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International audience; Granitoid magmatic sheets emplaced syntectonically during growth of the Orlica-nienik mantled gneiss dome (Central Sudetes, European Variscan belt) were examined by means of structural geology, quartz, and anisotropy of magnetic susceptibility (AMS) fabric studies. Magmatic emplacement was localized along the eastern transpressive margin of the high metamorphic core and below the low-grade detached mantle rimming the southern margin of the dome. In the first area, the magmatic sheets were emplaced parallel to the dilated subhorizontal foliation of an anisotropic pre-orogenic block. The resulting melt-host rock multilayer localized the transpressive zone along which the bulk of magma was emplaced. The AMS study shows an across-strike fabric zonation underlying a strongly transpressive solid-state deformation in the hanging-wall, magmatic subhorizontal fabric in the foot-wall and a transitional fabric in the center of the intrusion. In contrast, along the southern dome margin, magmatic sheets intruded along steep foliations of weakly metamorphic mantle rocks, and are affected by recumbent folding and subhorizontal shearing. The bulk of the sheets present a shallow-dipping magmatic foliation and an along-strike magmatic lineation. The variations of the quartz and magnetic fabrics are attributed to superimposition of pure shear-dominated ductile thinning followed by simple shear-dominated detachment onto the original steep fabric. AMS modeling confirms the role of variation of orientation of pre-intrusive anisotropy during progressive deformation on the resulting fabric pattern and helps explaining observed variations in fabric orientations and symmetries. This study highlights contrasting mechanical behavior of syntectonically emplaced magmas in different parts of a growing crustal-scale mantled gneiss dome. Keywords

International audience; Continental core complexes are generally interpreted to result from extensional doming due to gravity-driven upflow of lower crust. In contrast, the Vepor Dome is characterized by the lack of inverted density profile and relatively cold metamorphic field gradient which precludes an activation of Rayleigh-Taylor instability. Instead, the crustal structure of the Vepor Unit is marked by dense and weak metapelitic lower crust and light and strong granitoid upper crust inherited from Variscan nappe stacking. It is shown that the Cretaceous Eo-Alpine tectonic evolution of the Vepor Dome is controlled by the dynamics of two neighboring mechanically strong continental blocks, i.e., the overthrusting of the suprastructural Gemer Unit from the south and the underthrusting of the Fatric basement from the north. Structural, metamorphic and geochronological data from the Vepor Unit imply two main phases of the convergent process: (1) Lower Cretaceous crustal thickening due to overthrusting and internal deformation of the Gemer Unit together with upper crustal folding in the Vepor Unit led to the progressive development of the orogenic front parallel pressure gradient. The instantaneous response of the lower crustal and low-viscosity metapelites led to an along-strike lower crustal flow accompanied by prograde Barrovian-type metamorphism. (2) As the south vergent underthrusting of the Fatric basement propagated to greater depths during the Upper Cretaceous, the convergent process switched from top driven to bottom driven, and the exhumation of the lower crust occurred via polyharmonic folding. Overall doming of the Vepor Unit induced upper crustal detachment faulting and eastward unroofing of the dome.

International audience; The structure of the Moldanubian domain is marked by felsic granulites of Ordovician protolith age forming the cores of domes that are separated from mid-crustal Neoproterozoic and Palaeozoic metasedimentary rocks that occur in synclines by a late Ordovician to Silurian metabasic unit. Reflection and refraction seismic sections combined with gravity inversion modelling suggest the presence of a low density layer at the bottom of the crust (interpreted as felsic granulite) overlain by a denser layer (interpreted as amphibolite) with layers of intermediate density at the top (interpreted as metasedimentary rocks). It is proposed that the granulite domes surrounded by middle crustal rocks reflect transposed horizontal layering originally similar to that preserved in the deep crust and imaged by the geophysical surveys. This geological and geophysical structure is considered to be a result of Visean gravity redistribution initiated by radioactive heating of felsic crust tectonically emplaced at the bottom of a Palaeozoic orogenic root. The radioactive layer with heat production of 4 mu W m-3 corresponds geochemically and isotopically to Ordovician felsic metaigneous rocks of the Saxothuringian domain that have been emplaced at Moho depth under thickened crust during late Devonian-early Carboniferous continental subduction. Part of the continental crust continued to be subducted and produced fluids/low-volume melts which directly contaminated and enriched the local lithospheric mantle by lithophile elements, most notably Cs, Rb, Li, Pb, U, Th and K. Thermal incubation of 10-15 Myr was sufficient to heat and convert the underplated felsic layer into granulites via dehydration melting and melt segregation. The process of melt loss was responsible for the removal of radioactive elements and for switching off the heat at the beginning of the exhumation process. At the same time, the metasomatized underlying mantle was heated producing characteristic ultrapotassic magmas. Gravitational instability was then induced by the density contrast between the light granulites and the overlaying denser mafic lower crustal layer and a viscosity drop related to thermal weakening and partial melting of the latter.

International audience; Potassium-rich magmatism represents subordinate magma volumes worldwide but has been observed in many geodynamic settings. Most potassic magmas are thought to derive from very-low degrees of melting of metasomatized mantle lithologies. We performed piston-cylinder experiments to determine trace element partition coefficients between incipient potassic silicate melts and phlogopite ± pargasite peridotite in the spinel (1 GPa) and garnet stability fields (3 GPa). Most of the rare Earth elements (REEs) are compatible in pargasite but incompatible in phlogopite. Although garnet remains the mineral phase most efficiently fractionating heavy from light REEs (DYbgrt/melt/DLagrt/melt > 750), orthopyroxene can also significantly fractionate REEs, e.g., achieving DYbopx/melt/DLaopx/melt > 100 at 3 GPa. Mineral-liquid partition coefficients vary by about one order of magnitude between incipient melts derived from the spinel and garnet stability fields. We thus show that trace element partitioning at the onset of melting is controlled more by pressure (through melt composition) than by the extent of melting. With increasing pressure, Rb and Ba exhibit different behaviors in phlogopite, with DBaphl/melt > DRbphl/melt at 1 GPa and the opposite at 3 GPa. At 1 GPa, a decrease of melt polymerization (lower NBO/T) with increasing melt fraction translates into a significant decrease of most phlogopite partition coefficients. Finally, we show that resolvable inter-element fractionations do occur when phlogopite- (and pargasite)-bearing peridotite are melted, indicating that trace element ratios are not always faithfully representative of that of their sources but bear the imprint of varied P-T conditions of melting and contrasted pre-metasomatic histories. This self-consistent partition coefficient dataset thus gives a new scope to understand the complex petrogenesis of K-rich magmas in orogenic settings.

