Initial Reports of the Deep Sea Drilling Project - Volume 87

This volume covers Legs 87A and 87B of the cruises of the Drilling Vessel Glomar Challenger. The location, directions, and rates of lithospheric plate convergence for most of the active margins of the world are well established by geophysical data. Yet even after 12 Glomar Challenger voyages devoted to drilling active margins, the expression of lithospheric convergence in the geologic features of the setting remains poorly understood. On a larger scale, the mechanisms by which plate convergence causes mountains to rise above subduction zones and the origin of continental rocks are a matter of conjecture. With these broad questions in mind, Leg 87, the last DSDP expedition dedicated to drilling active margins, was designed to investigate two very different regions: the Nankai Trough and the Japan Trench. The transect in the Nankai Trough was selected to investigate: (1) subduction dynamics, namely, the physical properties of sediments; (2) kinematics and structural evolution of the forearc; and (3) sedimentation processes, both a determination of the relative importance of gravitational and tectonic processes in the deformation of the trench slope and a refinement of concepts regarding sedimentation in deep-sea fans. Additionally, the magnetostratigraphy, biostratigraphy, and tephrastratigraphy, as well as heat flow, paleoceanography, geochemistry of clathrates, and nature of pore water, were all distinct aspects of this investigation. The main objective in drilling the Japan Trench was to investigate the history of vertical motion of the continental margin near the trench and to penetrate a basal unconformity. Subsidence, probably associated with tectonic erosion, persisted throughout the Miocene, diminished in the Pliocene, and may have been followed by a slight amount of uplift since the beginning of the Pleistocene. Is this history unique to the previously drilled sites, or is it representative of the entire forearc? Two sites were drilled in the Nankai Trough (582-583) and one in the Japan Trench (584). Leg 87A began in Yokohama, Japan, and ended in Yokohama, Japan, taking place between June-July, 1982. Leg 87B began in Yokohama, Japan, and ended in Hakodate, Japan, taking place between July-August, 1982.