During Leg 205 of the Ocean Drilling Program (ODP) two boreholes were drilled into active hydrologic formations on the Costa Rica margin west of the Nicoya Peninsula. One borehole penetrated through the overriding plate into the décollement at ODP Site 1255. The other borehole penetrated through the subducting sediment section and plate into permeable igneous basement at ODP Site 1253. These two boreholes were sealed and instrumented with a borehole observatory (CORK), allowing pressure, temperature, fluid flow velocity, and fluid chemical composition to be measured within the formation. The 1.5-yr pressure and two-year temperature, fluid velocity, and fluid chemical composition records collected to date have provided a basic knowledge of formation properties, although fluids within the boreholes had yet to reach steady state with the surrounding formations. The data also provide evidence that tectonic forcing related to subduction results in measurable transients in pressure, temperature, fluid velocity, and fluid composition within the décollement. While the initial data provide a baseline for approaching several important scientific and technical questions, they have raised new key questions that will be addressed from additional continuous borehole data. The investigators will a 6-day submersible operation to retrieve instruments and stored data that will provide a continuous record of formation temperature, pressure, fluid flow rate, and chemical composition for an additional five years, from the time of the last visit in 2004 to 2009. This project will result in an extremely valuable long-term synchronous record of hydrologic, geochemical, and geodynamic activity at this subduction zone, and establish a technical and scientific foundation for future borehole studies in a broad range of tectonically and hydrologically active settings.
