggests that collision/subduction processes dominate up to these depths. Below the 80 Km, the vertical compression indicates that the negative buoyancy dominates with respect to the collision/subduction processes, favoured by the continental nature of the subducted slab.
The approximation between Iberia and the Internal Zones has been accommodated in the folds and thrusts with NW-SE shortening developed during the middle-to-late Miocene in the External Zones. Furthermore it has to be accommodated in the thrusting toward the SE of the External Zones over the flyschs, and of the latter, over the Internal Zones. Therefore, the External Zones have the position of a tectonic accretionary prism.
Ibañez JM, Ph. D. Thesis, Univ. Granada, 220 pp, (1990).
In the Alboran Domain of Southern Spain, late orogenic extension is superimposed on Alpine collision. This study focuses on a suite of mafic dykes exposed near the city of Malaga. At present, these dykes have an E/W strike and near vertical dips. They are intruded into andalusite schist with a moderately W-dipping foliation. Initial results from Ar-Ar whole rock analysis suggest that the dykes were probably intruded into the surrounding schists at approximately 30 Ma but were largely reset by a greenschist grade metamorphic event terminating at around 18 Ma.
Samples for magnetic analysis were cored at 11 locations. Results of rock magnetic experiments done in conjunction with thermal and AF demagnetizations demonstrate that the remanence in many of the dykes is composed of one to three components. The lowest temperature component is in the direction of the present day magnetic field. The intermediate temperature (IT) component, carried by pyhrrotite with a blocking temperature of 340°C, has a declination that is rotated approximately 140° and an inclination of -45°. This component was probably acquired around 18 Ma when radiometric data indicates that the rocks cooled below 350°C. The high temperature (HT) component is carried by magnetite with a blocking temperature of 575°C and was probably acquired at 30 Ma when Ar-Ar results indicate closure of hornblende. This component is also rotated approximately 140° but its mean inclination is nearly horizontal (-10°).
Possible explanations for this shallow inclination include: (a) intrusion in the Permo-Triassic, (b) rotation of the dykes from an initially inclined orientation and (c) late rotation about a N/S axis. Our analysis suggests that a simple rotation of 40° about a N/S axis normal to the dykes, brings the inclination of the HT component into statistical agreement with both the IT component and the expected inclination for the Oligocene. This rotation also brings the foliation in the surrounding schists to approximately horizontal.
We conclude that the original regional foliation was probably sub-horizontal and therefore related to the ductile phase of extension. The dykes were intruded sub-vertically into the schists with an original NW/SE trend and have undergone as much as 140° of clockwise rotation about a vertical axis and 40° of rotation about a N-S horizontal axis. The rotation of 140° occurred after the formation of the IT component and is probably related to emplacement of the Alboran Domain onto the Iberian margin.
The Subbetic Zone is an external, unmetamorphosed, thin skinned fold and thrust belt of Alpine age, which forms part of the Betic Cordillera, southern Spain. Miocene tectonics emplaced a 2 km thick pile of Mesozoic and Tertiary sediments northward onto the Iberian foreland. Previous palaeomagnetic studies have revealed differential vertical-axis rotations of the order of 60° clockwise or more. These rotations are large for a mountain belt. Palaeomagnetic and structural studies in the El Chorro and Velez Blanco areas were undertaken with the aim of understanding at what point during orogenic evolution the large vertical-axis rotations accumulated in this obliquely convergent mountain belt.
The El Chorro area, in the Western Subbetic, is a 16 km2 structural culmination. Palaeomagnetic results show that the imbricate thrust sequence has been subject to differential rotations both within and between thrust sheets. It likely that folding, thrusting and rotation occurred when the rocks from this area were close to the deformation front of the mountain belt, and vertical axis rotation is attributed to a thrust sheet pinning mechanism.
The Velez Blanco area covers approximately 400 km2 of the Eastern Subbetic close to the boundary with the Internal Zones. Structural analysis shows that normal faults developed during Jurassic rifting were influential in the development of Miocene structures associated with shortening. In this region, rotation gradually changes from zero to 60° clockwise rotation from NE to SW, without rapid changes across individual structures. This pattern of rotation correlates with a change in strike of thrust traces and stratigraphic contacts from N-S to almost E-W, and is suggestive of late transcurrent shear in the internal parts of the mountain belt.
Thrusting is mostly SSE-directed, but ESE-directed thrusting is common in the unrotated zone, suggesting that fault lineations indicative of transport directions have been rotated. If lineation data has been rotated then the original thrusting direction was N50°W. Combined with estimates of dextral motion parallel to the strike of the mountain belt, it is estimated that the local plate motion vector was N70°W, between the Alboran plate and Iberia.
There are two likely mechanisms responsible for rotation in this region. 1) Rotation during thrusting accumulated clockwise rotation though the thrust sequence. 2) Transcurrent shear in the internal parts of the mountain belt occurred later, and led to further clockwise vertical axis rotation.
The two marine gateways (Rifian Corridor through Morocco and Betic Corridor through Spain) that connected the Mediterranean to the Atlantic Ocean, progressively closed in the late Miocene by tectonic processes related to Africa-Iberia collision. Finally, this led to a complete isolation of the Mediterranean and the deposition of enormous evaporite units during the so-called "Messinian salinity crisis".
We will present a high-resolution integrated (magneto-bio-cyclo) stratigraphy for various late Miocene sections covering large part of the Mediterranean (from Morocco to Cyprus). Astronomical tuning of the sedimentary cycle patterns to calculated curves of insolation allows us to accurately determine the synchrony or diachrony of the recognised tectonic and climatic events. We will show that tectonic processes in the Atlantic gateways have major consequences for the paleoceanography of the Mediterranean. Furthermore we will show that the onset of evaporite formation is a remarkably synchronous event all over the Mediterranean, astronomically dated at 5.96 Ma.
Sedimentary facies, clay minerals, and benthic foraminiferal assemblages in deep-water formations of the Subbetic Zone (Spain) display significant changes in the Late Cretaceous. Calculated subsidence curves for the Subbetic Mesozoic sedimentary sequences point either to a pronounced uplift or to a drowning event around 85 Ma. Data from other localities around the Iberian microplate and from adjacent areas in the eastern Atlantic (off Morocco, Gulf of Guinea) with settings on different types of lithosphere, revealed the same general pattern of a major break or uplift in, or strongly enhanced rates of, subsidence around this time (e.g. variations depending on stratigraphic resolution, the time-scale applied, porosity and paleo-waterdepth estimates). At most localities, the subsidence events correlate with changes in the bulk sedimentary and clay mineral composition. Notably, where uplift is indicated, the relatively monotonous, smectite-dominated clay mineral assemblages are replaced by assemblages indicating either massive erosion of residual deposits or first-cycle wheathering of crystalline rocks, i.e. kaolinite, chlorite, and illite. The opposite trend was observed at localities characterized by increased subsidence. Subduction or collision-related related high-pressure metamorphism occured at about 90 - 80 Ma in the western Mediterranean region (i.e., the Internal Zone of the Betic Cordillera, the Moroccan Rif and the Kabylies) and preceeded the regional Alpine metamorphism. A Late Cretaceous metamorphic event is also known from the Benue Trough (central Africa). Literature on the timing of these metamorphic events suggests a synchroneity of high-pressure metamorphism with the subsidence changes. We assume that the subsidence history of the circum-Iberian basins was largely controlled by the convergence of the northern West African plate margin and the Iberian microplate, however, with certain impact of the opening of the Atlantic. Plate tectonic reconstructions suggest that the northward movement of Africa was largely controlled by the opening of the South Atlantic during the mid-Cretaceous. However, a change in the plate motion vector of Africa at 85 Ma, with a faster eastward movement afterwards may indicate that Africa became locked towards the North. Collision of Iberia and Africa is thought to be responsible for the uplift events in basins lying in the direction of Africas movement. Aside from plate tectonic implications, the closure of the seaway connecting the Tethys with the Central Atlantic probably had dramatic effects on oceanic circulation. The almost continuous tropical to subtropical circulation along the Tethys and the Central Atlantic is considered an influential precondition of mid-Cretaceous greenhouse climate. Blocking or deviation of the warm, westward-flowing Tethyan surface waters may have engendered led to a significant change in the global circulation pattern, thus potentially leading to climate cooling from 80 Ma on.
Timing of deformation within the North Pyrenean igneous complexes is debated. While some studies propose that deformation is linked with Hercynian magmatism, others consider it is Cretaceous. In the Agly Massif (Pyrénées Orientales, France) easternmost complex, two units are distinguished, separated by a subvertical south dipping reverse mylonitic shear zone. The Northern unit consists of the Saint-Arnac granite and its surrounding schists affected by contact metamorphism. A muscovite from these schists yields an Hercynian age of 273±3 Ma. This unit was not affected by later deformation. The Southern unit corresponds to HT-LP Hercynian metamorphic rocks. Metamorphic grade ranges from charnockite (Ansignan, 800°C/4-5 kbars) to very-low grade metamorphism (chlorite zone). These rocks are penetratively affected by gently, NE dipping ductile normal shear planes that thin the older metamorphic pile and occur in the same thermal conditions (300 to 500°C) throughout the unit. The associated lineation constantly trends N30, as in the nearby Mesozoic series of the Bas-Agly synclinal. U-Pb dating of one fraction of monazites yields a concordant age of 307±3 Ma. This Hercynian age for the protolith is confirmed by the Ar cooling age of an amphibole at 272±3 Ma. The preservation of these hercynian ages is compatible with the thermal conditions of the deformation. We also dated biotite from undeformed charnockite, muscovite from a mylonitized pegmatite, biotite and muscovite from mylonitic gneisses, and biotite from pelitic schists. All these 39Ar/40Ar ages cluster between 100 and 110 Ma implying a Cretaceous thermal event. These results are close to metamorphic ages obtained in the Bas-Agly synclinal (90-110 Ma), and to emplacement ages of peridotitic bodies along the left-lateral North Pyrenean fault.Formation of the Agly complex took place in at least two successive stages. The first stage, during Hercynian orogeny, corresponds to the emplacement of the Saint-Arnac granite in the upper crust, and of the Ansignan charnockite in the middle crust. The second stage affects only the southern unit under an unusually high thermal gradient. It probably corresponds to Cretaceous normal faulting in a transtensional environment associated with motion along the North Pyrenean fault. These conclusions can be extended to most of the igneous complexes of the Axial and North Pyrenean zones.
