Structural evolution of multiple ductile shear zone system in the Ryoke belt, Kinki Province

The ductile shear zones which developed in the Kinki Province of the low P/T Ryoke metamorphic belt, are mainly divided into the Ryoke southern marginal shear zone (RSMSZ) and Ryoke inner shear zone (RISZ). The RSMSZ is located at relatively lower structural level than the RISZ. The RSMSZ in the Kayumi district is regarded as subhorizontal ductile shear zone with westward directed sense of shear. The mylonitization related to the formation of the RSMSZ occurred at the temperature conditions of 470-350 °C (LT, low temperature to MT, medium temperature) during 83-60 Ma. The stretching direction which is represented by the mylonitic lineation is mostly oriented to EW. Strain analysis suggests that in the RSMSZ a NS directed stretching presumably occurred before the LT and MT mylonitization. On the other hand, in the RISZ (the Kasagi, Oikawa, Hakusan-Joryu and Takehara district) the mylonitization characterized by NS stretching occurred at 350-600 °C (LT to HT). A top to the southward directed sense of shear dominates for the LT and MT mylonitization in the RISZ. The HT mylonitization occurred under coaxial plane strain regime.
The temporal change of stretching direction from NS to EW in the RSMSZ appears to coincide with the change of subduction direction of the oceanic plate from NS to EW at ca. 85 Ma, suggested by the deformation temperature and mineral isotopic age in the RSMSZ. However, the kinematic data suggest that during the LT and MT mylonitization, the stretching directions were spatially partitioned into NS in the RISZ and EW in the RSMSZ. The partitioning of stretching direction is different in the different structural level in the continental crust. The spatial variation of stress field, which is indicated by dyke-orientations in the Ryoke metamorphic belt may be responsible for the spatial partitioning of stretching direction (heterogeneous flow).