WHAT ARE THE POSSIBLE SEDIMENTARY-TECTONIC-CLIMATE LINKAGES ALONG THE MIOCENE-TO-RECENT SOUTHWEST JAPAN MARGIN?

Scientific drilling* in the Nankai region of SW Japan recovered cores and cuttings of sediments deposited in trench, slope, and forearc basin settings during a time of change in both global climate and regional tectonics. Along with the Kumano 3D seismic volume, these materials have allowed us to evaluate the relative importance of sedimentation on the mechanics of the accretionary system. Late Miocene to mid-Pliocene warming coincided with the development of a regional unconformity that cut deeply into the preexisting accretionary prism. Then, with Pliocene cooling, and then later during Pleistocene cooling, slope basins formed along out-of-sequence thrusts and associated folds. The largest of these out-of-sequence faults evolved into the Megasplay Fault which bounds the seaward side of the Kumano forearc basin, and at times caused a pattern of landward-vergent sedimentation as the underlying prism was exhumed and eroded. Thus, one interpretation of the system is that the Nankai wedge was first subcritical as sedimentation decreased during a relatively warm period of the Miocene-Pliocene, and then achieved criticality followed by super-criticality as sediment flux to its slope and/or toe increased. However, prior to the Late Miocene the margin underwent highly oblique subduction as the Izu-Bonin (IZB) arc migrated from west to east. Although still controversial, U-Pb zircon provenance analysis from below the unconformity corroborates the “mobile” IZB model. Rotations within the wedge continued into the Early Pliocene and subduction of seamounts and adjacent sediments followed. Thus, another interpretation of the Nankai margin is that tectonic rotations and changes in the incoming plate drove the spatiotemporal strain and sedimentary patterns. One obstacle to testing the alternate hypotheses of climatic vs. tectonic controls on the SW Japan margin is that the major provenance signatures appear relatively fixed since Miocene time, but in either hypothesis the mechanical impact of sedimentation on the accretionary wedge is pronounced.

*as part of the series of NanTroSEIZE expeditions during the Integrated Ocean Drilling Program and International Ocean Discovery Program