This study investigates the tectonostratigraphy and metamorphic and tectonic evolution of the Caledonian Reisa Nappe Complex (RNC; from bottom to top: Vaddas, Kåfjord, and Nordmannvik nappes) in northern Troms, Norway. Structural data, phase equilibrium modelling, and U-Pb zircon and titanite geochronology are used to constrain the timing and pressure–temperature (P–T) conditions of deformation and metamorphism during nappe stacking that facilitated crustal thickening during continental collision. Five samples taken from different parts of the RNC reveal an anticlockwise P–T path attributed to the effects of early Silurian heating (D1) followed by thrusting (D2). At ca. 439 Ma during D1 the Nordmannvik Nappe reached the highest metamorphic conditions at ca. 780 ∘C and ∼9–11 kbar inducing kyanite-grade partial melting. At the same time the Kåfjord Nappe was at higher, colder, levels of the crust ca. 600 ∘C, 6–7 kbar and the Vaddas Nappe was intruded by gabbro at > 650 ∘C and ca. 6–9 kbar. The subsequent D2 shearing occurred at increasing pressure and decreasing temperatures ca. 700 ∘C and 9–11 kbar in the partially molten Nordmannvik Nappe, ca. 600 ∘C and 9–10 kbar in the Kåfjord Nappe, and ca. 640 ∘C and 12–13 kbar in the Vaddas Nappe. Multistage titanite growth in the Nordmannvik Nappe records this evolution through D1 and D2 between ca. 440 and 427 Ma, while titanite growth along the lower RNC boundary records D2 shearing at 432±6 Ma. It emerges that early Silurian heating (ca. 440 Ma) probably resulted from large-scale magma underplating and initiated partial melting that weakened the lower crust, which facilitated dismembering of the crust into individual thrust slices (nappe units). This tectonic style contrasts with subduction of mechanically strong continental crust to great depths as seen in, for example, the Western Gneiss Region further south.

International audience; Quartzite samples of high purity with a grain size of ∼200 μm have been experimentally deformed by coaxial shortening in a solid medium apparatus at 900°C and at confining pressures ranging from 0.6 to 2 GPa. Most samples have been shortened by ∼30% with 0.1 wt.% added H2O. The samples deformed dominantly by crystal plasticity (dislocation creep), and there is a systematic decrease of flow stress with increasing confining pressure. Strain rate stepping tests yield stress exponents of n∼1.4. The strain determined from individual grain shapes matches that determined from bulk shortening. In addition to plastic strain, mode I cracks developed in all samples, principally in the grain boundary regions. Recrystallized material, visible through cathodoluminescence colours, forms by two mechanisms: (1) progressive subgrain rotation and (2) cracking, nucleating small new grains. After high-angle boundaries have been established, grain boundary migration takes place, and a distinction of new grains nucleation origin (subgrain rotation or cracking) is impossible. At higher pressure, there is more recrystallized material forming in the deformed samples, and it is inferred that the inverse pressure dependence of flow stress is caused by enhanced grain boundary migration at higher pressure, consistent with previous studies.

International audience; A multidisciplinary approach integrating potential field analysis with geological and geochemical data provides new insights into the understanding of the crustal structure and evolution of the Mongolian collage. Magnetic and gravity data demonstrate the inconsistency between the geologically defined terranes and the geophysical domains in the southwestern part of the Mongolian collage. The combination of potential field analysis and modelling with whole rock geochemistry and isotopic mapping of Carboniferous–Permian granitoids indicates the presence of a homogeneous lower crust composed of a felsic to intermediate juvenile material beneath geophysically heterogeneous upper crust. This feature is interpreted as a result of a trench-directed lower crustal emplacement of an arc type crust underplating deformed Paleozoic oceanic crust. The potential field data also confirmed the occurrence of two orthogonal late Devonian and Permian–Triassic deformation upper crustal fabrics at the scale of the southwestern Mongolian collage. The prominent magnetic highs correspond to the tectono-metamorphic domains and magmatic provinces. The gravity anomalies highlight a periodicity of the signal correlating with alternating Permian–Triassic high and low strain zones, forming a zone of major deformation wrapping around the hinge of Mongolian orocline. The geometry and kinematics of dextral and sinistral transpressive faults are explained to result from the reactivation of Permian–Triassic deformation zones in the Cenozoic stress field.

The Barrovian inverted metamorphism of the Svratka dome developed within two nappes derived from the Brunia continent that was thrust beneath the Moldanubian orogenic root. The metamorphism increases from biotite–chlorite zone in the basement to very closely spaced staurolite, kyanite and sillimanite zones at the top of the nappe pile. The sequence of mineral growth, chemical zoning of garnet, and pseudosection modelling indicate prograde paths from 4.5 kbar/510 °C to 5.5 kbar/540 °C in the garnet zone, from 6 kbar/530 °C to 7 kbar/600 °C in the staurolite zone, and from 3.5 kbar/510 °C to 8.5 kbar/650 °C in the kyanite zone. The age of monazite inclusions in garnet and staurolite is interpreted to reflect prograde metamorphism at 338 ± 7 Ma and 336 ± 7 Ma, respectively. An older matrix monazite crystal is interpreted as dating prograde crystallization at 345 ± 7 Ma, whereas a younger monazite group records recrystallization at/or down to 334 ± 7 Ma. While these petrological and geochronological data are consistent with data from an inverted metamorphic sequence of the southern Thaya dome, the spacing and distribution of metamorphic zones, nappe thicknesses, and late structures are different in the two domes. An antiformal stack of imbricated basement sheets and the extreme attenuation of metamorphic isograds at the top of the nappe pile in the Svratka dome are explained by a relatively cold overthrusting Moldanubian domain, formed mainly of middle orogenic crust. The homogeneous thickening of the hinterland-dipping basement duplexes and the regular spacing of metamorphic isograds in the Thaya dome are explained by a hot overriding Moldanubian domain, which in this region has a high proportion of exhumed lower orogenic crust and formed a hot mid-crustal channel.

Highly deformed banded phengite-biotite metagranite from the Snieznik dome in the Bohemian Massif has been modified locally to have stromatic, schlieren or nebulitic textures typical of migmatites. This occurred mostly along subvertical deformation zones at eclogite-facies conditions, at a scale of several centimetres to several metres, mostly parallel to the foliation. The transition from banded to migmatite types of orthogneiss is marked by an increase in the amount of phases interstitial along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and by increasing consumption of recrystallized K-feldspar grains by fine-grained plagioclase and quartz, as well as myrmekite (intergrowth of Pl􏰄Qz). The new minerals are in textural equilibrium with phengite. The myrmekite, quartz and feldspars can be coarse-grained (grain size 􏰅0􏰀5cm). These features are considered to be the result of grain-scale melt infiltration that caused dissolution-reprecipitation along grain boundaries in the presence of phengite. The infiltration was pervasive at the grain scale, but localized at hand-specimen to outcrop scales. All the rock types have the same mineral assemblage of Grt􏰄+Ph+􏰄Bt+􏰄Ttn+􏰄Kfs+􏰄Pl+􏰄Qz+Rt+Ilm; they have similar garnet, phengite and biotite compositions, and based on mineral equilibria modelling we infer equilibration at a pressure of 1􏰀5-1􏰀7GPa and a temperature of 690-740􏰂C. Because the rocks are inferred to be H2O-undersaturated and above the temperature conditions of the wet solidus, infiltration must have involved a hydrous melt, as opposed to an H2O fluid. Stability of melt-bearing mineral assemblages and mineral compositions are almost independent of the melt proportion in the system, thus explaining the identical assemblage and mineral compositions observed in all the migmatite types. This precludes the estimation of the amount of melt infiltrated. Migmatite textures, however, suggest that variable degrees of melt-rock interaction occurred, being low in the banded migmatite types and higher in the nebulitic and schlieren types. Retrograde equilibration was largely restricted to retrograde zoning in phengite, garnet and plagioclase, and crystallization of biotite around phengite and garnet, presumably in a continuous reaction consuming melt. This may have occurred down to 􏰁0􏰀7-1􏰀0 GPa. According to Sr-Nd isotope data, the infiltrating melt is probably derived from similar rocks, structurally beneath the observed ones. The infiltration may have facilitated exhumation of a 2 km wide structural domain from 􏰁1􏰀7 to 􏰁0􏰀7 GPa, within which are the subvertical deformation zones along which the infiltration occurred.

Baaran and Baytag “terranes” form important part of the Trans-Altai Zone of the Central Asian Orogenic Belt in the SW Mongolia. The Trans-Altai Zone represents an oceanic domain built by Ordovician to Devonian ophiolites covered by Devonian–Carboniferous oceanic sediments. This oceanic assemblage was intruded by a spectrum of Carboniferous volcanic rocks ranging from basalts to rhyolites (325.6 ± 1.4 Ma to 351.0 ± 5.9 Ma; LA-ICP-MS U–Pb dating on zircon) that display similar whole-rock trace-element and Sr–Nd isotopic signatures regardless their silica contents and the “terrane” they belong to. They are normal-K calc-alkaline rocks whose NMORB-normalized multielement patterns show an arc-like enrichment in LILE (Large Ion Lithophile Elements) and deep negative TNT (Ta, Nb, Ti) anomalies. Based on the relatively high Nb/Yb and Th/Yb ratios, the arc was probably formed on immature continental rather than typical oceanic crust. Low (87Sr/86Sr)i (0.7043–0.7047), high e Nd 350 (+5.2 to = +5.8) and the observed in-situ Hf isotopic variation in dated zircons ( e Hf t = +7.4 to +13) are all attributed to a partial melting of primitive and youthful (Neoproterozoic–Ordovician) arc-type lower continental crust (metamorphosed intermediate–acid igneous rocks and/or arc-derived greywackes), augmented with, and most likely facilitated by, contemporaneous arc-related basaltic intrusions. A distinct possibility remains the previously formulated model assuming that the Carboniferous magmatism in Gobi-Altai and Trans-Altai zones sampled the juvenile Neoproterozoic to Cambrian arc crust relaminated under the Ordovician–Devonian oceanic crust. In any case, the studied arc association, together with lithological sequence of ophiolites, can be closely correlated with the Dulate arc in eastern Junggar. It is proposed that the two arc sequences form a single belt, c. 500 km long, that represents one of the main known arc structures in the Central Asian Orogenic Belt.

Lu-Hf and Sm-Nd garnet-whole rock geochronology combined with petrographic observations, minero-chemical variations, thermodynamic modelling and structural data was used to constrain the P–T–t–d evolution of eclogites from the Marianske Lazně Complex (Bohemian Massif). Boudins of mostly isotropic eclogite with relict steep eclogite-facies fabric are affected by steep migmatitic foliation, which is followed on a regional scale by the development of almost pervasive, predominantly SE-dipping, extensional foliation. The structural succession shows continuous transition from eclogite to garnetiferous migmatitic amphibolite and to amphibolite migmatite. A least retrogressed sample of eclogite shows clusters of fine-grained inclusion-poor garnet, omphacite relicts surrounded by a fine-grained clinopyroxene-plagioclase symplectite with minor amphibole, biotite-plagioclase intergrowths after white mica, kyanite with plagioclase-spinel coronas and accessory rutile. Rare potassic white mica occurs as inclusions in omphacite. A more retrogressed eclogite, with no omphacite or kyanite relicts, contains inclusion-poor garnet surrounded by amphibole-plagioclase corona in a matrix dominated by plagioclase-amphibole symplectite with minor clinopyroxene. In places, the symplectite is overgrown by coarse-grained amphibole. Peak P–T conditions, inferred from combined conventional thermobarometry and phase-equilibria modelling and based on inclusions of white mica (up to 3.33 Si p.f.u.), matrix omphacite (Jd33–36) and garnet core (Alm33–38Prp38–42Grs22–25Sps1) compositions are ~25 kbar at 650–750 °C. A HT overprint occurred at ~14–18 kbar and >800 °C based on coexisting clinopyroxene (Jd18–24), plagioclase (An18–35), and amphibole (Na(B) Lu-Hf and Sm-Nd garnet geochronology has been applied to both samples, an older age (c. 390 Ma) obtained by the Lu-Hf method is interpreted as the timing of HP metamorphism, while c. 15 Ma younger ages were obtained by the Sm-Nd method. As temperatures for the HT overprint exceed the empirically and experimentally determined closure temperature of the Sm-Nd system the Sm-Nd ages are interpreted to date cooling following the HT overprint. Combined together, contrasting eclogite and amphibolite-facies migmatite fabrics, the mineral textures, calculated P–T conditions, and distinct Lu-Hf and Sm-Nd ages, provide a complete P–T–t–D path characterised by rapid (~15 Ma) transition from HP subduction, crustal thickening to extensional HT shearing. This unconventional exhumation path does not fit to models of monocyclic exhumation in a subduction channel proposed for the Munchberg and ZEV Devonian HP units to the west. In contrast, the post-peak extensional low angle shearing shortly after subduction and collision resembles more the geodynamic model typical for the Iberian subduction system both in time scales and sequence of tectonic events.

