INTRODUCTIONThe 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 (e.g., Scholz, 1998; Moore and Saffer, 2001). 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, downdip limits of the seismogenic zone.
The escape of fluids to the surface from depth (return flow) supports a deep biosphere, contributes methane for gas hydrate formation, affects seawater chemistry for selected elements, and is intimately linked to deformation, faulting, and the evolution of the décollement. The distillation and loss of some volatiles and fluid-soluble elements from the shallow slab not only record reactions and processes within the seismogenic zone, but they also play a central role in the supply of residual volatiles to the deeper Earth and change the composition of the slab delivered to the depths of magmatism beneath volcanic arcs. Processes operating in the shallow subduction zone thus affect the way the slab contributes to continent-building magmatism, explosive volcanism, ore formation and, ultimately, the evolution of the mantle through time (collectively known as the subduction factory in many geoscience planning efforts). The subduction signature recorded in the chemistry of arc volcanics constrains the nature and sometimes the volume of the sediments transported through the seismogenic zone to the depths of magmatism. The arc thus acts as a flow monitor for the transport of sediments to depths greater than those that can be drilled or imaged seismically.
The Ocean Drilling Program (ODP) has identified deformation at convergent margins, fluid flow in the lithosphere, and subduction zone geochemical fluxes as important aspects of the JOIDES Long Range Plan (1996). The Initial Science Plan for the Integrated Ocean Drilling Program (IODP) includes an initiative focused on the seismogenic zone. The Central American convergent margin (see Fig. 1) has been a focus area for a number of national and international programs studying the seismogenic zone and subduction factory for several reasons. First, it is one of the few modern subduction zones that is subducting a significant carbonate section and thus provides an opportunity to investigate CO2 cycling through convergent margins. Second, along strike from Nicaragua to Costa Rica, the style and extent of seismicity and plate coupling changes. Third, along the same section, the style of arc volcanism changes as do volumes and the chemistry of the arc lavas. Changes in both the seismicity and volcanic chemistry have been proposed to result from changes in the balance between sediment underplating, erosion, and subduction (collectively referred to here as sediment dynamics), perhaps related to changing bathymetry, thermal structure, and hydrological behavior along the margin.
Leg 205, building on Leg 170 coring and logging while drilling at the same sites, is designed to investigate the thermal structure and hydrological activity of the Costa Rica segment through a combination of downhole measurements and long-term sampling of fluids as well as monitoring of fluid pressure and temperature at critical horizons. First observations of temporal variations of fluid and gas chemistry will be available once the fluid samples has been recovered about 1 yr postcruise. During the leg, we will also drill and core a minimum of 100 m into the subducting altered oceanic crust to characterize those basement fluxes to the volcanic arc. Once completed, Costa Rica will be the first convergent margin for which drilling-based studies of subducting sediment and basaltic crust, sediment dynamics, forearc structure, and prism hydrology and deformation can be linked to both an active volcanic arc as well as a hazardous seismogenic zone.
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