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

Abstract

Bulk mineral assemblages of sediments and igneous basement rocks on the incoming CocosPlate 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, 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 (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 consists 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. 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 1008C, 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.

Lithologic control of frictional strength variations in subduction zone sediment inputs

“…14). Some samples from Costa Rica exhibit both high friction and low calcite content of <20%; these samples likely contain a significant amount of amorphous silica (Spinelli and Underwood, 2004;Kameda et al, 2015b). In general, the lithologic contrast between the hemipelagic sediments and the underlying oozes and chalks observed offshore both Nicoya and Osa Peninsulas are also associated with a significant strength contrast (Fig.…”

Alteration and dehydration of subducting oceanic crust within subduction zones: implications for décollement step-down and plate-boundary seismogenesis

“…The clay phases within deeper portions of this drillhole were originally mixed-layer C/S or chlorite, meaning that these deeper sections are unlikely to have undergone bulk dehydration. Although the bulk-rock saponite content of the samples obtained during this drilling was not determined, some 20-40 wt% of saponite was identified within the upper 70 m of basement section (i.e., depth from the bottom of the hole upward) of the presubduction Cocos Plate at the Costa Rica margin (Kameda et al 2015). This suggests that breakage can occur within this saponite-bearing dehydrating section, triggered by dehydration-induced weakening of the oceanic crust.…”

Texture Development of Clay‐Rich Sediments Across the Costa Rica Subduction Zone

“…The composition of the pelagic calcareous ooze, in contrast, differs largely. Kameda et al () did not detect clay and Spinelli and Underwood () determined trace amounts to 6 wt. % clay, while our samples provide 15–19 wt.…”

“…Consequently, the samples in this study show a lower calcite content than the other studies (Spinelli & Underwood, : 83–92 wt. %; Kameda et al, : 85 wt. %; this study: 54–71 wt.…”

“…The quantitative mineralogical composition of the sediments from the incoming Cocos Plate has been subject to several studies (e.g., Kameda et al, ; Spinelli & Underwood, ). Regarding the hemipelagic sediments, these studies are largely comparable to what we found.…”