The role of upper mantle in lithosphere deformation: Insights from stress/strain evolution of a strike-slip fault

It has been under continuous debate whether the upper crust or the upper mantle plays a major role in supporting tectonic stress and determining lithosphere deformation. The strength calculations from lithospheric mineral composition, thermal field and experimental rheology generally suggest a stronger upper mantle, while other evidences, including earthquake depth distribution and postseismic/postglacial relaxation, suggest a weak upper mantle. The role comparison between upper crust and upper mantle becomes even more non- straightforward as faults and shear zones are taken into account. To improve our understanding of the upper mantle's role in lithospheric stress and deformation, I explore how lithosphere rheology structures affect stress/strain evolution of a strike-slip fault using visco-elastic numerical models. The model incorporates a strike-slip fault in the upper crust, which experiences cyclical stick-slip events, and various lower crust and upper mantle rheological structures, including horizontally homogenous upper mantle and heterogeneous mantle with weak shear zones. The results show that a stronger lower crust may couple the upper crust and upper mantle and transfer the effect of upper mantle to the stress/strain evolution of a stick-slip fault. Horizontally homogenous lower crust and upper mantle exert little stress on the upper crust before fault slip events, and exert backward- drag force right after fault rupture. If weak shear zones exist, however, the lower crust and upper mantle exert active drag force on the upper crust before fault rupture. This may cause a relatively lower postseismic stress in the lower crust and upper mantle, thus lower postseismic surface velocities. The magnitude of the coupled stress depends on the shear zone rheology and bulk viscosity of lower crust and upper mantle. These results indicate that the role of upper mantle in lithosphere deformation not only depends on bulk mantle rheology, but also rheology of lower crust and shear zones.