讃岐山脈西部の"中央構造線"

"Median Tectonic Line" in the Western Part of the Sanuki Mountain-range, Shikoku, Japan

NAKAGAWA Tsukasa

廣島大學地學研究報告

Geological report of the Hiroshima University

On the geology of Japan, the "Median Tectonic Line" is one of the most important and complicated faults and, therefore, it has been studied from various points by a number of authors such as YABE (1915), YEHARA (1936, 37), KOBAYASHI (1941) and others. Now, it is believed that this tectonic line might have been created by various modes of displacement happened at several times. In Shikoku, it runs longitudinally along the boundary between the Izumi group of the Inner Zone and the Sambagawa crystalline schist of the Outer Zone.
In this district, the "Median Tectonic Line" can be divided into the following three phases:
1.Pre-Shôbudani Kawataki Phase (thrust Fault)
2.Pre-Shôbudani Ikeda Phase (reverse Fault)
3.Late Pleistocene Shôbudani Phase (thrust Fault)
The first and the third phase of the "Median Tectonic Line" are well developed and recognized in the western and the eastern part of the district, respectively. On the other hand, in the central part the second and the third one are alternated horizontally with each other at the distance of about 2.5 km in elongation.
The former two of the "Median Tectonic Line" are distinctly covered by the Pleistocene formation, but the latest one is represented by a thrust fault in some places where the Izumi group or the Sambagawa crystalline schist thrusts up on the Pleistocene bed with a distinct fault plane having N75°E± strike and N20°-30° dip. The first one is a thrust fault and its plane is N 80° E and S 40°-44°, but they are N 75°-85° E and N 60°-80° in the second.
In the district, we can also see the following two types of west side down NNW-SSE faults which cut off the "Median Tectonic Line":
Pre-Shôbudani Phase
Post Late Pleistocene Shôbudani Phase
The former was formed in the later stage of the Pre-Shôbudani phase's "Median Tectonic Line", and is covered by the Pleistocene bed. While, the latter cuts off the "Median Tectonic Line" at the Shôbudani phase. These faults might have been due to the block movements of the Sanuki mountain.
Judging from the geological map of the district and the nature of NNW-SSE faults, it is de-emed that the original "Median Tectonic Line" must be represented by a thrust fault from the south side. In this respect, the "Median Tectonic Line" at the Kawataki phase may be the most oldest.
Pleistocene formation can be divided into two members called lower and upper gravel beds. The older or former is characterized by the presence of well sorted pebbles derived from the Sambagawa crystalline schist region, but they are absent in the latter. From the lower member, numerous plant remains were collected and they are probably assgined to "Middle or Upper Pleistocene" in age. It seems that the change of the Pleistocene litho-facies was influenced by the movement of the "Median Tectonic Line" at the Shôbudani phase.

jpn

廣島大學理學部地學教室

[ISSN] 0073-2303

[NCID] AN00213676