Seminar Josselin Gremmel

Abstract

Oblique tectonic systems, characterised by the combination of strike-slip and compressional or extensional components, are widespread on Earth and have become more commonly described as our understanding of geologic structures advances. These systems, including their end-members transpression and transtension, have been extensively studied in the brittle crust, focusing on seismic implications and specific structures such as pull-apart basins, en-echelon folds, and fault arrays. However, research on oblique tectonics in the ductile domain of the middle to lower crust reveals structural complexity that makes interpretation of fabrics challenging, especially in the context of hot orogens. A wide variety of structures and strain patterns are observed during their evolution, notably due to a strong strain partitioning. The investigation of the development of crustal fabrics formed by oblique tectonic flow in a transtensional regime example (Maures-Tanneron Variscan massif, France) and a transpressional regime example (Dumont d’Urville basin, Antarctica) has provided a detailed description of strain shape and partitioning evolution inside these specific tectonic frameworks. By combining structural analysis through detailed field mapping, microstructural observations and finite strain calculations, with thermobarometric and geochronological analyses, it has been possible to precise the nature, organisation and evolution of deformation. The synthesis of these results highlights the duality of deformation styles and structures orientations between transpressional and transtensional systems. This study also provides new insights on the role of these oblique regimes in the exhumation of orogenic roots and the influence of rheological contrast on strain partitioning.