The Valais domain of the Western Alps comprises a narrow zone of intensely deformed rocks bordered by two first order tectonic features , namely the Penninic front (PF) and the Houiller front (HF). While their outstanding importance, thanks to the ECORS-CROP seicmic traverse (e.g. Roure et al., 1996), is out of discussion their tectonic significance is still a matter of debate. Recent publications argue for thrusting as well as normal faulting along both of them and significant sinistral strike-slip movement has also been proposed.Our observations, based on detailed structural mapping of the area between the Cormet de Roseland (PF) and the Pt. St. Bernard pass (HF), revealed that all observed deformation phases are post high-pressure/low temperature metamorphism (Schürch, 1987; Goffé & Bousquet, 1997) affecting parts of the Valais domain (including the Pt. St. Bernard nappe). D1 and D2 caused both internal thrusting (e.g. internal Valaisan over Pt. St. Bernard nappe) and isoclinal folding with roughly N-S trending fold axes and stretching lineations. D1 and D2 are interpreted to be due to sinistral transpression in a NNE-SSW-striking corridor (e.g. Choukroune et al., 1986), taking up the 195 km of N-directed displacement of the Adriatic microplate relative to stable Europe as deduced by Schmid et al. (1996) for the time interval between 50 and 35 Ma in the eastern Central Alps. In the Pt. St Bernard pass region a greenschist facies shear zone, exhibiting top-to-the SE directed normal faulting, partly cuts the contact between the internal Valaisan and the Pt. St. Bernard nappe and overprints all D2 structures. After 35 Ma the Adriatic indenter was displaced by some 124 km towards WNW, relative to stable Europe (e.g. Laubscher, 1991). In our working area this convergence is evidenced by a further phase of folding (D4) and thrusting, displaying a strain gradient from SE to NW, i.e. towards the Penninic front, which is seismically imaged down to a depth of 15 km.Late (post 5 Ma) top-to-the SE directed normal faulting (D5) crosscuts the existing nappe stack at a low angle. Since this late normal fault overprints the HF north of Moutiers, its tectonic significance (syn D2 thrust) was studied further to the south.
Choukroune P, Ballèvre M, Cobbold P, Gautier Y, Merle O & Vuichard J-P, Tectonics, 5, 215-226, (1986).
Goffé B, Bousquet R, Schweiz. Min. Pet. Mitt, 77, 137-147, (1997).
Laubscher H, Eclog. Geol. Helv, 84, 631-659, (1991).
Roure Fet al(editors), Mém. Géol. France, 170, 113 pp, (1996).
Schürch ML, PhD thesis Univ. Geneva, 157 pp, (1987).
Schmid SM, Pfiffner OA, Froitzheim N, Schönborn G & Kissling E, Tectonics, 15, 1036-1064, (1996).
A field study of the metasedimentary rocks of the Valaisan domain from the Engadine window (Switzerland) to the Petit St Bernard (France) shows that all this domain was subjected to high pressure low temperature conditions during the alpine orogeny. The P-T conditions increase from the east to the west. The metasediments of the Engadine window and of the Grisons are characterised by blueschist facies conditions (12-13 kbar, 350-400°C, Bousquet et al., 1998) trough occurrences of Mg-carpholite and chloritoid. Toward the west in the Petit St Bernard and Versoyen units the metamorphism conditions become higher and reach conditions of the eglogite facies (15-16 kbar, 500°C, Goffé & Bousquet, 1997. This metamorphism is around 35 Ma in age. In the Tauern window equivalent metamorphic conditions of same age (32-36 Ma) occurs in metapelites (Kurz et al, 1998). The deformation associated to the HP metamorphic event is oriented NO-SE all over the domain. This direction is compatible with the direction of convergence between Europe and Apulian plates in the beginning of the Tertiary time (65-35 Ma). In Eastern and Central Alps sedimentary rocks metamorphised under the blueschist facies occur under the Austroalpines nappes over a surface of 300*20 km2 (from the Tauern to the Grisons) with 5 to 10 km of thickness. This considerable volume of metamorphic rocks is in contrast with those of the Petit St Bernard and the Versoyen units that form a small slice with a thickness of 2 to 5 km (ECORS-CROP). We interpreted the difference of volume and of metamorphic conditions from the east toward the west by a change of the subduction type: In the east we assume the formation of a large wedge with a thickness of 40-50 km. In this wedge the rocks undergone blueschist metamorphism and were exhumed by a large scale detachment. This wedge was active at least to 35 Ma that implies that the austroalpine nappes overlying later the Valaisan domain.In the west, we have not arguments for the formation of a large wedge and we assume that the Petit St Bernard and the Versoyen units undergone in subduction with the slab. We propose, in this case, that the exhumation was made by a mechanism as described by Chemenda et al. (1995).
Bousquet R, Oberhänsli R, Goffé B, Jolivet L & Vidal O, J. Met. Geol, 16, 657-674, (1998).
Goffé B & Bousquet R, S.M.P.M, 77, 137-147, (1997).
Kurz W, Neubauer F & Dachs E, Tectonophysics, 285, 183-209, (1998).
Chemenda A I, Mattauer M, Malavieille J & Bokun A N, E.P.S.L, 132, 225-232, (1995).
3D analysis of the top of Hercynian basement in northern Dauphiné (southern Belledonne, La Mure, Taillefer, Grandes Rousses, Rochail, Emparis, Muzelle, Meije and Combeynot massifs) was carried out using two methods: correlation of 130 E-W profiles to build a 3D model of this surface, and construction of a 3D geological map of the studied area. Stratigraphic arguments demonstrate that the studied surface was flat and horizontal during Triassic times. Its present geometry results from Mesozoic extension (Tethys) plus Tertiary shortening. The model shows several folds: some are northwestward or westward recumbent and are parallel to the regional trends of the Belledonne, Grandes Rousses and Taillefer massifs. They are associated with SE to E-dipping thrusts. Some other are oblique or perpendicular and are associated with S-dipping or SW-dipping thrusts. Both trends can merge to produce domes (i.e. Rochail massif). Microtectonic data in the Mesozoic cover indicate that N-S to NE-SW and E-W structures were built during different phases of deformation and give their relative chronology: N-S shortening occurred between two phases of E-W or NE-SW shortening. In western Oisans, N-S shortening is regarded as a consequence of the Pyrenean-Provence orogeny (Sue et al., 1997). An eastward view of the 3D geological map of eastern Oisans strongly suggests that the Meije and Combeynot massifs are E-W, northward recumbent folds whose axis are dipping to the east. This is consistent with fold axis measured in the Jurassic cover of the Combeynot massif. The Combeynot E-W fold is covered by the Paleogene "Flysch des Aiguilles d'Arves", which rests on the basement and on the folded Mesozoic cover on the SE side and on the NW side of the Lautaret pass, respectively. Despite some post-nummulitic reactivation, this shows that the Combeynot fold is older than Late Eocene, and is thus related with Pyrenean-Provence shortening. The main Alpine, E-W shortening phase deformed both the pre-Nummulitic, E-W structures and the Mesozoic, N-S extensional pattern. Basement folding, thrusting and uplift affected preferentially the shallow parts of Jurassic tilted blocks (half-horsts) which acted as buttress with respect to top-to-the-west shearing. The shallow parts of the main (presumably listric) Jurassic faults were not inverted.
Sue C., Tricart P., Dumont T. & Pêcher A., C. R. Acad. Sc. Paris, 324, 847-854, (1997).
We reconstruct the retrograde metamorphic history of three units in the Alps: the country rocks of the Alpe Arami peridotite (Central Alps), the Gran Paradiso nappe (Western Alps) and a tectonic mélange in the Voltri massif (Ligurian Alps).All three areas underwent high-pressure metamorphism followed by rapid exhumation. Recorded peak pressures are 1.6 GPa in the Alpe Arami country rocks, 1.3 - 1.5 GPa in the Gran Paradiso and 1.5 GPa in eclogitic blocks of the Voltri mélange. Structures and mineral assemblages in the matrix of that mélange were formed under greenschist facies conditions, indicating that the eclogitic blocks were incorporated after peak metamorphism. The three areas show a conspicuous difference in their thermal evolution during exhumation. The Alpe Arami country rocks underwent cooling and were subsequently reheated to about 600 ºC. The Gran Paradiso massif cooled to below 425 ºC and was then reheated to about 550 ºC. The rocks of the Voltri mélange show no evidence of any late-stage thermal overprint.Any geodynamic model for the Alpine orogeny should account for the above orogen scale differences and similarities in PTt history. High-pressure metamorphism of lower crustal rocks suggests the subduction of continental lithosphere. An interplay of erosion and tectonic exhumation is needed to explain subsequent fast exhumation rates. The late thermal overprint has a complex geometry. Its peak lies in the Central Alps (the classical Lepontine dome), but a slightly earlier and less pronounced thermal peak affected rocks in the Western Alps. In the Ligurian Alps no significant late thermal peak was recorded. If the heat source was essentially the same for all three areas, it must have evolved through time to be hotter and focussed further to the east. Several tectonic scenarios are currently under investigation and we envisage break-off of the subducted lithospheric slab as a possible cause. This would enhance exhumation and cause a flux of hot asthenospheric material into the orogenic root over a large area. Break-off is likely to migrate laterally along the slab rather than occurring at once over the entire along strike length of the slab, which could explain lateral migration of the heat pulse.
This research is (in part) supported by The Netherlands Geosciences Foundation (GOA) with financial aid from the Netherlands Organisation for Scientific Research (NWO). Microprobe analyses were carried out at the EU Geochemical Facility at Bristol University (UK) with funding from TMR (contract ERBFMGECT980128).
Eclogite facies rocks from the Sesia zone, the Dora Maira massif, and Cape Corse were analyzed to better constrain the age of high-pressure rocks in the Alpine orogenic belt. A series of about 40 titanites, monazites and rutiles were dated by the U-Pb method, and about 30 Rb-Sr measurements were performed on different size fractions of minerals. For a first eclogite marble from the Sesia zone an age of 278 ± 2 (2<sigma>) Ma is defined by concordant fractions of titanite. For two other eclogite marbles, the same mineral yield ages at 241-226 Ma and 201-180 Ma, respectively. Phengite Rb-Sr isochrons for the same three Sesia marbles lie at 54.8 ± 0.9 Ma (4 points), 50.2 ± 0.8 Ma (5 points) and 47.9 ± 0.5 (5 points). The preservation of old, essentially > 200 Ma old titanite in all three eclogite marbles rises the question on the age of eclogite metamorphism in the Sesia zone, previously considered to be Alpine. Widespread Al-rich titanite is apparently part of the stable eclogite paragenesis in the marbles, but not a single titanite fraction yields an Alpine age, and U-Pb isotope signatures have been preserved through Alpine orogeny. This survival of pre-Alpine titanite through Alpine eclogite metamorphism would also imply that new-growth of titanite during the eclogite event is very minor with the bulk of grains (>90%) originating from an external protolith. At the actual stage of knowledge, we cannot propose any mechanism that would produce such a rock and therefore, the question on the age of eclogite metamorphism in the Sesia zone should seriously be re-considered. In consequence, crystal-chemistry and metamorphic history of Sesia titanites is less constrained than previously thought. In contrast to the U-Pb chronometer in titanite, the Rb-Sr chronometer in phengites records new-growth during Alpine metamorphism between 55 and 48 Ma. These ages would reflect greenschist facies metamorphism possibly produced by initial collision of the African with the European plate during the Paleocene. Monazite and rutile from the White Schists of the Dora Maira massif yield a precise crystallization age of 35.1 ± 0.6 Ma, confirming earlier U-Pb ages from the same geological unit. Six muscovite fractions from this sample are in strong isotopic disequilibrium giving Rb-Sr ages between 31 to 5 Ma. For two eclogitic gneisses from North Corsica, ages of 37.6 ± 0.8 Ma are obtained for a phengite isochron, and 27.6 ± 0.5 and 27.8 ± 0.5 Ma for biotite. Together with these data, the old titanite ages from the Sesia zone opens the possibility that Alpine orogeny in the western Alps is characterized by a single, short-lived deep subduction event at 35-40 Ma.
