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doi:10.2204/iodp.proc.334.104.2012 Site U13791Expedition 334 Scientists2Background and objectivesIntegrated Ocean Drilling Program (IODP) Site U1379 (proposed Site CRIS-4A) was drilled into the upper slope of the Costa Rica margin, 34 km offshore Osa Peninsula and 16 km from Caño Island along seismic BGR99 Line 7 (Fig. F1). Site U1379 overlies the seismogenic zone as defined by the aftershock sequence of the 2002 M 6.4 Osa earthquake (S.L. Bilek, pers. comm., 2003; I.G. Arroyo, pers. comm., 2008) in an area where the plate boundary is 4.5 km below seafloor. Geodetic measurements indicate this area to be located above the locked portion of the plate boundary (LaFemina et al., 2009). At Site U1379, the temperature at the plate boundary was interpreted to be >150°C (Grevemeyer et al., 2004). More recent modeling implies much cooler intraplate temperature with values of ~90°C (Harris et al., 2010). Site U1379 was designed as a “pilot hole” in preparation for proposed deeper Costa Rica Seismogenesis Project Program B drilling at this location. The primary purpose of drilling Site U1379 was to determine the nature, composition, and physical properties of the upper plate basement. According to present geological and geophysical data interpretation of the Costa Rica margin, the upper plate basement provides the input material to the seismogenic zone. Interpretation of the on-land geology of Osa Peninsula suggests that the upper plate basement could be very heterogeneous in lithology and age. In fact, the outcropping unit is a mélange, the origin of which seems to be related to seamounts accreted between the early Eocene and the middle-late Miocene (Vannucchi, et al., 2006). Because of this heterogeneity Expedition 334 planned a double penetration of the forearc basement at Sites U1378 and U1379. Tectonic erosion is the process characterizing the Costa Rica subduction zone, and documentation of the margin subsidence/uplift is a key objective to infer the volume of material removed from the upper plate and transported along the plate boundary. Our strategy was to drill the slope sequence in two locations, the middle and the upper slope, and compare results to reconstruct the stratigraphy of the sediments and to infer its variations and subsidence/uplift histories along the transect. At Site U1379, a ~890 m thick slope sediments overlie the upper plate basement as shown in the seismic lines by the prominent reflectors and increasing P-wave velocity (Figs. F1, F2). Because of the shallow water depth at this site, high-resolution benthic paleontology is expected to provide an excellent record of vertical tectonism related to estimation of the thickness of the subduction channel. Additional objectives included
Stress measurements at this site and onshore strain measurements may show stress transients in the currently locked plate boundary offshore and beneath the Osa Peninsula, respectively. The seismic interpretation of Site U1379 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 750 (8°40.8496′N, 84°2.0169′W) at 125 meters below sea level (mbsl). Stratification in slope sediment is coherent, and the section is therefore likely to yield a complete sedimentation history. The seismic reflectors within the uppermost 800 m of the section show good continuity. An angular unconformity is inferred to occur at ~550 meters below seafloor (mbsf). The uppermost 550 m of the slope sediment sequence shows clear horizontal reflections. The underlying sequence shows reflectors that gently dip seaward (toward the southwest). 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 bottom reflectors of the slope sediments gently dip landward and, moving northeast, abruptly terminate against the basement in correspondence with one of the high-amplitude reflectors cutting through the upper plate. This is clear evidence of normal displacement along these latter structures. The velocity increases from <2.3 m/s in sediment to >3.6 m/s in the basement. The surface of the basement top at Site U1379 is smooth and subhorizontal. |
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