Olivine-induced viscous anisotropy in fossil strike-slip mantle shear zones and associated strain localization in the crust
Résumé
We propose that strain localization in plate interiors, such as linear belts of intraplate seismicity, may arise from spatial variations in viscous anisotropy produced by preferred orientation of olivine crystals (CPO or texture) inherited from previous deformation episodes in the lithospheric mantle. To quantify this effect, we model the deformation of a plate containing a fossil strike-slip mantle shear zone at different orientations relative to an imposed shortening, but no initial heterogeneity in the crust. The fossil shear zone is characterized by different orientation and intensity of the olivine CPO relatively to the surrounding mantle, which is isotropic in most simulations. The anisotropy in viscosity produced by the CPO, which remains fixed throughout the simulations, is described by an anisotropic (Hill) yield function parameterized based on second-order viscoplastic self-consistent (SO-VPSC) models. The results indicate that lateral variations in viscous anisotropy in the mantle affect the strain distribution in the entire lithosphere. Reactivation of the mantle shear zone and strain localization in the crust above it occur for compression at 35-55° to the mantle shear zone, with a maximum at 45°. The magnitude of strain localization depends on (i) the contrast in viscous anisotropy and, hence, on the CPO orientation and intensity in the mantle domains, (ii) the boundary conditions, and (iii) the feedbacks between mantle and crustal deformation. For a strong olivine CPO, when the boundary conditions do not hinder shear parallel to the fossil mantle shear zone, strain rates within it are up to 30 times higher than in an isotropic surrounding mantle or up to 200 times when the surrounding mantle is anisotropic, which results in strain rates up to 10 times or up to 100 times higher in the crust above the fossil shear zone. Frictional weakening in the crust faults increases strain localization in the entire lithospheric column. High strength contrasts between the mantle and the ductile crust result in less efficient coupling, with strong localization in the mantle and lower crust, but weak in the brittle upper crust. Decrease in the crust-mantle strength contrast enhances coupling and produces more homogenous strain distribution with depth, as well as a time-dependent evolution of strain localization, which reaches a peak and decreases before attaining steady-state. Comparison of seismic anisotropy, regional stress, and focal mechanism data in linear arrays of intraplate seismicity, like the New Madrid and South Armorican seismic zones, to our models' predictions corroborates that olivine CPO preserved in fossil lithospheric-scale shear zones may be key for the development of such structures.
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