Valais (North Penninic) and Briançonnais (Middle Penninic) cover nappes exposed in Eastern Switzerland between Prättigau (N) and Oberhalbstein (S) were sheared off during the Early Tertiary from southward-subducted mixed oceanic/continental lithosphere and accreted to the orogenic wedge. Their structural evolution can be subdivided in three steps: During D1, the Bündnerschiefer units, derived from the partly oceanic Valais trough (northern sub-basin of Alpine Tethys) were accreted forming south-dipping "horses" indicating top-north-directed shear sense. During this event, Fe-Mg carpholite grew in veins at 7±1 kbar and below 370±20°C. Bündnerschiefer accretion must have started soon after 50 Ma, in order to allow southward subduction of the European plate margin bordering the Valais basin to the North (HP and UHP metamorphism in the Adula-Cima Lunga nappe) already at about 43 Ma (Gebauer, 1996).
During D2, the bulk shear-sense changed from top-north to top-southeast, D1 folds were refolded, and the nappe stack was thinned by extensional, top-SE shear zones. These were shallower than the older thrusts but dipped in the same direction, leading to superposition of structurally deeper thrust sheets towards southeast over higher thrust sheets, and to "extensional folding" of the thrusts. One of the shear zones is connected to the Turba mylonite zone further south which predates the Bergell granodiorite intrusion (30 Ma). D2 is thus constrained to ca. 35 to 30 Ma. Northward shearing resumed during D3, around or after 30 Ma. A top-north, out-of-sequence thrust and shear zone formed at the base of the Bündnerschiefer complex when the latter was underthrust by European margin units. North-vergent folds connected to this Penninic basal thrust refolded the Bündnerschiefer a second time.
The origin of the shear-sense reversal from top-N thrusting to top-SE normal shearing is ambiguous; it may reflect gravitational spreading following slab breakoff.
Gebauer, D, Geophysical Monograph, 95, 307-329, (1996).
In the internal domains of recent collision belts, ductile deformations partly obliterate the records of pre-collisional tectonism in old basement rocks. The recognition of pre-collisional tectono-metamorphic events in Alpine type belts is of importance and helps to gain a better understanding of orogenic processes only associated with the Alpine tectonics. In the Eastern part of the Swiss Alps, the occurrence of heterogeneously deformed rocks leads to the distinction between pre-Alpine and Alpine metamorphic events.
In mafic rocks of the Suretta basement, two high pressure events are identified as being related to two different geothermal regimes occurring during pre-Alpine and Alpine times, respectively. A first eclogite facies overprint and related exhumation occurred before emplacement of the Roffna rhyolite (U-Pb zircons ages: 268 Ma). This first phase, related to the high-pressure metamorphic event, occurred at ca. 700°C and at least 2.0 GPa and is followed by deformation under high grade amphibolite conditions. These conditions are compatible with pre-Alpine HP-HT conditions already described in several Alpine units.
The sequence of Alpine deformations can be divided into four stages. The D1 ductile deformation is linked to progressive stacking of the Adula, Tambo and Suretta nappes towards the NNW during the Late Eocene subduction of the Valais trough. In mafic rocks of the basement, the schistosity S1 is marked by glaucophane, garnet and epidote, characteristics of blueschist facies conditions. This high-pressure metamorphism occurred at PT conditions around 1.0 GPa and 400-450°C. Similar HP-LT conditions were already described in Mesozoic and Permian rocks. This early Alpine schistosity S1 cross-cut a pre-Alpine foliation, including eclogite to amphibolite facies assemblages, located only in the core of mafic lenses. Deformation D2 is a ductile and heterogeneous deformation phase linked to an E-W stretching lineation marked by the appearance of blue-green amphibole, chlorite and epidote, related to the transition from blueschist facies to greenschist facies. Metamorphic rocks record decreasing pressure, from 0.9 to 0.5 GPa at constant temperatures around 400-450°C. D2 is responsible for the large scale structures on the top of the Suretta nappe which consist of recumbent SE vergent folds with very low angles between fold axes mainly N70 directed and E-W stretching lineations. This syn-collision extension is connected with the development of extensional structures under greenschist facies conditions and is responsible for an important part of nappe pile exhumation. Late deformation phases, D3 and D4, are associated with Oligo-Miocene vertical movements and last dextral transpressional movements along the Insubric Line during Miocene under ongoing retrograde conditions. They did not modify the nappe pile geometry.
The Periadriatic fault is the most obvious map-scale feature in the Alps. It separates the Penninic and Austroalpine nappes, intensively reworked in the Alpine orogenesis, from the Southern Alps which, in contrast, show no penetrative Alpine metamorphic or structural overprint. In its central portion, a major change in the otherwise almost constantly EW-striking direction of the Periadriatic fault occurs. Here, the fault is not a single straight structure, but instead consists of a number of distinct segments, namely the Giudicarie, Mauls, Passeier and Jaufen faults. The temporal and structural relationship between these faults is a key issue in understanding the tectonic history of this part of the Alpine chain.
A detailed structural study of these fault segments, along with dense sampling for fission-track dating, has been carried out in order to reconstruct the deformation history of this system and to try to evaluate the timing and amount of sinistral displacement on the Giudicarie fault, which is still controversial. Based on the preliminary results of this study, it appears that two main tectonic phases can be distinguished: a) A thrusting event around 30-32 Ma (Mueller, 1998) with transport direction towards 100-110°C, in which the Austroalpine system overrode the Southern Alps. This early event is largely recorded by basement and limestones mylonites along the Giudicarie and Mauls faults. b) A later sinistral transpressive displacement with a W side up component, mainly characterized by structures near the ductile-brittle transition.
These brittle-ductile fabrics overprint the mylonites related to top-to-SE thrusting but displacement was also partitioned onto an important system of transcurrent faults in the Southalpine domain (Prosser, 1998). During this late event, the shape of the Periadriatic fault was modified, but the offset was not the ca. 70 km required for an initially straight Periadriatic fault. The value is more on the order of the ca. 10 km estimated for the sinistral displacement of the Jaufen mylonites across the Passeier brittle fault. In this interpretation, the Jaufen mylonites have their continuation as the microstructurally and kinematically similar mylonite zone exposed below Thurnstein castle, west of Meran. Fission-track dating is in progress to confirm this working hypothesis. The distribution of tonalitic lamellae along the Giudicarie fault is also in agreement with this estimate. They outcrop continuously along the Periadriatic fault from Mauls to Dimaro, with only a single gap of slightly more than 10 km immediately south of Meran, along strike of the Passeier fault. This geometry is consistent with their tectonic elimination by sinistral strike-slip faulting on the Passeier fault. To the north, a direct structural connection has been established between the Brenner and the Jaufen fault. This provides good constraint on the timing of phase b), since it must postdate the main exhumation phase of the Tauern Window at ca. 18-20 Ma.
Mueller W, Isotopic dating od deformation using microsampling techniques: the evolution of the Periadriatic Fault System (Alps): unpublished dissertation, ETH Zuerich, (1998).
Prosser G, In: Evolution of the deep crust in the Central and Eastern Alps Abstracts volume, (1998).
An obscure and well known fact in Eastern Alpine geology is the lack of large quantities of calc-alkaline andesitic/dacitic rocks, although an active continental margin did exist from Late Cretaceous to Early Tertiary times. Most of the present-day occurrences are associated in minor dikes related to magmatism along the Periadriatic lineament which took place between 30 and 40 Ma (e.g. von Blanckenburg & Davies, 1995). Precise provenance analysis (geochemical and geochronological investigation) was carried out on calc-alkaline andesite and dacite pebbles from synorogenic conglomerates of the Eastalpine foreland basin. K/Ar whole rock, Ar/Ar hornblende and apatite fission track ages of 25-40 Ma with a prominent peak around 30 Ma confirm a clear relationship to Periadriatic magmatism. These ages proof that at least parts of the Periadriatic magmatic belt, today entirely drained to the south into the Adriatic Sea, were drained in northerly directions into the northern foreland basin. Large amounts of colourless, euhedral, volcanogenic zircon crystals in Molasse sandstones of Lower Egerian to Eggenburgian age (sedimentation age approximately 28-18 Ma) yield fission track cooling ages around 30 Ma. They indicate a substantial erosion of Periadriatic volcanic lithologies in Late Oligocene and Early to Middle Miocene times. In contrast to that, rounded and brownish zircons of the same sandstones reveal Mesozoic fission track ages and represent erosion of Austroalpine crystalline beasement units. We believe that the former colourless zircons are remnants of a volcanic chain which traced the Periadriatic lineament nearly along its entire length. The volcanogenic crystals provide evidence for intense syn- to postcollision calc-alkaline volcanism in the Eastern Alps and a more widespread distribution of volcanic lithologies than at the present-day. Predominantly stratovolcanic edifices once topped the Periadriatic plutons (e.g. Adamello, Rensen, Rieserferner plutons) and were completely eroded during the postorogenic geomorphological evolution.
von Blanckenburg F & Davies JH, Tectonics, 14 (1), 120-131, (1995).
