Comparing the effects of viscous anisotropy in fossil lithospheric shear zones with different past kinematics

Abstract:

Seismic anisotropy in intraplate domains testifies for fossil crystal preferred orientations (CPO) of olivine in the lithospheric mantle associated with past tectonic events. In strike-slip shear zones both the [100]- and the [010]-maximum of the olivine CPO are horizontal, respectively parallel and normal to the shear zone trend, whereas in extensional shear zones the [100]-maximum follows the down-dip lineation and the [010]-maximum is normal to the low angle shear plane. When does viscous anisotropy of olivine CPO in such fossil mantle structures result in their reactivation? What are the fundamental contrasts in strain distribution in the lithosphere?

To answer this question we modelled a heterogeneous continental plate containing either a fossil strike-slip or an extensional mantle shear zone using 3D elasto-viscoplastic finite-element models, in which the anisotropic viscosity due to olivine CPO in the mantle is parameterized using Hill (1948) yield function with parameters obtained by fitting second- order viscoplastic self-consistent (SO-VPSC) polycrystal plasticity simulations. The mechanical responses of strike-slip and extensional mantle shear zones is compared for varied orientations of the strike and dip angle of the fossil mantle shear plane relative to an imposed horizontal extension or compression, for initial homogenous crust, similar olivine CPO, mantle and crust compositions, and boundary conditions. The strongest reactivation is observed for a fossil strike-slip mantle fabric oriented at 45° to the imposed solicitation. However, the strain distribution in the mantle lithosphere is more sensible to the direction of past strike-slip shear zones than of extensional ones. The latter are more easily reactivated in extension than in compression, as lithostatic pressure resists thickening of plate in the latter case. The presence of a moderate dipping fossil extensional mantle shear zone with a strong down-dip olivine fabric might lead to enhanced thinning (or thickening) during stretching (or shortening) of the lithosphere relative to a model with the same geometry but with an isotropic mantle shear zone.