Effects of grain size reduction for oceanic detachment fault formation and inversion

Oceanic detachment faulting is one of the dominant styles of seafloor deformation at mid-ocean ridges. Previous studies have shown that the formation of oceanic detachment faults is mainly controlled by brittle/plastic weakening of the oceanic lithosphere. Here, we investigate numerically potential effects of ductile weakening controlled by grain size reduction on the oceanic detachment faults formation as well as on their subsequent inversion during the Wilson cycle. We employ 3D thermo-mechanical numerical models with a composite rheology consisting of diffusion and dislocation creep. In our model, oceanic crust deforms in a brittle manner and its strength is controlled by fracture-related strain weakening and healing. In contrast, the lithospheric mantle deforms according to the dry olivine flow law, as a mixture of grain size-dependent diffusion and dislocation creep. Numerical results show that ductile weakening induced by grain size reduction could indeed notably influence both the style of detachment faulting and the fault dipping angles. We systematically investigate the influence of the thermal structure, the thickness and rheological properties of the oceanic crust, initial grain size and asymmetric spreading rate on the characteristic oceanic detachment fault pattern. In addition, we also study effects of these parameters on the final inversion of detachment faults during induced intra-oceanic subduction initiation.