Lithospheric thinning due to hydration and melting at oceanic transform plate boundaries

Abstract

Transform plate boundaries, one of the key elements of plate tectonics, accommodate lateral motions and produce large earthquakes, but their nature at depth remains enigmatic. Using ultra-long offset seismic data, here we report the presence of a low-velocity anomaly extending down to ~60 km depth beneath the Romanche transform fault in the equatorial Atlantic Ocean. Our result indicates the presence of deep penetration of water leading to extensive serpentinization down to 16 km, followed by a shear mylonite zone down to 32 km over a low-temperature water induced-melting zone, elevating the lithosphere-asthenosphere boundary and hence thinning the lithosphere significantly beneath the transform fault. The presence of a thinned lithosphere and the melt underneath could lead to volcanism, migration and mixing of the water-induced melt with the high-temperature melt beneath the ridge axis, and small-scale convections beneath transform boundaries. Hence, a thinned lithosphere will have a major impact on the dynamics of ridge-transform system, and will influence the evolution of fracture zones and oceanic lithosphere.

CO2 degassing in the mantle triggers deep earthquakes at the Mid-Atlantic Ridge

“…We constructed five 1-D P-wave velocity models (Extended Data Fig. 3a), derived from an active-source wide-angle seismic refraction profile 45 , which provides velocity constraints at depths down to ~60 km below sea level. They include models beneath the northern flank, the transform valley, and the southern flank of the Romanche TF, a low-velocity model beneath the transform valley, and an average velocity model from north to south across the Romanche TF (Extended Data Fig.…”

“…Our results show that most deep axial events are located between 10 and 20 km, whose depth shifts are smaller than the average depth uncertainties (~2.6 km) in all cases (Supplementary Table 2). However, the fastest model (Model 1), which is derived from the southern Romanche TF 45 , leads to some shallow events in the crust beneath the axial valley (Extended Data Fig. 6d).…”