ABSTRACTThe character of the incoming plate subducting at convergent margins and the processes affecting it as it passes below the shallow forearc may play a major role in the nature and extent of hazardous interplate seismicity, as well as the magnitude of volcanism and 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 a key agent in transport of chemical species. The mineralogy and chemistry of any subducted sediments and their dehydration reactions 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.
Science objectives for Leg 205 have two primary foci. The first is the igneous and alteration history of the basement at reference Site 1039R on the incoming plate. The second is characterizing and monitoring the three hydrological systems: in basement at Site 1039R, in the uppermost section of the subducting sediment section at Site 1040R, and along the décollement and upper conduit at Site 1040R. These goals will be accomplished by (1) limited coring of selected intervals, (2) downhole temperature measurements, (3) logging at Site 1039R, and (4) installation of long-term observatories (CORKs) to monitor temperature and pressure and to sample fluids and gases in each of the hydrologic systems. Time series of fluid composition in the sealed-off intervals will be obtained by using osmotic fluid samplers. These samplers will be recovered for analysis of the water samples 1 to 2 yr postinstallation.
Science objectives for Site 1039R are: (1) quantifying the amount of carbonate in the subducting sediment and uppermost altered basaltic crust to evaluate carbon recycling through the arc; (2) determining the distribution of metalliferous carbonates above the sill and above basement to construct element fluxes into the trench and to constrain their flux out of the basement; (3) determining the extent of sill emplacement and their contribution to the bulk composition of the subducting igneous crust; (4) determining the igneous and alteration mineralogy, petrology, and geochemistry in the uppermost 100 m of the upper crust and determining subduction fluxes therefrom; (5) determining physical properties in the core and borehole, relevant to fluid flow and deformation such as porosity, density, permeability, fracture distribution, orientation, and strength; and (6) installing a modified CORK within the cored upper basement section to sample fluids and to monitor temperature and pressure.
Science objectives for Site 1040R are: (1) determining physical properties of the décollement horizon from structural experiments on whole-round samples to constrain hydrological modeling and permit integration of fluid flow and deformation models; (2) determining chemistry of pore fluid profiles from décollement whole rounds to compare with profiles measured during Leg 170 and to evaluate possible heterogeneity; (3) determining pressure, temperature, and composition of fluids and gases along the décollement and evaluating any possible changes through time for hydrologic modeling; (4) using selected elements, element ratios, and isotopic compositions in the fluids from the décollement in an attempt to constrain dehydration reactions at the updip and, perhaps, downdip limits of the seismogenic zone; (5) installing three modified CORKs: one to sample fluids and monitor temperature and pressure in the uppermost underthrust sediments and two in the décollement; (6) determining pressure, temperature, and fluid composition in the zone of compaction dewatering beneath the décollement to constrain pathways of fluid return to the surface and to evaluate the effects of this flow system on element fluxes; and (7) collecting whole-round samples from the décollement under appropriate conditions for postcruise microbiological investigations to determine the resident microbial ecology of the zone for comparison with eventual microbial experiments on fluids collected from the décollement.
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