Structures of Fault Zones in the Brittle-plastic Transition Zone of the Continental Earth's Crust

Abstract

Occurrences of fault rocks were analyzed along an exhumed brittle-plastic fault zone in the Earth's crust, the Hatagawa Fault Zone (HFZ) 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 a rupture size of an inland earthquake as large as M7. The cataclasite zone was formed at temperatures above 220 t, and its activity had terminated by 98.1 ± 2.5 Ma. Mylonite zones with a sinistral sense of shear are heterogeneously distributed for the entire 45 km length along the HFZ. The temperatures calculated by two-feldspar thermometry from most of the mylonite zones are above 360°C. On the other hand, there is a mylonite zone with a length along the HFZ of approximately 6 km where the temperatures calculated are below 360°C. Microstructures of the fault rocks in this zone indicate that the deformation condition was at the brittle-plastic transition. The distribution of fault rocks along the HFZ suggests that only limited areas of the brittle-plastic transition were plastically deformed when the present exhumed level was in the brittle-plastic transition zone. Such heterogeneity of plastic deformation in the brittle-plastic transition zone can result in a significant stress concentration and the nucleation of large earthquakes. The cataclasite zone along the HFZ was possibly formed by the propagation of an earthquake nucleated in the brittle-plastic mylonite zone.