Ocean Drilling Program, United States

To gain a better understanding of the mechanical and chemical behavior of accretion and underplating and tectonic erosion and to determine how deformation and dewatering are distributed throughout an accretionary prism, it is essential to establish the flow pattern of materials through subduction systems. Leg 170 consists of a program of drilling at four primary sites (proposed Sites CR-1 through CR 4) on the Costa Rica convergent margin to investigate mass- and fluid-flow patterns through the accretionary prism and will integrate structural analysis and sediment, fluid, and chemical mass balance calculations. The objectives are to determine (1) the relative importance of frontal accretion, underplating, out-of-sequence thrusting, sediment subduction, and subduction erosion; (2) the timing, rate, and modes of the accretionary prism development; (3) the importance of fluids in both strengthening and weakening the prism, particularly in the presence of underthrust carbonates; and (4) the fate of subcrustally subducted sediments and the associated fluxes. Drilling on the Costa Rica margin could provide us with the first good estimates of the total material and chemical fluxes through a subduction system, because of the ideal conditions (the capping sediment apron and the lack of trench turbidites), extensive seismic imaging of the accretionary complex, and the opportunity to constrain the deeper parts of the sediment cycle, which are reflected in the forearc fluids and arc volcanic rocks. Because of the global importance of understanding material and fluid fluxes through subduction zones, it is critical that these systems become well understood. The specific objectives of the drilling program are to determine the age and nature of the accretionary wedge beneath the slope apron; the rate of accretion of the wedge; the nature of the lower part of the apron and time sequence of formation; the physical properties of the material above, within, and below the top-prism reflector; the relative importance of underplating vs. out of-sequence faulting; the evidence of fluid stratigraphy and flow distribution within the wedge materials; and the nature of apron material relative to deep-sea hemipelagic sediment.