Tephrochronology of coastal terraces and their tectonic deformation in South Kanto

Abstract

South Kanto, environs of Tokyo and Yokohama, is composed of the Quaternary marine and fluvial deposits covered by volcanic ash (tephra), which have offered the standard series for studying the chronology of Quaternary in Japan.
In the present paper, the author summarizes the tephrochronological study of the coastal terraces from middle to late Quaternary at first and then clarifies the history of their tectonic deformation.
(1) The coastal terraces in the late Quaternary are divided into four groups as shown in the lower part of Fig. 1. Each is considered to have been constructed more or less in high sea level stages of the late Quaternary. Of these terraces the marine Shimosueyoshi terrace is more widely distributed (Fig. 5) and may be concluded to have formed in the last great interglacial stage, dated at about 120, 000-130, 000 years B.P. from the fission track method. The post-Shimosueyoshi terraces are called the Obaradai and the Misaki terraces. They are widely developed as fluvial terraces in inland area, nevertheless the marine surfaces are limited in the coastal region (Miura peninsula and its contiguous region). The pre-Shimosueyoshi terraces (so-called Tama terrace groups) are rather difficult to discriminate each other according to their poor distribution, but are able to be precisely distinguished from each other by using the tephrochronology (as shown in Fig. 2 and Table 1). And the ages of them are tentatively estimated as in Fig. 3 by extrapolating the rate of tephra accumulation in the late Pleistocene into earlier times. Of these. terraces one of the oldest may be the Kônan surface distributed in the west of Yokohama and is dated back to about 400, 000 years B.P.
(2) Tectonic deformation of the above mentioned terraces is able to discuss from surveying the altitude of the top of each marine formation. Fig. 4 shows the height distribution of the Numa surface (formed in the Holocene high sea level stage) and Fig. 5, that of the recurred landforrn of the Shimosueyoshi surface. Generally speaking, there can be formed the active upwarping area in the southern half and the relatively inactive or downwarping area in the northern. The boundary line between them runs from Hadano basin to Yokohama to the east. The northward tilting of the Shimosueyoshi surface south of this line attains to the highest degree in the western part of South Kanto, Ooiso hills, where it is accompanied by faulting and remarkable flexure to the north. And the tilting becomes the more slightly in degree to the east. The similarity of the deformation of the above two terraces, also corresponds to the seismic deformation of the great Kanto earthquake of 1923. It is probable that the Quaternary deformation in the southern half of South Kanto might mainly originate from the seismic activity which repeatedly occurred along the Sagami tectonic line.
From Fig. 5 is remarkable the three relatively local deformations, those are, the basin-forming movement of central Sagami plain and the northern Tokyo Bay, and the downwarping at the west of Yokohama, which are not conspicuous in the deformation of the Numa surface and also at the 1923 Kanto earthquake.
The rate of tilting is rather uniform during at least 105 years by comparing the gradient of several surfaces of different ages from the middle Pleistocene to Holocene. However, the direction and the rate of movement are different at places.