INTRODUCTION

The character of the subducting plate at a convergent margin and the processes affecting it as it passes below the shallow forearc may be a major determining factor in the nature and extent of hazardous interplate seismicity, magnitude of volcanism, and chemistry of lavas produced in the overlying volcanic arc. Subducting sediments and ocean crust along with their associated volatile components passing through shallow subduction zones (0–50 km) profoundly affect 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 contained within both fault zones and underthrust sediments at the trench affect early structural development and serve as a key agent in transport of chemical species. The mineralogy and chemistry of subducted sediments and the dehydration reactions during the subduction process may control the physical properties of the deeper subduction interface and, hence, downdip limits of the seismogenic zone.

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 evolution of the décollement. Distillation and partial loss of volatiles and fluid-soluble elements from the shallow slab not only record reactions and processes within the seismogenic zone but also play a central role in supplying residual volatiles to the deeper Earth and changing the composition of the slab delivered to magmatism depths 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, evolution of the mantle through time (collectively known as the "subduction factory" in many geoscience documents). 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 seismically imaged.

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 Joint Oceanographic Institutions for Deep Earth Sampling (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 (Fig. F1) is a focus area for a number of national and international programs studying the seismogenic zone and subduction factory for several reasons:

It has been hypothesized that changes in both seismicity and volcanic chemistry 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.