The Khantaishir Magmatic Complex (KMC) (south–central Mongolia) exposes a section of a magmatic system consisting of deep crustal, ultramafic cumulates (coarse-grained Amp gabbros and hornblendites; c. 0.35–0.5 GPa) to shallower crustal levels dominated by Amp–Bt tonalites ( c. 0.1–0.2 GPa). The magmatic rocks were emplaced during most of the Cambrian ( c. 538–495 Ma) and are mostly geochemically primitive (Mg# ~ 50), Na-rich and metaluminous. The (normal-) calc-alkaline signature and characteristic trace-element enrichment in hydrous-fluid mobile large-ion lithophile elements (LILE) relative to high-field strength elements (HFSE) suggest an origin within a magmatic arc. Multiple intrusions of basic magma derived from a subduction-modified depleted mantle developed by fractional crystallization and/or accumulation of (Ol, Cpx) Amp + Bt, later joined by Pl. Magma mixing with, or without, exchange of xenocrysts between compositionally dissimilar melt batches was also important. Over time, partial melting of older, lower crustal metabasic rocks became increasingly significant, again with a strong subduction signature. The lack of zircon inheritance in the magmatic products and rather high zircon e Hf t values (all >+3, but for most samples > +8) as well as whole-rock Sr–Nd isotopic compositions imply that the arc was not founded on mature continental crust. It was probably located at the margin of the Baydrag microcontinent, dominated by accreted metabasic rocks of an older (early Tonian?) island arc covered by a thin layer of subordinate metasediments containing detrital zircons with Tonian and ill-defined Palaeoproterozoic ages. The KMC represents a small vestige of an extensive Cambrian–Ordovician subduction system (termed here the Ikh-Mongol Arc System), bordering the western margin of a chain of Precambrian microcontinents (Tuva-Mongolia, Zabkhan and Baydrag) that, together with accreted Neoproterozoic marginal basins (the Lake Zone), defines the external part of the Mongolian orocline.

Paleomagnetic investigations of Early Permian and Triassic magmatic rocks from Catalan Coastal Ranges provide three magnetic directions which are interpreted as primary magnetizations and partly as overprints. The succession of magnetic directions is interpreted in terms of a succession of large-scale clockwise rotations, which bring the whole assembly of Pyrenees, Catalonia, Corsica-Sardinia and Maures-Esterel massifs from the southeast of the Massif Central to the west during Permian. This major movement is associated with clockwise rotation of northern limb of the Iberian orocline during Latest Carboniferous and Early Permian (~ 305–290 Ma). Subsequently, Permian large-scale dextral transtensional–extensional shearing operated along Aquitanian Shear Belt between the Massif Central in the northeast and the Iberian Variscan massifs in the southwest. The latest phase of the rotation in the Late Permian-Early Triassic is associated with alkaline magmatism probably linked to Neo-Tethys opening activity in Western Europe.

New geological, geochemical and geochronological data characterize architecture and geodynamic evolution of the southern Baydrag continental margin in Mongolia. The structurally deepest Zamtyn Nuruu Complex is composed of highly deformed orthogneisses and amphibolites intruded by syn-tectonic diorites and gabbros. Geochemical affinity of the amphibolites varies from continental-arc to within-plate tholeiites, while the Grenvillean (948 ± 6 and 941 ± 11 Ma) orthogneisses show features typical of magmatic-arc origin. The syn-metamorphic gabbros and diorites intruded coevally with the regional thermal event at 542 ± 4 Ma. Zircon Hf isotope and whole-rock geochemical data point to a primitive source of mafic rocks, most likely formed in Early Cambrian arc developed on Grenvillean basement. The overlying Alag Khadny subduction melange contains eclogites and amphibolites with MORB affinity while lenses of orthogneisses (953 ± 12 Ma: zircon, 939 ± 5 Ma: monazite) reveal late- to post-collisional signature. The subduction event is dated by zircon at ca. 538 ± 20 Ma. The matrix of the melange consists of metapelites and carbonates, most likely fragments of Neoproterozoic to Early Cambrian carbonate platform scrapped off the subducting plate during the mid-Cambrian accretion. The hanging-wall Khan-Taishir–Erdene Uul ophiolite is formed by Neoproterozoic pillow lavas, red cherts and associated ultramafic rocks, with gabbros dated at 973 ± 12 Ma. Taken together, the new data allow placing south Mongolian continental fragments into a geodynamic model of Rodinia formation, fragmentation and Palaeo-Pacific subduction initiation. The Zamtyn Nuruu Grenvillean arc is interpreted as a result of Mirovoi Ocean subduction beneath Rodinian margin, while the Early Cambrian diorites and gabbros testify initiation of supra-subduction magmatism of Palaeo-Pacific plate beneath continental fragment of Gondwanan, most likely Tarim affinity. These two events were separated by the development of Neoproterozoic passive margin probably associated with Rodinia fragmentation.

International audience; A structural and geochronological 40Ar/39Ar study was performed in kilometre-scale middle and lower crustal lens-shaped domains dominated by a preserved subvertical foliation, surrounded by horizontally foliated migmatites. These domains occur within the Moldanubian nappe overlying the Brunia microcontinent at the eastern margin of the European Variscides. Three main deformation phases were recognized: subvertical S2 fabric trending NW–SE in lower crustal rocks and NE–SW in mid-crustal rocks. It is reworked by HT/MT horizontal fabric S3 along margins of crustal domains and in surrounding migmatites. S3 bears a prolate NE lineation parallel to the S2–S3 intersection in the lower crustal domain. In the middle crustal units, L3 is weak, connected to oblate strain and trends NE–SW parallel to the S2–S3 intersection. D4 non-coaxial shear deformation is mainly localized at the boundary between the Moldanubian nappe and Brunia and bears strong top to the NNE shear criteria. In order to constrain kinematics of the D3 deformation, strain modelling was performed to show that the Moldanubian hot nappe was frontally thrust over the Brunia indentor. The renewed D4 tangential movement only heterogeneously reactivates the horizontal S3. This evolution is recorded in 40Ar/39Ar amphibole cooling ages, which show two statistically significant Carboniferous peaks at ~342 and 332 Ma, which are also reflected by published detrital muscovite 40Ar/39Ar ages in the adjacent foreland basin. This geochronological record is correlated with progressive erosion of the topographically elevated upper crustal part of the Moldanubian nappe during D3 frontal thrusting, followed by greenschist facies D4 transpressive reactivation and subsequent erosion of high-grade parts of the nappe.

