Time-dependent effects in elastoviscoplastic models of loaded lithosphere

Abstract

Summary We conducted a plane strain finite element analysis of topographic loading on a 40-km-thick, compositionally homogeneous, wet olivine oceanic lithosphere. The rheological model was elastoviscoplastic (EVP) and accommodated elastic deformation, steady-state dislocation creep, as well as frictional slip via a Drucker–Prager yield surface. From the model results, we analyse the development of thrust and normal faulting zones that evolve with the growth of a 150 MPa spatially uniform topographic load, as well as the interaction between frictional slip and creep in the normal faulting zone that exists at about 20 km depth beneath the load. The spatial extent of this zone is shown to be a function of the load growth rate, and during loading, the lowest 3 km of this region demonstrates simultaneous pressure-sensitive frictional slip and pressure-insensitive, temperature- and strain rate-dependent creep. This dual-mechanism deformation is similar to descriptions of semi-brittle deformation because of the combined cataclastic and crystal-plastic nature that characterizes semi-brittle flow. The models also show a feature that we refer to as palaeoslip. Within the two normal faulting regions in the lithosphere (and after the load has reached its maximum), part of each region maintains active faulting while nearby the faulting has long since ceased. The active faulting is enabled by the migration of stress due to post-loading creep in high temperature regions of the plate. These three phenomena cannot be realized in elastic-plastic or viscoelastic rheologies.