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Physical properties and mechanical state of accreted sediments in the Nankai Trough, Southwest Japan Arc

Daniel E. Karig
Daniel E. Karig
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January 01, 1986

The general shape and mechanical behavior of an accretionary wedge can be modeled with a pressure dependent Coulomb rheology, but measurements of physical properties and structural geometries in the Nankai (southwest Japan) wedge demonstrate that a simple, constant property model is inadequate. Porosity in the Nankai prism decreases downward slightly faster than in the correlative trench section, reflecting an increase in effective mean stress as well as a change from uniaxial to biaxial strain. The coefficient of cohesion (C0) is observed in DSDP cores to increase with decreasing porosity. The dip angles of a conjugate shear fracture set observed in these cores, as well as the frontal thrust, show that σ1 dips about 5° toward the trench at depths of 400 m to 1000 m, and the coefficient of internal friction (μm) is about 0.36, and the fracture strength at 1000 m can be approximated by τ = 1MPa + 0.36 σn′. Pore fluid pressures at this position are estimated to be moderately greater than hydrostatic (λ = 0.75), leading to a coefficient of sliding friction along the basal décollement of about 0.2

In contrast to the linear surface slope of the non-cohesive, constant property wedge model, the toes of accretionary wedges are generally convex upward. A minor fraction of this convexity can be explained by the effect on the force balance of an arcward decrease in average porosity. Moreover, instantaneous horizontal shortening in the Nankai wedge is more strongly concentrated toward the wedge toe than that in a non-cohesive constant-property model. These observations require an arcward and downward increase in average Coulomb parameters of wedge sediments, largely because of the reduction in average porosity and its effect on μm.

Discontinuities in surface slope, which mark zones of persistent thrusting, represent material at a different state of failure than that within the thrust sheets. Concepts developed in soil mechanics suggest that these zones progress through peak strength into a lower strength residual state with a further reduction in porosity and major fabric reorganization.

Although the porosity within these broad shear zones appears less than that outside, some parts of the zones remain zones of weakness. The central parts of these zones may actually become strengthened with continued strain and water loss. This effect would be enhanced if fracture permeability leads to preferential water flow along the fault zones because this would require a pore pressure decrease into the fault zone and thus higher effective normal stresses in that zone.

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GSA Memoirs

Structural Fabrics in Deep Sea Drilling Project Cores From Forearcs

J. Casey Moore
J. Casey Moore
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Geological Society of America
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January 01, 1986




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