Seismic properties of gabbroic sections in oceanic core complexes: constraints from seafloor drilling

The physical properties of rock within Oceanic Core Complexes (OCC) provide information about the history of magmatism, deformation and alteration associated with detachment faulting and unroofing of their gabbroic sequences. New core and logging data from Atlantis Bank (AB) OCC are compared with prior deep sea drilling data from this site on the Southwest Indian Ridge, as well as from Atlantis Massif (AM) OCC on the Mid-Atlantic Ridge. The average seismic velocity at each site is typical for gabbroic rock. Downhole core and log variations are found to depend on both porosity and alteration. Porosity at millimeter to meter scale impacts P-wave velocities (6.0–6.4 km/s) in the upper several hundred meters at both OCC. Below ~ 600–800 m depth, in situ velocities are higher (6.6–6.9 km/s) for the remainder of the 1.5 km drilled to date, except in zones where alteration is significant. Variability of 0.5–1 km/s over depth intervals of a few meters is persistent where mineral alteration exceeds 10–20%, whereas variability drops to ± 0.1 km/s for intervals affected by little to no alteration. Seismic anisotropy is negligible overall, with just a few intervals showing consistent fast azimuth and magnitude of a few percent. The depth extent of alteration, probably related to the zone deformed below the detachment when active, is ~ 600–800 meters at both AB and AM.