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doi:10.2204/iodp.proc.344.201.2015

Introduction

The Costa Rica Seismogenesis Project (CRISP) was designed to understand the processes that control fault zone behavior during earthquake nucleation and rupture propagation at erosional subduction zones. The CRISP study area, located offshore the Osa Peninsula of Costa Rica, is part of the active and long-lived subduction erosion from Guatemala to Costa Rica (Ranero et al., 2000a, 2000b; Vannucchi et al., 2004). This area is characterized by low sediment supply, fast convergence rate, abundant plate interface seismicity, and change in subducting plate relief along strike (Expedition 334 Scientists, 2012a; see also the “Expedition 344 summary” chapter [Harris et al., 2013a]). Arcward of the trench, the lower slope consists of a 10–12 km wide frontal prism, where a modern sediment apron overlies older sediment that may have been deposited in a fore-arc basin setting (A. Sakaguchi, pers. comm., 2013). The first phase of the CRISP focuses on sampling sediment, fluid, and crustal rock because fluid and associated diagenetic reactions affect hydrological parameters (e.g., permeability and pore pressure) and may regulate the mechanical state of the plate interface at depth. To this aim, seven sites were drilled on this margin during Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344, which targeted sites on the upper and incoming plates (Fig. F1).

Because the isotopic composition of strontium does not undergo biological fractionation (Mook, 2001), it has proven to be valuable in establishing fluid-rock reactions, sources, and fluid mixing (e.g., Teichert et al., 2005; Torres et al., 2004; Solomon et al., 2009; Joseph et al., 2012). Potential source materials that may alter a de facto seawater ⁸⁷Sr/⁸⁶Sr ratio of interstitial fluids are continental detritus (⁸⁷Sr/⁸⁶Sr = ~0.7119–0.7133), biogenic calcite (⁸⁷Sr/⁸⁶Sr = ~0.7075–0.7092), volcanic ash (⁸⁷Sr/⁸⁶Sr = ~0.706–0.704), and oceanic crust (⁸⁷Sr/⁸⁶Sr = ~0.703) (Veizer, 1989). Here we report on pore water strontium isotopic composition of subsamples collected from the seven sites drilled during Expeditions 334 and 344.

Study sites

Five sites were drilled on the overriding plate and sampled the upper and middle slope regions and the prism toe. Two additional sites were drilled in the incoming Cocos plate (Fig. F1). Complete descriptions of the drilling results are given in Expedition 334 Scientists (2012a) and in the “Expedition 344 summary” chapter (Harris et al., 2013a).

The shallowest site along the CRISP transect, Site U1379, was drilled into the upper slope of the Costa Rica margin, 34 km offshore the Osa Peninsula. This site is thought to overlie the locked portion of the subduction zone seismogenic zone in an area where the plate boundary is 4.5 km below seafloor. Sediment from Site U1379 is divided into five lithostratigraphic units (Fig. F2A).

Site U1378 was drilled into the middle slope of the Costa Rica margin, 41 km offshore the Osa Peninsula (Fig. F1). This site is located above the unlocked portion of the plate boundary, as indicated by interplate earthquake relocation and geodetic measurements (LaFemina et al., 2009). The margin here consists of an upper plate framework wedge underlying ~550 m of slope sediment. Drilling at this site penetrated a landward-dipping reflector interpreted as a normal fault cutting through the whole upper plate, which may be directly linked with the plate boundary. The sediment at Site U1378 is dominantly composed of a monotonous sequence of silty clay to clay that alternates with widely interspersed centimeter-scale sandy layers, and was divided into three main lithostratigraphic units (Fig. F2).

Site U1380 is a complementary site to nearby Site U1378 where only the upper part (~400 [meters below seafloor [mbsf]) of the slope sediment was drilled during Expedition 334. Site U1380 was drilled to investigate the deeper portions of the upper slope sequence and underlying wedge sediment. The site was cored between ~395 and 480 mbsf during Expedition 334 but was abandoned before science goals were achieved because of hole instability. Hole U1380C was cored during Expedition 344. The sedimentary succession recovered from the framework wedge revealed an alternating terrestrially sourced, turbiditic upper slope (Units I and III) to shelf (Unit II) sequence, eventually being influenced by deltaic-derived sediment (Fig. F2).

Farther north, Site U1413 was also drilled in the middle slope region within a 3-D seismic volume collected in 2013 (Bangs et al., 2013). Three lithostratigraphic units (Fig. F3) can be distinguished in the sediment of Site U1413. The frontal sedimentary prism at the base of the slope was drilled at Site U1412 (Fig. F1). The primary goal of this site was to penetrate the décollement and investigate the fluid flow regime within the sediment as well as the oceanic crust. Three units were distinguished in the sedimentary rock (Fig. F3). Unfortunately, due to hole instability, the décollement and underthrust sediment were not sampled at Site U1412.

To understand seismogenic processes in convergent margins we need to fully characterize the sediment and oceanic crust entering the seismogenic zone. To this aim, two sites were drilled on the subducting aseismic Cocos Ridge. Incoming sediment thickness at Site U1414 is 380 m, and a much thinner section (<100 m) was recovered at Site U1381 located higher on Cocos Ridge. The upper 50 m of Site U1381 is composed of a predominantly monotonous sequence of silty clay to clay (Unit I), which is underlain by a more pelagic sequence (Unit II) characterized by abundant biogenic components (Fig. F4). The contact between the basement and the overlying sediment was recovered at ~95 mbsf in Hole U1381B and at ~104 mbsf in Hole U1381C.

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