Change in fabric of melange in the Shimanto Belt, Japan: Change in relative convergence?

Abstract

Systematic changes in fabrics of melanges have been detected in rocks of the Shimanto Belt, Japan. Several shear indicators, including S‐C structures, Riedel shears, and folds, in the melanges indicate consistent sense of shear both in outcrop and microscopic scales. The deformation mechanisms, independent particulate flow and pressure solution, suggest that the deformation started under unlithified conditions and progressed through lithification due to strain hardening. After lithification, cataclastic deformation occurred locally, but most of the earlier deformation features are preserved. This process appears to have taken place along the decollement beneath the off‐scraped accretionary prism. Sinistral reverse sense of shear is observed in the Campanian melange, whereas a dextral reverse sense of shear is preserved in the early Eocene melange in the Shimanto Belt. This change in fabric is interpreted to be linked to a change in relative convergence between the subducting and overriding plates. Three different plate models have been proposed for the Late Cretaceous to Early Tertiary western Pacific margin: (1) subduction of the Kula‐Pacific ridge along the continental margin, (2) subduction of the Pacific Plate since 85 Ma, and (3) subduction of the aborted Kula‐Pacific ridge after 43 Ma. The observed change in fabric of melange is consistent only with the third model, which predicts a change in relative convergence from sinistral to dextral at the appropriate time.