Localisation of plastic flow in the mid-crust along a crustal-scale fault: Insight from the Hatagawa Fault Zone, NE Japan

摘要

This study examines plastic flow in fault rocks exposed along the Hatagawa Fault Zone (HFZ) of NE Japan. The fault zone, developed in 110 Ma granitoids, ceased activity by 98.1 ±2.5 Ma. Three different fault rocks (mylonites with microstructures A and B, and cataclasite) are exposed along the fault. Micro-structure A formed at the brittle-plastic transition. The temperature conditions for microstructure B were higher than those for microstructure A; those for the cataclasite were lowest. Microstructure A is exposed in limited areas (maximum length extent of approximately 6 km) along the HFZ. The distribution of microstructure A is considered to represent the latest-stage localised zones of plastic flow, associated with strain weakening accompanied by dynamic recrystallisation of feldspar, suggesting the restriction of plastic displacement to certain intervals at depth ranges with P-T conditions of the brittle-plastic transition. Many zones of localised deformation containing crush zones are observed in rocks with microstructure A, suggesting that numerous fractures nucleated due to the ductile fracturing of highly deformed fine-grained feldspar in the outcrop extent of microstructure A. The nucleation of large earthquakes was possibly promoted by interaction between fractures that nucleated by ductile fracture and stress concentrations associated with the restricted development of plastic displacement.