Search Results (6)

In collision belts, the upper plate is generally less deformed than the lower one that underwent syn-metamorphic ductile shearing, and frequently late-collisional crustal melting. Concerning the Variscan orogeny, it is widely accepted that the Armorica microcontinent represented the upper plate of the collision system. In France, the Central-North-Armorican Domain belonged to this upper plate whose southern margin in the Pontivy–Coray area exposes metamorphic rocks. There, structural and metamorphic studies indicate that an early tectono-metamorphic event (M0-M1) with biotite–garnet–staurolite–kyanite assemblage, crystallized at 0.9 GPa and 500 °C, is characterized by a top-to-the NW shearing. This event was followed by an HT event (M2) at ca 800–900 °C, coeval with a domal structure. In micaschists, monazite yields an LA-ICP-MS age at 351 Ma ascribed to M2. M0-M1-M2 events developed before the Late Carboniferous pluton emplacement at ca 315 Ma (M3 event). The tectono-metamorphic succession documents that Armorica was not a rigid block but underwent a synmetamortphic ductile deformation during the Famennian–Tournaisian (360–355 Ma) collision redefined here as the late episode of the “Bretonian orogenic phase”, whereas the pre-Famennian Bretonnian episode is ascribed to oceanic subduction. These new data allow us to reassess the geodynamic evolution of this part of the Variscan orogen. Full article

Lithospheric slices preserving pre-Alpine metamorphic imprints are widely described in the Alps. The Variscan parageneses recorded in continental, oceanic, and mantle rocks suggest a heterogeneous metamorphic evolution across the Alpine domains. In this contribution, we collect quantitative metamorphic imprints and ages of samples that document Variscan tectonometamorphic evolution from 420 to 290 Ma. Based on age distribution and metamorphic imprint, three main stages can be identified for the Variscan evolution of the Alpine region: Devonian (early Variscan), late Devonian–late Carboniferous (middle Variscan), and late Carboniferous–early Permian (late Variscan). The dominant metamorphic imprint during Devonian times was recorded under eclogite and HP granulite facies conditions in the Helvetic–Dauphinois–Provençal, Penninic, and eastern Austroalpine domains and under Ep-amphibolite facies conditions in the Southalpine domain. These metamorphic conditions correspond to a mean Franciscan-type metamorphic field gradient. During the late Devonian–late Carboniferous period, in the Helvetic–Dauphinois–Provençal and central Austroalpine domains, the dominant metamorphic imprint developed under eclogite and HP granulite facies conditions with a Franciscan field gradient. Amphibolite facies conditions dominated in the Penninic and Southalpine domains and corresponded to a Barrovian-type metamorphic field gradient. At the Carboniferous–Permian transition, the metamorphic imprints mainly developed under amphibolite-LP granulite facies conditions in all domains of the Alps, corresponding to a mean metamorphic field gradient at the transition between Barrovian and Abukuma (Buchan) types. This distribution of the metamorphic imprints suggests a pre-Alpine burial of oceanic and continental crust underneath a continental upper plate, in a scenario of single or multiple oceanic subductions preceding the continental collision. Both scenarios are discussed and revised considering the consistency of collected data and a comparison with numerical models. Finally, the distribution of Devonian to Triassic geothermal gradients agrees with a sequence of events that starts with subduction, continues with continental collision, and ends with the continental thinning announcing the Jurassic oceanization. Full article

The Southern Alps are the retro-vergent belt of the European Alps that developed from Late Cretaceous subduction to Neogene times. The most prominent Alpine thrusts and folds, nowadays sealed off by the Adamello intrusion, were already developed before the continental collision and clasts derived from the eroded pre-collisional wedge can be found in the Cretaceous foredeep sequences. In contrast, the thermal state attained by the Southern Alps during the long-lasting Alpine evolution is still unknown. This contribution provides evidence for Alpine metamorphism in the northern part of the central Southern Alps. Metamorphic conditions are determined for the alkaline Edolo diabase dykes that emplaced in the exhumed Variscan basement rocks before being deformed during the Alpine convergence (D3). The Alpine foliation in the Edolo diabase dykes is marked by actinolite, biotite, chlorite, epidote, albite, and titanite and it developed under greenschist facies conditions at temperature of 350–420 °C and pressure ≤0.2 GPa. The T/depth ratio indicates a minimum of 50–60 °C/km that is compatible with thermal gradients characteristic of arc settings. Based on radiometric ages from the literature, these conditions were attained during the Alpine subduction. Full article

