Three-dimensional S-wave velocity structure of oceanic core complexes at 13N on the Mid-Atlantic Ridge

Abstract

13°N on the Mid-Atlantic Ridge is regarded as a type site for oceanic core complexes (OCCs). Within ~70 km along the spreading centre, it hosts four OCCs in different stages of their life cycle making this an ideal location to determine how OCCs are formed, and what drives the hydrothermal circulation that sustains the vent fields associated with them. Here we describe the results of S-wave seismic tomographic modelling within a 60 x 60 km footprint containing several OCCs, the spreading centre and both flanks. A grid of 17 wide-angle seismic refraction profiles was shot within this footprint and recorded by a network of 46 ocean-bottom seismographs (OBS). Approximately 6200 S-wave arrival travel times have been modelled, constraining primarily the velocity-depth structure of the upper-to-mid-crust. Depth slices through the resulting 3-D S-wave velocity (Vs) model reveal the OCCs located at 13°20’N and 13°30’N to each have a region of relatively low Vs (<3 km s-1 ) beneath its detachment, and a higher Vs (>3 km s-1 ) in the inter-OCC basin and regions surrounding the detachments. Using the equivalent 3-D P-wave velocity (Vp) model of Simão et al. (2020), the corresponding Vp/Vs model is calculated to investigate lithology, permeability and the existence of any off-axis magmatic intrusions that may drive fluid flow. The Vp/Vs model clearly shows that the crust beneath the deep lava-floored inter-OCC basin is characteristically oceanic (Vp/Vs ratio of <1.85) in velocity-depth structure, in contrast to the OCCs themselves which have a Vp/Vs ratio of >1.85, suggesting that they formed under magma poor (tectonic) conditions. The Vp/Vs model also shows that the OCCs are not connected, at least to mid-crustal level. Alternatively, if the OCCs lie on the same detachment surface, that surface would have to undulate >3 km in amplitude over a distance of <20 km for these OCCs to appear to be unconnected. Our 3-D Swave and Vp/Vs models thus support MacLeod et al.’s (2009) model of localized OCC evolution. Our S-wave velocity model also suggests that the Irinovskoe (13°20’N) and Semyenov (13°30’N) vent fields have different hydrothermal circulation drivers, with the Semyenov field being driven by magma intrusion(s) and the Irinovskoe field being driven by the spreading centre thermal gradient and pervasive flow along open permeability within the detachment footwall, perhaps further opened by roll-over to lower dip angle as it exhumes at the seabed.