HR: 08:30h
AN: T41A-03    [PDF]
TI: Overpressures and Lithification State of Underthrust Sediments Along the Nankai Accretionary Margin: Implications for D\'{e}collement Evolution
AU: * Morgan, J K
EM: morganj@rice.edu
AF: Rice University, Dept of Earth Science, 6100 Main Street, Houston, TX 77005
AU: Ask, M V
EM: Maria.Ask@ce.luth.se
AF: Lule\aa University of Technology, Division of Rock Mechanics, SE-971 87, Lule\aa, SE-971 87 Sweden
AB: Porous sediments within the frontal portions of active accretionary prisms are subject to concurrent consolidation and diagenesis during burial. The competing effects of these two processes determine the in-situ porosity and effective strength of the sediments that govern the internal structure and deformation behavior of the prism, and ultimately, the mechanical strength and evolution of the underlying d\'{e}collement. New evidence from experiments carried out on Ocean Drilling Program cores collected below the frontal d\'{e}collement zone along the Nankai accretionary margin, suggests that the underthrust section exists in a state of enhanced lithification, capable of resisting rapid consolidation and downcutting of the d\'{e}collement. This condition has implications for the early evolution of the d\'{e}collement, and subsequent behavior near the up dip limit of the seismogenic zone. Uniaxial reconsolidation tests were conducted on four samples from similar stratigraphic levels below the basal d\'{e}collement and seaward proto-d\'{e}collement. Initial sediment yield occurred close to predicted in-situ effective vertical stress states for three of four samples tested, indicating a landward increase in consolidation state. Compared to hydrostatic stress conditions, these results imply a progressive increase in excess pore pressures from ~0.35 MPa at the seaward reference, to more than 3 MPa beneath the active prism. However, significantly higher effective vertical stresses, coincident with the peak strengths of the samples, were required before the sediments experienced measurable porosity reduction and renewed consolidation. This behavior indicates enhanced sediment lithification, a result of bonding or cemention of the grain contacts. Furthermore, the ratio of effective vertical stress at peak strength and at consolidation, referred to as sediment "sensitivity", increases down dip. This implies increased resistance of the subducting strata to low shear stress perturbations, as might occur during aseismic slip along the d\'{e}collement; this may explain why the d\'{e}collement thickens by preferential incorporation of accreted rather than subducting strata. Sensitive sediments, however, can undergo dramatic failure in response to rapid increases in shear stress, e.g., seismically induced. Instantaneous collapse of the sediment matrix can lead to transient increases in pore pressure that further weaken the sediment, allowing downcutting of the d\'{e}collement. The lithified condition of the underthrust package and associated deformation response, may explain the apparent downstepping of the d\'{e}collement fault along the Muroto transect, near the up-dip limit of the seismogenic zone.
DE: 3020 Littoral processes
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 8010 Fractures and faults
DE: 8020 Mechanics
SC: Tectonophysics [T]
MN: 2003 Fall Meeting