Rokko Movements and its Appearance

Abstract

Many geologists are giving attentions to the zonal arrangement of geological and geophysical features of the Japan island arc system, in which two recently active belts along the Izu-Mariana and Ryukyu Arcs cut the Honsyu Arc almost at right angles. These two active belts are thought to be the late Cenozoic orogenic belts (Matsuda et al., 1967). Southwest Japan lies between them and have behaved rather cratonically during Cenozoic times unlike Northeast Japan. J. Makiyama (1956) described this region as “quasi-cratonic”.SW Japan is divisible into two zones, Inner Zone in the north and Outer Zone in the south, separated by the Median Tectonic Line. Three series of the Cenozoic sediments lie scattered on the basement rocks of Inner Zone; the First Setouti Series belonging to the middle Miocene, the Setouti Volcanic Series of the upper Miocene, and the Second Setouti Series which is the Plio-Pleistocene in age (Huzita, 1962). Setouti means “Inland Sea” area.1) Fig. 1 shows the depressional zones which formed the First Setouti Series, and those of the lower part of the Second Setouti Series are also shown in Fig. 3. They have a similar tendency in the structural trends. Fig. 4 shows the horizontal directions of the maximum principal stress axes (compressive) which are inferred from the structural trends of the latter.2) Fig. 5 shows the main structures controlling the distribution of the upper part of the Second Setouti Series. Fig. 6 is the stress directions inferred from above structures. A distinct change of the stress state in SW Japan seen between Figs. 4 and 5 may have occurred in the middle of Pleistocene, but it did not make distinct angular unconformity in the Second Setouti Series.3) The faults affecting the Second Setouti Series may be classified into two systems as shown in Figs. 7 and 8. Fig. 7 is the thrust system appeared in the earlier stress field, and Fig. 8 is the fault pattern occurred in the later state of regional stress. Some of the older thrusts have been rejuvenated as strike-slip faults.4) The later movements have produced various types of structures in the basement rock-bodies depending on the different behavior of each body for the same regional stress. For example, weak warping associated with strike-slip faults has occurred in the Paleozoic Tamba terrain, but comparatively intense deformation of folding type with thrusting has affected the granitic regions (Fig. 2). The trends of these structures are almost N-S and subparallel with the Izu-Mariana or Ryukyu Arcs.5) The structures due to the later movements have been superimposed on the older structures parallel to the E-W Honsyu Arc. Both these structures control the present geomorphology of SW Japan.6) In contrast to Inner Zone occupied by granitic batholithes and plutons, Outer Zone has older rocks showing distinct linear features aligned almost perpendicular to the trends of the younger structures. Rejuvenation of the Median Tectonic Line due to the later movements has resulted in strike-slip as shown in Fig. 8.7) The later crustal movements are clearly expressed in the “Kinki Triangle” occupying the centre of SW Japan (Figs. 2, 6). The western edge of the Triangle is formed by the Rokko mountain range, so the term “Rokko Movements” is proposed to these movements (Ikebe & Huzita, 1966)