Asymmetric Interseismic Deformation and Structure Along Evolving Strike-Slip Faults: the Role of Lateral Contrasts in Lithosphere Rheology

Major faults often juxtapose materials with different physical properties, and recent studies suggest that elasticity contrasts may cause asymmetric interseismic velocity profiles which are often associated with such structures. Here, we explore how lateral contrasts in lithosphere rheology influence the growth and evolution of fault systems, using deformation models incorporating damage evolution in the brittle upper crust and various linear and nonlinear viscous rheologies at depth. We assess whether rheological contrasts cause asymmetric shear and damage zone structure, whether evolving strike-slip faults migrate toward and propagate along rheological interfaces, and how these effects may influence interseismic surface velocities. We find that reasonable contrasts in lithosphere elasticity have only a minor effect on the symmetry of damage or the propagation of faults. Contrasts in effective plate thickness or viscosity structure attract propagating faults, which tend to locate about 1 to 5 km from the interface, in the thinner plate (or the side with lower viscosities). Brittle damage is more extensive on the thin-plate (or low-viscosity) side of the fault. In these models, the highest interseismic surface strain rate (and the fault zone) often coincide with the position of a lower crust / upper mantle shear zone, which may be offset from the interface by up to 10 km. We will demonstrate how models with reasonable contrasts in viscoelasticity can explain marked asymmetry in interseismic GPS velocities (for example, across the North Anatolian Fault in the Marmara Sea), and that extreme contrasts in elasticity are not required. We will also show that models of asymmetric interseismic deformation in which the fault trace is assumed to coincide with the material interface may lead to an erroneous interpretation of GPS surface velocity data.