Abstract

The Shimanto accretionary complex of eastern Kyushu records a progressive deformational history that reflects the gradual underthrusting of sediments beneath the prism and their eventual accretion at depth to the base of the complex. Initial deformational style is controlled by grain size and porosity of the sediments. Stratal disruption of coarser grained, highly porous sandstones is accommodated by ductile intergranular flow, whereas finer grained, less porous sandstones and mudstones deform by localized shearing and brittle failure. As further reductions in porosity accompany lithification, both sandstones and mudstones continue to deform in a brittle manner. With progressive increases in pressure and temperature accompanying tectonic burial, shear bands and minor isoclinal folds are generated, deforming earlier formed fabrics and indicating a south-southeasterly shear direction. With increasing strain, these folds undergo rotation, and their axes approach that of the transport direction. A pronounced stretching lineation is typically associated with regions where these folds have been rotated, suggesting intense noncoaxial layer-parallel shear. These zones of higher shear strains may be related to thrust-fault propagation during accretion of material to the base of the prism.

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