Gas Hydrate Dissociation Triggering Mass Wasting on the Costa Rica Continental Margin

The continental margin off Costa Rica is an area of intense deformation and slope instability as recent mapping and sampling efforts have demonstrated. Several processes are simultaneously modifying the overall stability of the continental slope. Convergence-related seamount subduction and subduction erosion are the primary reasons for the large scale removal of slope sediments and consequent collapse of the lower continental slope. A second type of slope failures and slides of smaller scale was found to be related to the dissociation of methane hydrate near the sea floor. Several small scale failures of this type were discovered and sampled during recent expeditions of the SFB574 research group in Kiel. One exceptionally well imaged slide NE of Jaco Scar on the upper continental slope was surveyed and sampled in detail to evaluate a possible connection to shallow BSR occurence in this area. The slide was discovered on stacked wide-angle reflection seismic data acquired in 1999. In this area of the Costa Rica margin slope sediments are characterized by the occurrence of laterally continuous BSRs forming a laterally continous patch, where the BSR can be traced and mapped over several tens of kilometers. At the landward termination of the patch the BSR is converging towards the seafloor at a depth of about 580 m - coincident with the local minimum water depth for hydrate stability. The headwalls of a series of slides are located at this depth, suggesting a close genetic relationship with hydrate breakdown.. Data from ongoing sedimentological, geochemical and geotechnical studies on core material recovered immediately in front of the headwall of the slide will be presented. This data together with geophysical information such as Parasound and bathymetric data is utilized to constrain the mechanics, geometry and mass balance of these slides.