Ocean Drilling Program Leg 195 consisted of three science segments. The first segment was devoted to coring and setting a long-term geochemical observatory at the summit of South Chamorro Seamount (Site 1200), which is a serpentine mud volcano on the forearc of the Mariana subduction system (Fig. F1). The second segment was devoted to coring and casing a hole on the Philippine Sea abyssal seafloor (Site 1201) and the installation of a broadband seismometer for a long-term subseafloor borehole observatory. On the third segment, an array of advanced piston corer/extended core barrel holes was cored at Site 1202 under the Kuroshio Current in the Okinawa Trough off Taiwan.
The drilling and observatory installation program at South Chamorro Seamount was designed to (1) examine the processes of mass transport and geochemical cycling in the subduction zones and forearcs of nonaccretionary convergent margins; (2) ascertain the spatial variability of slab-related fluids within the forearc environment as a means of tracing dehydration, decarbonation, and water/rock reactions in subduction and suprasubduction zone environments; (3) study the metamorphic and tectonic history of nonaccretionary forearc regions; (4) investigate the physical properties of the subduction zone as controls over dehydration reactions and seismicity; and (5) investigate biological activity associated with subduction zone material from great depth.
The seismic observatory in the Philippine Sea is an important component of the International Ocean Network seismometer net. By filling a large gap in the global seismic station grid, the observatory will help increase the resolution of global tomographic studies, which have revolutionized our understanding of mantle dynamics and structure. Moreover, the observatory will allow more precise study of the seismic structure of the crust and upper mantle of the Philippine plate, as well as better resolution of earthquake locations and mechanisms in the northwest Pacific subduction zone.
Drilling at Site 1201 was also designed to provide more precise basement age constraints for models of backarc spreading in the Philippine Sea as well as high-quality sediment sections that could be used to reconstruct the history of microplate motion, climate change, eolian transport, and arc volcanism in the region.
Drilling at Site 1202 was designed to obtain a high-resolution sediment record under the Kuroshio Current to study global climate change, sea level fluctuation, local tectonic development, and terrestrial environmental changes in East Asia over the past 2 m.y. The Okinawa Trough is one of the few locations in the Pacific where the seafloor under the Kuroshio Current lies above the carbonate compensation depth, allowing the calcareous microfossil record to be preserved, and it is the only location with a high sedimentation rate, allowing high-resolution studies.
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