THE DEVELOPMENT OF PHYLLONITIC FAULT ROCKS ALONG THE MEDIAN TECTONIC LINE, JAPAN AND THEIR RHEOLOGICAL SIGNIFICANCE

The Median Tectonic Line (MTL) in SW Japan has been active since mid-Cretaceous with estimated sinistral displacements of between 200 and 1000 km. It separates a low-P/high-T belt of subduction-related granitoids with subordinate metasediments (Ryoke Belt) from high-P/low-T accretionary complex metasediments (Sambagawa Belt). Ryoke Belt rocks are variably mylonitized in a zone 1-2 km thick N of the MTL and preserve typical greenschist-facies textural assemblages formed during Cretaceous top-to-south sinistral movements. The remainder of the fault zone (the ‘core’ region) is up to 60 m wide and displays a wide variety of fault rocks mostly derived from Ryoke protoliths. In the core, the following progressive sequence of fault rock overprinting is recognised: mylonite/ultramylonite - cataclasite - foliated cataclasite - phyllonite (here defined as a phyllosilicate-rich fault rock with apparently mylonitic textures). Textural and geochemical relationships suggest that this sequence results from the syn-kinematic influx of a chemically active fluid phase into the more permeable, finer grained parts of the fault zone immediately following cataclasis. Thus the foliation develops initially along pre-existing brittle fault zones and forms due to the effects of fluid-assisted reaction softening (retrograde growth of sericite, chlorite) combined with the onset of widespread diffusive mass transfer (solution seams, fibrous overgrowths). The foliated cataclasites/phyllonites preserve textures almost identical to those produced in recent analogue experimental studies designed to assess the effects of fluids and phyllosilicates in fault zones. These studies predict a marked long-term weakening effect and this is consistent with the observed localisation of all subsequent movements into the foliated core of the MTL. Our findings suggest that the extensive development of phyllosilicate foliations overprinting cataclasites in fault core regions may be a primary indicator of long term, fluid-related weakening of fault zones.