中央日本南部の第四紀地殻運動 地殻運動の変化と場の移動

Abstract

Various opinions on the Quaternary tectonism previously pointed out are discussed and summarized as follows:1) The tectonic culmination in the middle Quaternary is not at least an extensive phenomenon at this area.2) The uniform rate of deformation in the Quaternary seems to be accepted in the order of less than 105yrs, but not likely in the order of more than 106yrs.3) Although the accelerated rates towards the present are often suggested by many recent studies, these may be an apparent feature due to a fluctuation in a relatively short time span within a long-term displacement, or due to a change from a long-wave (or extensive) deformation to a short-wave (or limited) deformation. And also may result from a growing process, as typically expressed with formative advance of fault, even in a same stress condition.4) The SW Japan has been under the regional stress field of WNW-ESE compression through the Quaternary, being originated from the subduction of oceanic plates.5) The shallow depression along the median zone of SW Japan had come into existence simultaneously with the subduction along the Nankai Trough from the middle Pliocene. This may suggest the rejuvenation of the Philippine Sea Plate from that time.6) The leading edge of the underthrusting slab of the Philippine Sea Plate locates at the outer zone in the active segment of the Median Tectonic Line, whearas it extends to the inner zone in its inactive segment (Fig. 4A). These indicate that the outer zone underthrust at very low angle (ca 10°) by the plate behaves as a rigid bloack. As such belts as the outer zone, the longitudinal depression zone and the upwarping mountain of the inner zone have gradually shifted to the northwestward with the times (Fig. 4), these systematic migrations of tectonic province with miscellaneous changes of its process are likely caused by an advance of the underthrusting slab.7) The landward tilting terraces along the Pacific coast of SW Japan are considered to be caused by reverse faults which have emerged as accretionary prism on the upper part of continental slope from the middle to late Quaternary.