地すべりの分布と地盤構造に関する古地磁気学的研究 ＩＩ 南関東地域

Abstract

The Hayama-Mineoka ophiolite belt, in which landslides have frequently occured, is exposed in the southern portion of Boso and Miura Peninsulas about 50 km south of Tokyo. Ophiolite complexes occur in a narrow uplifted zone in the direction of ESE-WNW. Pillow and dyke basalts are exposed throughout this zone. 39Ar-40Ar age of Mineoka basalts ranges from 40 to 50 Ma. These ages show the dates of eruption at the ocean floor.The paleomagnetic investigation indicates that the underlying basalts layer is tilted by about 28° nearly southward and the direction of the paleostress was NNW at that time of the formation of these ophiolite complexes. For the structual model derived from paleomagnetic investigation, the gravity and magnetic anomalies are theoretically calculated and found to be consistent with the observed anomalies.The relation between the topography characterizing landslide occurence and the geography of Hayama-Mineoka ophiolite belt has been investigated. It is recognized that in Boso and Miura Peninsulas landslides have been observed appreciably at the area composed of igneous rocks in Mineoka and Hayama belt. It is noted that the frequency in landslide occurence is high in the southern slope and low in the northern slope of Mineoka Hill.The obvious correlation has not been found between the distribution of landslides and water vein streams and also an angle of inclination of topography. It is speculated that ophiolite complexes was remarkably fractured by the NNW stress when it was uplifted on the land, and that the change of paleostress direction caused the release of stress in the cracks of underground basalts layer. The occurence of landslide in this district may be related intimately to the shape of underground rock body.