Exhumation processes of the Sambagawa metamorphic rocks, as inferred from quartz microstructures and mineral radiometric ages

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The results obtained in this research have been summarized below.(1)We have set up an automated measurement system of c-axis orientation of deformed quartz, using a Zeiss petrographic microscope, digital camera and Macintosh personal computer. This was made possible by Prof.R.Heilbronner, who stayed at Hiroshima University for 1 month in 2003, and investigated a lot of microstructures in quartz schists from the Sambagawa belt, central Shikoku with this method. The results will be submitted for a special volume of Tectonophysics.(2)The boundary and difference in the nature of terrane between the Sambagawa and Northern Chichibu belt have not been clear. In this study, we have found that the Kiyomizu Tectonic Line is the boundary fault between the two tectonic belts, which are completely different in both lithology and geologic structure.(3)We have conducted detailed geological survey and microstructural analyses of deformed quartz schist. As a result, it has become clear that an extensive folding and faulting occurred, and existing well-defined quartz c-axis fabrics were destroyed by solution-reprecipitation of quartz assisted by micro-faulting during the final exhumation stages under low temperatures in this area.(4)I hired Dr.N.Shigematsu as a research associate for three month in 2004. He was engaged in deformation experiments of K-feldspar single crystals, using a modified Griggs apparatus installed at Hiroshima University. We have found that K-feldspar single crystals deformed by mechanical twinning and intracrystalline slip, but also sometimes failed by faulting at the physical conditions (T=800゜C, strain rate=10^<-5>/s), suggesting that the experimental conditions can be correlated with those of the brittle-ductile transition. Also, these experimental conditions can be correlated with the temperature conditions of ca.400゜C at natural strain rate, which interprets that feldspars start to plastically deform around these temperatures in nature.