FAULT TOPOGRAPHY AND RATE OF FAULTING ALONG THE MEDIAN TECTONIC LINE IN THE DRAINAGE BASIN OF THE RIVER YOSHINO, NORTHEASTERN SHIKOKU, JAPAN

Abstract

The Median Tectonic Line is one of the most spectacular features in physiography of Southwest Japan, ; and lies as a boundary between the Upper Cretaceous Izumi group of the Inner Zone and the Palaeozoic crystalline schist of the Outer Zone. It trends continuously and almost straight in ENE direction, especially in the middle and western part. The writer investigated mainly fault topography and geology along the Median Tectonic Line in the lower drainage basin of the Yoshino. The results are briefly summarized as follows. (1) There are several faults which run parallel or en echelon along the so-called Median Line. All of these faults are not necessarily active, but have been dislocated in the rightlateral sense. The witer, therefore, named these faults the Median Tectonic Line fault system. In areas intervening between neighbouring two strands of these faults, several NE-trending faults of dip-slip type develop as shown in Fig. 1&2. (2) At Awa-Ikeda, horizontal and vertical displacements along the Ikeda fault amount to about 200 meters or more and 40-50 meters, respectively, during the past 30, 000 years (Okada, 1968). At Ikenoura, Mima-Town (Fig. 4), horizontal displacement along the Mino fault is 200-230 meters during about 25, 000 years. At Kami-kirai, Ichiba-Town (Fig. 8), hori-zontal and vertical displacements along the Chichio fault are 50 meters and 5-6 meters, re-spectively, in the recent 10, 000 years. From these data, the mean rate of right-lateral displace-ment along the Median Tectonic Line fault system is estimated at 6-9 meters per 1, 000 years. (3) Fault planes of the Median Tectonic Line, as far as observed at many outcrops in this area, generally dip 60°-80° N or almost vertically. Some features preserved in the shat-tered zones indicate the right-lateral faulting, accompanying with slight uplifting of the northern blocks. (4) In this area, generally speaking, northern blocks of these active faults have been up-lifted. However, rates of vertical displacement along one strand of these active faults are locally different, and recent faulting accompanied with wavy deformation or transcurrent buckling. Vertical displacement along the whole length of the Median Tectonic Line from central Japan to Kyushu shows the scissoring on both sides, that is, the northern blocks of the fault have been uplifted in its eastern part and vice versa in its western part. (5) As to stream offsets along the faults, it indicate that the longer are upper streams from faults, the larger are stream offsets, which amount to 1.5 or 2 kilometers in maximum (Fig. 10). It is concluded to have been dislocated at almost uniform rates at least during the latter half of the Quaternary. (6) No great earthquake has occurred along the Median Tectonic Line in the historical time, though there are many geological evidences of quite recent fault displacement probably accompanying with great earthquakes. Tectonic creep along the fault lines is not yet found in the investigated area. It is, therefore, highly possible that a great earthquake may occur along the Median Tectonic Line in near future.