A geological map is an indispensable tool for understanding the structure of the Earth’s crust but high-quality geological maps are usually lacking in remote areas of mountainous Central Asia covered by vast deserts. The progress in remote sensing and geographical information system (GIS), as well as the advancement in analytical methods, have generated new challenges in producing modern geological maps in such regions. The presented 1:50,000 geological map along the Sagsai River summarizes new and more accurate geological data from the geologically interesting region at the contact of the supracrustal and deep crustal Tugrug and Tseel units forming the metamorphosed accretionary wedge on the S and SW slopes of the Mongolian Altai. These geological units are formed by the lower Palaeozoic volcano-sedimentary sequences affected by Devonian Barrovian metamorphism ranging from lower greenschist to granulite facies. This metamorphic basement was subsequently intruded by the post-orogenic late Carboniferous Sagsai Pluton. The presented map shows complex relationships between different crustal levels of the metamorphosed accretionary wedge and post-tectonic intrusion evaluated using a GIS, numerical processing of remote sensing data as well as field documentation and laboratory studies.

International audience

International audience; We present a numerical model of the main phase (370-335 Ma) of the Variscan orogeny in the central part of the Bohemian Massif. The crustal deformation in our model is driven by radiogenic heating in the felsic lower crust, the lateral contraction of the Moldanubian domain due to convergence with the Saxothuringian plate (in the early stage of orogeny), and the indentation of the Brunovistulian basement into the weakened orogenic root (in the late stage). Our model explains the main geological events inferred from the geological record in the Moldanubian domain: formation of the orogenic plateau and onset of sedimentation at about 345 Ma, rapid exhumation of the orogenic lower crust at about 340 Ma and subsurface flow of crustal material (similar to 335 Ma and later). The results of our modeling suggest that delamination of the lithosphere, often invoked to explain the high temperature metamorphism in the orogenic lower crust of the Bohemian Massif, is not the only physical mechanism which can transfer a sufficient amount of heat to the crust to trigger its overturn.

Continental crust is formed above subduction zones by well-known process of “juvenile crust growth”. This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Laurussian or later Eurasian continent along the “Alpine Himalayan collisional/interior orogens” in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along “circum-Pacific accretionary/peripheral orogens” at the border of the PaleoPacific and modern Pacific oceanic plate.

Tectonic-magmatic reworking of accretionary wedges is a key process responsible for differentiation and stabilization of continental crustal in accretionary orogens. This generic problem can be exemplified by magmatic evolution of the Chinese Altai which represents a high-grade core of the world's largest accretionary system, namely the Central Asian Orogenic Belt (CAOB). In the Chinese Altai, voluminous SilurianDevonian granitoids intruding a greywacke-dominated Ordovician flysch sequence. These intrusions are classically interpreted to originate from predominant (70‒90 %) juvenile (depleted mantle-derived) magma. However, their close temporal and spatial relationship with the regional anatexis of flysch rocks, allows us to examine the possibility that they were mainly derived from flysch rocks.

International audience; The Morocco Variscan belt is considered to be the south-western continuation of the European Carboniferous orogen on the NW Gondwana margin. The Rehamna Massif, a paleozoic massif located on the Meseta domain which is part of this belt, is affected by three main deformation events (D1, D2 and D3) of variable intensity and geometry, associated with a Barrovian metamorphism. The first forms a flat-lying metamorphic foliation S1, which is deformed by WSW–ENE trending F2 folds with associated sub-vertical S2 cleavage, then heterogeneously reworked by NNE–SSW trending F3 folds with an S3 cleavage moderately to steeply dipping to ESE. Each deformation lead to the development of a new foliation and the crystallisation-deformation relationships show that biotite, chlorite, garnet, chloritoid and staurolite grew in the S1 fabric, and that chloritoid and staurolite continued their growth in the S2 and S3 fabrics. Two types of andalusite porpyroblasts located around granitic intrusions were identified: some are clearly post-tectonic whereas others are presumably coming from an early event. This study combines crystallisation-deformation relationships, in addition to the results of mineral chemistry and mineral zoning, with pseudosection modelling and geochronological datas. Based on the resulting P–T–d paths, three main tectonic events have been recognized: 1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. 2) Continuous shortening resulting first in continuation of burial, then in the development of a syn-convergent extrusion of metamorphosed units to form a dome elongated E–W. This was responsible for syn-convergent detachment of the Ordovician upper crustal sequence. 3) The next episode of convergence took place in a ESE–WNW direction orthogonal to the previous one and is characterized by the accretion of the Rehamna dome to the continental basement in the west. Existing Ar/Ar dating shows that the first and the second deformations occurred during the Late Carboniferous to Early Permian (315–290 Ma) and that the third deformation took place during the Early Permian (290–275 Ma). Geochronology with in situ U/Pb dating on monazites is performed in order to date the individual phases of metamorphism and constrain the timing of the deformations events.

International audience; Numerous talc, magnesite and siderite ore deposits occur in the Central Western Carpathians (Slovakia) along theso-called Gemer-Vepor Contact Zone, which marks the contact between two major blocks of Variscan basement,the Vepor and Gemer units. During the polyphase deformation of Cretaceous age, the Gemer Unit was first thrustover the Vepor Unit. This is well documented by the development of subhorizontal and prograde metamorphic foliationreaching up to amphibolite facies conditions in the Vepor and by the development of subvertical greenschistfacies cleavages in the Gemer Unit. The subsequent exhumation of the Vepor Unit along large-scale detachmentzone is documented by the development of subhorizontal lower grade cleavage in the Vepor Unit. Finally, the ongoingnorthward propagating convergence with the Gemer Unit led to the development of the sinistral transpressionalTrans-Gemer Shear zone. Two types of shear zones with contrasting metasomatic record have been recognized inthe Vepor Unit. Mg-enriched shear zones heterogeneously develop within the Carboniferous granitoids resulting information of Mg-chlorite–muscovite–quartz phyllonites as well as Mg-chlorite–kyanite-bearing schists. Comparedto the composition of granitoids, these mylonites-phyllonites are depleted in alkalies and enriched in magnesium,iron and manganese, which is most likely related to the influx of fluids along the shear zones. The equilibriummineral assemblage chlorite, kyanite, muscovite, rutile and quartz corresponds to 420C at 4 kbar. In contrast,the second type of shear zones developed within chloritoid-kyanite schists of the Veporic Permian cover. Theseshear zones display metasomatic alteration characterized by a strong Fe-enrichment resulting in formation of Fechlorite-muscovite-quartz phyllonite. The unusual enrichment in either Mg or Fe suggests either heterogenous fluidcomposition or two separate metasomatic events in the studied area. Indeed, the two types of shear zones are associatedwith two distinct deformation events. The Mg-enriched first type shear zones are associated with progrademetamorphic evolution related to burial of the Vepor Unit whereas the Fe-enriched second type shear zones arerelated to the Vepor exhumation. Therefore, the formation of these metasomatic rocks reflects different stages ofthe polyphase Cretaceous evolution, rather than heterogenous composition of fluids. The sources of metasomaticfluids are discussed in relation to the nearby ore deposits.

International audience; The Ochtiná Unit is situated in the ENE-WSW-trending contact zone between two crustal-scale nappes, the upper Gemer Unit and the lower Vepor Unit, in the Central Western Carpathians, Slovakia. The Ochtiná Unit consists mainly of Carboniferous phyllitic schists and sandstones enclosing lenses of diverse lithological nature and contrasting metamorphic history. Peak PT conditions obtained by means of phase equilibrium modelling from lenses of amphibolite and chloritoid schist in this unit indicate 500-600 °C and 4-6.5 kbar and 500-520 °C and 9-11 kbar, respectively. These PT conditions contrast not only with the greenschist-facies metamorphism of dominant phyllite but also with each other documenting two distinct metamorphic field gradients related to Variscan and Alpine metamorphic events. Geochemical data reveal an affinity of the amphibolite lenses to similar Variscan rocks in the basement of the upper Gemer Unit and of the chloritoid schist to similar Alpine rocks in the cover of the lower Vepor Unit. Such heterogeneous lithological and metamorphic record is consistent with a block-in-matrix rock arrangement and the Ochtiná Unit is interpreted as deep seated tectonic mélange. The mélange evolved via repeated slip along the rheologically weak sediments of the Ochtiná Unit during the building and collapse of the Eo-Alpine orogenic wedge of the Central Western Carpathians. Deformation record indicates that the mélange separates two distinct structural domains marked by a decoupled behaviour, i.e. the orogenic suprastructure represented by the Gemer Unit and the infrastructure represented by the Vepor Unit. With this respect, the Ochtiná Unit represents an unusual example of a suprastructure-infrastructure transition zone with its position being controlled by the mechanical weakness of this sedimentary horizon and not by the temperature-dependent rheological transition.

International audience; The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafi c lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called “diffuse cryptic suture zone.”

International audience; The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called "diffuse cryptic suture zone."

International audience; This paper presents quantitative microstructural and crystallographic preferred orientation (CPO) analyses of an Alpine amphibolite facies shear zone developed in the Fibbia metagranite (Gotthard massif, Central Alps). The weakly deformed metagranite and orthogneiss at the margins of the shear zone are characterized by a bulk strain partitioning between harder coarse-grained monomineralic aggregates, derived from quartz and K-feldspar porphyroclasts, and softer fine-grained plagioclase-bearing shear bands. A characteristic feature is a dilatant fracturing of strong quartz and feldspar aggregates. CPOs and microtextures suggest that quartz and K-feldspar aggregates are dynamically recrystallized via dislocation creep while plagioclases show evidences of fluid-assisted diffusive mass transfer and grain boundary sliding. In the mylonite and ultramylonite shear zone core, the porphyroclasts-derived quartz and K-feldspar layers are broken-down to produce a polyphased matrix that is characterized by a homogeneous micron-scale grain size and regular/random distribution. Here, the deformation of the whole aggregate occurs via a fluid-assisted dissolution-precipitation creep and grain boundary sliding, referred as a fluid-assisted granular flow. We propose a model of shear zone formation associated with the nucleation of shear zone followed by lateral widening of the sheared domain. The lateral broadening of the shear zone is driven by (1) the increase in fluid pressure in permeable albite-oligoclase shear bands that results in expulsion of fluids to the shear zone margins and hydraulic fracturing of strong aggregates, and (2) the thermodynamic re-equilibration via metasomatic reactions of the shear zone walls.

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International audience; A microstructural and metamorphic study of a naturally deformed medium- to high-pressure granitic orthogneiss (OrlicaSnieznik dome, Bohemian Massif) provides evidence of behaviour of the felsic crust during progressive burial along a subduction-type apparent thermal gradient (similar to 10 degrees C km-1). The granitic orthogneisses develops three distinct microstructural types, as follows: type I augen orthogneiss, type II banded orthogneiss and type III mylonitic orthogneiss, each representing an evolutionary stage of a progressively deformed granite. Type I orthogneiss is composed of partially recrystallized K-feldspar porphyroclasts surrounded by wide fronts of myrmekite, fully recrystallized quartz aggregates and interconnected monomineralic layers of recrystallized plagioclase. Compositional layering in the type II orthogneiss is defined by plagioclase- and K-feldspar-rich layers, both of which show an increasing proportion of interstitial minerals, as well as the deformation of recrystallized myrmekite fronts. Type III orthogneiss shows relicts of quartz and K-feldspar ribbons preserved in a fine-grained polymineralic matrix. All three types have the same assemblage (quartz + plagioclase + K-feldspar + muscovite + biotite + garnet + sphene +/- ilmenite), but show systematic variations in the composition of muscovite and garnet from types I to III. This is consistent with the equilibration of the three types at different positions along a prograde P-T path ranging from 700 degrees C (types II and III orthogneisses). The deformation types thus do not represent evolutionary stages of a highly partitioned deformation at constant P-T conditions, but reflect progressive formation during the burial of the continental crust. The microstructures of the type I and type II orthogneisses result from the dislocation creep of quartz and K-feldspar whereas a grain boundary sliding-dominated diffusion creep regime is the characteristic of the type III orthogneiss. Strain weakening related to the transition from type I to type II microstructures was enhanced by the recrystallization of wide myrmekite fronts, and plagioclase and quartz, and further weakening and strain localization in type III orthogneiss occurred via grain boundary sliding-enhanced diffusion creep. The potential role of incipient melting in strain localization is discussed.

International audience; During the ascent, emplacement and post-emplacement deformation of igneous rocks, two or more phases of deformation that overprint each other are often depicted. These overprints, when magnetic minerals are present, are recorded in magnetic fabric. In this contribution, overprints are studied by means of numerical modeling, following several basic scenarios common to igneous rocks. Biotite and amphibole that occur often together in igneous rocks are considered as carriers of the anisotropy of magnetic susceptibility. Modeling shows that (1) a constrictional fabric with a low degree of anisotropy as commonly recorded in magmatic rocks may result from a deformation overprint and not necessarily from an extensional/transtensional regime, and (2) that the constrictional AMS fabrics originates from orthogonal superimposition of a deformation event on an AMS fabric inherited from earlier magma emplacement history. Therefore, the interpretation of a constrictional fabric must be performed with caution. Numerical modeling may provide a suitable help in strengthening the interpretation of real magnetic fabric data.

