2000 Volume 106 Issue 6 Pages 385-396
This paper describes the lithology and collapse direction analysis of landslide deposits distributed along the active trace of the Median Tectonic Line (MTL), extending ENE-WSW at the Funato area in central Shikoku, Southwest Japan. The results together with topographical characteristics around the Funato area suggests about 30 m of right lateral displacement along the active trace of the MTL.The landslide deposits are distributed along the eastern side of Setodani Valley for 200 m (E-W)×150 m (N-S), overlying the fault surface of the MTL. The landslide deposits originated from sedimentary rocks of the Izumi Group to the north across the MTL, and are divided into two types (Types I and II) based on their lithological facies. The Type I deposits are characterized by boudinaged sandstone blocks contained in sheared argillaceous matrix, which were formed at the first stage of a landslide activity overlying the fault zone of the MTL. The Type II deposits are characterized by randomly oriented sandstone blocks, and were formed by the subsequent re-collapse of Type I deposits. Results of structural analysis show that the collapse directions of both Type I and Type II deposits are from N17°-31°W to S 17°-31°E. However, evidence of landslides, such as relict concave topography, are not found to the northwest (N17°-31°W) of the landslide deposits. A small ridge of these deposits extending from ENE blocks half the mouth of the Setodani Valley. From these observations, the following possibilities are suggested. (1) The source region of the landslide, which is inferred to be present to the northwest, has been lost by erosion or slope collapse. (2) The ridge composed of landslide deposits can be regarded as a "shutter ridge" which has been displaced from the east to its present position. In this case, the right lateral displacement along the active trace of the MTL after the formation of the Type II deposits is estimated to be about 30 m maximum (E-W length of the shutter ridge). This event must have ocurred after the deposition of fluival deposits (7120±260 yrs. B.P. ), since the Type II deposits cover the active fault surface, which had previously cut the fluival deposits.
