An integrated microstructural and geochemical study of fault and fluid related processes in the Median Tectonic Line, Japan.

Sharon Jefferies1, Bob Holdsworth1 and Graham Pearson2.

1Reactivation Research Group, Dept. Geological Sciences, University of Durham, Durham, U.K.

2Dept. Geological Sciences, University of Durham, Durham, U.K.

S.P.Jefferies@durham.ac.uk

The Median Tectonic Line (MTL) extends across SW Japan and has been active since mid-Cretaceous. It is regarded as one of the world’s largest seismogenic faults, with displacement estimates ranging from a few hundred to a thousand km. There have been no previous studies to investigate the role played by chemically active fluids along the MTL, and their likely interaction with deformational and metamorphic fault zone processes. Here we present preliminary results of a study conducted on the MTL in Mie Prefecture. Fieldwork and sample collection were carried out across the fault zone at two areas roughly 13 km apart; Tsukide to the east and Miyamae to the west. At each site, the transects started to the north within Ryoke "protolith" mylonites and then moved south through zones of fractured mylonites, cemented cataclasites, cataclasites, phyllosilicate-rich mylonites, foliated cataclasites, foliated gouge (MTL core), and into semipelitic Sambagawa schist to the south. Coupled geochemical and microstructural investigations were carried out via optical microscopic studies and X-ray fluorescence analysis of major and trace elements. The isocon plot of Grant (1986) was applied to assess the apparent mobility of elements within the MTL fault zone. All fault rocks were normalised to protolith compositions using samples collected in unaltered wall rocks well away from the MTL. With decreasing distance to the MTL fault core, the degree and trends of alteration were established. These are linked to features such as the disappearance of protolith minerals and the emergence of new phases. Across the fault zone steps in the degree of alteration and changes in mineralogy are correlative, and it is likely that this coupling influences the evolution of the dominant deformation regime with time. One such important process is framework collapse and the replacement of load bearing framework silicates with intrinsically weaker fine-grained phyllosilicate aggregates. The combined approach of investigating mineralogical, microstructural and geochemical changes across the MTL has allowed us to establish the effect of chemically active fluids on a spectrum of fault zone rocks. Our approach also attempts to link specific alteration steps to particular microstructural changes as a means of better understanding possible localisation and weakening mechanisms. Our findings illustrate the important influence of chemically active fluids on fault zone rheology in large crustal-scale reactivated fault zones.