Oceanic transform faults are a major category of plate boundary, yet the mechanisms accommodating slip across the plate interface are poorly constrained. Some fault sections have large, quasi-periodic earthquakes, while other sections act as barrier zones yet have abundant microseismicity. I will present observations of dredge samples from these faults that preserve evidence of seawater circulation, syn-deformation reactions, and brittle and ductile deformation to temperatures >900 °C. In the deep part of the fault, the formation of high strain, ductile mylonites may be driven by reactions with seawater. In the shallow part of the fault, the formation of hydrated breccias from pillow basalts may be key to controlling the mode of slip. Seawater infiltration in oceanic transform faults thus provides a mechanism to drive spatiotemporal variations in slip behavior within otherwise uniform oceanic lithosphere.

1. Deformed rocks from transform faults interacted with seawater up to >900 °C.
2. Grain size reduction and hydrous mineral formation allow brittle and ductile deformation over a broad temperature range of 300–1,000 °C.
3. Formation of hydrated breccias may control the shallow slip distribution.
4. Seawater circulation provides a mechanism to modify slip behavior within the oceanic lithosphere.

Jessica Warren is an associate professor in the Department of Earth Sciences at the University of Delaware. Her research explores the rheology and geochemistry of the Earth’s upper mantle, with a focus on peridotite samples collected at mid-ocean ridges and transform faults. She holds degrees in Natural Sciences from the University of Cambridge and a PhD from the MIT/WHOI Joint Program. She has participated in and lead numerous field expeditions, including seagoing expeditions in the Pacific, Atlantic, and Indian Oceans.