Deep structure of the southern Kerguelen Plateau (southern Indian Ocean) from ocean bottom seismometer wide‐angle seismic data

Abstract

Wide‐angle seismic data collected during the Kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern Kerguelen Plateau and support a new interpretation of the origin of the plateau. Velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22‐km‐thick crust composed of ∼1.6 km of sedimentary cover, ∼5.3 km of upper crust, ∼11.0 km of lower crust, and a 4‐ to 6‐km‐thick reflective zone immediately above Moho. Velocities in the upper crust (from 3.8–4.5 km/s at top to 6.0–6.5 km/s at bottom) are consistent with the basaltic nature of this layer, the top of which was sampled during the Ocean Drilling Program. Velocities in the lower crust increase continuously from 6.60 km/s at the top to 6.90 km/s at 19.5 km depth. The reflective zone at the base of the crust identified by wide‐angle reflections is observed only along the NNW‐SSE direction. It consists of alternating high‐ and low‐velocity layers with an average velocity of 6.70 km/s in the NNW‐SSE direction and ≥6.90 km/s in the perpendicular direction. Strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8.60 and 8.00 km/s, in the NNW‐SSE and E‐W directions, respectively. The absence of high velocities at the base of the crust that characterizes many large‐volume mafic provinces, the reflective lower crust, and anisotropy in upper mantle suggest that the southern Kerguelen Plateau represents a stretched continental fragment overlain by basaltic flows isolated from the Antarctic margin during the early opening of the Indian Ocean.