Structure, strain partitioning and exhumation mechanism during the late stage oblique tectonic evolution of the Variscan Tanneron massif (SE France)

Oblique tectonic, including transpressional and transtensional movements, is a common feature observed at active plate boundaries. Despite being often inferred at the orogen scale, the interpretation of local structural observations in the context of oblique tectonic regimes remains challenging, especially in ductile terrains. During the last stage evolution of orogenic belts, obliquity is expected to play a key role in controlling strain partitioning and exhumation patterns of deep crustal rocks, as well as the development of brittle structures in the upper crust.

Here, we present new structural data and finite strain analyses of migmatitic rocks exposed in the Tanneron Massif in SE France. This massif, representing the most internal part of the Maures-Tanneron Variscan belt segment, was structured between 320 and 300 Ma during the late stage Variscan orogeny. This late-stage deformation, synchronous to partial melting of the middle crust, the large-scale folding of the migmatitic units and their exhumation has been interpreted to take place in a regional transpressive regime.

New detailed structural mapping carried on in two sectors of the massif highlight different strain patterns with dome-like structures in a migmatite unit to the East, and sub-vertical shear-zones with stretching lineation to the West. However, in these two sectors stretching is dominant and finite constrictional fabrics are ubiquitous. The regional lineation, corresponding to the maximum stretching direction of these L-tectonites is parallel to the large scale and local fold axes. In addition, narrow continental Carboniferous basins oriented roughly parallel to the main ductile fabric opened inside the massif contemporaneously to the exhumation of the L-tectonites. Therefore, our results suggest that local transtension might best describe the tectonic regime associated with the late-stage evolution of the massif. We will discuss these results for the Tanneron massif in the light of a series of preliminary 3D thermo-mechanical numerical models designed to investigate the horizontal and vertical partitioning of deformation in a hot orogen subject to regional oblique deformation.