UPLIFT TECTONICS OF THE HIGH-PRESSURE SANBAGAWA METAMORPHIC BELT

在为期2年的研究项目中，进行了实地调查和室内工作，获得了以下新成果：1)白垩系三巴川高p /T变质岩产于日本西南部喜喜半岛西部、中部和东部的岛满托带晚白垩世弱变质增生杂岩之上。2)三川变质岩在构造上则位于四国中部和东部的一父带侏罗纪弱变质增生杂岩之下。它们之间的边界为一条低角度正断层。这些关系表明，三巴川变质岩形成了一个亚水平推覆体，由低角度断裂与构造上、下伏单元隔开。3)这些并置单元之间的变质等级差异表明，边界断裂具有明显的压力间隙，其压力间隙超过几千巴。4)日本西南部的三巴川带可以重新定义为三巴川变质岩因构造上覆侏罗纪增生杂岩的剥离而暴露的大地构造窗口区。5)三八川推覆体厚度小于2km，表明其独特的隆升构造来自于俯冲带深处的变质域。6)从库拉-太平洋脊俯冲的时间来看，三巴川变质岩的隆升是由大陆边缘脊俯冲引起的楔形挤压机制主导的。这些成果不仅更新了对日本大地构造史的总体认识，而且为造山学，特别是科迪勒拉型造山带的形成过程的研究提供了一个新的方面，因此值得注意。上述部分结果已经口头提出，并在本报告其他地方的科学文章中发表。少

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