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doi:10.2204/iodp.proc.334.103.2012 Site U13781Expedition 334 Scientists2Background and scientific objectivesIntegrated Ocean Drilling Program Site U1378 (proposed Site CRIS-3B) was drilled into the middle slope of the Costa Rica margin, 38 km offshore Osa Peninsula and 24 km from Caño Island along seismic BGR99 Line 7 (Fig. F1). 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; I.G. Arroyo, pers. comm., 2008; LaFemina et al., 2009). Within the Costa Rica Seismogenesis Project, this location is considered as complementary to the upper slope site located above the seismic portion of the plate boundary. The primary purpose of drilling Site U1378 was to determine the nature, composition, and physical properties of the upper plate basement, which is considered to be the main source of material in a erosive plate boundary system. Moreover, seismic sections show that this site is above the seaward edge of one of the high-amplitude, landward-dipping reflectors interpreted as normal faults. These landward-dipping reflectors appear as a characteristic feature of the Costa Rica forearc along the Expedition 334 transect. Operations at Site U1378 were designed to penetrate one of these reflectors to understand their nature. In order to quantify the material input into the seismogenic zone of the plate interface, it is important to constrain volume and timing of subduction erosion/accretion. Our strategy was to drill the slope sequence at least in two locations and compare results to reconstruct the stratigraphy of the sediments and document the margin subsidence/uplift. Specifically Site U1378 was located in an area of the middle slope with a relatively thick sequence of slope sediment in order to better decipher, for example, the occurrence of discontinuities. The margin here consists of ~750 m thick slope sediments overlying upper plate basement as defined in the seismic lines by prominent reflectors and increasing P-wave velocity (Figs. F1, F2). Additional objectives included
The seismic interpretation of Site U1378 is based on the prestack depth-migrated seismic BGR99 Line 7 processed by C.R. Ranero (Fig. F1). The site is located at common midpoint 2500 (8°35.5414′N, 84°4.6306′W) at 527 meters below sea level (mbsl). The seismic stratigraphy of the sedimentary slope at Site U1378 shows a good continuity of reflection events (Fig. F2). The uppermost 100 m of the slope sediment sequence shows clear horizontal reflections. The underlying 100 m of sediment forms the weakest reflective part of the sequence. From there the sequence grades downward in a more highly reflective zone that is basically continuous between 500 and 600 meters below seafloor (mbsf). 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 a high-amplitude reflector interpreted as the top of the upper plate basement. 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. The surface marking the basement top is irregular and cut by several landward high-amplitude reflectors interpreted as faults. The basement top is displaced a few tens of meters across these faults. Around Site U1378, the slope sediments are thickened, but not visibly deformed by the faults. Thickening occurs in correspondence to the fault cutting the basement reflector, suggesting normal displacement. Some growing strata visible along BGR99 Line 7 also confirm this normal sense of displacement. At Site U1378, the temperature at the plate boundary was interpreted to be at least 140°C (Grevemeyer et al., 2004). Recent modeling of the intraplate temperature interprets it to be between 60°C and 90°C (Harris et al., 2010). Visual observations and the towed ocean bottom instrument detected abundant mud mounds and vent communities within a few kilometers of the site (McAdoo et al., 1996; Weinrebe and Ranero, 2003). Many of these structures collect fluids from areas with thermogenic hydrocarbon sources (Hensen et al., 2004) and they seem to be related to the landward-dipping reflectors interpreted as active faults (Ranero et al., 2008). Judging from the continuity of the landward-dipping reflectors and the presence of thermogenic hydrocarbon in fluids, any fluid advection along these structures originates in areas of at least 4 km depth. |
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