Multi-layer lithospheric extension: implications for Mesozoic rifting in the Alps

Duretz, Thibault and Mohn, Geoffrey and Schenker, Filippo and Schmalholz, Stefan (2015) Multi-layer lithospheric extension: implications for Mesozoic rifting in the Alps. In: Abstract Volume, 13th Swiss Geoscience Meeting 13 th Swiss Geoscience Meeting, Basel, Switzerland.

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The Alpine belt have undergone a polyphase geological history spanning the Devono-Carboniferous Variscan orogeny, the Permian post-Variscan extension, the Mesozoic rifting and ultimately the Late Cretaceous to Tertiary Alpine orogeny. In particular, Permian post-orogenic processes are responsible for strongly pre-structuring the continental crust/lithosphere prior to the Mesozoic rifting. This event was characterized by the emplacement of acid and mafic intrusions at all crustal levels and is associated to high-temperature metamorphism. Therefore, before Mesozoic rifting the continental crust was likely considerable mechanically heterogenous due to a variety of different rock types and the dependence of mechanical strength on temperature. These pre-rift mechanical heterogeneities and the thermal inheritance likely played a key role for the development of the Liguro-Piemontese and Valaisan domains. We investigate the role of pre-rift inheritance on the development of rifted margins using two-dimensional thermo- mechanical models of lithospheric extension. In the model we represent the pre-rift mechanical heterogeneities with a mechanical layering. We study the control of mechanical layering on the extension of the continental lithosphere. The mechanical layering causes a multi-stage depth-dependent extension. In the first rifting phase, lithospheric deformation is decoupled: while the crust undergoes thinning by brittle (frictional-plastic) faults, the lithospheric mantle accommodates extension by symmetric ductile necking. In a second rifting phase, deformation in the crust and lithospheric mantle is coupled and marks the beginning of an asymmetric extension stage. Low angle extensional shear zones develop across the lithosphere and exhume subcontinental mantle. Furthemore, crustal allochtons and adjacent basins develop coevally. We describe the kinematic and geometrical features as well as the thermal evolution predicted by the numerical models. We discuss the implication of these first-order features in the context of the Alpine geological history.

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