Regional distribution and sedimentation history of the incoming sediments in the Nankai Trough

The Nankai Trough is formed by subduction of the Philippine Sea plate to the northwest beneath the Eurasian plate at a rate of ~4 cm/y (Seno et al. 1993). Historically, large earthquakes along the Nankai subduction zone have occurred with a recurrence interval of 100-200 years. Since incoming sediments may constrain strength of the upper plate and subsequent seismogenesis at depth, a study on sedimentary structure and physical properties of the Nankai Trough sediments is crucial to figure out a mechanism of megathrust earthquake generation along the Nankai subduction zone. In this study, we aim to figure out regional distribution of the incoming sediments in the Nankai Trough and reconstruct its sedimentation history. We interpreted single channel and multichannel seismic reflection profiles that have been acquired in the Nankai Trough margin by Japan Agency for Marine-Earth Science and Technology (JAMSTEC) since the year of 1997. We focus on seismic reflection structure and stratigraphy of the incoming sediments in the Nankai Trough. Based on seismic characteristics, we could identify 5 major seismic units from top to bottom: (1) trough turbidite fill, (2) upper Shikoku Basin sediments consisting of hemipelagic mud and volcanic ash, (3) middle Shikoku Basin sediment of volcaniclastics, (4) lower Shikoku Basin sediments consisting of turbidites and hemipelagic mud, and (5) oceanic crust of basalt. In particular, we recognize 3 different turbidte sediments within the lower Shikoku Basin (LSB) unit: LSB-T1, -T2, and -T3. The shallow LSB-T1 is widely distributed in the east Nankai Trough including offshore Kumano Basin, with pinch-out off Cape Muroto of Shikoku Island. The middle LSB-T2 is confined to a region off Cape Ashizuri of Shikoku Island. The deep LSB-T3 shows a local distribution off Kumano Basin. Variation of oceanic basement highs appears to have influenced those turbidite sedimentation along the Nankai Trough. In this talk, we will show regional distribution of the turdidites within the LSB, and discuss implications on the subduction processes in the Nankai Trough.