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QUATERNARY FAULTING ALONG THE MEDIAN TECTONIC LINE IN THE CENTRAL PART OF SHIKOKU

1973; Association of Japanese Geographers; Volume: 46; Issue: 5 Linguagem: Inglês

10.4157/grj.46.295

2185-1735

Atsumasa Okada,

earthquake and tectonic studies

In Shikoku, the Median Tectonic Line running nearly from east-northeast to west-south west separates Ryoke metamorphic complex overlain by late Cretaceous Izumi group and partly with Quaternary sediments to the north from Sambagawa crystalline schist to the south. Topographical contrast is very conspicuous along this tectonic line, especially in the central part of Shikoku, where the northern rim of the Shikoku Mountains is bordered by the magnificent Ishizuchi fault scarp abruptly descending northwards to the Seto Inland Sea depression. The area along the Median Tectonic Line is a belt of high tectonic instability in the Quaternary Period. The writer investigated the various features of active faulting and associated phenomena in this area to clarify the modes of faulting and the processes of tectonic relief occurred along the Median Tectonic Line since the Neogene, especially in the Quaternary. Active faults along the Median Tectonic Line are remarkably linear and are arranged en echelon. They generally dip northward with high-angled or nearly vertical fault planes, whereas thrust planes of the Median Tectonic Line in a narrow sense decline northward with rather low angles of 30° to 35° in the western part of Shikoku. The active fault system has been displaced predominantly in the right-lateral sense at mean rates of 5 to 10m/103 C14 years in the late Quaternary. Active lateral faulting dates from the beginning of the Quaternary, and has repeatedly acted at approximately constant or gradually accelerated slip rates. Vertical displacement upthrown to the south has been accompanied at rates of about 0.4 to 0.8m (or more than these) /103 C14 years, forming the Ishizuchi fault scarp. It is inferred that the vertical displacement also has acted in the similar processes to the lateral one and has amounted to about one thousand several hundreds of meters. Transcurrent buckling or wavy deformation associated with the lateral displacement is recognized in the northern down thrown blocks, judging from the topographic features and the distribution patterns of late Quaternary deposits. Quaternary faulting of these modes along the Median Tectonic Line is quite different from its earlier history of predominantly dip-slip displacement caused by approximately northsouth shortening and has acted since the early Quaternary under the regional stress field of nearly east-west compression in Southwestern Japan.