The central High-Atlas is an intracontinental thrust belt resulting from the northward displacement of the African craton relative the Moroccan meseta, and leading to a moderate thickening of the crust under this range. This crustal thickening, which elevates both the Precambrian and the Paleozoic up to 4000 m, can not explained, as was previously admitted, by subvertical reverse faults. Here, we try to show this belt derived from the recent, mainly post miocene, activity of thrusts with ramps and flats, cross cutting the basement and causing thick-skin imbricates within it. The geometry of the basement imbricates is restored using the balanced cross-sections method, with the help of seismic reflexion profiles, on the J. Mgoun - J. Tazika transverse section. The thick-skin thrusting leads to fault-bend anticlines, more or less accentuated by duplexes, which rise the basement. At least two large basement culminations are observed within the High-Atlas (north of Ouarzazate). Some minor reverse faults upheave the basement within the foreland basin itself (Ouarzazate basin). Along the Atlas front, the basement thrusts appear as dextral "en echelon" culminations, more and more younger towards the East. The southern belt front is sinuous because of the occurrence of two tectonic styles: when the crustal thrusts propagate into the cover towards the foreland, their displacement progressively deadens, giving way to a piggy-back (in sequence) fold and-thrust belt carried over the foreland basin (the range is therefore wide); on the contrary, the basement thrust may lead to a wedge structure with backthrusting of the cover (the range is then narrow). The structures from the northern side of the High-Atlas may result from the northward propagation of another basement thrust, antithetic of the preceding ones, and also emerging into the cover as a wedge. At its roof, the cover is deformed according to the detachment fold mode (Atlas of Marrakech and Demnate). Both the great strike-slip and normal faults, often generated during the variscan orogeny (Tizi-n'Test Fault Zone) and the Triassic-Liassic extensional event, thus appear as passively carried over the great thick-skin basement thrusts. Lastly, the shortening calculated for different cross sections of the southern front, varies from 8 to 20 km. This variation can be related to differencial distribution over both northern and southern fronts of a quite uniform shortening.
A review of the geological and geophysical data from the Northwestern Mediterranean and 6-step map reconstructions are used in order to investigate the geodynamic evolution of the area and the mechanisms with generate the opening of the Mediterranean. Based on cross-sectional reconstructions of two transects across the Valencia trough and Provençal basins, the proposed evolution model assumes mass preservation and constant rates of tectonic processes. According to the presented data, the late Paleogene to Present evolution of the Northwestern Mediterranean is mainly marked by two major stages:
Lower Oligocene-middle Serravallian (34-13 Ma) stage of calk-alkaline volcanism and strong tectonic activity in which the Northwestern Mediterranean basin formed from the coeval development of: a) a Rupelian-late Burdigalian extensional rift system in the whole basin area which, locally, derived to formation of oceanic crust, and b) a Late Oligocene-early Serravallian fold-and-thrust belt in the southeastern parts of the Valencia trough.
Late Serravallian-Quaternary (13-0 Ma) stage in which the volcanism became alkaline and the tectonic activity attenuated in a such degree that it did not substantially changed the structure of the Northwestern Mediterranean basin.
We suggest that this evolution is related to the roll-back of the subduction of the Apulia-Africa plate beneath the Iberian and Eurasian plates. This process appears to be quite continuous until the late Burdigalian-early Serravallian (16.5-13 Ma) when the disappearance of major tectonic processes and the calk-alkaline volcanism suggest the break down of the subducting slab. The detachment of the subducting slab would explain the alkaline volcanism and the additional lower lithospheric stretching that has been inferred in the Northwestern Mediterranean from the Middle Miocene.
The diffuse convergent boundary between the Eurasian and African plates in the Western Mediterranean is associated to a zone with seismic activity more than 300 km wide. The two plates have undergone recent NW-SE convergence. However, low and high angle normal faults developed in the Betic Cordillera since the Miocene. The extensional deformations in the region occurred simultaneously with the uplift of the cordillera. The Granada Depression is a late Miocene to present basin. In the southeastern sector of the Granada Depression, the present-day stresses have been determined using earthquake focal mechanisms. Paleostresses have been analysed from the study of the orientation and kinematics of microfaults.
The major structures developed in the area since the Tortonian are large NE-SW open folds and normal faults with predominant NW-SE strikes. In addition, a low angle normal fault located in the basement and with a top-to the SW hanging wall displacement was active up to present-day. High angle normal faults with NE-SW and E-W strikes cutting Quaternary rocks also are observed. Both geological surface data and earthquake focal mechanisms indicate a present-day regional NE-SW extension, with triaxial to prolate stress ellipsoids. The <sigma>1 axis is vertical at the surface, but plunges in depth towards the SW. Nevertheless, the stress field is heterogeneous, with local variations in stress over time and sometimes even acting simultaneously different stresses in adjacent areas. The most frequent change consist of pluridirectional to NE-SW extension, and NW-SE subhorizontal compression, favored by the regional tectonic setting. Strike-slip faults are scarce in the region even though they would be the most likely structure to be expected in a setting dominated by SW-NE extension and NW-SE compression.
The seismicity is concentrated in the upper crust. It may correspond to the activity of low- and high-angle normal faults similar to the surface faults, although they can not be correlated with them. The lower cut off of this seismicity, located between 14 and 16 km, probably coincides with the 300o C isotherm and shows that the thermal gradient of the area is low (about 20oC/km).
These data suggest that the region has undergone compressional deformations, probably ductile and located at depth in the crust, as a response to the NW-SE plate convergence. These deformations may produce thickening of the lower crust and uplift of the Cordillera. However, in the upper crust there develop a simultaneous crustal thinning in a trend orthogonal to that of plate convergence, which reduces the effect of the crustal thickening deformations.
The Basque-Cantabrian basin (western prolongation of the Pyrenees) is an example of Inversion Tectonics, where sedimentary basins related to the opening of the Bay of Biscay were deformed by folding and thrusting processes during the Pyrenean orogeny. This work deals with the structure of the central part of the Basque Arc (Rat, 1962; Feuillée & Rat, 1971), corresponding to the northernmost structural domain of the Basque-Cantabrian basin. At the cartographic scale and from north to south, the main structures of the Basque Arc are: the Northern Biscay Anticlinorium, the Biscay Synclinorium and the Bilbao Anticlinorium.
The area studied comprises the closure of the Biscay Synclinorium and a complete traverse of the Northern Biscay Anticlinorium. The subvertical and N125°E-trending Azkoitia fault marks out the boundary between both structural domains. An imbricate thrust system with large-scale and NE-verging folds characterizes the structure of the northern domain, where three allochthonous units have been distinguished: the Aia, Pagoeta and Azpeitia nappes, from bottom to top. The Pagoeta and Aia nappes are flat-dipping thrust-sheets. They only involve cover lithologies detached within Triassic evaporites and propagate NE-wards into shallower Cretaceous and Eocene beds, respectively. In his turn, the Azpeitia nappe is bounded by a steeper basal thrust and also comprises the Hercynian basement; besides, this trailing nappe shows a penetrative and vertical to steep-dipping cleavage related to upright to large NE-wards overturned folds. In contrast, the Biscay Synclinorium is formed by younger Middle to Upper Cretaceous sedimentary and interlayered volcanic rocks, which did not develop significant imbrications of the stratigraphic sequence.
The reactivation of former synsedimentary faults played a fundamental role in the development of subsequent compressive structures. We consider that the Azpeitia nappe developed from a rotated block which was bounded by a stepply dipping normal fault to the north. During the process of Inversion Tectonics of the basin, this fault acted as a buttress producing intense folding and an associated back-thrust, the Azkoitia fault, in the hangingwall. In this interpretation, the wedge of Hercynian basement in the Azpeitia nappe is explained by the activation of a footwall shortcut. The palinspastic reconstruction shows a minimum overlapping of 4.7 km for the Azpetitia over the Pagoeta nappe and 18.5 km for the Pagoeta over the Aia nappe; the displacement for the Aia nappe cannot be established from surface data alone, as the foreland is located offshore, below the Cantabrian Sea (Pinet et al., 1987).
Feuillée P & Rat P, Hist. struct. Golfe Gascogne, V1, 1-48, (1971).
Pinet B, Montadert L, Curnelle R, et al, Nature, 325, 513-516, (1987).
Rat P, Inst. Est. Pirenaicos, 9-26, (1962).
Sardinia Channel is an assymmetric Tortonian rift located south-east of Sardinia. The channel appears to have evolved through a series of compressive and tensile events, in a way very similar to the Tyrrhenian Sea. Studies of seismic data from the region have postulated an orogenic structure belonging to the Appennine-Maghrebide arc, inverted by extension.
A rifting process opened a channel in a crystalline basement, previously thrusted southward onto the African margin sedimentary cover. This basement is overlain by sediments similar to those of the Calabro-Peloritano-Kabylian group (CPK). A series of important seismic reflectors dip toward Sardinia, stretching as far as the Sicily-Tunisian Plateau to the south-east. They were considered as ancient thrusts, equivalent to the thrusts, that typically divide the CPK basement (Tricart, Torelli, 1994).
Some outcrops on the channel margins were filmed and sampled by the Cyana submersible during the Sarcya and Sartucya surveys (1994,1995). These outcrops reveal that the basement has been subjected to obviously less intensive metamorphism and deformation than in the CPK group. They might represent structuraly higher fragments of the CPK, originally located backward, latter emplaced along the arc front by NW-SE trending strike-slip faults needed by kinematic reconstructions.
The rifting remobilized the previous structures, giving way to a large variety of directions visible both in the fractures and the morphology. However, the survey observations were not able to evidence tectonic inversion, even though certain scarps of the southern margin appeared initially to be denudation surfaces.
Numerous landslides have removed the post-rift morphology on the southern margin. They are associated with rises and swellings that might be correlated with a renewed compressional context. This regime could continue to the present day although the channel zone is currently aseismic.
Tricart P., Torelli L. et al., Tectonophysics, 238, 317-329
The North Pyrenean Fault (NPF) is a subvertical and E-W trending structure which is marked out by scattered outcrops of orogenic lherzolites, granulites and migmatites. The Cretaceous volcanism and metamorphism of the Pyrenees are also closely linked to the NPF. It is now widely admitted that the NPF developed from the thinned lithosphere located at the transcurrent plate boundary between Europe and Iberia during pre-Albian times. Thus, the precise location of the NPF is fundamental to establish the geodynamic evolution of the Pyrenees and the Bay of Biscay. The NPF is well-defined along more than 300 km, through the Eastern and Central Pyrenees, but its surface outline vanishes in the western Pyrenees.
We report the finding of scapolite-bearing marbles in the Basque Cantabrian basin, the western prolongation of the Pyrenees. The marbles are located in the southern limb of the Biscay Synclinorium, one of the major structures in the Basque-Cantabrian basin and come from the Cenomanian marls and calcarenites which are largely represented in the Biscay Synclinorium. Some scarce blocks of white, coarse-grained marbles contain large (~ 12 cm) crystals of tremolite; the most reliable protoliths of such blocks are Jurassic limestones, since they are found in the cap-rock of small diapirs of Triassic evaporites that pierce the southern limb of the Biscay Synclinorium. Textural features of metamorphic minerals (scapolites, tremolites or diopside), that are idiomorphic and show a random orientation, indicate a static growth after the formation of the marble layering.
The scapolite-bearing marbles together with the presence of a small spinel-lherzolite bodies in a near outcrop, are solid evidences for the prolongation of the NPF through the Basque-Cantabrian basin, along the southern limb of the Biscay Synclinorium. With this new proposal for the western prolongation of the NPF, the Cretaceous volcanism of the Basque-Cantabrian basin is laid to the North of the NPF, like on the rest of the Pyrenees. Nevertheless, since the structure of this region is the result of folds and thrusts with northwards vergence, the NPF should be shifted more than 30 km to the South in relation to the location of the scapolite-bearing marbles.
