Symmetric and asymmetric lithospheric extension: Relative effects of frictional‐plastic and viscous strain softening

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Abstract
Strain‐dependent rheologies may play a critical role in the deformation of the lithosphere and particularly in the development of focused shear zones. We investigate the effects of strain softening on lithospheric extension using plane strain thermomechanical finite element model experiments. Parametric softening is specified as a linear decrease of the effective internal angle of friction, the effective viscosity, or both in the model rheologies. The sensitivity of deformation to the choice of softening parameters is investigated in cases where the crust is either strongly or weakly coupled to the mantle lithosphere. Results are classified according to the symmetry (S) or asymmetry (A) of the deformation of the upper and lower lithosphere during rifting. Strain softening is required for rifting asymmetry but is not always sufficient. A range of model tectonic styles occurs including pure and simple shear modes with focused shear zones. Mode selection is mostly determined by the feedback between two primary controls, the “dominant” rheology and the parametric strain‐softening mechanisms listed above. Softening of the dominant rheology promotes asymmetric extension of that part of the lithosphere controlled by the dominant rheology. Model results are consistent with the proposed primary controls and the factors that contribute to these controls. In particular, decreasing and increasing the rifting velocity can change the mode by changing the dominant rheology. Asymmetry is strongest in coupled models which include a decrease in the internal angle of friction and have low rifting velocities.