Quasi-3-D Seismic Reflection Imaging and Wide-Angle Velocity Structure of Nearly Amagmatic Oceanic Lithosphere at the Ultraslow-Spreading Southwest Indian Ridge

We present results from 3-D processing of 2-D seismic data shot along 100m spaced profiles in a 1.8km wide by 24km long box during the SISMOSMOOTH 2014 cruise. The study is aimed at understanding the oceanic crust formed at an end-member mid-ocean ridge environment of nearly zero melt supply. Three distinct packages of reflectors are imaged: (1) south facing reflectors, which we propose correspond to the damage zone induced by the active axial detachment fault: reflectors in the damage zone have dips up to 60 degrees and are visible down to 5km below the seafloor; (2) series of north dipping reflectors in the hanging wall of the detachment fault: these reflectors may correspond to damage zone inherited from a previous, north dipping detachment fault, or small offset recent faults, conjugate from the active detachment fault, that served as conduits for isolated magmatic dykes; and (3) discontinuous but coherent flat-lying reflectors at shallow depths (<1.5km below the seafloor), and at depths between 4 and 5km below the seafloor. Comparing these deeper flat-lying reflectors with the wide-angle velocity model obtained from ocean-bottom seismometers data next to the 3-D box shows that they correspond to parts of the model with P wave velocity of 6.5-8km/s, suggesting that they occur in the transition between lower crust and upper mantle. The 4-5km layer with crustal P wave velocities is interpreted as primarily due to serpentinization and fracturation of the exhumed mantle-derived peridotites in the footwall of active and past detachment faults.