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

Abstract

We 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.

Internal Structure of the Oceanic Lithosphere at a Melt‐Starved Ultraslow‐Spreading Mid‐Ocean Ridge: Insights From 2‐D Seismic Data

“…While the fault damage zones at mid‐ocean ridge systems documented in the geological literature are narrow (<400 m), for example, (Hayman & Karson, ), our results confirm findings of Momoh et al () and indicate that a thicker damage zone is likely even for short‐offset faults (present offset at young D1 axial detachment fault is estimated as <4 km; Cannat et al, ). Following on the discussion in Momoh et al (), we propose that this young yet thick damage zone may have formed in three ways: (1) due to distributed simultaneous deformation over a thick domain, (2) as a finite damage zone due to strain localizing on a succession of subparallel individual fault segments, or (3) by a combination of both styles. Numerical experiments incorporating the proportion of plate divergence taken by magmatic diking versus tectonic extension (the M factor of Buck et al, ) predict that when the proportion of melt supply is <50%, the fault system tends to migrate toward the hanging wall.…”

“…The nature of the south‐dipping reflectors in the axial valley indicates that the highly reflective parts are laterally offset from the inferred trace of the emergence of the active detachment fault (e.g., Figures b–d). Similar reflectors were found in the narrow 3‐D box (Momoh et al, ) as shown in Figures and S8. Surrounding the very reflective parts of the south‐dipping reflectors are weakly reflective events with similar dips (Figures c and ).…”

“…(a) SMOO 2 on the western boundary of the melt‐poor corridor. (b) SMOO 3 within the corridor, bordering the narrow 3‐D box on the west presented in Momoh et al (). (c) SMOO 33, the combination of three seismic profiles shot in close proximity, 200 m east of the 3‐D box.…”

“…The top panel shows the depth‐converted section of the combined lines (SMOO 33) using the refraction velocity model (Momoh et al, ) obtained on a coincident profile (Figure ) with the bathymetry overlain. The inferred breakaway (B) and emergence (E) of present and past detachments are as indicated in Figures and .…”

“…In the hanging wall next to this damage zone, the seismic crust (defined by the compressional wave velocity model <7.5 km/s) is thicker (5 km) than further away from it (4 km). This crust has been described as a downward gradient of serpentinization with tectonic damage and small magmatic intrusions (Momoh et al, ). Several dipping and subhorizontal reflectors were also imaged in the crustal portion of the hanging wall of the active detachment fault and shallow mantle of the axial valley region and interpreted as minor offset faults and magmatic rocks trapped in the ultramafic basement (Momoh et al, ).…”