Hydrogeological responses to incoming materials at the erosional subduction margin, offshore Osa Peninsula, Costa Rica

Bulk mineral assemblages of sediments and igneous basement rocks on the incoming Cocos Plate at the Costa Rica subduction zone are examined by X-ray diffraction analyses on core samples. These samples are from Integrated Ocean Drilling Program Expedition 334 reference Site U1381, similar to 5 km seaward of the trench. Drilling recovered approximately 100 m of sediment and 70 m of igneous oceanic basement. The sediment includes two lithologic units: hemipelagic clayey mud and siliceous to calcareous pelagic ooze. The hemipelagic unit is composed of clay minerals (similar to 50 wt.%), quartz (similar to 5 wt.%), plagioclase (similar to 5 wt.%), calcite (similar to 15 wt.%) and similar to 30 wt.% of amorphous materials, while the pelagic unit is mostly made up of biogenic amorphous silica (similar to 50 wt.%) and calcite (similar to 50 wt.%). The igneous basement rock consists of plagioclase (similar to 50-60 wt.%), clinopyroxene (similar to>25 wt.%), and saponite (similar to 15-40 wt.%). Saponite is more abundant in pillow basalt than in the massive section, reflecting the variable intensity of alteration. We estimate the total water influx of the sedimentary package is 6.9 m(3)/yr per m of trench length. Fluid expulsion models indicate that sediment compaction during shallow subduction causes the release of pore water while peak mineral dehydration occurs at temperatures of approximately similar to 100 degrees C, 40-30 km landward of the trench. This region is landward of the observed updip extent of seismicity. We posit that in this region the presence of subducting bathymetric relief capped by velocity weakening nannofossil chalk is more important in influencing the updip extent of seismicity than the thermal regime.