The ancient crust of the western Pacific plate is the primary input to the >2000-km-long Izu-Mariana subducting margin. Ideally, this input would be constrained by drilling several holes through the sedimentary section and deeply into oceanic crust along the length of the trench. Because of the great expense and time it takes to drill in ~6000-m water depth, we are limited in practice to a few drill holes and extending this information regionally using sedimentation and plate-motion models, along with seismic stratigraphy. Understanding the context of sedimentation and plate history in the western Pacific is thus important in order to maximize the information gained from a small number of reference sites, such as Leg 185 Sites 801 and 1149.
The crust subducting into the Mariana Trench is characterized by seafloor divided into the Jurassic East Mariana and Pigafetta Basins (Fig. F2). Based on magnetic anomaly lineations, this region was thought to contain the Earth's oldest in situ oceanic crust formed at fast-spreading rates (160 km/m.y. at Site 801). The basic goal of Leg 129 was to sample Jurassic oceanic crust. Earlier attempts to recover Jurassic sediments and basement in the western Pacific had been thwarted by extensive mid-Cretaceous volcanics and sills and by problems with the drill string sticking in chert horizons. While drilling two holes (ODP Sites 800 and 802) that encountered Cretaceous basalt, Leg 129 was the first to succeed in recovering Jurassic oceanic basement in the Pacific Ocean, and Hole 801C rocks are still the oldest sampled in the ocean basins, at 167 ± 5 Ma (Pringle, 1992).
Complete sedimentary sections were sampled during Leg 129 at three sites in the East Mariana and Pigafetta Basins (ODP Sites 800-802) (Fig. F2). Before this leg, the recovery from nine DSDP and ODP sites averaged <50 m each (Fig. F2). Taking Hole 801C as typical of the region, the sedimentary stratigraphy consists of Cenozoic brown pelagic clay overlying Coniacian to Campanian cherts and porcelanite, Albian seamount volcaniclastics, and Bajocian to Valanginian radiolarites (Fig. F6) (Lancelot, Larson, et al., 1990). This sedimentary history reflects the plate history, which begins in the Southern Hemisphere in a zone of high biological productivity, as recorded by the Jurassic radiolarites (Fig. F7) (Lancelot, Larson, et al., 1990). The plate then moved southward until the Early Cretaceous when it began to move northward again, collecting volcaniclastics from the nearby Cretaceous Magellan Seamounts and then more siliceous sediments as it again entered the high-productivity zone 5°-10° south of the paleoequator. The Cenozoic was characterized by very slow accumulation of deep brown pelagic clays, with minimal biogenic input, as is expected for the open-ocean environment. This history is typical for the western margin of the East Mariana Basin and the Pigafetta Basin. This stratigraphy, particularly the clay/chert and volcaniclastic intervals, can be traced regionally from seismic records (Abrams, et al., 1992). Although recovery was generally low (<30%), Leg 129 provided adequate sampling of the different sedimentary components to characterize the sedimentary geochemical flux into the Mariana Trench (Plank and Langmuir, 1998). This contrasts with the lack of a single, continuously drilled sedimentary section along the entire 1000 km of the Izu-Bonin margin to the north.
Previous drilling in the Nadezhda Basin, seaward of the Izu-Bonin Trench, was about as successful as drilling to the south prior to Leg 129. The chert horizons plagued drilling during Leg 20, which placed five holes in the region, none of which was to hit basement except DSDP Hole 197, where only 1 m of undatable tholeiite was recovered (Fig. F6). Thus, the age of the M-series magnetic anomaly had never been tested in this region. Along the Izu-Bonin Trench, magnetic anomalies predict that the oceanic crust decreases in age from Jurassic (>M18) in the south to Early Cretaceous (M11) at Site 1149 (Fig. F2). It was unknown if the extensive mid-Cretaceous volcanism that took place in the south extended north into the Nadezhda Basin.
Average recovery of sediments in the Nadezhda Basin was extremely low (<15 m) for previous DSDP sites, again because of sticking problems and spot coring. Leg 20 cores indicate an upper ash- and diatom-rich clay unit overlying a brown pelagic clay and Cretaceous chert and chalk (Fig. F6). The paleolatitude history for Site 1149 predicts a longer duration beneath equatorial zones of high biological productivity, and, therefore, extensive chert and chalk sequences (Fig. F7). Water-gun seismic profiles collected during a presite seismic survey show a prominent reflection at ~0.2 s two-way traveltime (TWT) that corresponds to the chert horizon and another prominent reflection at 0.42 s TWT (Fig. F8) that corresponds with probable basement. Postcruise investigations will determine if these distinctive reflections can be identified throughout the Nadezhda Basin.
Site 1149 lies within the same spreading compartment as Site 801, along a flow line in crust ~35 m.y. younger (Fig. F2), formed at a slower spreading rate than Site 801 (100 mm/yr full rate). Roughly 100 km from the trench, Site 1149 lies seaward of the main faulting of the plate as it bends into the subduction zone (Fig. F2). The main objective at Site 1149 was to drill through the inferred 400-m-thick sedimentary sequence and into basement subducting along the Izu-Bonin margin, which would enable a comparison with the fluxes to the south into the Mariana Trench.