The kinematics and origin of the western alpine arc is contoversial with several distinct tectonic models proposed. This is a primary arc with a strike swing of approximately 90° which was generated by the collision between the Apulian indentor and the European plate margin. Some of the proposed models are here examined by comparing the geometry and kinematics of the external alpine arc to the results of sand-box experiments. In these models 30 mm of layered dry sand were used to simulate the upper crustal sedimentary sequence of the European foreland. In the first set of experiments, an arcuate thrust belt was generated by pushing a rigid indentor (representing Apulia) into the sand in a straight, diagonal, curved or rotational path. A further set of experiments investigates the potential of simple variations in the mechanical stratigraphy of the foreland to produce arcuate trends in a linear thrust belt (i.e. with no indentor), specifically (a) the local presence or absence of an easy-slip horizon, and (b) the presence of a "high" or "low" obstruction. By modelling the large-scale indentor-related kinematics and the effects of local stratigraphic variations separately like this, the significance of local stratigraphic variations in the formation of the alpine arc, as suggested by many authors, can be evaluated. It is concluded that in comparison to the movement vector of the indentor, local variations in mechanical stratigraphy have had a negligable effect on foreland arc kinematics. In addition, the evolution of the alpine arc indicates that the movement path of Apulia varied temporally, and so different stages of arc evolution may be best represented by different experiments.
The Valensole conglomerates were deposited during late Miocene-Pliocene times in a continental foreland basin of the Western Alps. Syndepositional deformation linked to the thrusting of the Digne nappe occurred before the sedimentation ceased in Late Pliocene (Villafranchian). After that time, the northern part of the plateau was deformed and tilted as shown by the fan-like shape of the Quaternary fluvio-glacial terraces in the Durance Valley (Gabert, 1979; Dubar, 1984). - Neogene folding occurred in the northernmost part of the plateau and on both sides of the Bléone valley. In the field, NW-SE folds affect the Miocene and Pliocene sequences and display a typical fault propagation geometry in association with top-to-the-SW shearing of the Meso-Cenozoic cover in the foreland of the Digne nappe. - These Alpine structures are truncated by the Villafranchian surface, which is incised by fluvial erosion but which can be reconstructed by interpolation of its preserved parts. Its SW dip increases towards the NE, which is the highest part of the plateau. Estimates of fluvial incision of the plateau, calculated from a DEM, also increase towards the NE. These criteria demonstrate the Quaternary tectonic activity of the NE part of the Valensole plateau. In addition, the altitude and the fan-like shape of the fluvio-glacial terraces on the left side af the Durance Valley, between the Bléone and the Asse rivers, are compatible with an uplift and a tilt of the northern plateau, although erosional processes (incision and regressive erosion) must also be considered. The Villafranchian surface seems offset along the Asse valley, which also suggests recent fault activity. - The post-Villafranchian most uplifted zone lies above a basement anticline reached by the "Les Mées" drill-hole (Dubois & Curnelle, 1978). It is likely that the uplift is linked to the activity of this structure. Moreover, the Mesozoic sequence of the latter was significantly eroded before the deposition of the Miocene conglomerates, indicating that the Quaternary uplift reactivates an Eocene ("Pyrenean") positive structure. This reactivation is compatible with the recent N-S compressive stress orientation proposed by Bergerat (1985) and regarded as Ligurian in origin, but it could also occur in an Alpine compressive setting.
Bergerat F, Mem. Sc. Terre Univ. Curie, 85-07, (1985).
Dubar M, Bull. Ass. Fr. Et. Quat, 134-138, (1984).
Dubois P & Curnelle R, C. R. Som. S. G. F, 4, 181-184, (1978).
Gabert J, Bull. Ass. Fr. Et. Quat, 3, 101-108, (1979).
The Penninic front (PF), also referred to as Pennine Front or Frontal Pennine thrust in the literature, is a first order structure juxtaposing Penninic nappes against Helvetic-Ultrahelvetic nappes. Seismic reflection studies (NFP20, ECORS-CROP) revealed a moderately (30°) south to southeastward dipping zone of strong reflectivity down to a depth of 30 km which has been interpreted in terms of major thrusting along the PF (e.g. Nicolas et al., 1990). In a more recent study Seward & Mancktelow (1994), based on a fission-track study, argue for reactivation of the PF in the Neogene to Quarternary with a normal-fault component in the area southeast of the Mont Blanc external massif.
Further towards the SW, in the Roselend pass area where the ECORS-CROP seismic line transected the PF, structural observations clearly reveal top-to-the WNW-directed thrusting. With respect to the internal deformation history of the North Penninic (i.e. the Valaisan) units in this area WNW-directed thrusting corresponds to the 3rd phase of deformation overprinting earlier phases established during sinsitral transpression in an ENE-trending corridor (e.g. Ricou & Siddans, 1986).
Still some 20 km further to the southwest, near Moutiers, a late NE-SW trending normal fault cuts all structures at a low angle. This led to some brittle normal-fault overprinting of the PF as, for instance, visible in the "Col du Bonnet de Prêtre" area. Yet, in areas unaffected by this late event the PF is a sinsitral strike-slip zone together with an east side up component separating Subbrianconnais paleogeographical units from Ultradauphinois units. Concerning the internal deformation of the Subbrianconnais these movements are related to the first two deformation phases.
WNW-directed thrusting, as compared to the Roselend pass area, no longer takes place at the PF but has occured along the Ultradauphinois-Dauphinois contact carrying the Ultradauphinois and the Subbrianconnais, and thus the PF, in its hangingwall.
In the Pelvoux area Tricart et al. (1997) decribe W-directed thrusting overprinted by possibly still ongoing normal faulting along the PF and the Houiller front.
Nicolas A., Polino R., Hirn A., Nicolich R. & ECORS-CROP working group, Soc. Géol. Fr. Mém., 156, 15-27, (1990).
Ricou & Siddans, Geol. Soc. Special Publ., 19, 229-244, (1986).
Seward D & Mancktelow N.S., Geology, 22, 803-806, (1994).
Tricart et al., 3rd Alpine workshop, 136-137, (1997).
The investigated area represents the most internal part of the Valais domain and is limited by the Houllier Front in the SE. Tectonically it comprises the Versoyen and the Pt. St. Bernard units. The first unit is made up by serpentinites, basalts and gabbros (substratum) together with breccias and quartzitic schists (syn-rift sediments) unconformably overlain by post-rift sediments. The Pt. St. Bernard unit on the other hand represents a detached cover nappe mainly comprising Liassic limestones and shales. Both units have been subjected to HP-LT metamorphism during alpine orogeny (Schürch, 1987; Goffé & Bousquet, 1997).The whole area is affected by five phases of deformation. Three of them are observable in the entire Valaisan domain, whereas two of them (D3 and D5) are only locally preserved.The three major phases of deformation affecting the whole area are in agreement with observations further to the NW (e.g. Lancelot, 1979; Fügenschuh, 1998). The first two phases involve isoclinal folding on all scales and led to the formation of an intense schistosity. Fold axes are roughly N-S trending and a stretching lineation subparallel to the fold axes can be observed. Shear sense indicators, although rarely preserved, unequivocally yield top to the north directed transport during stages D1 and D2. D1 folding also involves the contact between the Pt. St. Bernard and the Versoyen unit. D3 is characterised by a greenschist-facies mylonitic shear zone indicating top-to-ESE directed normal faulting. Genetically related drag folds with fold axes subparallel to the ESE-WNW trending stretching lineations of the mylonite and subhorizontal axial planes can be observed in the footwall next to the mylonite. The fourth phase (D4) is of regional importance again and displays a clear strain gradient from SE (Houllier Front) to NW (Penninic Front). In the SE this phase is characterised by open folds and a weak spaced cleavage, whereas in the NW folds get tighter. There, isoclinal D4 folds are cut by the coevally active Penninic Front, which shows WNW directed thrusting in the Cormet de Roseland area.The youngest event (post 5 Ma) within the investigated area is top-to-the SE directed brittle normal faulting along the Houllier Front with the "zone houllière" forming the hangingwall. Based on zircon and apatite fission track data a vertical offset on the order of 2-3 km can be deduced.
Goffé & Bousquet, SMPM, 77, 137-147, (1997).
Lancelot, thèse, 3. cycle Paris, (1979).
Fügenschuh, Geol. Rdsch., submitted, (1998).
Schürch, thèse n. 2257, Genève, (1987).
In the western Alps, most kinematic models have considered that, since the late Cretaceous, the convergence between the Adria and European plates has been dominantly accomodated by both thickening and horizontal translation of tectonic units. Some models have also inferred large rotations about vertical axis but no data are presently available to support this mechanism. In order to test this hypothesis, we have conducted a paleomagnetic study on the Briançonnais zone of the Western Alpine Arc. About 300 samples on 35 sites were sampled in upper Jurassic rocks (Ammonitico Rosso facies) of the southwestern Alpine Arc, in an area extending from the Briançon city to the North, to the Ligurian Alps to the South East. In these rocks, natural remanent magnetization (NRM) is usually weak, between 5.10-4 A/m and 1.10-3 A/m. Magnetization is dominantly carried by magnetite, and to a minor extent, by hematite. Thermal and alternating field demagnetization of the NRM reveal three components of magnetization. The first component, with maximum unblocking temperature around 200°C is close to the present-day magnetic field, and can be interpreted as a recent viscous overprint. The second component (ITC) with unblocking temperatures between 200°C and 450°C, is well defined at all the sites and always shows a reverse polarity. The last component is observed above 450°C. Due to the strong increase of viscosity above 450°C, this component is difficult to isolate and no reliable statistical direction can be obtained. Fold tests at local (site) and regional scale are negative for the ITC. Therefore we conclude that this component of aimantation was acquired after the late Oligocene-early Miocene folding phase. On the other hand, late Alpine normal fault bounding tilted blocks, has been evidenced in the study area (Sue and Tricart, 1998). Removing tilting related to this extension, induces a significant clustering of mean site directions. We therefore infer that the ITC is a remagnetization acquired before the late Alpine extension, probably during late Oligocene, early Miocene post-metamorphic cooling. Mean direction for the ITC points to the South East in the Ubaye and Briançon-Guillestre area, and to the East for Liguria.
This implies a counterclockwise rotation of about 45° of internal units of the western Alps relative to stable Europe since the Miocene. Rotation may reach up to 90° in the south eastern part of the arc in the Liguria. These rotations reveal not only local motions of small areas but show that rotation is an important mechanism in post-Oligocene evolution of the western Alpine Arc.
Sue C, Tricart P, Eclogal. Geol. Hel., in press
The Swiss Molasse foreland basin consists of Oligocene to Miocene marine and alluvial sediments, predominantly sandstones, mudstones and conglomerates which record the erosion of the Swiss Alps. Zircon fission track dating on molasse sandstones yields age spectra of the different tectonic units formerly exposed in the source region and therefore provides a powerfull tool to deduce the tectonomorphological evolution of the hinterland.
