HR: 1340h
AN: T13B-0462    [Abstracts]
TI: 3-D Depth Interval Velocity Model Analysis and Interpretation of the Nankai Trough Margin (Southwest Japan).
AU: * Costa Pisani, P
EM: patrizia@hawaii.edu
AF: Geology and Geophysics Dept., University of Hawaii, 1680, East West Rd, Honolulu, HI 96822 United States
AU: Kramer, G
EM: gkramer@ees.nmt.edu
AF: Earth and Environmental Science Dept., New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 United States
AU: Moore, G F
EM: gmoore@hawaii.edu
AF: Geology and Geophysics Dept., University of Hawaii, 1680, East West Rd, Honolulu, HI 96822 United States
AU: Tobin, H J
EM: tobin@nmt.edu
AF: Earth and Environmental Science Dept., New Mexico Tech, 801 Leroy Place, Socorro, NM 87801 United States
AB: The 3-D Muroto seismic transect images the Nankai Trough margin south of Shikoku island, Japan, where the Philippine Sea plate subducts at a rate of 2-4 cm/y beneath the accretionary prism. The ODP drill sites of Legs 190-196 show the basaltic basement of the incoming plate overlain by the Shikoku Basin Lower Miocene-Lower Pleistocene hemipelagic mudstone, and the Pleistocene-Recent Nankai Trough turbidites. A geological meaningful interval velocity volume has been modeled using 3-D pre-stack depth migration (PSDM) calibrating the seismic volume with known depths at the drill sites, and extended to the entire seaward part of the Muroto transect. Our final PSDM velocity gradients closely resemble the drill holes logging and core velocities. Hence, the PSDM velocity field is representative of the "in-situ" velocities. In the trench area, the velocities increase with depth within the turbidites (1525-1580 m/s), as expected for a section with a normal compaction trend (decreasing porosity with depth). In the Upper Shikoku Basin Facies (USBF) hemipelagic unit (1580-1860 m/s), the P-wave velocity increases downhole although the porosity remains approximately constant within the entire unit. A conclusion of Leg 190 is that velocity in this interval can be controlled by cementation. In the upper part of the Lower Shikoku Basin Facies (LSBF) hemipelagic unit (1860-1920 m/s) the velocity increases until about 50-60 m below the top of LSBF, where a velocity inversion of about 50 m/s is recorded. The velocity inversion can be connected to an anomalously high fluid content at that level in the LSBF unit. We interpret this depth (5200 m below the sea level) as the stratigraphic equivalent of the d‚collement that occurs landward in the imbricate thrust zone. Below the velocity inversion zone, the PSDM velocities increase again with depth in the rest of the LSBF and the Volcaniclastic units (1870-2160 m/s). This is in agreement with a decrease in porosity versus depth in the underthrusting sediments. At about 5500 m, the velocity sharply increases to 3500-3800 m/s at the top of the oceanic crust. Landward in the accretionary prism, the PSDM vertical gradient maintains a similar trend to the one described in the trench, though the velocity values are slightly higher. The average PSDM vertical gradient above the d‚collement decreases from 0.99 s^-1 seaward of the trench axis to 0.87 s^-1 landward near the frontal thrust. The velocity inversion described in the trench extends almost to the first 30 km of the d‚collement (first 1200 cdps). The velocity inversion is about 150 m/s at the 808 drill site location, and it increases landward to 200-300 m/s (cdps interval 800-1600). The vertical gradient in the underthrusting section decreases from 0.97 s^-1 in the basin to 0.72 s^-1 beneath the frontal thrust. In this unit, the lateral velocity gradient between the basin and the deformation front is 0.02 s^-1, corresponding to a 10% thinning in the direction of the subduction. This has been related to dewatering processes due to compaction of the underthrusting sediments. Velocity variations occurring along strike of the Muroto transect are generally negligible and a similar velocity field pattern is observed all along the data set. More in detail, the southwest sector of the Muroto transect around inline 215, shows lower velocities in the underthrusting section than the northeast sector around inline 284, on the order of 100 to 200 m/s.
DE: 0935 Seismic methods (3025, 7294)
DE: 3025 Marine seismics (0935, 7294)
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 5102 Acoustic properties
DE: 5114 Permeability and porosity
SC: Tectonophysics [T]
MN: Fall Meeting 2005