Fabric-based criteria to distinguish tectonic from sedimentary mélanges in the Shimanto accretionary complex, Yakushima Island, SW Japan

Abstract

Fabric analyses based on the anisotropy of magnetic susceptibility (AMS) provide a criterion with which to distinguish tectonic from sedimentary mélanges in the Tertiary Shimanto accretionary complex, Yakushima Island, SW Japan. Sedimentary mélanges are characterized by fabric overprinting associated with compaction and diagenesis. The magnetic ellipsoids for such mélanges represent a cleavage/bedding-parallel magnetic foliation, similar to that obtained for coherent sedimentary layers in nearby rocks. The tectonic mélanges and tectonically formed duplex structure are characterized by grain rotation, whereby the grain short-axis is rotated about axes oriented parallel to the intersection between S- and C-planes and the long axis of pencil cleavage, perpendicular to the shear direction. The AMS data obtained for tectonic mélanges and duplex structure show a less intense magnetic foliation compared with the sedimentary mélanges, with Kmax (the maximum magnetic susceptibility) axes oriented parallel to the intersection between S- and C-planes and the long axis of pencil cleavage, and Kmin (the minimum magnetic susceptibility) axes showing a girdle distribution about Kmax. These contrasting paths of fabric development may reflect different stages of mélange formation during the overall history of sediment deposition, burial, accretion, and uplift. Sedimentary mélanges are likely to be strongly compacted, given that they formed soon after sedimentation. In contrast, tectonic mélanges and duplex structure contain a shear-induced fabric because they develop at a relatively late stage, once the sediments have been sufficiently compacted for such a fabric to develop, possibly at levels near the maximum burial depth.