27 p.; International audience; [1] The contribution of lateral forces, vertical load, gravity redistribution and erosion to the origin of mantled gneiss domes in internal zones of orogens remains debated. In the Orlica-Snieznik dome (Moldanubian zone, European Variscan belt), the polyphase tectono-metamorphic history is initially characterized by the development of subhorizontal fabrics associated with medium- to high-grade metamorphic conditions in different levels of the crust. It reflects the eastward influx of a Saxothuringian-type passive margin sequence below a Teplá-Barrandian upper plate. The ongoing influx of continental crust creates a thick felsic orogenic root with HP rocks and migmatitic orthogneiss. The orogenic wedge is subsequently indented by the eastern Brunia microcontinent producing a multiscale folding of the orogenic infrastructure. The resulting kilometre-scale folding is associated with the variable burial of the middle crust in synforms and the exhumation of the lower crust in antiforms. These localized vertical exchanges of material and heat are coeval with a larger crustal-scale folding of the whole infrastructure generating a general uplift of the dome. It is exemplified by increasing metamorphic conditions and younging of 40Ar/39Ar cooling ages toward the extruded migmatitic subdomes cored by HP rocks. The vertical growth of the dome induces exhumation by pure shear-dominated ductile thinning laterally evolving to non-coaxial detachment faulting, while erosion feeds the surrounding sedimentary basins. Modeling of the Bouguer anomaly grid is compatible with crustal-scale mass transfers between a dense superstructure and a lighter infrastructure. The model implies that the Moldanubian Orlica-Snieznik mantled gneiss dome derives from polyphase recycling of Saxothuringian material.

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International audience; In a Barrovian metamorphic sequence, garnetiferous mica schists document a heterogeneously developed superposition of sub-orthogonal fabrics and multiple garnet growth episodes. In the variably deformed domains, four types of garnet porphyroblasts have been defined based on inclusion trail patterns. Modelled garnet zoning in the MnNCKFMASHTO system indicates a prograde evolution from 4-4.5 kbar and 490-510 C to 5-6 kbar and 520-550 C in the earliest subhorizontal fabric progressing towards 6.5-7.5 kbar and 560-590 C in the subsequent subvertical foliation. This fabric is heterogeneously deformed into a shallow-dipping retrograde foliation associated with garnet resorption. In situ electron backscatter diffraction measurements of ilmenite inclusions in individual garnet grains yield precise data on included planar and linear elements. Consistent orientations of internal foliations, lineations and foliation intersection axis sets indicate a superposition of three sub-orthogonal foliation systems. Weak variations of internal records with increasing intensity of deformation suggest that a moderate buckling stage occurred, but apparent lack of porphyroblast rotation is interpreted as a result of dominant passive flow. Coupling the orientation of internal fabric sets with P-T estimates is used to complement the tectono-metamorphic evolution of the thickened crust. We demonstrate that garnet porphyroblasts preserve features which reflect large-scale tectonic processes in orogens.

International audience; A new model of the structure and composition of the Variscan crust in the Bohemian Massif is proposed based on 3D gravity modelling, geological data, seismic refraction (CEL09) and reflection (9HR) sections. The Bohemian Massif crust is characterized by a succession of positive and negative anomalies of about 6080 km wavelength for nearly constant Moho depths. The south-western part of the Bohemian Massif displays a large negative Bouguer anomaly corresponding to high grade rocks (granulites and migmatites) of the Palaeozoic crustal root represented by the Moldanubian domain. The adjacent Neo-Proterozoic Bruno-Vistulian microcontinent displays an important gravity high reflecting mafic and intermediate medium grade metamorphic and magmatic rocks. The deep crustal boundary between the root domain and the Bruno-Vistulian microcontinent is represented by a strong gradient located 50 to 70 km westwards from the surface boundary between these units indicating that the high density basement rocks are covered by a thin sheet of low density granulites and migmatites. North-west from the Moldanubian domain occurs an important gravity high corresponding to the Neo-Proterozoic basement of the Tepla-Barrandian Unit limited in the north by southeast dipping reflectors of the Tepla suture which is characterized by high density eclogites and ultramafics. The footwall of the suture corresponds to low density felsic crust of the Saxothuringian basement. The reflection and refraction seismics and gravity modelling suggest a complex lithological structure of the Moldanubian domain marked by a low density 5-10 km thick lower crustal layer located above the Moho, a 5-10 km thick heavy mafic layer, a 10 km thick mid-crustal layer of intermediate density and a locally developed 2-5 km thick low density layer at the surface. The low density lower crust correlates well with low P-wave velocities in the range 6.0-6.4 km s(-1) in the CEL09 section. This complex geophysical structure and surface geology are interpreted as a result of Carboniferous partial overturn of low density lower crust and high and intermediate density crust in the area of central root and by viscous extrusion of low density orogenic lower crust over the high density Bruno-Vistulian continent. Comparison of these data with geophysical profiling of the Andean and the Tibetan plateaus suggests that modern orogenic systems reveal comparable deep crustal geophysical pattern. Based on these similarities we propose that the Variscan root represents a deep crustal section of above mentioned plateaus, which may have develop by the same orogenic process. (C) 2010 Elsevier B.V. All rights reserved.

International audience; In the Orlica-Snieznik Dome (NE Bohemian massif), alternating belts of orthogneiss with high-pressure rocks and belts of mid-crustal metasedimentary-metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE-SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat-lying foliation. The metamorphic evolution of the mid-crustal metasedimentary rocks involved successive crystallization of chlorite-muscovite-ilmenite-plagioclase-garnet, followed by staurolite-bearing and then kyanite-bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow-dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat-lying fabric. With reference to a P-T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X-Fe are compatible with a P-T path from 3.5-5 kbar/490-520 degrees C to peak conditions of 6-7 kbar/similar to 630 degrees C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at similar to 4 kbar/similar to 630 degrees C and to andalusite-cordierite stability at 2-3 kbar indicating exhumation from 25 km to around 9-12 km. Subsequent exhumation to similar to 6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary-metavolcanic rocks, and gneissic and high-pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp-like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high-pressure rocks in lobe-shaped and volumetrically more important anticlines. Converging P-T-D paths for the metasedimentary rocks and the adjacent high-pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow-dipping fabric affects both the metasedimentary-metavolcanic rocks and the gneissic and high-pressure rocks, and indicates that the similar to 15-km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.

