THE NATURE AND DISTRIBUTION OF BOTTOM SIMULATING REFLECTORS AT THE COSTA-RICAN CONVERGENT MARGIN

Bottom simulating reflectors (BSRs) at the base of the hydrate stabili ty zone are often observed in marine seismic reflection data. The comp ressional (P-)wave velocity structure of a BSR offshore Costa Rica was analysed by applying a full-waveform inversion technique. The resulti ng velocity profile indicates that the BSR is caused by a thin low-vel ocity layer, suggesting the presence of at least a small amount of fre e gas in the sediment pore space. In undisturbed sediment sections, BS Rs offshore Costa Rica are observed over much of the continental margi n. They frequently occur at shallow depths beneath the seafloor, usual ly similar to 100-400 m; at some locations BSRs appear to intersect th e seafloor. This was interpreted as an indication that hydrates in the study area form partly from methane which was produced beneath the hy drate stability zone and migrated upwards. In the study area, the vert ical movement of the hydrate stability zone relative to a given point in the sediment column, one of the potential factors leading to BSR fo rmation or suppression, is controlled by both sedimentation and vertic al tectonism. Both processes may hence play a role in controlling BSR distribution. BSRs are absent in areas which have been affected-by slu mping, except where the sedimentary section above the BSR remained int act during slumping. This indicates that slope failure can cause the d estruction of BSRs.