MODE OF CRUSTAL MOVEMENT IN THE LATE QUATERNARY ON THE SOUTHEAST COAST OF SHIKOKU. SOUTHWESTERN JAPAN

Abstract

On the southwest coast of Muroto Promontory, the southeastern tip of Shikoku, develop very magnificent coastal terraces, gradually descending toward northwest (Figs. 1 & 2). while on the east coast featureless steep scarps stretch northwards and, then, pass into ria-coast, submerged into the Kii Strait. The coastal terraces in this region are classified into two levels, which are called Muroto-misaki Terraces (M) and Hane-saki Terraces (H) in the ascending order. Terraces on each level are separated further into two or three subordinate levels (Fig. 3). Most of these terrace surfaces are abraded flat rocky ones, usually covered by thin beach gravel beds. At some places, especially at mouths of comparatively large rivers in the northwestern part of the coast, however, terrace surfaces on both levels are composed of marine and fluvial gravel beds more than 20 meters in thickness. Narrow abraded lowlands lower than 15 meters above sea level extend along the coast and merge in alluvial plains of comparatively large rivers, which consist of gravel and sand layers thicker than 30 meters. From these facts is it inferred that the coastal region has been deeply dissected and then relatively submerged three times in process of formation of these coastal terraces. On the south coast of Shikoku, it has been elucidated that severe earthquakes originating below the oceanic floors have occurred at an interval of about 120 years in the historic time and accompanied acute upheaval of the promontory, which amounts have diminished northwestwards, while in pre-seismic periods chronic subsidence increasing toward the promontory has been detected by precise levellings. In consequence of such pre- and post-seismic land deformations, Muroto Promontory has been upheaved, since amounts of acute upheaval accompanied by earthquakes have exceeded those of pre-seismic chronic subsidence. Terrace surfaces on each level are generally flat and smooth, but are interrupted by many minor scarps lower than 5 meters. A. Watanabe (1961) explained, as the authors agree, that such characteristic features of terrace surfaces had been modelled in succession of intermittent small upheavals as was accompanied by the great earthquake in 1946. And further, heights of raised beaches preserved on coastal terraces have a positive and negative correlation respectively with amount of post-seismic acut eand pre-seismic chronic displacements of their neighbouring bench marks, as shown in Figs. 4 and 7. It is considered, therefore, that upheaval of coastal terraces has been mainly influenced by land deformation as have been accompas vied by great earthquakes. It is proper to put a question why submergence has taken place three times in process of formation of coastal terraces on the coast where upheaval has predominated. The Japanese Islands have suffered from remarkable crustal movements throughout the Holocene, but velocity of upheaval of coastal regions has been generally far less than that of the post-Glacial eustatic rise of sea level. Accordingly, even actively upheaved coasts may have been submerged in consequence of the post-Glacial eustatic rise of sea level. From this point of view, it should be recognized that the physiographic development of Muroto Promontory has been achieved in composite process of crustal movement and eustatic change of sea level. Judging from their physiographic development, it is reasonable to assume that Muroto-misaki Terraces were formed at a higher sea level in the Riss-Wurm Interglacial, which commenced about 90, 000 years B. P. (H. E. Suess, 1956; R. W. Fairbridge, 1961).