Deep hydration and lithospheric thinning at oceanic transform plate boundary
Transform faults accommodate the lateral motions between lithospheric plates, producing large earthquakes. Away from active transform boundaries, former oceanic transform faults also form the fracture zones that cover the ocean floor. However, the deep structure of these faults remains enigmatic. I will present the ultra-long offset seismic data from the Romanche transform fault in the equatorial Atlantic Ocean that indicates the presence of a low-velocity anomaly extending to ~60 km depth below sea level. Three-dimensional thermal modelling suggests that the anomaly is probably due to extensive serpentinization down to ~16 km depth, overlying a hydrated, shear mylonite zone down to 32 km depth. The water is considered to be sourced from seawater-derived fluids that infiltrate deep into the fault. Below 32 km depth is interpreted to be a low-temperature, water-induced melting zone that elevates the lithosphere–asthenosphere boundary, causing substantial thinning of the lithosphere at the transform fault.
