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Geologie N° 2.Qxp ABSTRACTS GÉOLOGIE DE LA FRANCE, N° 2, 2007 55 ABSTRACT Variscan kinematics of the Ural and Tienshan junction area ALEXEIEV Dmitriy V. * Late Paleozoic evolution of the Ural-Tienshan junc- the Kazakhstan continent with Tarim in the latest tion area was controlled by the processes of conver- Carboniferous and with Turan in the Permian time. gence and multiply collisions between Kazakhstan, Baltica, Turan, and Tarim continents. Subduction in Principal structural assemblages and deformation ages the Valerianov arc since middle Visean till beginning within the Ural–Tienshan junction area can be defined as of the Late Carboniferous (during ~335-315 Ma) was the following. D1) Accretionary thrust folded belt of the responsible for convergence between Kazakhstan South Tienshan (since ~330 Ma in the West and since and Baltica. Duration of arc magmatism for ~20 Ma ~315 Ma in the East); D2) Laramide-style thrust-folded suggest that minimum several hundreds of kilometers belt in the SW and S of Kazakhstan in Karatau and Middle of an oceanic crust was consumed there that time. Tienshan, with major thrusts, directed toward the conti- nent (since ~315 Ma); D3) North-South trending folds in Collision of Kazakhstan started in the middle of the the W and SW of Kazakhstan, mainly Karatau ridge, over- Bashkirian age (~315 Ma). It resulted in: a) formation printing D2 structures (Permian?); D4) Left-lateral strike- of sub-meridional divergent thrust folded belt in Urals, slip faults and plunging folds within entire Tian Shan, due b) Laramide-style deformations in the Southwest and to oblique collision of Kazakhstan and Tarim (mainly Late South of the Kazakhstan continent, and c) iniciation Permian and early Mesozoic), and D5) East-West of a north-dipping subduction zone in the South of the trending compressional structures and diagonal strike-slip Kazakhstan continent within Central Tienshan. faults due to shortenning in the North-to-South direction Ongoing subduction in the South lead to collision of (Latest Triassic and Jurassic). * Geological Institute RAS, Russia GÉOLOGIE DE LA FRANCE, N° 2, 2007 57 MECHANICS OF VARISCAN OROGENY Transected Folds with opposite patterns in Terena Formation (Ossa Morena Zone, South Portugal) ARAÚJO António Alexandre * ROCHA Rodrigo Camargo * The Terena Formation is a Lower Devonian flysch the Ossa-Morena Zone and interpreted as the effect of a located in the Estremoz-Barrancos Sector (central part left lateral shearing, contemporary of the regional of the Ossa-Morena Zone, South Portugal).Taking flatening.The cartographic right transection can be advantage of the age of this unit which is contemporary explained by the generation of a near N-S trench, as the with the beginning of the Variscan orogeny, it was pos- effect of the first deformation phase (trending also N-S sible to make a geometrical study of the evolution of the but only present at lower structural levels). Several sedi- early stages of deformation. mentary features indicate that the genesis of this trench and the deposition of the Terena flysch are almost con- At the regional scale, the Terena Formation outcrops in temporaneous (strong lateral variations of facies, olis- the core of a first order syncline from the second defor- tolits of Silurian rocks). The trench shape conditioned the mation phase, with nearly 100 km long, 10 km wide and deposition of flysch facies during Lower Devonian and having an unusual “Z” shape. The relation of this first the orientation of the first order D2 syncline. The right order fold with the foliation, shows that the central part of transection results from the superposition of the NW-SE the macroscopic structure, trending nearly N-S, is tran- upright S2 cleavage on this major regional structure. sected by S2 cleavage (NW-SE), showing a right tran- section pattern. The cartographic right transection is The mesoscopic folds, observed on the upper levels of opposite to the left transection pattern showed by D2 the sedimentary sequence were not influenced by the folds at a mesoscopic scale, in the axial sector of the topographic anisotropy of the basin, and therefore they major synclinal. The transection of mesoscopic folds is developed a left transection, according to the regional coherent with geometrical patterns usually described in deformation mechanisms. * Centro de Geofísica de Évora, Portugal 58 GÉOLOGIE DE LA FRANCE, N° 2, 2007 ABSTRACT New insights of the Variscan late-orogenic extension in South-Brittany: the Sarzeau Shear Zone AUGIER Romain * TURRILLOT Paul *, FAURE Michel * Together with the Quiberon Shear Zone, the Sarzeau brittle structures that are stochastically parallel. Top to Shear Zone (SSZ, Morbihan) witnesses of the late the ESE ductile shear zones were then progressively Carboniferous extensional collapse of the Variscan exhumed and replaced by shallow dipping cataclastic thickened crust. The SSZ is a km-scale normal fault shear zones when they reached the brittle field then that exhibits a kinematic and dynamic