Ocean Drilling Program, United States

The character of the incoming plate subducting at convergent margins and the processes affecting the plate as it passes below the shallow forearc may play a major role in the nature and extent of hazardous intraplate seismicity as well as the magnitude of volcanism and the chemistry of lavas produced in the overlying volcanic arc. The fate of incoming sediments and ocean crust and of their associated volatiles as they pass through the shallow levels of a subduction zone (0-50 km depth) has profound effects on the behavior of the seismogenic zone, which produces most of the world's destructive earthquakes and tsunamis. Fluid pressure and sediment porosity influence fault localization, deformation style, and strength and may control the updip limit of the seismogenic zone. Fluids within both fault zones and sediments underthrust at the trench affect early structural development and are key agents in transport of chemical species. The mineralogy and chemistry of any subducted sediments and their dehydration reactions, governed by the thermal structure of the plate during subduction, may control the physical properties of the deeper subduction interface and, hence, the updip and downdip limits of the seismogenic zone wherein interplate earthquakes are generated. The mineralogy, composition, and volatile content of the slab, transformed during its progress through the shallow subduction zone, will govern the flux of fluids or melts from slab to mantle wedge, which is an important control on the extent of mantle melting and formation of arc lavas. Costa Rica is an important area for studies of the seismogenic zone and subduction factory for several reasons. As one of the few modern arcs subducting a carbonate-rich sediment section, Central America permits study of CO2 recycling through a subduction zone. Changes along strike in seismicity, plate coupling, and volume and composition of the arc lavas (between Nicaragua and Costa Rica) appear to correlate with changes in sediment dynamics. The balance between sediment accretion, underplating, erosion, and subduction may ultimately result from changing bathymetry, thermal structure, or hydrological behavior along the margin. Science objectives for Leg 205 had two primary foci, both related to seismogenic zone and subduction factory questions. The first was to determine the igneous and alteration history of the uppermost part of the downgoing plate at reference Site 1253, along with the inferred distribution of fracture permeability in the core and borehole. The second was to characterize and monitor two of the three hydrological systems inferred from Leg 170 results: in basement at Site 1253 and along the decollement (or upper fault zone) at Sites 1254 and 1255. These goals were accomplished by (1) targeted coring of selected intervals, (2) downhole temperature and pressure measurements, (3) logging at Site 1253, and (4) installation of long-term observatories (CORK-IIs) to monitor temperature and pressure and to sample fluids and gases in each of the hydrologic systems. In the decollement zone, instruments were also deployed to attempt to measure fluid flow rates. Temporal variation of fluid composition in the sealed-off intervals will be obtained using osmotic fluid samplers. The samplers and temperature loggers will be recovered for analysis 1 to 2 years after installation, pressure data will be downloaded, and new samplers and temperature probes will be installed.....