Sedimentation in forearc basins, trenches, and collision zones of the western pacific: A summary of results from the ocean drilling program

The comprehensive strategy pursued by the Ocean Drilling Program in the western Pacific Ocean resulted in significant improvements in our understanding of arc-related geologic processes and products. Many types of depositional environments were examined, including intraoceanic forearc basins, backarc basins, trenches, and ridge-trench collision zones. Most of the ODP sites were chosen to address tectonic, geochemical, or hydrologic objectives, but a wealth of new sedimentologic information was generated at the same time. We now have a better appreciation of how intraoceanic forearcs are affected by diverse depositional processes (e.g., debris flows, turbidity currents, and vertical settling of tephra, hemipelagic mud, and pelagic ooze). Detailed turbidite stratigraphy deduced from FMS borehole imagery shows that the Izu-Bonin forearc basin is characterized by a noncyclic basin-plain style of sedimentation, rather than a channelized submarine fan or a channelized slope apron. Detrital modes for volcaniclastic sands in the Izu-Bonin forearc are consistent with an undissected-arc provenance; subdivisions of volcanic texture and glass type provide additional details and support the idea of extensive subaerial production and mixing of epiclastic detritus. Evolution of the Tonga forearc has been affected by a complicated interplay of tectonic subsidence, eustacy, and arc volcanism. Stratigraphic evolution of the Nankai accretionary prism produced a classic upward-coarsening and upward-thickening mega-cycle representative of transition from abyssal hemipelagites to trench turbidites. Turbidite sands within the outer trench-wedge facies display indicators of northwest-directed paleoflow; those flows probably experienced a phase of upslope movement onto the outer wall and deflection back toward the Nankai trench axis. Sand petrography and clay mineralogy collectively support a mixed detrital provenance for the Nankai trench wedge; the main source area is located in the Izu-Honshu collision zone. Because of high geothermal gradients, clay-mineral diagenesis within the Nankai prism has progressed to advanced stages at relatively shallow depths. The initial occurrence of illite/smectite (I/S) mixed-layer clay, for example, occurs approximately 555 m below the seafloor at a temperature of 65°C. Ridge-trench collision in the Vanuatu subduction zone has affected rates of trench sedimentation, pathways of sediment dispersal, formation and destruction of sediment ponds on the forearc slope, and facies partitioning at the décollement. The collision zone contains a well-defined accretionary prism, and intervals of kaolinite-rich clay serve as preferred sites for propagation of the décollement.