Studies on deformation mechanisms of regional metamorphic rocks by means of microfabric analyses

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The main purpose of this study is to know processes and mechanisms of plastic deformation of rocks by means of quantitative fabric analyses that are carried out by the x-ray texture goniometer. First, we devised and developed a computer program for measurement and data-processing through many trial experiments on micas, chlorite, hornblende, olivine, and quartz in deformed rocks. We established a highly reliable method which gives more accruate values than those from current methods, and only takes a half to one third of measuring time on a current apparatus. Results of measurement are represented quantitatively in terms of types and degrees of preferred orientation. Main results are as follows. 1. As deformation mechanisms for slates, pressure solution is dominant at a lower temperature condition, while preferred recrystallization is dominant at a higher temperature condition. The strain ellipsoids are oblate and are correlative to S fabric. 2. As deformation mechanisms for the high P/T Sambagawa metamorphic rocks, dislocation creep together with preferred recrystallization is dominant. The strain ellipsoids are generally prolate. 3. As deformational mechanism for low P/T Ryoke metamorphic rocks, diffusion creep is dominant. The strain ellipsoids are generally prolate. 4. The deformation of ultramafic tectonites, which may give a clue to know the deformational mechanism in the upper mantle, is inferred to be due to dislocation creep from the fabric pattern of olivine. In future, measurements should be carried for rocks collected from wider areas, and the relationship between amounts of strain and fabric types should be established. Comparative studies on deformation mechanisms among main orogenic belts in the world are also important to know the machanisms of plastic deformation in the deeper crust and upper mantle.