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doi:10.2204/iodp.proc.334.105.2012 Site U13801Expedition 334 Scientists2Background and objectivesIntegrated Ocean Drilling Program Site U1380 (proposed Site CRIS-10A) was drilled into the middle slope of the Costa Rica margin, 38 km offshore Osa Peninsula and 24 km from Caño Island. The site is located along Seismic BGR99 Line 7, at common midpoint 2350 (8°35.9980′N, 84°4.4037′W) at 515 meters below sea level (mbsl) (Fig. F1). Site U1380 was the contingency site for Site U1378, and these two sites share the same objectives. The basement at Site U1380 has a thinner sedimentary slope cover than does Site U1378, and the loss in stratigraphic resolution was traded for the easier access to the basement. This site was chosen because it is located above the unlocked portion of the plate boundary, as indicated by interplate earthquake relocation and geodetic measurements (S.L. Bilek, pers. comm., 2003; LaFemina et al., 2009). The temperature at the plate boundary was interpreted to be at least 140°C at Site U1380 (Grevemeyer et al., 2004), but more recent modeling implies lower temperatures than previously inferred (i.e., between 60° and 90°C; Harris et al., 2010). Within the Costa Rica Seismogenesis Project (CRISP), characterization of the aseismic region of the margin complements characterization of the seismic portion of the plate boundary and above upper plate. The primary purpose of drilling Site U1380 was to determine the nature, composition, and physical properties of the upper plate basement and to understand the nature of the landward-dipping seismic reflectors that characterize the Costa Rica forearc in the CRISP region. Additional objectives included
The margin here consists of upper plate basement underlying slope sediments about 550 m thick (Fig. F1). The seismic sections show that this site is located above the seaward edge of one of the high-amplitude reflectors interpreted as a displacement surface. Site U1380 was designed to penetrate this reflector. The seismic interpretation of Site U1380 is based on the prestack depth-migrated section of Seismic BGR99 Line 7 processed by C.R. Ranero (Fig. F1). Seismic stratigraphy of the sedimentary slope at Site U1380 shows good continuity of reflection events. The uppermost 100 m of the slope sediment sequence shows clear horizontal reflections. The underlying 300 m of sediments grade downward in a more highly reflective zone. The reflectors in this interval gently dip seaward, and the sequence is clearly cut upslope (toward the northeast) by a younger sequence, forming an angular unconformity. The lowermost part of the sedimentary sequence is sharply marked by an irregular, high-amplitude, seaward-dipping reflector interpreted as the top of the upper plate basement. Several landward-dipping, high-amplitude reflectors interpreted as faults cut the surface marking the basement top. At Site U1380, one of these high-amplitude reflectors, interpreted to be a fault, is present at ~800 meters below seafloor (mbsf). This reflector does not clearly displace the basement top. The velocity increases from <2 m/s in the sediment to 2.8 m/s in the fault zone to >3.3 m/s in the basement. Visual observations and the towed ocean bottom instrument detected several tens of mud mounds and vent communities within a few kilometers of the site (McAdoo et al., 1996; Weinrebe and Ranero, 2003). Many of these structures seem to be related to active faults (Hensen et al., 2004; Ranero et al., 2008). Judging from the continuity of the landward-dipping reflectors interpreted as faults, any fluid advection along these structures originates in areas of at least 4 km depth. |
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