Development of the Hatagawa Fault Zone clarified by geological and geochronological studies

The occurrence of mylonite and cataclasite, mineral assemblages of cataclasite, and the K-Ar ages of surrounding granitic rocks and dikes were studied to examine the possibility that the Hatagawa Fault Zone (HFZ), NE Japan was experienced under the conditions of the brittle-plastic transition. The Hatagawa Fault Zone is divided into three structural settings: mylonite zones with a sinistral sense of shear and a maximum thickness of 1 km, a cataclasite zone with a maximum thickness of about 100 m, and locally and sporadically developed small-scale shear zones. Occurrence of epidote and chlorite, lack of montmorillonite in cataclasite, and the coexistence of cataclasite and limestone mylonite suggest that the cataclasite was deformed at temperatures higher than 220°C. Crush zones in the mylonite near the cataclasite zone were recognized in one outcrop; they have a structure concordant with the surrounding mylonite and some fragments in them are dragged plastically. Granodiorite porphyry dikes near the HFZ intruding into cataclasite and mylonite with a sinistral sense of shear exhibit no deformational features. K-Ar ages of hornblende from host granitic rocks and from one granodiorite porphyry dike are 126 ± 6 to 95.7 ± 4.8 and 98.1 ± 2.5 Ma, respectively. These indicate that the fault activity gradually changed from mylonitization to cataclasis within 28 m.y., and suggest that the HFZ underwent a brittle-plastic transition during its activity.