ボーリング調査で明らかになった丹那盆地の変形構造

Abstract

Tanna basin is one of tectonic depressions caused by the downwarping of the western slopes of Taga and Yugawara volcanoes in the northern part of Izu peninsula. Owing to this movement, the thick accumulation of sediment has filled up this basin during the late Quaternary. The Tanna fault that caused the 1930 Kita-Izu Earthquake (M 7.3) traverses this basin in N-S direction and has displaced the sediment with left lateral slip. The subsurface information of this basin gives us data on the geological history of the basin and the tectonic features of the fault since the late Pleistocene.In December 1980, Geological Survey of Japan carried out the drilling investigation in the northern part of Tanna basin to get the stratigraphical and tectonic information of the basin fill sediment. Four borings of 20 m depth were made at intervals of 20m along the nearly E-W direction so as to cross the fault trace. Obtained core samples were investigated thoroughly from the stratigraphical, chronological and paleoecological view point.As a result, it is revealed that the sediment to the depth of 20 m below the basin surface accumulated during the last 20, 000 years. And it is also found that the sediment has suffered from the small scale deformation of pressure ridge shape, as shown in Fig. 6, by the movement of the Tanna fault. However, such deformation was not observed from the geological section along the railway tunnel which penetrates into the volcanic rocks just below Tanna basin. It is thought that the small scale deformation is only a local disturbance limited to the upper horizon of the basin fill sediment and has no relation to the structure of the basin.Throughout these studies, it is revealed that the deformation in Tanna basin is composed of two different scale deformations associated with the fault movement during late Quaternary period. The larger one is the downwarping of a few kilometers in extent and the smaller one is the pressure ridge of a few tens meters in width in the upper part of the basin fill sediment.