Evolution and architecture of continental passive margins: the role of structural softening

Duretz, Thibault and Petri, Benoit and Mohn, Geoffrey and Schmalholz, Stefan and Schenker, Filippo and Müntener, Othmar (2016) Evolution and architecture of continental passive margins: the role of structural softening. In: Abstract Volume, 14th Swiss Geoscience Meeting 14th Swiss Geoscience Meeting, Geneva, Switzerland.

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Numerous geological and geophysical studies have drawn attention to the multi-stage and depth-dependent character of lithospheric thinning. During rifting, a variety of structures are generated to accommodate lithospheric thinning (normal faults, low angle detachments, extensional shear zones, extraction faults), leading to complex passive margin architectures (e.g. necking zones, extremely thinned continental crust, mantle exhumation, continental allochthons). It is generally accepted that the lithosphere bears mechanical heterogeneities inherited from previous tectonic events (i.e. tectonic inheritance). During deformation, the presence of mechanical heterogeneities may induce mechanical instabilities and trigger structural softening. The influence of such processes on the dynamics of lithospheric extension and passive margin formation is so far incompletely understood. We have designed two-dimensional thermo-mechanical models of lithospheric thinning to study the role of inherited mechanical heterogeneities on the development of passive margins. We represent first order heterogeneities by mechanical layering at the kilometre scale. The entire lithosphere incorporates several mechanically strong layers. The rheologies are voluntarily kept simple (temperature, stress, and strain rate-dependent visco-plastic) and we do not consider any material strain softening parameterisation nor damage model. Our numerical simulations demonstrate that mechanical layering induces depth-dependent multi-stage lithospheric extension. Firstly the overall deformation style is decoupled and symmetric: both upper crustal and lithospheric layers undergo thinning by brittle (frictional-plastic) faults independently, while lower crustal and lithospheric levels accommodate extension by ductile necking. Secondly a low angle extensional structure develops across the Moho causing the overall deformation style to be coupled and asymmetric. Consequently subcontinental mantle exhumation takes place along the low angle extensional structure. At later stages, crustal allochthons form at the edges of the conjugate passive margins and competent mid-crustal levels are laterally extracted from the hyper-extended domain These results stress the primordial role of mechanical heterogeneities and associated structural softening for lithospheric thinning. The incorporation of mechanical heterogeneities in lithospheric extension models can explain the formation of numerous observed geological features (necking zones, mantle exhumation, continental allochthons) regardless of the usage of a simple rheological model. Structural softening can hence alone give an explanation for the complex structuration and evolution of passive margins.

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