Multiscale structural analysis is carried out to explore the sequence of superposed pre-Alpine chloritoid–staurolite–andalusite metamorphic assemblages in the polydeformed Variscan basement of the upper Val Camonica, in the central Southalpine domain. The dominant fabric in the upper Val Camonica basement is the late-Variscan S2 foliation, marked by greenschist facies minerals and truncated by the base of Permian siliciclastic sequences. The intersection with the sedimentary strata defines a Permian age limit on the pre-Alpine tectono–metamorphic evolution and exhumation of the Variscan basement. The detailed structural survey revealed that the older S1 foliation was locally preserved in low-strained domains. S1 is a composite fabric resulting from combining S1a and S1b: in the metapelites, S1a was supported by chloritoid, garnet, and biotite and developed before S1b, which was marked by staurolite, garnet, and biotite. S1a and S1b developed at intermediate pressure amphibolite facies conditions during the Variscan convergence, S1a at T = 520–550 °C and P ≃ 0.8 GPa, S1b at T = 550–650 °C and P = 0.4–0.7 GPa. The special feature of the upper Val Camonica metapelites is andalusite, which formed between the late D1b and early D2 tectonic events. Andalusite developed at T = 520–580 °C and P = 0.2–0.4 GPa in pre-Permian times, after the peak of the Variscan collision and before the exhumation of the Variscan basement and the subsequent deposition of the Permian covers. It follows that the upper Val Camonica andalusite has a different age and tectonic significance as compared to that of other pre-Alpine andalusite occurrences in the Alps, where andalusite mostly developed during exhumation of high-temperature basement rocks in Permian–Triassic times. Full article

The composite Albian–Eocene orogenic wedge of the northern part of the Inner Western Carpathians (IWC) comprises the European Variscan basement with the Upper Carboniferous–Triassic cover and the Jurassic to Upper Cretaceous sedimentary successions of a large oceanic–continental Atlantic (Alpine) Tethys basin system. This paper presents an updated evolutionary model for principal structural units of the orogenic wedge (i.e., Fatricum, Tatricum and Infratatricum) based on new and published white mica ⁴⁰Ar/³⁹Ar geochronology and P–T estimates by Perple_X modeling and geothermobarometry. The north-directed Cretaceous collision led to closure of the Jurassic–Early Cretaceous basins, and incorporation of their sedimentary infill and a thinned basement into the Albian–Cenomanian/Turonian accretionary wedge. During this compressional D1 stage, the subautochthonous Fatric structural units, including the present-day higher Infratatric nappes, achieved the metamorphic conditions of ca. 250–400 °C and 400–700 MPa. The collapse of the Albian–Cenomanian/Turonian wedge and contemporary southward Penninic oceanic subduction enhanced the extensional exhumation of the low-grade metamorphosed structural complexes (D2 stage) and the opening of a fore-arc basin. This basin hemipelagic Coniacian–Campanian Couches-Rouges type marls (C.R.) spread from the northern Tatric edge, throughout the Infratatric Belice Basin, up to the peri-Pieniny Klippen Belt Kysuca Basin, thus tracing the south-Penninic subduction. The ceasing subduction switched to the compressional regime recorded in the trench-like Belice “flysch” trough formation and the lower anchi-metamorphism of the C.R. at ca. 75–65 Ma (D3 stage). The Belice trough closure was followed by the thrusting of the exhumed low-grade metamorphosed higher Infratatric complexes and the anchi-metamorphosed C.R. over the frontal unmetamorphosed to lowest anchi-metamorphosed Upper Campanian–Maastrichtian “flysch” sediments at ca. 65–50 Ma (D4 stage). Phengite from the Infratatric marble sample SRB-1 and meta-marl sample HC-12 produced apparent ⁴⁰Ar/³⁹Ar step ages clustered around 90 Ma. A mixture interpretation of this age is consistent with the presence of an older metamorphic Ph1 related to the burial (D1) within the Albian–Cenomanian/Turonian accretionary wedge. On the contrary, a younger Ph2 is closely related to the late- to post-Campanian (D3) thrust fault formation over the C.R. Celadonite-enriched muscovite from the subautochthonous Fatric Zobor Nappe meta-quartzite sample ZI-3 yielded a mini-plateau age of 62.21 ± 0.31 Ma which coincides with the closing of the Infratatric foreland Belice “flysch” trough, the accretion of the Infratatricum to the Tatricum, and the formation of the rear subautochthonous Fatricum bivergent structure in the Eocene orogenic wedge. Full article

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps. Full article