High-resolution upper crustal structure from OBH data at the TAG Hydrothermal Field, 26°N on the Mid-Atlantic Ridge

The Trans-Atlantic Geotraverse (TAG) segment at 26°N on the Mid Atlantic Ridge (MAR) is notable for hosting hydrothermal mounds and seafloor massive sulphide deposits. At the slow-spreading MAR, detachment faulting plays an important role in controlling the seafloor morphology. In this study, we investigate the seismic velocity in the upper crust at a finer scale than previously possible, and its relationship to fault structures.

We used short-offset ocean bottom hydrophone (OBH) data collected during the Meteor 127 cruise in 2016. The survey was designed mainly to study the hydrothermal mounds. We chose a NW-SE trending, 11-km long wide-angle seismic profile that crosses a detachment breakaway identified from AUV bathymetry and seismic reflection profiles. The source was a G-gun array of 760 c. inch towed at 6 m depth. The shot spacing was 12 s (15-20 m) with four OBHs at 1.3 km spacing.

A two-dimensional P-wave velocity model was generated by first-arrival travel-time tomography using the TOMO2D code. We used as our starting model the average 1D velocity depth function of a slice along our profile through Zhao et al’ s (2012) three-dimensional velocity model. Our final tomographic model reveals crustal velocities from 3.4 km/s to 5 km/s for the upper 600 m below seabed. Most of the profile lies beneath the eastern valley wall, where a corrugated detachment surface crops out. Beneath the detachment surface in our profile, we observed an increased velocity of 6.5 km/s at 1.5 km below seabed. Our velocity model suggests that the west-dipping normal fault exhumes lower crust of velocity up to 6.5 km/s.