The effect of brittle-ductile weakening on the formation of detachment faults at ultraslow spreading ridges

Large offset detachment faults form with exhuming mantle-derived rocks into the seafloor at the slow and ultralow spreading ridges. However, their formation mechanism still remains partly elusive.  The thick axial lithosphere of ultraslow spreading ridges detected by seismic studies may prevent the formation of detachment faults. Previous studies have proposed that only the combination of both serpentinization and grain size reduction in the mantle lithosphere can result in detachment faults which are consistent with the natural cases. Here, through 3D self-consistent magmatic-thermomechanical numerical models with both brittle/plastic strain weakening and grain size evolution, we systematically investigate effects of these coupled brittle-ductile weakening processes on the formation of detachment faults at ultraslow spreading ridges. Numerical results show that ultraslow ridges spontaneously break into shorter and warmer magma-rich (10-20% of the ridge length) and longer and colder magma-starved segments (80-90% of the ridge length). Small grain size formed in the deep root of detachment faults near the brittle-ductile transition depth at the magma-starved amagmatic segments. Then with mantle rocks exhumation into the surface, the decreasing temperature leads to the growth of small grain size, consistent with the deformation process of detachment fault systems in the amagmatic segments of the eastern part of the Southwest Indian Ridge. Through quantitatively exploring effects of grain size reduction and strain weakening, we obtained that strain weakening may be the primary factor to control the formation of detachment faults at the ultra-slow spreading ridges, although grain size evolution can also influence the spreading pattern in case of small (<= 1 mm) initial grain size of the lithospheric mantle. Furthermore, we also found that the weak ductile domain induced by the very small initial grain size (<= 0.1 mm) promotes the formation of detachment faults in the models without grain size evolution.