Full Waveform Inversion Reveals Trans-Tensional-Regime at the Romanche Oceanic Transform Fault in the Equatorial Atlantic Ocean

Summary Oceanic transform faults (TFs) are fundamental elements of plate tectonics and have traditionally been viewed as conservative strike-slip boundaries. Seafloor observations and numerical modelling 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. We use source-receiver reciprocity and downward continuation to emulate a split-spread ocean bottom cable survey geometry from one-sided surface streamer data, bringing the refracted waves ahead of reflections while accounting for rough seafloor topography. We then perform travel time tomography followed by FWI to the downward-continued data to derive a high-resolution crustal model. The resolution is about 0.7 km horizontally and 0.4 km vertically, down to 3.5–4 km depth from the seafloor. 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 on the valley floor. Moreover, we image inward dipping normal faults extending to ∼4 km depth, forming a flower-like structure. Regional earthquake data reveal strike-slip mechanisms along the transform and normal-faulting near the RTI, 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, accommodating local strain deformation.