Sandstones with a sedimentation age of 30 and 25 Ma show complex age spectra: The dominant age clusters between 130 and 90 Ma are characteristic of the Austroalpine unit thus indicating its former exposure in the hinterland of the Swiss Molasse basin. This is in agreement with the pebble content of the conglomerates (25 Ma sedimentation age), containing many granites partly of the lithotype of granites occurring in the Lower Austroalpine Err-Bernina nappe. A minor peak at 31 Ma is related to Periadriatic volcanism and testifies that the presently exposed Periadriatic intrusives were topped by volcanic edifices, completely destroyed today. Another small peak around 190 Ma may indicate a Triassic / Early Jurassic thermal event related to crustal thinning which led to spreading in the Penninic ocean in Middle to Late Jurassic times. Variscan ages are also present in the age spectra of the sandstones and can be either attributed to the erosion of the Austroalpine crystalline basement or of Cretaceous to Paleogene flysch units containing reworked Variscan basement.
19 and 13.6 Ma old Molasse sandstones again show the Austroalpine zircon fission track signature. However, sediment influx related to the Periadriatic volcanism is more pronounced. Prominent peaks of 27 and 22 Ma, derived from the 19 and 13.6 Ma old sandstones, respectively, correspond to the exhumation of the Penninic Lepontine dome. The difference between the zircon cooling ages and the sedimentation ages in the foreland basin is around 8 Ma in both cases. Under the assumption of a geothermal gradient of 40°C/km (for a rapid exhuming region), and taking a zircon closure temperature of 250°C, an exhumation rate of about 0.8 km/Ma can be infered from these data. Compared to the present exhumation rate of the Lepontine region (0.5 km/Ma, Hurford, 1986) the Miocene exhumation rates were significantly higher.
Hurford A, Contrib. Mineral. Petrol., 92, 413-427, (1986).
The Mortirolo fault (MF) marking the boundary between the Campo crystalline and the Tonale unit (Eastern Alps) has previously been interpreted as a pre-Alpine structure, supposedly contact metamorphosed by the Permian Serottini intrusion. However, detailed structural mapping of distinct shear zones within the intrusive rocks unequivocally establishes an important Alpine deformational and metamorphic overprint.
The SE-dipping mylonitic foliations of the MF show strongly developed gently SW-plunging stretching lineations. Shear sense indicators within the mylonites give a top-to-the-E sense of movement, which corresponds at Passo Mortirolo to a relative sinistral sense-of-shear. New crystallization of biotite and dynamic quartz recrystallization, predominantly by grain boundary migration, indicate minimum temperature conditions of ~400-450°C. The change in metamorphic conditions from Alpine greenschist facies in the footwall (N) to well-preserved high-T pre-Alpine rocks in the hanging wall (S) requires the kinematics of a normal fault. Biotite from a Permian granodiorite within the mylonitized footwall, has a Rb-Sr age of ~78 Ma (Del Moro & Notarpietro, 1987). This suggests that the MF may belong to the system of E- to SE-directed normal faults (e.g. Corvatsch, Schlinig and Pejo) responsible for Late Cretaceous extension in the Eastern Alps (Froitzheim et al., 1997, Müller, 1998). Subsequently, the MF zone was reactivated along N-/NNE-directed, steeply S-dipping thrusts, best documented in the hanging wall of the fault. The low-grade mylonites show strong dynamic recrystallization of quartz by subgrain rotation, brittle deformation of feldspar and formation of pseudotachylytes. Thus, syndeformational temperatures of ~300°C are estimated. Stepwise-heating 40Ar/39Ar dating of a pseudotachylyte and Rb/Sr dating by microsampling synkinematically recrystallized white mica from a mylonitized pegmatite demonstrate a Tertiary age for reactivation.The hanging wall and footwall of the MF are folded around an open regional WSW-ENE-striking antiform across Passo Mortirolo. The NE- to N-dipping northern limb, as exposed in Valtellina and Val Grosina, shows a dextral top-to-the-NE transport direction consistent with the folded sense on the MF. A superposed second set of NW-SE striking open folds results in a broad dome and basin interference pattern at outcrop and regional scale.
Locally, a high-T fabric is also preserved in the footwall of the MF that is petrographically similar to the pre-Alpine fabric of the Tonale unit. The distinction between an Alpine pervasively overprinted Campo unit (footwall of MF) and a Tonale unit preserving mainly pre-Alpine fabrics (hanging wall of MF), as previously assumed, cannot be uncritically applied. It is more appropriate to consider differing degrees of Alpine overprint, both in the hanging wall and footwall, variably affecting a similar pre-Alpine protolith.The geometr y and kinematics of the MF can be compared with the tectonic structure at the northern border of the Tonale unit east of the Bergell intrusion. Thus, the fault could be responsible for the westward thinning out of the underlying Austroalpine units, as observable on the map scale.
Del Moro A & Notarpietro A, Schw. Min. Petr. Mitt., 67, 295-306, (1987).
Froitzheim N, Conti P & Van Daalen M, Tectonophysics, 280, 267-293, (1997).
Mueller W, Ph. D. thesis ETH Zürich, (1998).
Eclogites and evidences for an Eoalpine high pressure (HP) metamorphism within the SE Oetztal Basement are only known since the last decade [Hoinkes and Thoeni 1987, Hoinkes et al. 1991]. HP metamorphism has been found in two different areas: (i) SE Texel group, 1.2 GPa minimum pressure in eclogite bearing rocks [Hoinkes et al. 1991] and (ii) 0.8-1.0 GPa in the Schneeberg Complex (SC) further to the NW [Konzett and Hoinkes 1996]. However, this study revealed a minimum pressure of about 0.8 GPa in basement rocks located in between the two above mentioned HP areas and thus suggests a common tectonic history of the whole area, which is further confirmed by petrological and structural evidences. Therefore, a minimum area of 150 km2 within the SE Oetztal Basement suffered Eoalpine HP metamorphism, followed by amphibolite facies overprint.Generally, in order to exhume rocks from depth greater than 20 km, tectonic denudation is considered to control exhumation [England 1981]. Thus structures related to the exhumation of Eoalpine HP rocks should predominate in the area. In fact, strong mylonitic foliation and stretching lineation can be related to the HP metamorphism by oriented omphacite and phengite grains. The same structural pattern is found in autochthonous Permomesozoic sediments within the SC, representing an upper time limit for deformation, while Rb/Sr cooling ages of biotite set the lower limit to a mean age of 79.7 Ma. In the research area Alpine deformation can be subdivided into 2 main events, a former Dn+1with the mentioned strong mylonitic foliation and a stretching lineation trending W-E, followed by a second event (Dn+2), characterised by a NNW trending stretching lineation and W-E striking isoclinal folds with an axial plane foliation trending NNW, forming the main foliation of the whole area. Rheological and petrological evidences indicate, that Dn+2 took place under amphibolite facies conditions. Rb/Sr dating on biotite (see above), which forms the main foliation, reveals Dn+2 to be of Early to Mid Cretaceous age. Concluding, HP metamorphism in the Texel group suggests, that the SE part of the Oetztal Basement represents an Eoalpine collision zone, involving already metamorphosed basement rocks. This Eoalpine subduction zone can be related to Eoalpine HP metamorphic rocks further to the east (e. g. Saualpe-Koralm). However, the mostly acidic character of subducted rocks in the Texel group exposes the intercontinental character of the collision zone, which needs a subduction model differing from those involving basic oceanic material.
England, P.C., EPSL, 56, 387-397, (1981).
Hoinkes and Thoeni, Terra cognita, 7, 96, (1987).
Hoinkes et al., Minerology and Petrology, 43, 237-254, (1991).
Konzett and Hoinkes, J. metamorphic Geology, 14, 85-101, (1996).
The Adriatic-African-plate motion during Oligocene released combined transpression and extrusion between the southwestern margin of the Tauern Window and the Periadriatic Lineament (PL). Major structural elements include steeply north dipping foliations progressively shallowing to the south and subhorizontal east-west oriented stretching lineation. These structures are related to coeval activity of major shear zones as the Defereggen-Antholz-Vals-Line (DAV) and backtrusting of Penninic Tauern Window units to the south onto the Austro-Alpine block.Vertical displacement components as inferred from fold pattern and strain geometries were accompanied with emplacement of granitoids (e.g., Rensen- Rieserferner-plutons). Studies on structures, textures and magma flow suggest that major vertical displacement occurred in northern parts of the wrench corridor and was associated with low vorticity flow. This accounts for pure shear dominated transpression simultaneously with east west extension and orientation of compressional flow apophyses of 20° - 40° in respect to stable Europe. This is interpreted to reflect direction of plate motion of the African plate. Kinematic model indicate that combined transpression an extrusion under this boundary conditions can explain backthrusting of the Tauern Window and uprise of the deeply seated Rensen pluton from depth of ca. 25 km depth. Highly sheared late aplitic dykes as well as low temperature shear planes indicate progressive rotation of shortening axes from NE to NNW. This corresponds to an anticlockwise rotation of the African plate and reactivation of the Periadriatic Lineament. We suggest that during NE convergence of the African plate the system of sinistral DAV and dextral Periadriatic Lineament operated simultaneously. Exhumation and backthrusting of Tauern window and exhumation of Austro-Alpine units as well as emplacement of deep seated magmas may be interpreted as reverse crustal scale flower structure.
Mesoalpine collisional system in the Eastern Slovakia comprises subduction-related units (Iòaèovce-Krichevo Unit - IKU), accretion- and offscraping-related units (Magura and Dukla Unit), transpression-related units (Pieniny Klippen Belt) and fore-arc and back-arc related basins (Central Carpathian Paleogene Basin - CCP and East Slovakian Basin). The IKU is built up mostly by metasedimentary rocks, which with respect to non-metamorphosed Mesozoic and Paleogene rocks of the adjacent Central Carpathian units appear to be core complex. The IKU shows lithological properties of an underplated slate belt, which in Alpine orogene is identify with the Penninic zone. The underplated slate series in the East Slovakian Basin basement are notable for scally fabrics and subduction/accretion style of deformation. The rock complexes of the IKU, including those of the Eocene age, underwent to MP/LT metamorphism, which is responsible for about 15 km depth of undeplating. The vertical displacement of core complexes started in the Oligocene with high volume of uplift rate and reached the zircon FT blocking temperature around 20 Ma. It was forced by strong hinterland extension within a broad dextral wrench corridor following the main litotectonic boundary (Klippen Belt). The collision was accompanied by the formation of the CCP basins. The CCP accomodates the destructive front of the Central Carpathian plate in the position of the constructed fore-arc basin. The basin began to develop with initial collaps and rapid subsidence being induced probably by subcrustal erosion of overriding plate above a zone of subduction. Initial subsidence pattern of CCP basin reflects a trenchward tilting with a fault-controlled deposition of mariginal slope fans. Landward migration of the basin was driven by a highstand eustasy inferred from overall of pelagic sedimentation, condensation (magnanese beds), relative abundance of planktonic biota and "Menilite" episodes. The upper sedimentary cycle of the CCP is developed as a prograding lowstand wedge with a complex deep-sea fan zones. The deep-sea fan system of the CCP shows organization responsible for geodynamic setting of active margin-fans (elongate shape, development of attached lobes as well as suprafan lobes). The marginality of the of the deep-sea fans tends to a sources inferred in the East Slovakian Basin floor. In the pre-Neogene time this area occurred under strong compression induced by the uplift and subsequent exhumation of an underplated unit (IKU).
