Structure of Fault Zones at the Brittle-Plastic Transition Zone of the Continental Earth Crust: A Case Study at the Hatagawa Fault Zone

Occurrence of fault rocks was analyzed along an exhumed brittle-plastic fault zone in the earth crust, the Hatagawa Fault Zone of NE Japan. A conspicuous cataclasite zone with a maximum width of 100 m extends continuously for at least 40 km along the HFZ, corresponding to an inland earthquake as large as M7. The cataclasite zone was formed at the temperatures above 220 degree in Celsius and the activity had terminated by 98 +/- 2.5 Ma with an activity without plastic deformation. Although mylonite zones with a sinistral sense of shear are distributed for the entire length of 45 km along the HFZ, the microstructural and deformation temperature analyses revealed a presence of the zone where deformation temperature was lower than 310 degree in Celsius and that it_fs length along the HFZ is approximately 6 km. In this zone, the microstructure of fault rocks indicates that the deformation condition was at the brittle-plastic transition. It can be considered that the depth of the brittle-plastic transition at lower deformation temperature zone was shallower than other part the fault zone. Such change in the depth of the brittle-plastic transition can result in a significant stress concentration and the nucleation of large earthquakes. There is a possibility that the cataclasite zone along the HFZ was formed by the propagation of the earthquake which was nucleated at the lower deformation temperature zone and this zone was the nuclei of inland earthquakes along the HFZ.