Seismic Evidence for Trans-Tensional-Regime at the Romanche Oceanic Transform Fault in the Equatorial Atlantic Ocean

Oceanic transform faults (TFs) are fundamental elements of plate tectonics and have traditionally been viewed as conservative strike-slip boundaries. Seafloor observations and numerical modeling suggest the existence of extensional stress, however how it manifest at depth remains unknown. Moreover, slow-slipping TFs are often associated with thin crust and possible exposures of serpentinised peridotite near the seafloor. Here we apply full waveform inversion (FWI) to a 12-km offset seismic dataset across the Romanche TF, the largest TF on the Earth. The TF along our profile contains 20-km-wide 6 km deep valley with inward steeply dipping bounding faults. Given the steep seafloor topography, we first enhance the refracted waves by applying source-receiver reciprocity and downward continuation to the surface streamer data to mimic an ocean bottom cable survey geometry. We then perform trace-normalized FWI to derive a high-resolution crustal model. Our results reveal low P-wave velocity in the upper 3 km, suggestive of basaltic origin, and no evidence for high velocities characteristic of serpentinised peridotite beneath the valley floor. Moreover, we image inward dipping normal faults extending down to ~4 km depth below the seafloor, forming a flower-like structure. Regional earthquake data reveal both strike-slip and normal-faulting, with strike-slip hypocenters aligning with interpreted faults. These features suggest that the Romanche TF resembles a trans-tensional regime with a deep-rooted strike-slip fault in the middle, and complex faulting in the transform valley, accommodating both plate-scale and local strain deformation.