Evidence of extension in the granulite facies lower crust of Calabria during the Hercynian orogeny.
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Structural, petrological and geochronological data from the Variscan chain of northern Sardinia are discussed, with the aim to improve our knowledge on the tectono-metamorphic evolution of the northern Sardinian basement and its role in the tectonic puzzle of the southern Variscan massifs. In northern Sardinia, which corresponds to the Inner zone of the chain, the metamorphic grade rapidly increases towards the NE from biotite to sillimanite+K-feldspar zone. The highest grade rocks cropping out in the Migmatite zone reached amphibolite facies conditions in the sedimentary and igneous-derived migmatite and eclogite/granulite facies in the enclosed metabasite lenses. Evidence of HP metamorphism has also recently been found more to the south in micaschist from the garnet zone. The D 1 deformation can be observed in the southern part of the Inner zone, whereas, moving progressively northwards, the D 2 deformation progressively obliterates the previous foliation. The other, less pervasive, deformation phases (D 3 , D 4 , D 5 ) are sometimes recognizable in the outcrop localities, but they are not as ubiquitous as the previous D 1 and D 2 . HP metamorphism in metabasites and schists is related to the collision and/or northwards-directed subduction of the continental crust, which was part of the northern Gondwana or peri-Gondwanian terranes in Early Palaeozoic times. The HP metamorphism, which is pre- to syn-D 1 , was followed by Barrovian metamorphism. The stacking of metamorphic sequences with the building up of the nappe pile was a consequence of the High Grade Metamorphic Complex exhumation. This was activated by NW-SE striking and top-to-the S and SW shear zones and faults with a major dip-slip component of movement, coeval with the late phase of the D 1 deformation. Transpressional deformation then developed until the High-Grade Metamorphic Complex continued to be exhumed and the Low- to Medium-Grade Metamorphic Complex was underthrust to the north. Finally, the extensional tectonic regime was accompanied by local occurrences of HT-LP metamorphism. Deformation and metamorphism in the Sardinian Variscides were accompanied by the intrusion of igneous suites of the Sardinia-Corsica batholith, which was emplaced during the Middle Carboniferous/Permian post-collisional phase.
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The basement of the Southern Alps (northern Italy) belongs to the southernmost part of the European Variscan mountain belt. In contrast to other areas of the Alps, the post-Variscan metamorphic and tectonic overprint is weak and therefore permits the unravelling of the Variscan tectonometamorphic evolution of this region. Overprinting criteria and the mapping of the penetrative structural elements (S2 and L2) allow three deformational events to be distinguished. Major Variscan deformation (D2) in the Southern Alps commenced in the Carboniferous and lasted until the end of the Late Carboniferous. Tectonic movement during D2 was north-directed and was accompanied and followed by greenschist facies metamorphism. Peak metamorphic conditions reached 450–550°C and 5–6.5 kbar in the Brixen area and decreased in a southeasterly direction. Tectonic movement during D2 followed subduction of the Plankogel terrane and collision of the Noric terrane (northernmost part of Gondwana) with Laurasia and is interpreted to result from late orogenic extensional collapse of the Variscan mountain chain. Extensional deformation caused exhumation of the basement rocks as evidenced by greenschist facies basement pebbles, which show no substantial retrograde overprint, in Late Carboniferous/Early Permian conglomerates.
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The eastern Central Alps consist of several Pennine nappes with different tectonometamorphic histories. The tectonically uppermost units (oceanic Avers Bündnerschiefer, continental Suretta and Tambo nappes, oceanic Vals Bündnerschiefer) show Cretaceous/early Tertiary W‐directed thrusting with associated blueschist facies metamorphism related to subduction of the Pennine units beneath the Austroalpine continental crust. This event caused eclogite facies metamorphism in the underlying continental Adula nappe. The gross effect was crustal thickening. The tectonically lower, continental Simano nappe is devoid of any imprint from this event. In the course of continent‐continent collision, high‐ T metamorphism and N‐directed movements occurred. Both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions. Crustal thinning commenced during the regional temperature peak. A final phase is related to differential uplift under retrograde P–T conditions. Further thinning of the crust was accommodated by E‐ to NE‐directed extensional deformation.
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Abstract Linking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation ( P–T–d ) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a D n ‐1 under‐thrusting event. The following exhumation during the D n mylonitic event, and contact metamorphism during D n +1 and D n +2 folding events, have also been modelled because they are essential to restore the previous tectono‐metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid‐deficient conditions, as well as the final late Carboniferous contact metamorphism up to T max of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating‐dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P / T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T / P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during D n ‐1 under‐thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.
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The Cabo Ortegal Complex is a composite allochthonous terrane that was thrust onto the western edge of Gondwana during the Variscan orogeny. It is formed of two main tectonic units: the Upper Tectonic unit, comprising rocks affected by high-pressure (P)-high-temperature (T) metamorphism, and the Lower Tectonic unit, which represents the resulting suture of the Variscan collision. The suture preserves remnants of strongly deformed and metamorphosed ophiolitic rocks overriding the parautochthon, and the lower Paleozoic sequence of the Ollo de Sapo antiform, regarded as the autochthonous sequence of the Iberian plate. The Upper Tectonic unit is formed by layered ultramafic, mafic, and quartzo-felspathic rocks that were buried at levels in excess of 50 km (-1.56 GPa) before the Variscan collision in a convergent plate boundary within the Rheic ocean domain ca. 490-480 Ma (Early Ordovician). They have been interpreted either (1) as an earlier thinned continental crust, underlain by a lithospheric mantle and oceanic spreading, or (2) as independent terranes, formed in different geodynamic settings (island arc, oceanic). Most structures observed in these rocks are ductile and are associated to their exhumation process. It started with the development of a persistent horizontal foliation in granulite facies conditions, which equilibrated in amphibolite facies conditions ca. 385 Ma, and ended in higher crustal levels with the progressive development of noncoaxial structures, such as east-verging asymmetrical isoclinal folds and thrusts, leading to the emplacement of the Upper Tectonic unit over the Lower Tectonic unit ca. 365 Ma (Late Devonian).
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