HR: 11:20h
AN: T42B-05    [Abstracts]
TI: Stratigraphic Control on Excess Pore Pressure at the Plate Boundary Fault of Nankai Trough
AU: * Underwood, M B
EM: UnderwoodM@missouri.edu
AF: Department of Geological Sciences, University of Missouri, Columbia, MO 65211 United States
AU: Bellew, G M
EM: gmb13e@mizzou
AF: Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO 65211 United States
AU: Loehr, J E
EM: eloehr@missouri.edu
AF: Department of Civil and Environmental Engineering, University of Missouri, Columbia, MO 65211 United States
AB: One of the unresolved issues regarding the subduction front of Nankai Trough is the control over stratigraphic position of the decollement. The Muroto Transect area (ODP Sites 808, 1173, 1174) is atypical of the regional system because hemipelagic strata within the lower Shikoku Basin accumulated above a basement high formed by back-arc spreading and late-stage seamount volcanism. The Ashizuri Transect area (ODP Site 1177) is more characteristic of the system as a whole. Seismic reflection and coring prove that sandy turbidites are common within the lower Shikoku Basin facies, except along the Muroto Transect. Consolidation tests demonstrate that the stratigraphic equivalent of the decollement at Site 1173 is slightly overconsolidated, whereas strata below the decollement horizon are heavily overconsolidated. Samples from Site 1177, in contrast, are slightly to heavily underconsolidated within and immediately below the stratigraphic equivalent of the decollement. Mudstones within the underlying Shikoku Basin turbidite facies show moderate to strong overconsolidation. Evidently, the turbidite facies drains very effectively through a process of compaction-driven fluid flow. There is a 40-m interval of mudstone above the turbidites, however, which acts as an aquitard. Stratigraphic compartments of overpressure build up long before the subducting strata reach the deformation front, especially where the uncemented sand packets pinch out. Tectonic consolidation and up-dip fluid flow beneath the decollement exacerbate the overpressured condition. The unusually high fluid pressures reduce effective stress and provide the optimal location for propagation of the plate boundary fault.
DE: 8045 Role of fluids
DE: 8105 Continental margins and sedimentary basins
DE: 5114 Permeability and porosity
DE: 3022 Marine sediments--processes and transport
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting