Oceanic transform fault seismicity and slip mode influenced by seawater infiltration

Oceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep. The mode of slip varies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures <600 °C, and others that slip through creep and microearthquakes at temperatures up to 1,000 °C. Rocks from both fast- and slow-slipping transforms show evidence of interactions with seawater up to temperatures of at least 900 °C. Here we present a model for the mechanical structure of oceanic transform faults based on fault thermal structure and the impacts of hydration and metamorphic reactions on mantle rheology. Deep fluid circulation is accounted for in a modified friction-effective pressure law and in ductile flow laws for olivine and serpentine. Combined with observations of grain size reduction and hydrous mineralogy from high-strain mylonites, our model shows that brittle and ductile deformation can occur over a broad temperature range, 300–1,000 °C. The ability of seawater to penetrate faults determines whether slip is accommodated at depth by seismic asperities or by aseismic creep in weak, hydrous shear zones. Our results suggest that seawater infiltration into ocean transform faults controls the extent of seismicity and spatiotemporal variations in the mode of slip.