6:15 PM - 7:30 PM
[SSS02-P08] Incoming materials for an erosional subduction zone, offshore Osa Peninsula, Costa Rica
Drilling recovered approximately 100 m of sediment and 70 m of igneous oceanic basement. The sediment includes two lithologic units: hemipelagic clayey mud (unit 1) and siliceous to calcareous pelagic ooze (unit 2). The hemipelagic unit is composed of clay minerals (~50 wt.%), quartz (~5 wt.%), plagioclase (~5 wt.%), calcite (~15 wt.%) and ~30 wt.% of amorphous materials, while the pelagic unit is mostly made up of biogenic amorphous silica (~50 wt.%) and calcite (~50 wt.%). The igneous basement rock is composed of plagioclase (~50-60 wt.%), clinopyroxene (~>25 wt.%), and saponite (~15-40 wt.%). Saponite is more abundant in pillow basalt than in the massive section, reflecting the variable intensity of alteration. Fluid expulsion models shows that sediment compaction during shallow subduction causes the release of pore water while peak mineral dehydration occurs at temperatures of approximately ~100℃. The subducting sediment has the potential to generate fluid overpressure at distances of between 5 and 70 km landward of the trench.