Thinning mechanisms of heterogeneous continental lithosphere

Benoit Petri (1) , Thibault Duretz (2, 1) , Geoffroy Mohn (3) , Stefan M Schmalholz (1) , Garry D. Karner (4) , Othmar Müntener (1)

1 UNIL - Université de Lausanne = University of Lausanne
2 GR - Géosciences Rennes
3 GEC - Laboratoire Géosciences et Environnement Cergy
4 Now at ExxonMobil Exploration Company

The mechanisms responsible for the formation of extremely thinned continental crust (<10 km thick) and lithosphere during rifting remains debated. Observations from present-day and fossil passive margins highlight the role of deep-seated deformation, likely controlled by heterogeneities within the continental lithosphere, such as changing lithologies, mechanical anisotropies and inherited structures. We investigate the mechanisms of lithospheric thinning by exploring the role of pre-existing heterogeneities on the architecture and evolution of rifted margins. We estimate pre-rift pressure conditions () vs. depth diagrams of crustal to lithospheric sections, to quantify rift-related modifications on inherited lithostatic pressure gradients. Two field examples from the Alpine Tethys margins in the Eastern and Southern Alps (SE Switzerland and N Italy) were selected to characterize: (1) the pre-rift architecture of the continental lithosphere; (2) the localization of rift-related deformation in distinct portions of the lithosphere; and (3) the interaction between pre-existing heterogeneities of the lithosphere and rift-related structures. These observations are compared with high-resolution, two-dimensional thermo-mechanical numerical models. The design of the models takes into account pre-existing mechanical heterogeneities representing the initial pre-rift architecture of the continental lithosphere. Extensional structures consist of high-angle and low-angle normal faults, anastomosing shear-zones and decoupling horizons. Such structures accommodate the lateral extraction of mechanically stronger levels derived from the middle to lower crust. As a result, the extremely thinned continental crust in Tethyan passive margins represents the juxtaposition and amalgamation of distinct strong levels of the crust separated by major extensional structures identified by sharp pressure gradients. Future work should determine the applicability of these results to other present-day and fossil rifted margins.

Mots clés: inheritance; rifting; rifted margin; lithospheric thinning; extraction tectonics

Domaines: Tectonique

Benoit Petri, Thibault Duretz, Geoffroy Mohn, Stefan M Schmalholz, Garry D. Karner, et al.. Thinning mechanisms of heterogeneous continental lithosphere. Earth and Planetary Science Letters, 2019, 512, pp.147-162. ⟨10.1016/j.epsl.2019.02.007⟩. ⟨insu-02044661⟩