The nature and importance of phyllonite development in crustal-scale fault cores: an example from the Median Tectonic Line, Japan

Like many large, crustal-scale faults, the Median Tectonic Line (MTL) in SW Japan 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 1–4 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, exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn-tectonic fluid influx over a range of deformation conditions. Exposures within 50 m of the central slip zone 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 such as white mica and chlorite. The grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid-assisted, stress-induced diffusive mass transfer mechanisms. 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, provided the network of phyllosilicate-rich fault rocks are able to form an interconnected thin layer of weak material on kilometre- to tens of kilometre-length scales.