Tectonic evolution in transtensional regimes: the example of the Variscan Tanneron massif (SE France)

Deformation of the oceanic and continental lithosphere induce by plates motion on Earth’s surface implies the existence of an oblique deformation component at plate boundaries. Oblique tectonic systems, including transpressional and transtensional regimes, represent complex oblique boundaries and in-between movements combining transcurrent with either convergent or divergent deformation. These oblique strain regimes could generate a wide variety of structures and strain patterns in the ductile domain of the middle to lower crust, that makes interpretation of fabrics challenging. Studied example of transtensional regimes in orogenic domains are still rare and deformation process in this context remains poorly understood. However, transtensional systems are expected to play a key role by accommodating tectonic forces within a rheologically and mechanically heterogeneous lithosphere, in particular during orogenic collapse and exhumation of high-grade metamorphic rocks by thinning the orogens.

Here we present a new multidisciplinary and multiscale study by combining (micro)structural, thermobarometric and geochronological analyses in the Variscan Tanneron massif to reconstruct its late P-T-t-D evolution. The Tanneron massif represent the most internal part of the Maures-Tanneron Variscan belt (SE France), which was mainly structured during the late stage Variscan orogeny. The aim of this study was to precise the nature, organisation and evolution of ductile deformation and associated structures inside a transtensional tectonic regime. Structural and microstructural analysis (AMS and 3D finite strain ellipsoids calculation) of the Tanneron massif indicates that the late tectonic event that structured the massif is a general transtensional regime characterised by a strong subhorizontal stretching that originated through two phases. The first phase is a pure shear- dominated transtension with the development of a subhorizontal constrictional flow associated with L>S tectonites and minority gently-dipping foliations. The second phase is a simple shear-dominated transtension characterised by a plane strain flow (S-L tectonites) and the development of vertical foliations and dextral shear zones. Thermobarometric modelling (X-Ray compositional maps and mineral composition using EPMA) and in-situ U-Th-Pb dating (monazite and xenotime, LA-ICPMS) of the migmatitic units allow us to precise the tectonic evolution of this transtensional deformation regime. This regime represents a tectonic continuum between ~ 325 and 300 Ma and defines a progressive deformation event synchronous with the exhumation of high-grade metamorphic units. Deformation was initiated at high-temperature conditions associated with partial melting of the crust between 7.2 - 10.0 Kbar and 730 - 770 °C, then progressed mainly under subsolidus conditions during the retrograde path of the migmatitic units until low greenschist metamorphic facies conditions reaching 4.1 Kbar and 370 °C. Linking structural, microtextural observations and thermobarometric data, we propose a rheologically driven strain path partitioning during the progressive exhumation of this deep crust. During the second phase of the transtensional regime, deformation was localised preferentially in the hydrated meta-sedimentary units, rather than in meta-igneous rocks.