The remnants of the Triassic/Jurassic Meliata-Hallstatt ocean in Slovakian part of the Inner Western Carpathians are located in two units: (1) the non- or very low-grade metamorphosed Meliata Unit s.s. and (2) the dominantly HP/LT metamorphosed Borka Unit newly defined by Mello et al. (1997). Basic rocks in the Meliata Unit s.s. are represented by spilitized basalts of N-MORB signature associated with Middle Triassic radiolarites and limestones in Jurassic shaley matrix. Basic rocks in the Borka Unit are more variable and can be divided into five groups: (1) Basalts, dolerites and gabbros with BABB to N-MORB signature and only progressive metamorphic evolution to epidote-blueschist stage (2) Basalts geochemically close to BABB, formerly metamorphosed by HP/LT conditions and later retrogressed to greenschists (3) Basalts, rarely dolerites geochemically close to CAB formerly metamorphosed in HP/MT conditions with HP/LT overprint (4) Banded pyroclastics with CAB signature with carbonate intercalations retrogressed from HP/LT blueschists into greenschist conditions (5) Phyllonitized amphibolites geochemically close to N-MORB with former HP/LT and later greenschist overprint Differences in geochemical character and metamorphic evolution of these basic rocks follow from their different geodynamic setting and tectonic history: Only the groups (1) and (2) together with the spilites of the Meliata Unit s.s. are clearly related to the ancient Meliata-Hallstatt oceanic crust. More enriched types (BABB - more marginal and probably also older) were earlier involved into the subduction zone and underwent HP/LT metamorphism while the types typical for mature oceanic basins (N-MORB) are recently presented in non- or very weakly metamorphosed Meliata Unit s.s. Groups (3), (4) and (5) were originally located on the bottom of the overriding plate margin also involved to the subduction mechanism. Groups (3) and (4) originated as parts of an unknown magmatic arc while group (5) can represent probably primarily even older basement rocks.
Mello, Jet al, Explanations to the Geological Map of the Slovak Karst Mts., Dyoniz Stur Publishing, Bratislava, 1-256, (1997).
Most subvolcanic dykes known from the Villány Mts. (SW Hungary) are rift-related alkali basalts and belong to a Jurassic - Lower Cretaceous rift. In the previous years two new sets of dykes were found next to the villages Máriagyüd and Beremend. They both penetrate to Aptian-Albian limestone. The rock is highly altered, most igneous minerals are substituted for secondary phases, mainly calcite and clays (smectite, nontronite). Traces of an earlier porphyric texture can be recognized under the microscope. The most significant constituent is augite with a subservient amount of olivine, hornblende and plagioclase. Applying the isocon method (constant volume and constant Ti approach) it was proved that only Ti, V, Fe and Cr were immobile during the alteration processes. The possible paleo-tectonic setting of the basaltic rocks using Ti/V ratios were determined as a volcanic arc. Additionally, their LIL element concentrations are much higher than it would be expected for MORB-related rocks. The samples studied are high-K basalts, and probably represent a late product of a volcanic arc series.In the surrounding area only one, geochemically and petrologically similar rock type exists. Arc-related basalts were discovered next to Voin and Posega in Baranja (Croatia). They represent remnants of that volcanic arc which was situated on the northern margin of the Tethys ocean in the Upper Cretaceous. The significant geotectonical, petrological and geochemical similarities of the basaltic rocks from the Villány Mts. and from Baranja suggest that the basalt dykes of the Villány Mts. should belong to the same arc system. Traces of this volcanic activity have not been found in Hungary before.
Intermediate-depth seismicity of the Carpathian arc is mainly concentrated at its southeastern bending area, suggesting that the Miocene continental collision was not a frontal one, but started in the northern part of the Carpathians and successively proceeded towards the SE and S. New insights into this final stage of a collision zone are given by the combination of two views, from top down (kinematic data) and from bottom-up (seismicity, tomography).
Analysis of kinematic data reveals the tectonic evolution of the intra-Carpathian crustal blocks which formed the upper plate during middle Miocene subduction. Strike-slip zones laterally guided the northeast- and eastward movements of these blocks, while frontal accretion (and later collision) was accompanied by thrusting (Sperner et al., 1997). Continental collision during middle and late Miocene was followed by slab break-off. Similar to collision, break-off started first in the northern part and then proceeded towards the SE and S. Thus the northernmost slab segments already sank into the mantle, which would explain the lack of recent seismicity in that area. Only in the youngest, southeasternmost part of the Carpathian orogen a relic of this originally west-dipping subducted plate remained. In this region earthquakes occur under strong vertical extension in a small, almost vertical volume with depths between 70 and 220 km and a width of 30 km x 70 km. However, this seismogenic volume is not located beneath the Miocene collisional suture zone, but is shifted about 80-100 km towards the SE.
This offset is also visible from seismic tomography which shows a high-velocity body enclosing all subcrustal earthquake locations. Two different orientations of this body can be distinguished: at larger depths (> 130 km) it trends N-S, thus reflecting the orientation of the middle Miocene subduction zone beneath the Eastern Carpathians. At shallower levels its orientation is NE-SW and the downdip length of this part is identical with the distance between the earthquake zone and the Miocene collisional suture. Thus the southeastward offset of the slab relative to the west-dipping suture zone can be explained by post-collisional delamination of the lower lithosphere (Girbacea & Frisch, 1998). Lateral boundaries of the delaminating body have been pre-existing crustal fracture zones, the Intramoesian fault in the SW and the Trans-European Suture Zone in the NE. Both fracture zones show weak recent seismic activity. The delamination theory is supported by the spatial distribution of alkaline and calc-alkaline volcanism.
Sperner B, Girbacea R, Moser F & Zweigel P, Am. Assoc. Petrol. Geol., Int. Conf., Sept. 1997, Vienna, A56, (1997).
Girbacea R & Frisch W, Geology, 26 (7), 611-614, (1998).
Apatite fission track analysis was carried out on a set of samples from the early-middle Miocene (23-15 Ma) Mt. Cervarola Sandstones. The Mt. Cervarola Sandstones constitute one of the most extensive tectonic unit of the northern Apenninic chain generated in the Cenozoic by the west-dipping ensialic subduction of the Adriatic margin. This unit consists of a thick succession of turbidite sediments accumulated in a longitudinal foredeep migrating toward the east and progressively incorporated in the evolving orogen as a system of folds and thrusts. The emplacement of the Ligurian units of oceanic affinity over the accretionary wedge resulted in a substantial burial of the Cervarola unit. The eastward migration of both compressional and extensional fronts, the latter associated with the opening of the Tyrrhenian Sea directly behind the Apenninic chain, led to the unroofing of the northern Apennines.The apatite samples yield ages between 4 and 7 Ma and show mean confined track lengths around 14µ m with narrow distribution. All samples passed the <chi>2 test indicating that the single grain ages belong to a single population. The age data from all samples are significantly younger than the stratigraphic age and imply that the sediments were buried deep enough to totally anneal pre- depositional fission tracks, as a consequence of exposure to paleotemperatures exceeding 120°C. The fission track ages, then, indicate the time at which the rocks last cooled below ~100°C and record the time of exhumation of the Cervarola unit. Moreover, mean confined track lengths data suggest rapid cooling through the partial annealing zone.
The presence of geologic structures with an opposite vergence respect to that of the whole chain assumes a key role in the understanding of the tectonic evolution of the orogens. In the Northern Apennines (Italy) the innermost outcrops of the Tuscan Nappe are involved in one of these retroverging structures. The Northern Apennines is a fold and thrust belt derived from the collision between Corsica-Sardinia and Adriatic plates during the late Oligocene-Miocene times (Boccaletti et al., 1971; Alvarez et al., 1974). The different tectonic units, belonging both to the oceanic ligurian domain (Ligurian Units) and to the epicontinental tuscan margin (Tuscan Units) have been transported towards the East (Abbate et al., 1970). In the La Spezia area the Tuscan Nappe is folded in a west-facing plurichilometric structure, with a N140 trending axis and plunging a few degrees towards the North, characterised by a sub-horizontal axial plane. Even if this geologic structure has been recognized for a long time its origin is still debated. The most recent interpretations refer its development to a post-collisional extensional tectonic phase linked to the uplift of the Apuane Alps Metamorphic Complex (Gianmarino & Giglia, 1990; Carter, 1992). Geological mapping, accompanied by a detailed structural analysis, led to recognize, in the La Spezia area, a poliphase tectonic history characterized by three ductile tectonic phases. The analysis of the relations among bedding and tectonic foliations, supported by microstructural analyses and by a study of progressive deformation and finite strain determination on minor folds in the hinge zone of the major structure, led to confine the development of the plurichilometric fold to the first tectonic phase. It is regarded as a syn-collision fold due to a large-scale backthrust recognized few kilometers west of La Spezia fold by seismic profiles (Bernini et al., 1997).The study of crystallinity index and analyses on primary fluid inclusions trapped in syntectonic quartz and calcite veins constrained the development of the plurichilometric structure under anchimetamorphic conditions. The later termo-baric evolution has been studied through fluid inclusion analyses sampled in syntectonic composite veins related to the second deformation phase and indicate a retrograde metamorphic pattern that marked the beginning of the exhumation of the tectonic units.
Abbate E, Bortolotti V, Maxwell JC, Merla G, Passerini P, Sagri M & Sestini G, Sedimentary Geology, 4, 201-648, (1970).
Alvarez W, Cocozza T & Wezel F, Nature, 248, 309-314, (1974).
Bernini M, Del Ben A, Diviacco P, Finetti IR, Pipan M, Rogledi S, Torelli L, Zanzucchi G, Riassunti del convegno Nazionale Progetto CROP (Crosta Profonda) Trieste 23-24Giugno, (1997).
Boccaletti M, Elter P & Guazzone G, Nature, 234, 108-111, (1971).
Carter KE, Journ. of Struct. Geol, 14, 182-192, (1992).
Giammarino S, Giglia G, Boll. Soc. Geol. It, 109, 683-692, (1990).
