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