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

Abstract

AbstractWe present results from 3‐D processing of 2‐D seismic data shot along 100 m spaced profiles in a 1.8 km wide by 24 km 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° and are visible down to 5 km 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 (<1.5 km below the seafloor), and at depths between 4 and 5 km 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 6.5–8 km/s, suggesting that they occur in the transition between lower crust and upper mantle. The 4–5 km 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.