Seismic observations in the eastern Romanche ridge-transform intersection (Equatorial Atlantic) from microseismicity data and seismic tomography

Large-offset oceanic transform faults (OTFs) juxtapose warm ridge segments against cold lithosphere and induce extreme thermal gradients in the lithosphere as well as structural and petrological complexities around the Ridge-Transform Intersection (RTI). However, its real thermal structure and its effects on the earthquake and magmatic processes remain unknown. Here we present seismological observations from the eastern Romanche OTF in the equatorial Atlantic Ocean that offsets the slow-spreading (2 cm/ yr) Mid-Atlantic Ridge (MAR) by 900 km with an age offset of 45 Ma. A temporary network of 19 ocean-bottom seismometers, covering a one-month period, was deployed during the SMARTIES cruise in 2019. As a result, 514 microearthquakes were automatically detected and precisely located based on high-quality picked P- and S-wave arrival times, and they were then used to obtain high-resolution three-dimensional P- and S-wave velocity models and Vp/Vs models of the crust and upper mantle beneath the study region. We find that: (i) Microearthquakes along the Mid-Atlantic Ridges (MAR) are mostly observed within the axial valley, with focal depths of 5 -24 km below sea level. They show an undulating focal depth distribution from north to south, indicating significant variations in the lithosphere and thermal structure along the rift axis. A seismic gap in the crust and upper mantle is observed at the extreme oblique ridge section when approaching the RTI. (ii) Unexpectedly, an earthquake cluster is observed west of the ridge axis, showing a shallower focal depth than that observed beneath the axial valley, in an area where volcanic cones were mapped, suggesting that recent magmatic activity may be taking place off the ridge axis. (iii) Tomographic results reveal high Vp/Vs ratios, ~1.8-1.9, at depths of 0-10 km below seafloor (bsf) beneath the oblique bathymetric highs along the Romanche OTF, implying the possible serpentinization of the mantle rocks, which is consistent with the serpentinized peridotites sampled by the submersible dives. However, low Vp/Vs ratios, ~1.6, are widely observed at depths of 5-15 km bsf beneath the RTI and the large-scale oceanic core complex identified from the high-resolution bathymetry data. Most of the recorded microearthquakes seem more likely to occur in the area with low Vp/Vs ratios (~1.6).