The purpose of this study is to synthesize and provide as accurate and precise age-depth models as possible, utilizing calcareous nannofossils for age determination, for all of the cores recovered during ODP Leg 170 seaward of the Costa Rican Nicoya Peninsula on the subducted margin. Comprehensive calcareous nannofossil species range and distribution charts have been constructed from all the sites cored during Leg 170 from which the age-depth models have been synthesized.
The primary focus of Leg 170 was to core a series of sites off of the Costa Rican Nicoya Peninsula in an effort to understand what happens to the pelagic and hemipelagic sediments, associated pore waters, and oceanic basement that enter the subduction zone off the Costa Rican margin (Kimura, Silver, Blum, et al., 1997). The Costa Rica subduction margin was chosen because the volume of incoming sediments into the trench is thought to have been relatively constant over long periods of time and the influx of turbidites into the trench system appears to have been minimal.
Five sites were drilled during Leg 170 perpendicular to and across the Middle America Trench along a 15-km transect (Figs. F1, F2). Site 1039 was drilled seaward of the trench on the Cocos plate and acts as a reference for the age, thickness, lithology, physical properties, and fluid composition of the incoming sedimentary block into the trench. Site 1039 also reached basaltic crust.
Site 1040 (Figs. F1, F2) was drilled on the Caribbean plate immediately to the east of the Middle America Trench through the accretionary wedge, the décollement, and the underthrust sedimentary section to basement. This site was drilled to determine the age, the portion of the incoming section that is accreted, the physical and chemical properties, and the changes in chemistry and physical properties between the underthrust sedimentary and basaltic rocks and the reference section.
Site 1043 (Figs. F1, F2) was drilled with the same objectives as Site 1040 but was located closer to the trench axis, on the toe of the wedge, where the sediments overlying the décollement are not as thick.
Sites 1041 and 1042 (Figs. F1, F3) were also drilled on the Caribbean plate with the purpose of penetrating through the slope apron into the underlying prism to determine the age and makeup of the apron and prism sediments.
A bathymetric map of the study area is provided (Fig. F4) in order to illustrate the location and depth of all five Leg 170 sites relative to each other.