continuity from a accompanied by the formation of high angle normal deep, synmagmatic and ductile activity to a shallow fault in the hanging wall. brittle one.The SSZ is an example of an E-dipping flat- lying ductile extensional shear zone evolving into a low- Most of the last increments of extensional strain are angle brittle detachment. achieved through in the upper crust by the formation of a penetrative network of dominantly E-dipping normal A clear continuum of extensional strain from ductile to brittle faults. This N110 extensional stress field is also brittle regime is observed in both footwall and hanging responsible for the injection of N20 to N50 striking late wall. The brittle deformation is characterized by shallow Carboniferous granitic dykes interpreted as km-scale and steeply dipping normal faults associated with sub- tension gashes. The chronological and structural rela- vertical extensional joints and quartz veins. The earliest tionships with the Carnac pluton that crop out to the west brittle structures are low angle normal faults which are discussed. E of the SSZ, at least two end-members superimpose on, and reactivate, earlier ductile shear geometries of granite dykes of same strike can be distin- bands commonly located along pegmatite and aplite guished with (early?) 30 to 45° West dipping magmatic- dykes, but newly formed muscovite-rich low-angle structured ones and subvertical ones exhibiting no strain normal faults are also observed. The inversion of fault patterns. Towards the SSZ, a petro-structural analysis of slip data collected within and away from the main oriented sections showed that these dykes are carrying detachment zone (50 stations over a 45 x 20 km area) a magmatic planar and linear fabric upon which ductile shows a consistent N110 stretching for both ductile and and brittle extensional structures overprint. * Institut des Sciences de la Terre d’Orléans, France GÉOLOGIE DE LA FRANCE, N° 2, 2007 59 MECHANICS OF VARISCAN OROGENY Imaging the mantle roots of the Variscan orogen below the Paris basin: Insight from the P-wave velocity model PM0.5 AVERBUCH Olivier * PIROMALLO Claudia ** A growing number of recent seismological and geochem- As suggested in previous tomographical studies along ical studies have provided evidence for long-lived compo- ancient suture zones, these data argue for such sitional anomalies at all scales in the mantle. Such anomaly being the remnant of a Variscan LRH oceanic lithological contrasts can induce lateral seismic velocity slab that did not suffered any significant late orogenic heterogeneities in the mantle persisting over several hun- removal. As the latter apparently cuts across another dred million years, i.e. much longer than if they were exclu- superficial high-velocity anomaly located upon the Late sively due to temperature variations. Therefore, in old Devonian (ca 375 Ma) Ligerian-Massif Central (LMC) suture zones, mantle seismic tomography can significantly suture zone, it would imply an early orogenic process of help in defining ancient deeply buried slabs and thus in LMC slab break-off before the initiation of the opposite constraining geodynamical models for past orogenic sys- subduction of the LRH slab. tems. In northern France, the Paris basin corresponds to the Late Paleozoic-Mesozoic episutural basin of the Late As suggested by modelling experiments, this process Devonian-Carboniferous Variscan collisional belt. could be considered as a major driving mechanism for the exhumation of HP and UHP metamorphic com- The large-scale P-wave velocity model PM0.5 developed plexes along the LMC suture zone (unlike the low grade by Piromallo and Morelli (2003) allowed imaging the metamorphic LRH suture zone). As a whole, these new lithospheric mantle roots of the Variscan orogen below the data question the long-term evolution of subducted Paris basin, pointing to the existence of a significant high oceanic lithosphere and in the case of the Western velocity anomaly in the upper mantle. The latter fits in European Variscan belt, suggest the subduction of the map view, at ~150-200 km depth, the trace of the northern LRH oceanic slab below the previously thickened Variscan suture zone i.e. the Upper Carboniferous (ca Eovariscan crust to have played an important role in the 325 Ma) Lizard-Rheno-Hercynian (LRH) suture zone. late collapse of the belt inducing a significant amount of Moreover, it is surprisingly spatially correlated to the well- thermal erosion and softening of the overriding litho- known Paris Basin Magnetic Anomaly, however slightly sphere all over Carboniferous times. shifted to its SW extension. In cross-sections, the main high-velocity anomaly seems to extend through the lithos- Reference pheric mantle with a steep southern dip, to apparent Piromallo C. and Morelli A. (2003) - P-wave tomography of the depths greater than expected for a standard continental mantle under the Alpine-Mediterranean area. JGR 108(B2), lithosphere (about 350-400 km depth). 2065, doi:10.1029/2002JB001757.
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