Vertical tectonics and the origins of BSRs along the Peru margin

The distribution of bottom simulating reflections (BSRs), which mark the base of the gas hydrate zone on marine seismic records across the Peru margin, is examined with respect to tectonic uplift, sedimentation, and carbon content of sediment. Where accretion of only deep ocean pelagic sediment containing less than 1% carbon is seismically imaged, no BSR is observed. No BSR is apparent on seismic records of carbon-rich (4¯8%) sediment of rapidly subsiding basins. BSRs are abundant on records of carbon-rich sediment affected by tectonic uplift and/or rapid sedimentation. In areas of uplift, the bottom simulating reflectors contain small amounts of free gas in contact with sediment containing small amounts of gas hydrate. During uplift, free gas is released from gas hydrates by dissociation as the hydrate phase boundary shifts upward with respect to sea level. The strong effects of free gas in reducing seismic velocity and gas hydrate in increasing velocity can explain the relatively small total quantities of gas and hydrate indicated by observations of BSRs off Peru.