The Effect of Brittle-Ductile Weakening on the Formation of Faulting Patterns at Mid-Ocean Ridges

Although seafloor spreading is one of the most prominent plate tectonic processes, its development and faulting pattern diversity are incompletely understood. This study addresses how brittle-ductile weakening affects the formation and development of faulting patterns at spreading centers using 3D magmatic-thermomechanical numerical models. Grain size evolution and brittle/plastic strain-dependent friction coefficient weakening are fully coupled into models. A spectrum of faulting patterns, from asymmetric long-lived detachment faults developing by the same polarity to short-lived detachment faults by the flip-flop mode to symmetric conjugate faults, is documented in our models. Systematic numerical results indicate that fault strength reduction and axial brittle layer thickness are two pivotal factors in controlling faulting patterns and development modes. Through varying initial friction coefficients, we found that strong friction weakening with significant fault strength reduction results in very asymmetric detachment faults developing by the same polarity, while weak friction weakening leads to slight fault strength reduction, forming conjugate faults. In addition, the thermal structure, influenced by spreading rates, hydrothermal cooling, and mantle potential temperature, in turn, controls the axial brittle layer thickness and faulting patterns. To test a damage mechanism with a physical basis, we investigate grain size reduction at the root of detachment faults. We found that compared to the friction weakening of faults in the brittle/plastic layer, the development of oceanic detachment faults appears to be less sensitive to grain size reduction, which is mainly due to a relatively shallow axial brittle layer of the spontaneously accreting oceanic lithosphere in our models.