Decollement processes at the Nankai accretionary margin, Southeast Japan; propagation, deformation and dewatering

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doi: 10.1029/95JB00675
Author(s): Morgan, Julia; Karig, Daniel E.
Author Affiliation(s): Primary:
Cornell University, Department of Geological Sciences, Ithaca, NY, United States
Volume Title: Journal of Geophysical Research
Source: Journal of Geophysical Research, 100(B8), p.15,221-15,231. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0148-0227
Note: In English. 62 refs.; illus., incl. sect., 1 table, strat. col.
Summary: The décollement zone, expressed on seismic profiles and observed in drill cores from the Nankai accretionary margin off the southeast coast of Japan, reveals several unique characteristics which appear to distinguish it from thrust faults identified in the same setting. Physical manifestations of these include evidence for the asymmetric distribution of deformation structures about the décollement, the extension of this fault zone well in front of the tectonic deformation front, and the absence of features indicative of precursory shear, for example, folded sediments, shear bands, and penetrative mineral fabrics. These characteristics suggest that the mode of formation and evolution of this décollement zone may be unique from that of most thrust faults. We propose that the décollement zone propagates not as a shear fracture controlled by tectonic stress conditions but rather as a subhorizontal tension fracture propagating under high pore pressures. To test this possibility, physical property measurements and clay mineral fabrics were obtained for several samples from the Nankai décollement zone using computed tomography and X ray texture methods. Our findings suggest that deformation within the décollement zone is partitioned into a volumetric component, preserved as reduced porosities within coherent fragments, and a localized shear component, evidenced by mineral preferred orientations along discrete slip surfaces. We suggest that the reduced porosities result from the destruction of "cementation" in the sediments during the early stages of deformation and may arise from cyclic fatiguing of the sediment induced by fluctuating pore pressures. The nonpenetrative shear fabrics probably develop as the tectonic deformation front migrates seaward, and the weakened protodécollement subsequently accommodates shear displacements along discrete fractures. Copyright 1995 by the American Geophysical Union.
Year of Publication: 1995
Research Program: ODP Ocean Drilling Program
Key Words: 16 Structural Geology; Accretionary wedges; Asia; Continental margin; Decollement; Deformation; Displacements; Fabric; Far East; Fault zones; Faults; Fractures; Geometry; Geophysical profiles; Japan; Leg 131; Leg 132; Leg 196; Nankai accretionary margin; North Pacific; Northwest Pacific; ODP Site 808; Ocean Drilling Program; Pacific Ocean; Pore pressure; Porosity; Propagation; Seismic profiles; Strain; Structural analysis; Tomography; West Pacific
Coordinates: N322105 N322111 E1345646 E1345634
Record ID: 2003042427
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute.