琉球列島の第四紀後期の地殻変動に関する一考察

抄録

Pattern and rate of late Quaternary tectonic movement of the Ryukyu Islands, belonging to an active island arc, are discussed on the basis of the height distribution of the coral reef terraces and characteristics of active faults dislocating them. The main terrace (S terrace) and the Holocene terrace (H terrace) which are considered to have been formed at the high sea level of the Last Interglacial and Post-glacial ages, respectively, are used as reference surfaces for the estimation of vertical displacement. The Holocene terrace is rather easily identified as the lowest raised coral reefs fringing the present shoreline with abundant ¹⁴C dates from the reef complex. Recognition of the S terrace, however, is difficult, because of the absence of the radiometric dating except for Kikai-jima and other few islands. In this paper, therefore, the most extensive and well-preserved depositional surface in each island is tentatively assumed as the S terrace formed at 12-13×10⁴y.B.P. Active faults are investigated mainly by the air-photo interpretation. Results of the investigation of each island are listed in Table 1 and summarized in Figs. 1, 15 and 16.
As a whole, the Ryukyu Islands have uplifted since the Last Interglacial age at the latest, judging from the age-height diagram (Fig. 2). The height of the S terrace varies from about 200m to 30m, indicating a notable differential vertical displacement and the H terrace also ranges from 22m to few meters in height as shown in Table 1, Figs. 1 and 15. The Ryukyu Islands can be classified into three zones in terms of pattern of vertical deformation, as represented in a schematic diagram of Fig. 16.
The Zone 1 is characterized by a notable westward tilting from Kikai-jima, situated close to the axis of the Ryukyu Trench, toward the eastern part of Amami-Oshima. Such a tilting characteristic to “trench margin” may be an accumulated result of co-seismic deformation associated with the subduction of the Phillipine Sea Plate. The maximum rate of uplift reaches to about 2m/1, 000 years for 12-13×10⁴ years. In Zone II, which consists of main axis of the island arc and may correspond to the “outer arc rise”, an upwarping-like deformation can be recognized. A hinge line of different deformation areas is presumed at the western margin of the Kasari Peninsula of Amami-Oshima. In Zone III, composed of Tertiary and Quaternary volcanic rocks west of the volcanic front, the terrace height of each island shows rather irregular distribution, implying that each island has uplifted independently. Such a zonal arrangement of the deformation pattern is more obvious in the northern area than in the southern area. It may suggest the difference of activity and history of an active island arc between both the areas.
Active faults of “certainty I and II” are mapped in Figs. 3 to 14. Generally speaking, they run in NW or WNW (Fig. 17), approximately perpendicular to the trench axis as well as the island arc. Dip-slip faults, probably high-angle normal faults, are predominated, as far as ever known. Consequently, PHS max in the late Quaternary is estimated to be perpendicular to the strike of trench axis. Active faults not only result in the deformation of the coral reef terraces but also control the outline of island in the southern area (Fig. 10). They are more densely distributed on the coral reef terraces than on the ordinal marine terraces and on the hilly or mountainous lands. It is inferred that an existence of active faults may partly correspond to an existence of hard and resistant coral limestone. Activity of the faults usually belongs to “Class B” (10^-1m/1, 000 years) or “C” (10^-2m/1, 000 years).