HR: 0800h
AN: S51B-0163    [Abstracts]
TI: Along strike changes in basement topography and sediment thickness in the northern Shikoku Basin: Variable inputs to the Nankai Trough Seismogenic Zone
AU: * Ike, T
EM: tike@Hawaii.edu
AF: SOEST, Univ. of Hawaii, 1680 East-West Rd.,POST 813, Honolulu, HI 96822 United States
AU: Moore, G F
EM: gmoore@Hawaii.edu
AF: SOEST, Univ. of Hawaii, 1680 East-West Rd.,POST 813, Honolulu, HI 96822 United States
AU: Okano, T
EM: tokano@jamstec.go.jp
AF: CDEX, JAMSTEC, Natsushima 2-15, Yokosuka, Kanagawa, 237-0061 Japan
AU: Kuramoto, S
EM: s.kuramoto@jamstec.go.jp
AF: CDEX, JAMSTEC, Natsushima 2-15, Yokosuka, Kanagawa, 237-0061 Japan
AU: Taira, A
EM: ataira@jamstec.go.jp
AF: CDEX, JAMSTEC, Natsushima 2-15, Yokosuka, Kanagawa, 237-0061 Japan
AB: The very thick terrigenous sediments in the Nankai Trough and the northern Shikoku Basin mask significant basement irregularities that will soon be subducted into the Nankai seismogenic zone. We present a new seismic reflection data set across the northern Shikoku Basin that images remarkable variations in basement relief and corresponding variations in sediment thickness and type. The basement relief has a wavelength of ~5-15 km, with the amplitude of the peaks and troughs generally ~300-400m. Two significant troughs exist south of Shikoku that are each ~ 12 -15 km wide and 1900 m deep. These troughs strike NW-SE and are at least 40 km long. They are mostly filled with high amplitude continuous reflections that onlap the trough margins. We correlate these reflections with the mid-Miocene lower Shikoku Basin turbidite sequence drilled at ODP site 1177. Along strike approximately 125 km to the NE, off the Kii Peninsula, there is a significant basement high that is just entering the trench ~ 15 km from the base of the inner trench slope. It is roughly oval with a diameter of ~50km parallel to the trench axis and ~20 km perpendicular to the trench, with a peak that is 1400 m higher than the adjacent crust. This basement high (seamount) is covered by the hemipelagic Shikoku Basin sediments that thin from 900 m on the SW flank to 200 m at the summit. The Shikoku Basin turbidite section that fills the troughs to the SW is absent on the seamount and its flanks. Because of the variations in sediment thickness around this seamount, its bathymetric summit is only 800 m above the adjacent seafloor while the basement relief is 1400m. Between the seamount and Zenisu Ridge (~ 60 km NE), the basement deepens by 500 m and has subdued relief (compared to that SW of the seamount). Basement is overlain by the Shikoku Basin hemipelagic sequence and onlapped by young trench turbidites. These significant along strike variations in basement topography (1400m high seamounts and 1900m deep troughs), sediment thickness (200-2000m) and sediment type (hemipelagics vs turbidites) will soon be input to the Nankai seismogenic zone and are representative of the kinds of irregularities that have been input in the geologic past. Such variations must contribute to along-strike differences in seismogenic behavior.
DE: 3022 Marine sediments--processes and transport
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Seismology [S]
MN: 2004 AGU Fall Meeting