Microstructural and geochemical processes in long-lived reactivated crustal-scale fault zones: A case study from the Median Tectonic Line, SW Japan.

Abstract

The Median Tectonic Line (MTL) is a major, crustal-scale fault in Japan that separates the low-P/high-T Ryoke metamorphic belt of mainly subduction related granitoids from the high-P/low-T Sambagawa Belt of accretionary complex metasedimentary rocks. Like many large, crustal-scale faults, the MTL has a long history of movement having been active predominantly as a strike-slip fault since the mid-Cretaceous. Fault rock exposures in the core of the MTL preserve a history of deformation at a range of mid- to shallow-crustal depths. Ryoke mylonites 1-5 km north of the main contact record deeper level, Cretaceous top-to-the-south sinistral movements. The remainder of the fault zone core is surprisingly narrow (く50 m) exhibiting a wide variety of fault rocks that illustrate both the passage and interaction of syn-tectonic fluid influx over a range of deformation conditions. Exposures of the fault core at Miyamae, Mie Prefecture display a progressive sequence in fault rock evolution from ultramylonite →cataclasite → foliated cataclasite → phyllonite→breccia/gouge. This sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone. This leads to the replacement of load-bearing phases such as feldspar by fine grained, foliated aggregates of intrinsically weaker phyllosilicates and the onset of fluid associated diffusive mass transfer (DMT) in finest grained regions of the cataclastically deformed fault zone. Comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the MTL will lead to long-term weakening. An integrated field, microstructural and geochemical study at Miyamae and Tsukide, Mie Prefecture, has highlighted two distinctive domains: 1) the narrow fine-grained foliated fault core where strain is localised; and 2) a wider damage zone of variably fractured mylonites to the north of the fault core that progressively grade into Ryoke protolith mylonite at distances over -400 m north of the MTL central slip zone. The fine-grained foliated fault rocks within the core have experienced the most intense deformation, but show the apparently least altered geochemical signature. A model for the presence of a damage zone experiencing pervasive fluid flow and a fault core experiencing high fluid fluxes of channellized fluid flow structurally linked to a Ryoke protolith fluid reservoir is presented. A central slip zone 'seal' IS inferred to retard mixing with Sambagawa-derived fluids. The phyllonitic fault rocks in the core of the MTL posses a strong permeability anisotropy where transverse fluid flow is inhibited and focused fault-parallel fluid flow occurs. Foliated cataclasites are exceptionally well exposed in the MTL fault core at Anko, Nagano Prefecture. Processes of brittle fracture and cataclasis have led to the development of cm- to sub-mm spaced fracture systems defining a crude fabric. This fracture system established an initial architectural hierarchy that influenced the subsequent development of foliated cataclasite and gouge. Fluid influx at the onset of grain-scale brittle deformation led to precipitation of fibrous chlorite within the finest- grained sections of the fault core. This ultimately led to the development of a foliation within the ultra-cataclasite defined by an interconnected network of aligned phyllosilicate aggregates. The brittle reduction of grain-size and the ingress of a chemically active fluid phase promoted the operation of diffusive mass transfer mechanisms ('frictional-viscous creep') and reaction softening. Field and microstructural observations at Anko suggest that the foliated cataclasites are a shallower equivalent to the phyllonites found along the more deeply exhumed parts of the MTL at Miyamae.