The nature and possible significance of phyllosilicate-rich fault rocks in the core of the Median Tectonic Line, Japan.
Sharon Jefferies1, Christopher A.J. Wibberley2 and Bob Holdsworth1.
1
Reactivation Research Group, Dept. Geological Sciences, University of Durham, Durham, U.K.2
Dept. Geology and Mineralogy, Graduate school of Science, Kyoto University, Kyoto, Japan.The Median Tectonic Line (MTL) extends across SW Japan. It is regarded, as one of the world’s largest active faults, with displacement estimates ranging from a few hundred to a thousand km. Present day fault rock exposures in the core of the MTL preserve a history of movements at a range of mid- to shallow-crustal levels since the mid-Cretaceous. Deeper level deformation has been accommodated by the so-called ‘Ryoke mylonites’ exposed to the north of the main MTL contact. The Ryoke mylonites record Cretaceous top-to-south sinistral movements across a 1-2 km zone lying immediately north of the MTL. The remainder of the fault zone, as exposed in Mie Prefecture is surprisingly narrow. Here the fault zone exhibits a wide variety of fault rocks that illustrate the interaction and effects of both protolith heterogeneities and a range of deformation conditions. Localised within 12 m of the MTL fault core, phyllosilicate-rich mylonite and cataclasites exhibiting strong alteration and deformation as a result of reaction weakening can be found. Within the phyllosilicate-rich mylonite, lenses of quartz-cemented cataclasite occur. The phyllosilicate-rich mylonites display a typical mylonitic foliation with phyllosilicates and recrystallized quartz enveloping elongated clasts of feldspar. The following sequence is observed during exhumation of the fault zone contemporaneous with deformation; feldspar fracturing → fluid assisted alteration, dominantly feldspar-sericite reactions → cataclasis → fracturing. The phyllosilicate-rich mylonites become progressively brecciated to form cohesive cataclasites upon approaching the fault contact, with red (oxidised iron) discolouration marking alteration fronts around fracture zones indicative of meteoric water percolation. The cataclasite lenses are composed of ultramylonite deformed by cataclasis and brecciation. The lenses appear to be cemented by quartz and show a significantly lower percentage of phyllosilicate material. The lenses are enveloped by the surrounding phyllosilicate-rich mylonites suggesting a sequence of ultramylonitisation → cementation → cataclasis → mylonitisation and fluid influx.
The metapelitc Sambagawa schists to the south have been deformed into incohesive foliated gouge within 15 m of the MTL contact. Together with the fault rocks derived from the Ryoke mylonites, these form the narrow MTL fault core. Numerous oblique slip zones at different orientations crosscut the core zone, but the youngest zone is a discrete planar central gouge interpreted to represent the latest principal displacement of the MTL in Mie Prefecture. Within the core, lateral continuity is best maintained by the youngest structures. Evidence for recent dextral reactivation is only preserved in incohesive fault rocks localized to within 1 m of the central slip zone. This suggests that increasing localization of deformation and fluid infiltration has occurred at the presently exposed level of the MTL during exhumation and continued deformation.