Tectono-stratigraphy and low-grade metamorphism of Late Permian and Early Jurassic accretionary complexes within the Kurosegawa belt, Southwest Japan: Implications for mechanisms of crustal displacement within active continental margin

Abstract

We characterize the tectono-stratigraphic architecture and low-grade metamorphism of the accretionary complex preserved in the Kurosegawa belt of the Kitagawa district in eastern Shikoku, Southwest Japan, in order to understand its internal structure, tectono-metamorphic evolution, and assessments of displacement of continental fragments within the complex. We report the first ever documented occurrence of an Early Jurassic radiolarian assemblage within the accretionary complex of the Kurosegawa belt that has been previously classified as the Late Permian accretionary complex, thus providing a revised age interpretation for these rocks. The accretionary complex is subdivided into four distinct tectono-stratigraphic units: Late Permian mélange and phyllite units, and Early Jurassic mélange and sandstone units. The stratigraphy of these four units is structurally repeated due to an E–W striking, steeply dipping regional fault. We characterized low-grade metamorphism of the accretionary complex via illite crystallinity and Raman spectroscopy of carbonaceous material. The estimated pattern of low-grade metamorphism showed pronounced variability within the complex and revealed no discernible spatial trends. The primary thermal structure in these rocks was overprinted by later tectonic events. Based on geological and thermal structure, we conclude that continental fragments within the Kurosegawa belt were structurally translated into both the Late Permian and Early Jurassic accretionary complexes, which comprise a highly deformed zone affected by strike–slip tectonics during the Early Cretaceous. Different models have been proposed to explain the initial structural evolution of the Kurosegawa belt (i.e., micro-continent collision and klippe tectonic models). Even if we presuppose either model, the available geological evidence requires a new interpretation, whereby primary geological structures are overprinted and reconfigured by later tectonic events.