Is the South Kitakami Belt a klippe?

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  • Ehiro Masayuki
    Department of Earth Sciences, College of General Education, Tohoku University
  • Okami Kazuyoshi
    Department of National Resource Development, Faculty of Engineering, Iwate University

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Other Title
  • 南部北上帯はクリッペか?
  • ナンブ キタカミタイ ワ クリッペ カ

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Abstract

The pre-Tertiary of the Northeast Japan is divided into five tectonic belts, namely, the Abukuma, South Kitakami, Hayachine Tectonic, Kuzumaki-Kamaishi and Akka-Tanohata Belts from the west. The first and the last two are regarded as the Jurassic accretio nary complexes. For the purpose of re-examining the tectonic models that the South Kitakami Belt is a klippe or nappe rests on the Jurassic accretionary complexes, stratigraphy and geologic structure in and around the South Kitakami Belt were studied. The results are as follows: 1) The Matsugadaira-Motai Metamorphic Rocks have been inferred, by some authors, to be Jurassic accretionary complexes and have been metamorphosed by the overthrusting of the Palaeozoic sedimentary rocks of the South Kitakami Belt onto them. However in point of fact, they are the pre-Devonian basement rocks metamorphosed before the Devonian. 2) Several low-angle faults (observed or inferred) have been known in the South Kitakami Belt and along the southern margin of the Hayachine Tectonic Belt. Many of them, however, are essentially high-angle. And the stratigraphic studies indicate that the displacements along the low-angle faults are small. 3) The Hayachine Tectonic Belt is a boundary zone between the South Kitakami and Kuzumaki-Kamaishi Belts, and forms a broad anticline (anticlinal fan fold), associated with longitudinal high-angle reverse faults on both flanks. No pile-nappe structures exist in and around the Hayachine Tectonic Belt. 4) Studies on the xenoliths in the post-Jurassic dyke rocks in the boundary zone between the Hayachine Tectonic Belt and the Kuzumaki-Kamaishi Belt and in the central and southern part of the South Kitakami Belt suggest that the Palaeozoic to Mesozoic sedimentary rocks of the Hayachine Tectonic Belt and the South Kitakami Belt are underlain by metamorphic and plutonic basements, not the Jurassic accretionary complexes. From these facts it is concluded that the South Kitakami Belt is neither a klippe nor a nappe.

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