Evidence for a thick free gas layer beneath the bottom simulating reflector in the Makran accretionary prism

Abstract

Seismic reflection data from the Makran continental margin indicate the presence of a strong and widespread bottom simulating reflector (BSR). We apply a nonlinear full waveform inversion technique to multichannel reflection data from this area, to investigate the detailed velocity structure and hence the origin of the BSR. Our result shows an abrupt decrease in the compressional wave velocity from 2.2 to 1.3 km/s at a depth of 500 m below the sea-bed. The low velocity zone is unusually thick (~200–350 m), and may contain large quantities of free gas, similar to some of the recently drilled Blake Ridge sites of ODP Leg 164. The voluminous free gas may have been generated by the dissociation of gas hydrates as a consequence of the upward movement of the base of gas-hydrate stability field, relative to the sediment column, due to uplift and sedimentation in the accretionary wedge.