2-D Oceanic Core Complex Structure at the Semenov hydrothermal field on the Mid-Atlantic Ridge from New Wide-Angle Seismic Data

We presented results from a new seismic refraction experiment at the Semenov hydrothermal area at 13°30’ N on the western flank of Mid-Atlantic Ridge. The survey was carried out on Cruise JC254 on RRS James Cook in November 2023. Semenov is a typical ultramafic-hosted field consisting of five active and extinct hydrothermal sites (Semenov-1 to 5) associated with massive sulphide mounds (SMS), hosted on a 20-km-long oceanic core complex (OCC). The OCC offers an exceptional opportunity to observe deep-seated ultramafic rocks exposed on the seafloor by detachment faulting. Semenov field is an ideal location to investigate the link between OCC-related detachment and SMS deposit formation. Here, we aim to determine the Semenov OCC crustal structure, geometry, and extent of subseafloor SMS mineralisation.

Our seismic refraction survey revealed a detailed 2-D P-wave velocity structure beneath Semenov. We target the Semenov-3 and Semenov-4 hydrothermal sites sitting at either side of the seabed termination of the detachment fault. We focused on profiles crossing the OCC in E-W and N-S directions, shot with two GI guns (250G and 105I cubic inch) every 30 m. The data were recorded by a network of 18 ocean bottom nodes (OBX) at 0.4 to 1 km spacing, showing clear first-arrival refractions from beneath the OCC and the hanging wall. We expect to define the seismic structure down to 2 km beneath the seabed. The derived velocity model could give information to the lithology beneath the Semenov OCC-related detachment and possibly driving source for the hydrothermal circulation. Lastly, we compare our result with another detachment-related hydrothermal system at TAG, where the OCC is thought to be at the initial stage and the hydrothermal system is basalt-hosted.