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 signi cantly 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 in uence the evolution of fracture zones and oceanic lithosphere.

<p>Global tomographic models depict long-wavelength azimuthal anisotropy in the oceanic upper mantle, with a fast axis direction orthogonal to divergent plate boundaries. This anisotropy is usually attributed to the Lattice Preferred Orientation (LPO) of olivine due to asthenospheric mantle flow away from the ridge axis. In this work, we want to test an alternative hypothesis, whether this observed anisotropic signal could be partially explained by the presence of transform faults and associated fracture zones in the lithosphere. The transform plate boundaries represent sharp structures perpendicular to the ridge-axis with the wavelength (˜10 km), which is much smaller than the wavelength of seismic surface waves used to image the mantle (˜100 km). Therefore, transform faults could potentially result in an effective anisotropy in tomographic images through their Shape Preferred Orientation (SPO). We base our calculations on several thermo-chemical models that follow the observed ridge-transform geometry at different spreading rates. To produce the effective medium as seen by long-period waves, we use a non-periodic homogenization algorithm. The resulting seismic velocity field can be interpreted as the tomographic image that would be obtained after inverting long-period seismic data; it is smooth, fully anisotropic, and comparable to actual tomographic models.</p>