UPLIFT TECTONICS OF THE HIGH-PRESSURE SANBAGAWA METAMORPHIC BELT

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Field survey and in-door works have been conducted in the 2 years research project, and following new results were obtained. 1) The Cretaceous Sanbagawa high-P/T metamorphic rocks occur tectonically above the weakly metamorphosed Late Cretaceous accretionary complex of the Shimanto belt in western, central and eastern Kii Peninsula, Southwest Japan. The boundary between them is a low-angle thrust which was recognized for the first time in Southwest Japan. 2) The Sanbagawa metamorphic rocks, on the contrary, tectonically underlie the weakly metamorphosed Jurassic accretionary complex of the Chichibu belt in central and eastern Shikoku. The boundary between them is a low-angle normal fault. These relations suggest that the Sanbagawa metamorphic rocks form a subhorizontal nappe, bounded from tectonically over-and under-lying units by low-angle faults. 3) Difference in metamorphic grades among these juxtaposed units forming a sandwich, indicates that the boundary faults represent remarkabl … More e pressure gap more than some kilobars. 4) The Sanbagawa belt in southwest Japan can be re-defined as an area of geotectonic window where the Sanbagawa metamorphic rocks are exposed by removal of tectonically overlying Jurassic accretionary complex. 5) The thickness of the Sanbagawa nappe is less than 2km, and this suggests its peculiar uplift tectonics from the metamorphic domain in the deeper part of subduction zone. 6) Judging from the timing of the Kula-Pacific ridge subduction, uplift of the Sanbagawa metamorphic rocks was led by wedge extrusion mechanism induced by ridge subduction along continental margin. These results are noteworthy because they not only renewed the total understanding of geotectonic history of Japan but also gave a new aspect in the study of orogeny, in particular, of the formation processes of the Cordilleran-type orogenic belts in general. Parts of the above-mentioned results were already presented orally and published in scientific articles elsewhere of this report. Less