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doi:10.2204/iodp.proc.344.209.2018

Study sites

Site U1378 was drilled during Expedition 334 in the middle slope of the Costa Rica margin, ~38 km offshore the Osa Peninsula and 24 km from Caño Island (Fig. F1). This location is situated above an unlocked portion of the plate boundary, as defined by interpolate earthquake relocation and geodetic measurements, and the upper plate sediment representing the older framework wedge is overlaid by ~750 m of slope sediments (LaFemina et al., 2009; Expedition 334 Scientists, 2012a; Vannucchi et al., 2016). This older framework wedge, however, was not reached during drilling at Site U1378 or at complementary Hole U1380A because of unfavorable drilling conditions (Expedition 334 Scientists, 2012b). The sediment composition at Site U1378 is marked by a monotonous sequence of silty clay to clay alternating with widely interspersed centimeter-scale sandy layers. Hole U1380C was drilled during Expedition 344 as a complementary site to Site U1378. The goal for drilling Site U1380 was to reach the deeper portions of the upper slope sequence and underlying framework wedge sequences (see the Expedition 344 summary chapter [Harris et al., 2013a]). The sediment recovered from this site was characterized by an alternating, terrestrially sourced, turbiditic upper slope to shelf sequence (see the Mid-slope Site U1380 chapter [Harris et al., 2013d]). Given the complementary locations of Site U1378 and Hole U1380C, we combine the pore fluid data from the two sites to refer to general characterization of the Costa Rica upper slope.

On the incoming plate, Site U1381 was drilled in line with Sites U1378/U1380 to characterize the sediment and oceanic crust entering the seismogenic zone (Fig. F1). The upper 50 m of Site U1381 is composed of a monotonous silty clay to clay sequence and is underlain by a sequence characterized by abundant biogenic components and reflecting pelagic sediments (see the Input Site U1381 chapter [Harris et al., 2013b]). Site U1414 is also situated on the incoming plate but is ~1 km seaward of the deformation front offshore the Osa Peninsula and northwest of Site U1381 (see the Input Site U1414 chapter [Harris et al., 2013c]).

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