| Research Abstract |
The Izumi Sedimenatry Basin (ISB) along the Median Tectonic Line (MTL) has become of major interest lately as a typical strike-slip basin. The purpose of this study is to become clear formative mechanism for the ISB and deformational style in the ISB, attaining to about 300 km in length. A summary of the results obtained is shown below.
1. Division of ISB : A tectonic map showing the MTL fault system and the distribution of the Izumi Group (Upper Cretaceous) was compiled. According to this map, the ISB is divisible into at least 5 segments (A-E). The possibility of a basin having liked with each segment (subbasin) is envisioned, though the distribution of the Izumi Group is continuous from segment A (west Shikoku) to segment D (Izumi Mountains). This study has hence tried to determine ages of some segments : Until now, the fission track age of 77-72 Ma from the western part of the segment D, and Radioralia (Archeodictyomitra Sp, etc.) from the segment E (far east of Gojo) were obtained.
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(will be submitted)
2. Formative mechanism for ISB : A tectonic map in the segment D, west of a fault bend (Gojo bend) of the MTL, was also compiled. At this releasing bend of the MTL, duplex structures produced by left-slip movement have become clear (Geology, v. 18, p. 392-394, 1990 ; Structural Geology, no. 35, p. 65-70, 1990). The ISB can be regarded as a pull-apart basin formed by the duplexing at the MTL bend. The mechanism of pull-apart basin was further discussed (Structural Geology, no. 36, 1991, in press).
3. Deformational style in ISB : Deformation structures, such as disharmonic folds and small-scale echelon faults, produced by gravity-induced bedding-plane slippages, are well developed in the turbidite facies of the Izumi Group in the segment D. By measuring for them, the shear stranin (gamma= 0.3--1.4) along the bedding plane was obtained. Using a scanning electron microscope, the slip direction on slickensides was determined. Judging from their evidence and the migration of depocenter, the tilting of the filling Izumi Group in the basin is thought to have been caused by strike-slip duplexing along the MTL (Geological Society of Japan, Kansai Branch Proceedings, no. 109, p. 7, 1990 ; Geological Society of Japan, Annual Meeting, 97th, Abstracts, p. 346, 1990 ; will be submitted). Less
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