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doi:10.2204/iodp.proc.334.106.2012 Site U13811Expedition 334 Scientists2Background and objectivesThe primary objective of Integrated Ocean Drilling Program (IODP) Expedition 334 was to sample and quantify the input into the seismogenic zone of an erosive subduction margin. Although the main transfer of material to the seismogenic zone is implied to come from the upper plate, the characteristics of the downward-moving plate entering the Costa Rica Subduction Zone are essential to constrain the system. This situation is particularly true in the area of the Costa Rica Seismogenic Project (CRISP), as this area is characterized by the subduction of a prominent aseismic ridge, the Cocos Ridge. In this context, IODP Site U1381 (proposed Site CRIS-1A) served as a reference site. Site U1381 is a key site for characterizing the material input into the subduction zone, considering both the composition and mass of the sediment cover and basement rocks as well as the fluids. In erosive margins, fluids are thought to trigger the hydrofracturing processes of the upper plate and to control the strength of the plate boundary. According to existing models of erosive margins, sediments are inferred to play a minor role in the input to the seismogenic zone. The role of the igneous basement of the incoming plate is less well established. Here we target the fluids circulating in the upper oceanic crust, since a vigorous hydrologic system was discovered offshore Nicoya Peninsula during Ocean Drilling Program (ODP) Legs 170 and 205 (Kimura, Silver, Blum, et al., 1997; Morris, Villinger, Klaus, et al., 2003). The importance of the hydrological activity in the subducting oceanic plate is just beginning to be appreciated (Silver et al., 2000). The igneous crust of the incoming Cocos plate shows a significant along trench strike variability related to its origin at either the East Pacific Rise or the Cocos-Nazca spreading center. It is also characterized by a different hydrologic system compared to the crust subducted of the Nicoya Peninsula. Heat flux measurements revealed high heat flow in the Cocos Ridge area in contrast with the low values of the smooth crust offshore Nicoya Peninsula (Fisher et al., 2003; I. Grevemeyer, unpubl. data). The upper crust in the Cocos Ridge area appears to be well layered and probably very porous (von Huene et al., 2000). Thus the contribution from the lower plate to the fluid circulation could be significant, and its characterization has important consequences for the investigation of the fluid system at the aseismic/seismic boundary during CRISP Program B. Site U1381 is located along Seismic BGR99 Line 7 (Fig. F1), 50 km offshore Osa Peninsula and 43 km from Caño Island. Site U1381 is in a basement-relative high at common midpoint (CMP) 5750 (8°25.7150′N, 84°9.4690′W) at 2067 meters below sea level (mbsl). This location is critical because basement-relative highs, thought to act as major fluid discharge areas, have the best potential to record traces of vigorous fluid flow. The seismic section indicates a 120 m thick sediment section resting on high reflective basement interpreted as Cocos Ridge igneous crust. Paleomagnetic data constrain the age of this portion of Cocos Ridge to 14 Ma (Barckhausen et al., 2001). The sedimentary section has been interpreted as formed by pelagic and hemipelagic sediment. The sediment thickness along the seismic transect (southwest–northeast) is variable. This is in accordance with the relief of Cocos Ridge (basement). This relief can be partly correlated with normal faulting (CMP 4600 and 4800 in Fig. F1), even though faulting is not a particularly strong characteristic of the subducting plate in this region. |
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