Many destructive subduction-zone earthquakes nucleate beneath the sedimentary forearc basins above accretionary prisms. Such behavior certainly characterizes the Nankai prism off Japan?s Kii Peninsula where the Kumano basin sits above an out-of-sequence, or megasplay fault that is likely seismogenic. Nankai-prism behavior is largely governed by the frontal thrust system, incoming trench-sediment, and décollement zone. Yet, slope and forearc basin processes are also likely important to the mechanics of the margin. Because of drilling during IODP Expeditions 315, 316, and 319, it is possible to examine forearc basin evolution at a level not previously attainable. IODP drilling has revealed that the base of the Kumano basin roughly coincides with the Plio-Pleistocene boundary, raising the possibility that a change in sedimentation patterns affected forearc basin initiation. It is proposed to test the hypothesis that changes in sedimentation patterns from trench-fed sediment into the frontal accretionary prism to hinterland-fed slope and forearc basin deposition, coincide with changes in accretionary wedge mechanics. If climate-controlled sedimentation patterns do exert a large control on the Nankai margin tectonics, it would provide a good example of the controversial link between climate-driven sediment flux and tectonic deformation. We will test our hypothesis in the context of a margin-evolution model. Our model predicts that during the Plio-Pleistocene transition from low-to-high sediment flux, out-of-sequence thrusts and slope-basins developed along the Nankai prism in an effort to return the wedge to critical taper. The internal wedge deformation ultimately formed an incipient forearc basin. During the Pleistocene, uplift of the Japanese Alps caused significant sediment flux to the toe of the wedge and resulted in seaward advance of the accretionary prism, sediment underthrusting, and formation of the megasplay. Even greater sediment flux after the Mid-Pleistocene resulted in rapid infilling of the Kumano Basin, and loading of the seismogenic plate boundary. We will evaluate the key components of this descriptive model using a kinematic restoration of 3-D seismic reflection data, down-hole and core constraints on structural orientation and tilting histories, sedimentary provenance of key sections, and Coulomb wedge analytical solutions. Japan?s Nankai accretionary prism is the site of the 1944 and 1946 great earthquakes. Our work will generate needed datasets for the NantroSEIZE experiment, which seeks to study and monitor seismic deformation below the forearc basin, in contrast with predominantly aseismic deformation near the prism?s toe. The focus of the proposed research is of great interest to the GeoPRISMS program, leading to cross NSF programmatic impact. We will partner with Japanese colleagues, enhancing the international impact. Our education and outreach activities will be through support of a graduate student, and generating learning modules from our research. These modules will build on existing undergraduate tutorials, and be additionally tailored to UT-Austin programs that cater to predominantly minority-serving public school teachers, thereby increasing domestic public awareness about geological processes and natural hazards.

National Science Foundation (NSF)
Division of Ocean Sciences (OCE)
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Thomas Janecek
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University of Texas Austin
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