Exhumation of a Variscan orogenic complex: insights into the composite granulitic–amphibolitic metamorphic basement of south‐east Corsica (France)

Abstract

A structural, petrological and geochronological (U-Th-Pb of zircon and monazite) study reveals that the lower crust sequences of the Variscan high-grade basement cropping out between Solenzara and Porto Vecchio, south-east Corsica (France) have been tectonically juxtaposed along with middle crustal rocks during the extrusion of the orogenic root of the Variscan chain. We propose that a system of hightemperature, orogen-parallel shear zones that developed under a transpressive dextral tectonic regime caused the exhumation of the entire sequence. This tectonic complex is thus made up of rocks having undergone different P-T conditions (eclogite-?, high-pressure granulite facies and amphibolite facies) at different times, reflecting the progressive foreland migration of the orogenic front. The Solenzara granulites were derived from burial of continental crust to high-pressure (1.8-1.4 GPa) and high-to ultrahigh-temperature conditions (900-1000°C) during the Variscan convergence: U-Pb ELA-ICPMS zircon dating constrained the timing of this metamorphism at c. 360 Ma. The gneisses cropping out at Porto Vecchio are middle crustal-level rocks that reached their peak temperature conditions (700-750°C at <1.0 GPa) at c. 340 Ma. The diachronism of the metamorphic events, the foliation patterns and their geometry suggest that the granulites were exhumed to middle crustal levels through channel flow tectonics under continuous compression. The amphibolite facies gneisses of Porto Vecchio and the granulites of Solenzara were accreted through the development of a major dextral mylonitic zone forming under amphibolite facies conditions: in situ monazite isotope dating (ELA-ICPMS) revealed that this deformation occurred at c. 320 Ma and was accompanied by the emplacement of syntectonic high-K melts. A final HTLP static overprint, constrained at 312-308 Ma by monazite U-Th-Pb isotope dating, is related to the emplacement of the igneous products of the Sardinia-Corsica batholith and marks the transition from the Variscan orogenic event to the Permian extension.

Evolution of the Corsica–Sardinia Batholith and late-orogenic shearing of the Variscides

“…Stretching already-hot crust can result in decompression melting; however, metamorphic P-T-t paths in northern Sardinia and southern Corsica (Giacomini et al, 2006(Giacomini et al, , 2008 show that temperatures increased during the late Carboniferous, requiring additional heat sources. This can be explained either by a large thermal anomaly rooted in the sub-continental mantle, or by focused shear heating in an already attenuated, and presumably hot, crust.…”

Crustal exhumation during ongoing compression in the Variscan Maures-Tanneron Massif, France-Geological and thermo-mechanical aspects

“…345 Ma, e.g. Paquette et al, 2003) and ended when the IZ and EZ were fully decoupled and when transcurrent motions started to be recorded at 320 Ma Giacomini et al, 2008;Oliot et al, 2015). The D2 phase follows the nappe-stacking phase D1 and is characterised by progressive backthrusting of tectonic units in the core of the orogenic prism in a direction N140°+/-20 (top to SE sense of shear as shown by the black arrow in Fig.…”

“…Meanwhile the partially molten rocks of the IZ are strongly affected by transcurrent motions followed by lateral flow or extrusion in a direction sub-orthogonal to convergence. Giacomini et al (2008) and Casini et al (2012) independently propose that this transition occurred around ca. 310-315 Ma with final thinning of the chain.…”

Structure of the Variscan metamorphic complexes in the central transect of the Posada-Asinara Line (SW Gallura region, Northern Sardinia, Italy)

“…Whatever it is the geodynamic reconstruction, the mylonites of the PAL show geometry, kinematics and timing of shear activity comparable with those documented on shear zones cropping out in southern Corsica (Giacomini et al, 2008), in the Maures-Esterel massif (e.g. Corsini & Rolland, 2009) and in the External Crystalline Massifs of Western Alps (Guillot & Ménot, 2009).…”