HR: 15:50h
AN: T52E-09 INVITED
TI: The Evolution of the Decollement in the Nankai Trough Inferred from 3-D Seismic Reflection Data and ODP Leg 196 Drilling
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: Department of Geology and Geophysics, Univ. of Hawaii, 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
AB: In 1999 we acquired a 3-D seismic reflection data volume to trace the evolution of decollement thrust zone from its initiation at the toe of the Nankai trough accretionary wedge down dip into the seismogenic zone. The 3-D seismic data were acquired on the R/V Ewing, and imaged an 8 x 80 km volume of the Nankai Trough accretionary wedge off the Muroto peninsula, which overlaps with ODP Leg 131, 190, and 196 drill sites. The decollement and proto-decollement seismic reflections, with constraints from borehole physical property measurements, map physical properties of the fault zone from the deformation front into the seismogenic zone. A prominent reversed-polarity seismic reflection develops along the proto-decollement about 5 - 7 km seaward of the deformation front within the lower Shikoku Basin facies, 120 m below the boundary with the upper Shikoku Basin facies. This reflection extends beneath the wedge to Site 808, 3 km landward of the deformation front, where LWD data show that the reflection is caused by a decrease in density and seismic velocity at the base of the decollement corresponding to the contrast in porosity of the decollement ($\sim$28\%) and the underconsolditated underthrust sediments ($\sim$50\%). The interface forms within the bottom 3-4 m of the 19-m-thick decollement shear zone, implying that the decollement lies within the uppermost few meters of this underconsolidated and presumably overpressured section. The decollement reflection amplitude diminishes sharply landward from Site 808 beneath the accretionary wedge. Ten km landward of the wedge toe, the decollement reflection is a factor of 5 smaller than beneath Site 808. The reduction in amplitude indicates the underthrust section consolidates significantly to $<$30\% with a loss of approximately 2/3 of the available pore fluid within the interval at the base of the decollement. However, fluid distribution is uneven, as 2-3 km wide patches of relatively high-amplitude polarity-reversed decollement reflections emerge along the decollement and extend to the updip limit of the seismogenic zone, 30+ km down dip from the deformation front. These patches of locally high-amplitude, polarity-reversed reflections are indicators of highly-pressured fluids caused by either delayed consolidation within segments of the fault, or pulses of fluids that dilate the fault zone as they migrating along it.
DE: 5102 Acoustic properties
DE: 7223 Seismic hazard assessment and prediction
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
SC: T
MN: 2001 AGU Fall Meeting