2020 Volume 126 Issue 12 Pages 663-678
This study examined meso- and microscale structures in tectonic mélange of the Jurassic Chizu Complex in the Kuse-Kagamino area, Okayama Prefecture, Southwest Japan. The Chizu Complex consists of coherent clastic rocks (mudstone, sandstone, and alternating beds of mudstone and sandstone), and chaotic rocks of sedimentary origin. They have been deformed by shearing to form tectonic mélange. Four stages of deformation are recognized: (1) layer-parallel elongation and extensional fractures; (2) layer-parallel shearing and disaggregation; (3) formation of slaty cleavage subparallel to bedding; and (4) formation of crenulation cleavage, overprinting the slaty cleavage. The shearing of the second stage formed asymmetric structures such as a Y-P composite planar fabric and disaggregation of sandstone clasts. It is likely that disaggregation enhanced the deformation of the sandstone clasts by grain boundary sliding. The slaty cleavage of the third stage is defined by the preferred orientation of platy minerals and formed by layer-perpendicular flattening that resulted in the development of symmetrical pressure shadow around pyrite grains. The crenulation cleavage of the fourth stage indicates layer-parallel shorting. The stage one deformation affected poorly consolidated sediments at shallow depths, whereas the following three deformation stages affected consolidated sediments at greater depths. The asymmetric geometric relationship between Y shears and the P foliation of the second stage indicates top-to-the-NNE shear, after correcting for the rotation of Southwest Japan related to the opening of the Japan Sea.
