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Durham University

Department of Earth Sciences


Publication details for Professor Robert Holdsworth

Jefferies, S.P.,, Holdsworth, R.E.,, Shimamoto, T.,, Takagi, H.,, Lloyd, G.E., & Spiers, C.J. (2006). Origin and mechanical significance of foliated cataclastic rocks in the cores of crustal-scale faults: Examples from the Median Tectonic Line, Japan. Journal of Geophysical Research: Solid Earth 111: B12303.

Author(s) from Durham


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.