CRUSTAL STRUCTURE OF THE SOUTH FOSSA MAGNA, JAPAN, INFERRED FROM THE GEOLOGICAL DATA

Abstract

Crustal structure in the South Fossa Magna, Japan, a Tertiary geosynclinal area,. is inferred from study of stratigraphic profiles prepared for each stage of development and the modification of those profiles which must have resulted from subsequent crustal deformation. The general sequence of events was: 1) deposition of early Miocene Misaka formation; 2) crustal warping; 3) deposition of the late Miocene Fujikawa series with contemporaneous erosion of Misaka rocks in upwarped areas; 4) quartz diorite intrusion; and 5) formation of crystalline schist along the southern limb of the Tanzawa upwarp. It is concluded that the Mohorovicic discontinuity lies at a depth of 15 km to 20 km in the area and increases in depth northward. The Mohorovicic discontinuity must have relief of 10 km or more corresponding to the amplitude of Miocene folds. The quartz diorite magma was generated in the lower part of the crust during orogeny. M. Russell