The Alps, a thick-skinned double-vergence belt related to shallow E-ward subduction, show conspicuous structural and morphologic relief and largely consist of metamorphic basement nappes. Conversely the Apennines, a thin-skinned belt related to steep W-ward subduction, display modest relief and are mostly made of sedimentary rocks. Detritus with contrasting provenance signatures derived from the Alps and the Apennines is carried down the Po Plain, representing the peripheral basin of both orogenic belts.
Integrated quantitative petrographic and mineralogic analyses - and usage of a larger set of key indexes (i.e., Q= quartz; F= feldspars; L= lithics: v= volcanic, c= carbonate, t= terrigenous, ch= chert, m= metamorphic, o= serpentine) than the canonical QFL-type parameters plotted three by three on triangular diagrams - allow differentiation between mostly metamorphiclastic detritus derived from the core of the Alps and sedimentaclastic detritus derived from the Apennines. Four main compositional groups are recognized:
I) first-cycle alpine metamorphiclastic sands carried by left tributaries of the Po River in Piedmont (mean key indexes Q33 F17 Lc5 Lt1 Lm40 Lo3);
II) "evolved" alpine quartzose sands carried by the lower courses of the Po, Ticino, Adda, Borbore and Tanaro Rivers, documenting extensive recycling of pedogenized fluvioglacial Quaternary sediments (Q62 F15 Lv1 Lc4 Lt2 Lm12 Lo4);
III) carbonaticlastic sands carried by Apenninic tributaries of the Po River, reflecting recycling of largely calcareous synorogenic oceanic turbidites (Q17 F5 Lv1 Lc52 Lt17 Lch1 Lm3 Lo5);
IV) carbonaticlastic sands carried by the Brembo and Serio Rivers, derived from Mesozoic cover and Hercynian basement rocks exposed in the Southern Alps fold-thrust belt (Q16 F3 Lv12 Lc50 Lt6 Lch1 Lm13).
Heavy mineral suites are instead fundamentally influenced by the occurrence of ophiolitic sources. River sands derived from alpine greenschist to blueschist facies metamorphic ophiolites are characterized by abundance of epidotes and amphiboles (tremolite, actinolite, glaucophane), and markedly contrast with sands derived from unmetamorphosed apenninic peridotites, characterized by abundance of pyroxenes and Cr-spinel. Non-ophioliticlastic river sands derived from the core of the Alps are rich in garnet and hornblende. Suites vary surprisingly little from first-cycle to "evolved" detritus, and are similar also for rivers draining the Southern Alps, because Mesozoic cover rocks provide few heavy mineral grains. Even sands carried by Apenninic tributaries are garnet-dominated and show alpine affinities, being largely recycled form Cretaceous to Neogene turbidites largely fed in turn - directly or indirectly - by erosion of the growing alpine orogen.
A structural field study on brittle deformation in the Alpi Apuane (Northern Apennine, Italy) has been carried out in order to understand the late structural evolution of the region.Although in previous studies the presence of brittle faults has already been described especially in the border region of the Alpi Apuane, no detailed structural investigation of geometries and kinematics of faults has been carried out yet. Our contribution presents the first results of such a study in the NW part of the region near Carrara city where the well known Jurassic marble body (Carrara marble) outcrops. Our data reveal a polyphasic deformational history in which at least two main structural phases can be recognized. The older stage consists of conjugate sub-vertical fault plane systems which are in agreement with a roughly N-S trending horizontal <sigma>1 and a E-W horizontal <sigma>3. Locally further sets of dip-slip to oblique-slip faults are in agreement with a vertical <sigma>1 and a E-W horizontal <sigma>3; this seems to indicate the first brittle stage in the Carrara area to be a combination of normal and strike-slip faulting pointed out by constant <sigma>3 and permutation of <sigma>1 and <sigma>2. This interpretation is further supported by field analyses of striation curvatures and of alternating relations between strike-slip and normal striations on fault planes. A later event showing a vertical <sigma>1 and variable orientations in the direction of the <sigma>3 can be associated with faults cross-cutting the previous structures which are locally reactivated. This second generation of structures seem to be correlated with the "apenninic" (NW-SE striking) normal faulting well known in the internal part of the Northern Apennine. Although a direct dating of the described brittle structures cannot be performed, some considerations allowed to constrain the timing of the deformation in the regional geological framework.
The remnants of palaeotectonic structures are often used to explane some features of the peri-Mediterranean orogenic belts. The restoration of Mesozoic palaeotectonic setting of the south-Apennines segment of the African passive margin, including the Lagonegro basin generated by continental rifting since middle Triassic times (Scandone, 1975), is remarkably difficult being the chain affected by Neogene contractional and Quaternary strike-slip and extensional tectonics (Schiattarella, 1998). In such a complex mountain chain, the analysis of ancient volcanic layers may provide new insights about a reliable palaeotectonic scenario. In the lower part of the Scisti silicei Fm (upper Triassic - Jurassic) outcropping in the Agri Valley, and in the lower part of the Galestri Fm (lower Cretaceous) cropping out near Pignola village, Lucanian Apennine, we found for the first time volcanic layers interbedded with pelagic sediments (Di Leo et al., 1998).The unweathered portion of the volcanic levels is made of quartz and feldspars. In the TAS scheme the rocks are classified as dacites and rhyolites. To constrain the palaeotectonic setting associated to the volcanic beds we used the Y vs Nb and Y+Nb vs Rb discrimination diagrams proposed for felsics (Pearce et al., 1984; Twist and Harmer, 1987). In both diagrams the samples fall in the Volcanic Arc field indicating a more complex palaeogeographic scenario of the Lagonegro deep-sea basin relative to the classical view that states the basin was generated in an extensional regime and bordered by normal faults during the entire Mesozoic tectono-stratigraphic evolution. A continental transform zone may be envisaged as partly responsible for the basin evolution in a more general context of extensional tectonics. Such an anisotropy may have therefore conditioned the crustal and magmatic evolution.
Di Leo P, Giano SI, Mongelli G, Schiattarella M, Plinius, 20, 101-103, (1998).
Pearce JA, Harris NB W, Tindle AG, J. Petrol, 25, 956-983, (1984).
Scandone P, In: C. Squyres (ed. ) "Geology of Italy", The Earth Sciences Society of the Libyan Arab Republic, 305-315, (1975).
Schiattarella M, In: R. E. Holdsworth, R. A. Strachan and J. F. Dewey (eds) "Continental Transpressional and Transtensional Tectonics", Geological Society, London, Spec. Publ, 135, 341-354, (1998).
Twist D, Harmer RE J, J. Volc. Geothermal Res, 32, 83-98, (1987).
The Internal Albanides form the eastern part of Albania, are characterized by the presence of ophiolites and high intensity of the tectonic activity. They, tectonically were structured by some peaks of alpine foldings, caused by the movements of African plate and Adria block to eastwards and their collision with Europian lithospheric plate. The main folding phases, which have taken part in the structuring of the Internal Albanides are during the Jurassic, Early and Late Cretaceous and Early Tertiary.
1-Lias-Early Dogger is characterized by the subsidence of the greatest part of paleo-Mirdita, opening a narrow NW-SE direction basin and in the basin floor were uplifted the ophiolites (Early Kimmerian phase). Later on, during the Callovian and Oxfordian, followed the emplacement of ophiolites on the continental margins (as serpenite diapirs and by the mechanism of subduction) in the Mirdita- Subpelagonian ophiolitic belt. Finally these regions have been strongly folded and emerged, undergoing weathering processes (Middle Kimmerian folding). After closing the active period of ophiolites, during which they had a transformation role of the environs, starting with Kimmeridgian, begins the new era of transgressions. The pelagic sedimentation continues up to the end of the Valanginian (Peza et al. 1981,1983,1992).
2-During the Hauterivian (Mirditean folding), paleo- Mirdita was emerged and strongly folded again. Many great rock masses have been displaced during this time to westward, forming some overthrust nappes: Kurbnesh nappe, Vanas nappe, Perroi Varoshit nappe, in the northern part and Vithkuq-Ujebardha nappe, in the southern part of the zone. (Peza et al. 1981,1983,1995). The subsidence started with the Barremian, being the greatest transgression in the zone. The sedimentation continue up to the Middle Turonian by the platform deposits. D1 and D2 deformations of Carosi et al. (1996) and Bortolotti et al. (1996) are copied from other's papers and don't bring any new data about the problem.
3- Illyrian folding, (Middle Eocene), was very strong in the Albanides. Some overthrusts nappes of the illyrian origin are Shkrodra nappe, Devolli nappe, in the mostsouthern part of the Mirdita zone Kolonja nappe. The occurence of the bauxite of the Middle Eocene in the Kruja zone, represent the remnants of the ultramafic nappe during this time.
Bortolotti V.,Kodra A. et al., Ofioliti, 21(1), 3-20, (1996).
Carosi R.,Kodra A. et al., Ofioliti, 21 (1), 41-45, (1996).
Peza L.H., Marku D, & Pirdeni A., Permbledhje Studimesh, 2, 95-108, (1081).
Peza L.H., Pirdeni A. & Toska z., Bul. Shkenc. Gjeol., 4, 71-91, (1983).
Peza L.H. & Shkupi D., 29-th Intern. Geol. Congress, nr.II-6-2,p-44, paper nr.6553, (1992).
Peza LH, TERRA NOVA , abstr. supl. , 1, 7, 180, (1995).
The Milesian Peninsula, SW-Anatolia, Turkey, is situated between the valley of the Büjük Menderes to the north and the Akbük Gulf to the south. Rocks of the peninsula comprise several hundred metres of Miocene and Pliocene sediments (Schröder and Yalcin, 1993). The Miocene part of the stratigraphy consists of limestones of the Nergiztepe Formation and marls, conglomerates and minor pyroclastics of the Balat Formation. These units are overlain by thick, patly onkolithic limestones and minor marls of the Pliocene Akbük Formation. Metamorphic rocks of the Menderes Massif form the basement to the Neogene sedimentary rocks but these are only exposed some 10 km inland towards the east.
The morphology of the peninsula is partly fault-controlled and dominated by large limestone plateaus and inselbergs with their greatest elevation of up to 250 m along the prominent graben shoulder of the Büjük Menderes valley in the N. From here they step down as several flat-topped ridges, separated by shallow WSW-ENE- and NNE-SSW-trending valleys, towards the Akbük Gulf in the SSE. Results from detailed geological mapping revealed two systematic fault directions with the more prominent one running WSW-ENE, the other trending NNW-SSE. Some of the faults display scissor-like movements, with maximum vertical displacement in the west and flexures in the east. These faults have truncated the Neogene sediments and resulted in the systematic down-stepping of individual fault blocks from north to south within the peninsula. The orientation of these fault blocks is parallel to the Menderes graben in the north and the graben of the Akbük Gulf in the south.
The distribution of recent earth quake epicentres in the southeastern Aege