REGIONAL CHARACTERISTICS AND THEIR GEODYNAMIC IMPLICATIONS OF LATE QUATERNARY TECTONIC MOVEMENT DEDUCED FROM DEFORMED FORMER SHORELINES IN JAPAN

Abstract

In the Japanese coastal area, deformation patterns deduced from the height of former shorelines are classified into four types, A, B, C and D, each reflecting different response of tectonic regions to island arc movements. Each area has been progressively and acceleratedly deformed in a same pattern during the late Quaternary. Maximum rate of average uplift is 1.5m/1, 000 years for the Last Interglacial terrace and 4m/1, 000 years for the Holocene terrace.The landward tilting, type D, on the Pacific coast of Southwest Japan has been associated with great earthquakes occurring below the inner slope of the Nankai Trough. Type D area is separated from upwarping mountains by hinge lines along which subsidence has accumulated. Tectonic basins filled with younger sediments on the continental slopes are assumed to be depressed zones along the former hinge lines. Ages of deformed shorelines suggest that until the early Pleistocene seismic deformation had affected only the continental slopes and later propagated onto the coastal area in the late Pleistocene at the latest, resulting in the intensification of deformation of former shorelines. In contrast, type C deformation has predominated on the Pacific coast of Northeast Japan, which are 200km distant from the Japan Trench and seem to be still located landward of hinge lines of seismic crustal movement.