3D seismic reflection imaging and wide-angle velocity structure of nearly amagmatic oceanic lithosphere at the ultra-slow spreading Southwest Indian Ridge
IPGP - Îlot Cuvier
Séminaires Géosciences Marines
IPGP, Géosciences Marines
Mantle-derived peridotites observed at nearly amagmatic domains of the easternmost Southwest Indian Ridge are proposed to have been exhumed along the footwall of successive detachment faults. I shall present results from an integrated geophysical study of data collected during the SISMOSMOOTH 2014 cruise to understand the nature of the oceanic crust formed at this end-member mid-ocean ridge environment. The results presented here are from 3D processing of a cluster of 2D seismic data acquired along 100 m-spaced profiles in a 1.8 km wide by 24 km long box spanning the axial valley. Three distinct packages of reflectors are imaged: (1) south-facing reflectors which may correspond to the damage zone induced by the active axial detachment fault. We propose that this damage zone may widen as detachment fault offset increases and as the fault migrates toward the hanging wall plate. 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 events in the hanging wall of the detachment fault. These reflectors may correspond to locally enhanced serpentinization of the peridotite along damage zones related to the previous, north-dipping detachment fault. These may also have provided pathways for melt feeding sparse volcanic eruptions on the seafloor; (3) discontinuous but coherent flat-lying reflectors at shallow depths (< 1 km below the seafloor) and at depths between 4 and 5 km. Comparing these deeper flat lying reflectors with the wide-angle velocity model obtained from ocean bottom seismometers (OBSs) data next to the 3D box shows that they correspond to the lower crust and upper mantle, as defined on the basis of the seismic velocity (6.5-8 km/s). The 4-5 km crustal velocity layer is interpreted as primarily due to serpentinization and fracturation of the exhumed mantle-derived peridotites in the footwall of active and past detachment faults.