Kinematic analysis of shear zones in sandstone and mudstone of the Shimanto belt, Shikoku, SW Japan

Abstract

I describe kinematic criteria for sense-of-shear in unmetamorphosed interbedded sandstone and mudstone of the Tertiary Shimanto accretionary complex at Ariigawa, Kǒchi Prefecture. In certain zones parallel to bedding and up to several meters wide, sandstone layers with internal bedding-parallel turbidite laminations are broken into blocky rectangular fragments or inclusions. Many inclusions rotated counterclockwise and so indicate a sinistral sense-of-shear with respect to the present orientation of bedding. Asymmetric folds corroborate this interpretation. Amounts of rotation are variable and are inversely proportional to the axial ratios of fragments. Fragments with axial ratios greater than about 4–6 did not rotate, while those with ratios between 4 and 1 record progressively greater maximum rotations. These observations accord with theoretical and experimental analyses describing the behavior of rigid inclusions in a matrix subjected to simultaneous simple shear and pure shear. The data from the Shimanto suggest that: (1) the rotation-inducing deformation included a coaxial component and departed from strictly non-coaxial simple shear; and (2) total shear strains were of the order of 3–5.