International audience; This study answers the question of origin and evolution of a granulitic microstructure typically developed in felsic granulites of the European Variscan belt. It shows that the precursor of the Variscan felsic granulites was a high-pressure alkali feldspar-rich coarse-grained layered orthogneiss. Its S1 subhorizontal layering is defined by the alignment of alkali feldspar porphyroclasts alternating with monomineralic bands of quartz and bands rich in plagioclase and garnet. The alkali feldspar porphyroclasts contain inclusions of quartz, garnet, kyanite, biotite and rutile, reflecting peak P-T conditions of 1.6-1.8 GPa and 850 degrees C during S1 formation. Superimposed steep folds and steep cleavage, S2, are associated with recrystallization of alkali feldspar, plagioclase and quartz, and garnet chemistry modifications that correspond to 0.9-1.0 GPa and 800 degrees C. During exhumation, involving 0.8 GPa decompression and cooling, the probably perthitic alkali feldspar underwent an unusual process of heterogeneous decomposition along irregular reaction fronts forming a fine-grained matrix composed of plagioclase and K-feldspar grains. Regular grain distributions in the matrix, nucleation-dominated crystal size distribution and preservation of lattice orientation of the parental perthite crystals are all explained by a discontinuous precipitation process. This heterogeneous decomposition of alkali feldspar solid solution is controlled by chemically and strain induced grain-boundary migration. During exhumation and decompression, the fine-grained matrix underwent viscous deformation, forming the typical microstructure of the Variscan granulites. Random phase distributions, minor coarsening and feldspar textures are interpreted as a result of strain softening due to diffusion creep-accommodated grain-boundary sliding. Subordinate large quartz ribbons were rheologically stronger than the feldspar-dominated matrix due to the activity of different deformational mechanisms. Finally, in mid-crustal levels, the subvertical structure was overprinted by a perpendicular steep fabric associated with the growth of sillimanite, heterogeneous hydration and local partial melting, development of aggregate phase distributions and significant coarsening. This evolution is accompanied with the development of a strong lattice preferred orientation of quartz, K-feldspar and plagioclase, reflecting a switch to dislocation creep mechanism and a general hardening of the granulites under amphibolite facies conditions.

International audience; Reflection seismic section, field structural analysis and gravimetric modelling of orogenic lower crust in the core of a Carboniferous orogenic root reveal details of the polyphase process of exhumation. Subvertical amphibolite facies fabrics strike parallel to former plate margins that collided in the NW. The fabrics are developed in both mid-crustal and lower crustal high-pressure granulite units as a result of intensive NW-SE intraroot horizontal shortening driven probably by the west-directed collision. In granulites, the steep fabrics originated as a result of extrusion of orogenic lower crust in a similar to 20 km wide vertical ascent channel from lower crustal depths at 350-340 Ma. The large granulite bodies preserve older granulite facies fabrics documenting a two-stage evolution during the exhumation process. Surface exposures of granulites coincide with the absence of subhorizontal seismic reflectors at depth, suggesting preservation of the similar to 20 km wide subvertical tabular structure reaching Moho depths. Horizontal seismic reflectors surrounding the vertical channel structure corroborate a dominant flat migmatitic fabric developed in all tectonic units. This structural pattern is interpreted in terms of subhorizontal spreading of partially molten orogenic lower crust in mid-crustal levels (765 degrees C and 0.76 GPa) at 342-337 Ma. Large massifs of extruded and progressively dismembered felsic granulites disturbed mid-crustal fabrics in the surrounding horizontally flowing partially molten crust. The horizontal mid-crustal flow resulted in collapse of the supra-crustal Tepla-Barrandian Unit (interpreted as the orogenic lid) along a large-scale crustal detachment above the extruded lower crustal dome. The presence of felsic granulites at the bottom of the orogenic root is considered to be a key factor controlling the exhumation of orogenic lower crust in large hot orogens.

International audience; Newly discovered eclogites, Early Cambrian carbonates and chloritoid-bearing metapelites form the Tsakhir Uul accretionary wedge, which was thrust during the Early Cambrian over the Mesoproterozoic Dzabkhan-Baydrag continent. The rock association of the wedge forms a tectonic window emerging through the hangingwall Khantaishir ophiolite unit, which preserves a typical Tethyan-type ophiolitic sequence. The eclogites correspond geochemically to T-MORB modified by fluid circulation. They are composed of garnet, omphacite, amphibole, rutile ±muscovite ±quartz ±epidote and exhibit well-equilibrated matrix textures. Jadeite content of the omphacite reaches up to 45 mol.%, the Si content of muscovite is between 3.40 and 3.45 p.f.u., amphibole is winchite to barroisite, but reaches tschermakitic composition at some rims, and garnet composition is grs0.24–0.36, alm0.43–0.56, py0.05–0.18, sps0.00–0.18, inline image. The peak assemblage, together with the composition of garnet rims, omphacite, amphibole and muscovite, correspond in a pseudosection to 20−22.5 kbar and 590−610 °C. The tschermakitic rim of amphibole is interpreted as partial reequilibration on decompression below 16 kbar and ∼600−630 °C. Two muscovite separates from the eclogite yielded an Ar–Ar plateau age of 543.1 ± 3.9 Ma (1σ) and a mean age of 547.9 ± 2.6 Ma (1σ), whereas muscovite from an interbedded garnet-chloritoid micaschist yielded an Ar–Ar plateau age of 536.9 ± 2.7 Ma (1σ); these ages are interpreted as cooling ages. The P–T data, geochemistry of eclogites and cooling ages suggest an affinity between the Tsakhir Uul wedge and the Gorny Altai and the north Mongolian blueschist belt, which are believed typical for subduction of warm oceanic lithosphere and closure of small oceanic basins. Thus, the discovery of the Tsakhir Uul eclogites represents an important finding suggesting extension of the Early Cambrian subduction system of the Central Asian Orogenic Belt far to the east in a region where it was not expected.

International audience; We 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.

International audience; 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 so-called 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.

International audience; Optical microstructures and crystallographic preferred orientations were studied in naturally deformed granulite- to high-amphibolite facies quartzo-feldspathic rocks in southern Madagascar. The microstructures of coarse-grained granulite suggest that feldspar and quartz accommodated deformation by both dislocation and diffusion creep in the absence of melt. The extreme ductility of feldspar in dynamically recrystallized granulite is explained by activity of dislocation creep, in conjunction with stress-controlled intracrystalline diffusion. In the studied rocks, the considerable volume of quartz is not interconnected even at high strain. The lack of its interconnectivity in coarse-grained granulites and in amphibolite facies granoblastic platy quartz rocks is explained by an extreme stability of the load-bearing framework structure at high-temperatures. In dynamically recrystallized granulite, the feldspar viscosity decreases so that quartz becomes enveloped by a weak feldspar matrix which inhibits its coalescence and interconnectivity. We predict an important decrease in strength of quartzo-feldspathic granulites due to activity of diffusional creep and convergence of viscosity of recrystallized feldspar and quartz.