完新世段丘からみた相模湾・する河湾沿岸地域のネオテクトニクス

Abstract

Neotectonics of the coastal areas of Sagami and Suruga Bays in the South Fossa Magna region are diseussed with special reference to Holocene marine terraces of the area, where the Eurasian Plate is considered to have collided with the Izu Peninsula on the Philippene Plate along the Sagami and Suruga troughs.In the coastal areas of the Oiso Hills, the Miura Peninsula and the south tip of the Boso Peninsula, the adjacent areas northeast of the Sagami trough, there are several sets of Holocene marine terraces, suggesting remarkable upheavals. Active faults are developed throughout these areas, a notable one of which is manifested by the Kohzu-Matsuda fault known as the northern extension of the Sagami trough. Along the Kohzu-Matsuda fault, the upheaval of the Oiso Hills is recognized at the rate of about 3.4m in about 1, 000 years.The area of the Izu Peninsula is characterized by a downward movement of the west coast adjacent to the Suruga trough, and an upheaval of the east coast. A terrace of the maximum phase of the Jomon transgression is found only on the northeastern coast. On the southern coast, Holocene terraces of the highest level are dated from 3, 000 years B.P.On the west side of the Suruga trough, crustal movements are prominent on the coast of the delta of the Fuji River, which is regarded as the northern extension of the Suruga trough. That is manifested by an upheaval of the lwabuchi area in the west and a subsidence of the area of Ukishimagahara in the east. The amount of the dislocation in the area seems to be much larger than that of the Kohzu-Matsuda fault, and to decrease towards the Omaesaki point.