Seismic Investigation of the Décollement Offshore Costa Rica

The convergent margin offshore Costa Rica is the focus of the investigations within the SFB 574 "Volatiles and Fluids in Subduction Zones: Climate Feedback and Trigger Mechanisms for Natural Disasters." Within the subproject A2, the main objectives are the coupling and the mass transfer between overlying and downgoing plate. We present a detailed seismic study of the near trench plate boundary, just 3 km landward of the Middle American Trench, located between the Quepos Plateau and the Cocos Ridge. In this area, previously recorded dip-profiles showed a very bright décollement reflection of negative polarity within the first five kilometres landward of the trench. Along strike, the zero-offset reflection strength of the décollement shows a high lateral variability. To further investigate these lateral variations, we deployed ten ocean-bottom-stations on a profile of 1.5 km length during the cruise 173 of RV Sonne in July 2003. Due to the shallow depth of the décollement (around 700 m below seafloor), we are able to observe reflected waves at angles up to 50 degrees. This will enable us to describe the amplitude-variations with angle in detail and will allow us to further investigate the lithologic parameters causing this variations.