Ocean Drilling Program, United States

Ocean Drilling Program Leg 195 consists of two science segments. The first segment is devoted to coring and setting a long-term observatory at the summit of South Chamorro Seamount (Site MAF-4B), which is a serpentine mud volcano on the forearc of the Mariana subduction system. The second segment is devoted to coring and casing a hole on the Philippine Sea abyssal seafloor (Site WP-1B) coupled with the installation of a broadband seismometer for a long-term subseafloor borehole observatory. Drilling at the South Chamorro Seamount will (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 supra- subduction 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 station grid, the observatory will help increase the resolution of global tomographic studies, which have revolutionized 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 WP-1B will also provide more precise basement age constraints for models of backarc spreading in the Philippine Sea as well as high-quality sediment sections that may be used to reconstruct the history of microplate motion, climate change, aeolian transport, and arc volcanism in the region.