170 Scientific Prospectus


Essential observations for establishing the mechanisms of accretion and underplating, tectonic erosion, and deformation and dewatering must include (1) the rate (positive and negative) of prism growth as a function of incoming sediment volume and type; (2) the partitioning of frontal offscraping, underplating, internal prism deformation, and subduction erosion; and (3) the effects of fluids. Excellent control is required of material mass-balance and residence time in the prism, in addition to detailed structural geometry of the complex interior regions of forearcs. Such control and imaging is not well established in any convergent margin, despite expending a great amount of effort to understand these margins.

The scarcity of accurate mass-balance estimates is due to the complexity of both sedimentary and structural processes at convergent margins, the poor structural imaging of the deeper parts of forearc regions, and the need for reliable age estimates that generally require drilling. The presence of trench turbidites, varying dramatically in thickness both spatially and temporally, is a major obstacle in estimating material influx. Erosion and redeposition of accreted material are additional complications that can not be taken easily into account. A first step in addressing these problems is to locate an experiment along a convergent margin that lacks trench turbidites, has a slope cover preventing erosion of the accreted material, and has clearly imaged deep and shallow structural control.

If the convergence rate is known, the incoming sediment flux can be estimated closely and prism size will reveal the relative importance of sediment accretion or bypassing. The case of subduction erosion may also be documented with additional structural, stratigraphic, or biostratigraphic data to show subsidence or arcward retreat. The final requirement is availability of accurate emplacement dates for the accreted material.

The convergent margin off Costa Rica (Figs. 1 and 2) satisfies all the requirements necessary to determine accurate mass-balance and flow estimates, except for knowledge of the age and residence time of the prism material. The trench is devoid of turbidites here and the convergence rate is known. Recently acquired 2D and 3D seismic reflection data across the margin provide excellent control of the internal structure of the forearc, and they define boundaries between the accretionary prism and the overlying slope cover, as well as between the prism and the subducting plate (Fig. 2). These data show that the slope cover extends to within 3-5 km of the trench, so it protects the accreted mass from erosion and conserves its volume. Consequently, the growth rate of this prism can be calculated accurately when the emplacement age of the accreted material is determined by drilling through the basal slope cover and top of the prism.

This relatively closed system is also a superior environment in which to investigate the fluid and chemical fluxes in a subduction system. Comparison of physical (i.e., velocity and porosity) and chemical properties of sediments seaward of the trench with sediments that have been subducted or accreted can provide important information on the nature and rate of diagenetic processes in subduction zones. Aided by relatively rapid plate convergence and a stratigraphically consistent subducting sedimentary section, the sediment and pore-water chemistry can also be compared with the constituents of arc volcanic rocks using geochemical tracers. By their presence or absence in the volcanic rocks, tracers such as 10Be and Ba may be used to indicate the amounts of sediment accretion, amounts of sediment recycling to the volcanic arc, and subduction into the mantle (Tera et al., 1986; Plank and Langmuir, 1993).

Prominent lithologic or structural boundaries that produce high-amplitude seismic reflectors in the interior of this moderately accreting prism are within reach through drilling. The existing 3D seismic grid will allow exceptionally good correlation between seismic and borehole data as a result of accurate 3D imaging. Certain coherent intraprism reflectors identified in the seismic data appear to be unrelated to offscraping but are interpreted as faults (Shipley et al., 1992), which formed during out-of-sequence thrusting and underplating. These and other faults are likely pathways along which fluids escape from the prism. Identifying the physical properties, lithologies, and geologic processes that produce these impedance contrasts will provide essential information for the structural interpretation of this margin and fundamental data about the nature of seismic reflections in accretionary prisms.

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