Origin and mechanical significance of foliated cataclastic rocks in the cores of crustal‐scale faults: Examples from the Median Tectonic Line, Japan

Abstract

The Median Tectonic Line (MTL) is Japan's largest onshore fault and has been active since the mid‐Cretaceous. Foliated cataclastic fault rocks are exceptionally well exposed in the fault core at Anko, Nagano Prefecture. Following an early phase of mylonitization and exhumation during left‐lateral shearing, brittle fracture and cataclasis occurred leading to the development of centimeter‐ to submillimeter‐spaced, fault zone parallel fracture systems. These fracture systems established an initial architectural hierarchy that influenced the subsequent development of foliated cataclasites and gouge. Initially, fracture systems coalesced to form interconnected zones of fine‐grained ultracataclasite. Fluid influx at the onset of grain‐scale brittle deformation led to precipitation of fibrous chlorite within the ultracataclasites, ultimately leading to the development of an interconnected network of foliated, phyllosilicate‐rich cataclasites and gouges in the core of the MTL. The brittle reduction of grain size and ingress of a chemically active fluid phase simultaneously promoted reaction softening and diffusive mass transfer in the foliated ultracataclasites, leading to rate‐dependent “frictional‐viscous” flow at sub‐Byerlee friction values. Associated weakening is indicated by the preferential localization of deformation within the ultracataclasites. A protracted sequence of carbonate mineralization and cementation events is also recognized during the fault rock evolution and suggests episodic periods of fluid overpressuring. A crustal‐scale fault zone model is proposed, suggesting that the foliated cataclasites/gouges are weak in the long term and represent shallower crustal equivalents of phyllonitic fault rocks exposed in more deeply exhumed fault zones, including other parts of the MTL.