Evidence for a Deep Hydrologic Cycle on Oceanic Transform Faults

Oceanic transform faults represent abundant yet relatively unexplored components of the hydrologic cycle in the mantle lithosphere. Current models limit fluid circulation to 600 °C, the thermal limit of earthquakes recorded by teleseismic surveys. However, recent ocean‐bottom seismic surveys have located earthquakes at depths corresponding to >1000 °C in modeled thermal structure. To constrain the depth extent of brittle deformation and fluid infiltration, we analyzed peridotite mylonites dredged from the Shaka Transform Fault, Southwest Indian Ridge. Samples range from high strain mylonites that preserve ductile microstructures to lower strain mylonites that are fractured and overprinted by hydrothermal alteration. Microstructural analysis of the high strain samples reveals brittle deformation of pyroxene concomitant with ductile deformation of olivine and growth of amphibole. Porphyroclasts preserve healed fractures filled with fluid inclusions, implying repeated episodes of fracture, fluid infiltration, and healing. The association of hydration features with brittle structures points to seawater, rather than melt, as the fluid source. Textural analysis indicates that strain localization was initiated by grain boundary pinning and that olivine grain size was reduced to ~1 μm in the presence of amphibole. Comparing the amphibole stability field to thermometry estimates for the limit of recrystallization suggests that fluid flow extended to ~650–850 °C. Our results indicate that the hydrologic cycle extends past the brittle‐ductile transition and promotes strain localization via hydrolytic weakening and hydration reactions. We propose that seawater infiltration on oceanic transform faults is driven by the seismic cycle and represents a first order control on the rheology of the oceanic lithosphere.