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Shearing of partially consolidated sediments in a lower trench slope setting,Shimanto Belt,SW Japan
Institution:1. Department of Earth and Environmental Sciences, The Earth and Environmental Science System Research Center, Chonbuk National University, Jeonju 561–756, Republic of Korea;2. Geology Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, South Korea;3. Divisions of Earth and Environmental Sciences, Korea Basic Science Institute, Ochang 363–883, Republic of Korea
Abstract:Detailed mapping of a coastal platform in Shikoku, SW Japan, provides evidence for progressive deformation in partially lithified sediments. The Eocene sediments involved are interpreted as lower slope basin deposits. An assemblage of listric normal faults, sheath folds, broken formations and late-stage faulting has developed during the sediments' burial and uplift history. These structures are typical of many other areas in the Shimanto Belt of Shikoku. Despite the ‘soft’ sediment style of deformation, the consistency of the fold orientations relative to the regional foliation suggests that they are valid kinematic indicators. A sequence of extensional faulting overprinted by synchronous folding and shearing is recognized. This is interpreted as the response of the sediments to shape changes in the accretionary basement induced by shortening. A general model has been constructed for the evolution of the structures: it is proposed that early listric normal faults are subsequently deformed either by shearing along planar surfaces or by motion over frontal and lateral ramps. Back-rotation of sediments during progressive shortening near the front of the prism tightens the fold hinges and rotates the fold axes towards the local shear direction. Alternative sequences which could account for the observed geometries are also discussed.
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