Deformation history of low-grade schists in the Joetsu region, central Japan : Correlation between the Kawaba and Mizunashigawa metamorphic rocks

Abstract

Mesozoic to Paleozoic systems in the Joetsu region show complex geologic structure caused by crustal movements and sub-sequent Cretaceous to Neogene plutonism and volcanism. This study reconstructed a once continuous low-grade schist zone by tracing sporadically distributed schist bodies with a similar tectonic history. Their representatives are the Kawaba metamorphic rocks and Mizunashigawa metamorphic rocks. Both schists have similar deformation chracteristics such as assemblages, occurrence, fabric and deformation mechanism of deformed minor structures, and inverted shear regimes that form the deformed minor structures. In addition, deformation grade expressed by mean ductility at the formative stage of each deformed minor structure in the both schists shows sequential decrease with time. Earlier and later deformation phases recognized in the Mizunashigawa metamorphic rocks correspond to the earlier phase with high mean ductility and the later phase with low mean ductility in the Kawaba metamorphic rocks, respectively. The Kawaba and Mizunashigawa metamorphic rocks are regarded as remnants of a once continuous N-S trending schist zone before the disturbance of the zonal arrangement of the Joetsu region caused by Cretaceous-Neogene crustal movements, plutonism and volcanism.