Thermal and mechanical evolution of an orogenic wedge during Variscan collision: an example in the Maures–Tanneron Massif (SE France)

Abstract

Synthesis of structural, petrological and geochronological data for the Maures–Tanneron Massif and its integration in the framework of adjacent massifs (i.e. Sardinia and Corsica) has allowed us to propose a new model of evolution for the southern Variscan belt. After Siluro-Devonian subduction associated with high-pressure–low-temperature (HP/LT) metamorphism M0 (c. 10–15 °C km−1) and subsequent Carboniferous nappes stacking, the belt underwent strong reworking related to back-thrusting. Nappes stacking and back-thrusting were associated with typical Barrovian metamorphism M1 (c. 20–30 °C km−1) starting at 360 Ma that progressively evolved to higher temperature metamorphisms M2 (c. 40–60 °C km−1) and M3 (c. 60–80 °C km−1) during 330–300 Ma in the internal part of the belt. Progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root. Continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold-dome structures. The mass transfer is accommodated by orogen-parallel transpressive shearing synchronous with M3 during Late Carboniferous time. The orogenic wedge is characterized by two main tectono-metamorphic units decoupled by a major shear belt: an Internal Zone with migmatites and syntectonic granitoids, where HP relicts have been exhumed, and an External Zone that escaped the late HT event and preserved precious structures.

The Variscan orogeny: extent, timescale and the formation of the European crust

“…This event was followed by the development of the Axial Zone migmatitic dome at 330 -320 Ma, and upright folding in the Palaeozoic non-metamorphic series. A broadly similar pattern is proposed for the tectonic and metamorphic evolution of the Maures -Tanneron Massif further east (Schneider et al 2014). The model accounts for Siluro-Devonian subduction associated with HP/LT metamorphism, subsequent Carboniferous nappe stacking and back-thrusting.…”

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

“…1d, Schneider et al, 2014). Relatively to the actual position of the MTM, structures display a NW-SE orientation along which two major tectonic units appear: the External Zone to the West (EZ) and the Internal Zone to the East (IZ, in blue and pink, respectively in Fig.…”

“…However the MTM portion of the SEVB cannot be easily described by these processes nor those mentioned above (Schneider et al, 2014 and references herein). The late tensile regime of the MTM actually occurred perpendicular to convergence, and exhumation of the partially molten orogenic root actually initiated nearly 50 My earlier (around 340 Ma; Schneider et al, 2014;Oliot et al, 2015). Schneider et al (2014) recently argued based on structural, petrological and geochronogical data, that diapiric exhumation of the partially molten orogenic root likely began during ongoing convergence at ~345 Ma and continued for about ~20 My.…”

“…The late tensile regime of the MTM actually occurred perpendicular to convergence, and exhumation of the partially molten orogenic root actually initiated nearly 50 My earlier (around 340 Ma; Schneider et al, 2014;Oliot et al, 2015). Schneider et al (2014) recently argued based on structural, petrological and geochronogical data, that diapiric exhumation of the partially molten orogenic root likely began during ongoing convergence at ~345 Ma and continued for about ~20 My. Such critical transition between (2D) convergence to perpendicular extension remains rarely studied within a coherent framework between geological observation and theoretical numerical modelling.…”

“…This study thus provides a basis to be tested in future studies of threedimensional orogenic systems such as the MTM. The paper is therefore organised as follows: in section 2, we complement the geological synthesis of Schneider et al (2014) with a PTt compilation enriched with published data from Corsica and Sardinia and with recent geochronological data from the MTM . Arguments in favour of a compressional setting at the onset of exhumation are strengthened.…”