THE COASTAL TERRACES OF THE TANE-GA-SHINIA ISLAND AND THE MODE OF ITS CRUSTAL MOVEMENT

Abstract

In Tane-ga-shima Island, which is located in the northern-most part of the Nansei-shoto Islands and is giving its long axis in the same direction as the Ryukyu Arc, are recognized eight coastal terraces built mainly by wave abrasion. These coastal terraces are classified and correlated by the continuity of altitudes of their surfaces and tephra on the surfaces. They are widely distributed as marginal terraces surrounding hills which originated from abrasion platforms in the northern part, and as plateau-like ones in the southern part, but they do not occur in the northern-most and south-eastern parts. The island is divided into five fault blocks by four faults parallel to NW-SE direction, and extent of the development of terraces is different in each block. The author made the geomorphological survey of these terraces to con-sider the geomorphic history and crustal movements of the island. The results are summarized as follows; A) Altitudes of the coastal terraces classified into eight are 120m-180m on the first, 120m-170m on the second, 30m-210m on the third, 25m-130m on the fourth, 50m-90m on the fifth, 25m-75m on the sixth, 20m-30m on the seventh, and 5m-8m on the eighth terrace. They are more clearly distinguished in the southern and western parts than they are in the northern and eastern parts. B) The third, the fourth and sixth terraces are comparatively more widely developed and distributed in a larger area. However, terraces are not found in the northern-most portion of the island where they are submerged into the Osumi strait, and in the south-eastern portion they have not been preserved as terrace surfaces but are recognized only as level summits because of the erosiveness of the Miocene series. C) The characteristics of the terraces are almost same and they form mostly rock-cut terraces, although extents of their developments are different in each fault block. Marine deposits of the terraces composed mainly of ill-sorted gravels and boulders form very thin layers and rarely exceed ten metres thick even in waste-buried valleys. In the marine deposit of the fourth terrace are seen tests of Foraminifera and fragments of corals which indicate warm climatic conditions. And also raised coral reefs are observed on the lowest terrace in Mageshima Island which belongs to Tane-ga-shima Island. This terrace is correlated to the eighth terrace of Tane-ga-shima Island. From these facts the fourth and eighth terraces are to be correlated to the main transgressions; the former is to the transgression prior to the regression of the last glacial age, and the latter is to the transgression of the early Holocene. D) The crustal movements inferred from the displacement of the terraces are as follows;(1) Upwarping has been taking place with the axis in the vicinity of Yakutsu, southwest of the island, showing the older the terrace is the larger the displacement.(2) Activities of the four faults occurred mainly in the age between the formation of the third terrace and the fourth. These faultings made tilted blocks with fault scarps in the southern edge of each block, and shattered terrace gravels are found along fault lines. The fault which runs from Kerisaki to Tanowaki in the north of the island has been active before and after the construction of the fourth terrace.(3) Relations of the altitudes of the raised strandlines among the third (T3), fourth (T4) and sixth (T6) terrace are formularized as: T3=1.5T4+2, T4=1.4T6+30, and T3=2.5T6+30 in meters. These formulae indicate that there are positive correlations between the altitudes and ages of the terraces.(4) The tendency of the strike and dip of the Miocene series and the crustal movements in more recent geological time shown by the terraces are very similar to each other.(5) The facts mentioned in (1), (2), (3), and (4) lead to the deduction that two kinds of crustal movements have been taking place, upwarping and faulting.