The exhumation of eclogite facies granulites (Omp-Plg-Grt-Qtz-Rt) in the Rychleby Mts, eastern Czech Republic, was a localised process initiated by buckling of crustal layers in a thickened orogenic root. Folding and post-buckle flattening was followed by the main stage of exhumation that is characterized by vertical ductile extrusion. This process is documented by structural data, and the vertical ascent of rocks from a depth of c. 70 to c. 35 km is documented by metamorphic petrology. SHRIMP 206 Pb ⁄ 238 U and 207 Pb ⁄ 206 Pb evaporation zircon ages of 342 ± 5 and 341.4 ± 0.7 Ma date peak metamorphic conditions. The next stage of exhumation was associated with sideways flat thrusting associated with lateral viscous spreading of granulites and surrounding rocks over indenting adjacent continental crust at a depth of c. 35-30 km. This stage was associated with syntectonic intrusion of a granodiorite sill at 345-339 Ma, emplaced at a crustal depth of c. 25 km. The time required for cooling of the sill as well as for heating of the country rocks brackets this event to a maximum of 250 000 years. Therefore, similar ages of crystallization for the granodiorite magma and the peak of eclogite facies metamorphism of the granulite suggest a very short period of exhumation, limited by the analytical errors of the dating methods. Our calculations suggest that the initial exhumation rate during vertical extrusion was 3-15 mm yr )1 , followed by an exhumation rate of 24-40 mm yr )1 during further uplift along a magma-lubricated shear zone. The extrusion stage of exhumation was associated with a high cooling rate, which decreased during the stage of lateral spreading.

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Teplá-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Teplá-Barrandian middle crust along the socalled Teplá suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Teplá-Barrandian crust. The back-arc region -the future Moldanubian zone -was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid-to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Teplá suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes. To cite this article: K. Schulmann et al., C. R. Geoscience 341 (2009). # 2009 Published by Elsevier Masson SAS on behalf of Académie des sciences. RésuméConvergence paléozoïque de type Andin dans le Massif de Bohême. Le Massif varisque de Bohême est le résultat de la subduction, au Dévonien supérieur, de l'océan Saxothuringien sous la plaque continentale représentée à l'est par les zones actuelles

International audienceWe provide a detailed description of the structures along a 300 km long and 50 km wide transect across the Central Asian Orogenic Belt (CAOB) in southwestern Mongolia, covering the Precambrian Dzabkhan continental domain with overthrust Neoproterozoic ophiolites in the north (Lake Zone), a Silurian-Devonian passive margin association (Gobi-Altai Zone) and oceanic domain (Trans-Altai Zone) in the center, and a continental area (South Gobi Zone) in the south. Structural analysis suggests late Cambrian collapse of the thickened Lake Zone continental crust, leading to stretching of the lithosphere and followed by Silurian-Devonian formation of oceanic crust in the Trans-Altai domain. Subsequent emplacement of Devonian-Carboniferous and late Carboniferous magmatic arcs occurred on the Gobi-Altai and South Gobi Zone crusts, respectively, during E-W shortening. Finally, the entire system was affected by N-S convergence from the Permian to Jurassic, leading to heterogeneous shortening of the orogenic domain. The model best fitting these observations is one of generalized westward drift of the Tuva-Mongol-Dzabkhan-Baydrag ribbon continents during the Silurian-Devonian, associated with westward-subduction of the Mongol-Okhotsk Ocean and sequential growth of syn-convergent magmatic arcs. Back-arc basins opened during this period in the area of the western Paleoasian Ocean. The present-day shape of the CAOB in southern Mongolia was probably formed during Permian to Mesozoic anticlockwise rotation and folding of the Tuva-Mongol-Dzabkhan-Baydrag continental ribbons, combined with a strike-slip (transpressional) reactivation of ancient transform boundaries in the Paleoasian oceanic domain. All continental and oceanic crustal domains were reactivated and intensely deformed during this convergence in a style controlled by crustal rheology and a heterogeneous Permian magmatic-thermal input. The sequence of tectonic events is tested against published paleomagnetic data, paleogeographic reconstructions and tectonic models, leading to a revised model for the accretion of juvenile crust to a continental margin in the CAOB of southern Mongolia

New SHRIMP U-Pb and evaporation Pb-Pb zircon ages, together with a revision of the lithostratigraphy of "suspect" terranes in SW Mongolia, suggest that the collage of continental and oceanic units in this region resulted from recurrent magmatic reworking and deformation of Silurian-early Devonian proximal and distal passive margin sequences of the Paleo-Asian Ocean. The zircon ages from early Ordovician volcaniclastic rocks and syntectonic felsic dikes reveal an heterogeneous stretching of the Precambrian Dzabkhan microcontinent (Lake Zone basement) during the Ordovician, followed by the development of a carbonate platform on a proximal margin (Gobi-Altai Zone), serpentinite breccias and Silurian chert sequences on a distal margin and possibly also the formation of oceanic crust. The assumed early Neoproterozoic South Gobi continental zone may either represent an allochthonous block detached from Dzabkhan or, less likely, the conjugate margin of a Paleo-Asian continental rift. Early Devonian volcanism subsequently affected both types of margins with back-arc spreading centers and arcs located in the core of the future Trans-Altai Zone. During the late Devonian to early Carboniferous a Japan-type magmatic arc developed on the previously stretched continental crust of the Gobi-Altai Zone. This event was associated with shortening of the entire domain, exhumation of the deep arc core and formation of intramontane basins with Devonian and Carboniferous detrital zircons of the adjacent Lake Zone continent. Clastic, flysch-type sedimentation occurred on the former distal margin and in oceanic areas. During this early Carboniferous contraction event the continental and oceanic units were imbricated and accreted to the continent in the north. Subsequently, late Carboniferous volcanic arc sequences and a Japan-type magmatic arc developed on the Trans-Altai oceanic crust and the southern South Gobi Zone, respectively. Finally, a Permian thermal event was localized in the Gobi-Altai-Lake Zone contact domain and was responsible for formation of Permian grabens, bimodal volcanism and substantial melting of the accreted crust.

A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two-stage exhumation model of deep-seated rocks in the eastern sector of the Variscan belt. An early sub-vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18)20 kbar to depths equivalent to 8)10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8-9 kbar. Following the folding and vertical extrusion event, sub-horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE-directed Saxothuringian continental subduction. Fabrics that record the later subhorizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north-directed sub-horizontal channel. This sub-horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re-equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SEoriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.