HR: 14:30h
AB: In 1999 we acquired a volume of 3-D seismic reflection data to examine the physical properties that develop along the central Nankai Trough plate-boundary decollement thrust. These data image an 8 x 80 km area and examine the thrust across 70 km of the accretionary wedge from the proto-decollement in the Nankai Trough, down dip into the seismogenic zone, to more than 9 km sub-seafloor. Images of the decollement produced from the preliminary processing of the 3-D seismic data volume reveal a down-dip pattern of alternating regions of high- and low-amplitude decollement reflections that coincide with major down-dip changes in surface slope and thickness of the overlying accretionary wedge. The proto-decollement forms near the top of the hemipelagic sequence of the trench fill section, where little or no reflection is typically observed. Beneath the deformation front, within the proto-thrust zone, the decollement develops a high-amplitude reflection with polarity opposite that of the seafloor reflection. The decollement develops the reflection from an acoustic impedance contrast between the higher velocity/density accretionary wedge and a lower velocity/density underthrust section. The decollement reflection is not an inherent property of the decollement because the proto-decollement does not form along a reflective stratigraphic boundary seaward of the deformation front. Ocean Drilling Program drilling results at Site 808, which lies within the 3-D survey area, found a decrease in density of 0.2 g/cc at the decollement that probably produces the seismic reflection we observe beneath the toe of the accretionary wedge throughout our survey area. The development of a prominent decollement reflection beneath the wedge toe is consistent with rapid addition of overburden and slow consolidation of the underthrust section. The decollement reflection amplitude diminishes by more than a factor of two 11-30 km landward of the initial thrusts at the wedge toe. This is coincident with flattening of the overlying slope of the accretionary wedge, steepening of wedge thrusts, and diminished thrusting within the wedge. However, the decollement reflectivity strengthens markedly 30 km from the deformation front roughly coincident with a significant steepening of the overlying surface slope and thickening of the accretionary wedge. We speculate that first order variations in physical properties of the decollement result from contrasts developed between underthrust and overlying wedge sediments, which are largest where loading of the underthrust section is greatest and consolidation may be delayed.
AN: T52D-05 INVITED
TI: Character of the Central Nankai Trough Subduction Thrust: Preliminary Results from 3-D Seismic Reflection Imaging
AU: * Bangs, N L
EM: nathan@utig.ig.utexas.edu
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs, Rd., Austin, TX 78759 United States
AU: Shipley, T H
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs, Rd., Austin, TX 78759 United States
AU: Moore, G F
AF: University of Hawaii, Department of Geology and Geophysics, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Zhao, Z
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs, Rd., Austin, TX 78759 United States
AU: Gulick, S S
AF: University of Texas, Institute for Geophysics, 4412 Spicewood Springs, Rd., Austin, TX 78759 United States
AU: Nakamura, Y
AF: University of Tokyo, Ocean Research Institute, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Hills, D
AF: University of Hawaii, Department of Geology and Geophysics, 1680 East-West Rd., Honolulu, HI 96822 United States
AU: Kuramoto, S
AF: Geological Survey of Japan, 1-1-3 Higashi, Tsukuba, 305-8537 Japan
AU: Taira, A
AF: University of Tokyo, Ocean Research Institute, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
AU: Park, J
AF: Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Mochizuki, K
AF: University of Tokyo, Ocean Research Institute, 1-15-1 Minamidai, Nakano-ku, Tokyo, 164-8639 Japan
DE: 7230 Seismicity and seismotectonics
DE: 8005 Folds and folding
DE: 8045 Role of fluids
DE: 5102 Acoustic properties
DE: 3025 Marine seismics (0935)
SC: T
MN: Fall Meeting 2000