Fluvial Terrace Development and Crustal Movement along the Souzu and Matsuda Rivers, Southwestern Shikoku, Japan

Abstract

The purposes of this paper are to discuss the development of fluvial terraces along the Souzu and Matsuda Rivers in southwestern Shikoku using widespread marker tephras and to estimate the crustal movement in the region on the basis of deformed terrace profiles. First the author identified several tephras from the terrace and tales deposits, including the Zentsubo, Kubo, and Ogawa tephras to establish terrace chronology. The fission-track age of the Zentsubo tephra with the zeta calibration is 0.58±0.11 Ma. The Kousenji II tephra located below the Zentsubo tephra shows a fission-track age of 0.66±0.15 Ma. In terms of the similarities of mineral composition, age, and refractive indices of volcanic glass and hornblende, the Zentsubo tephra can be correlated with the Yufugawa pyroclastic flow deposits that erupted from the Yufuin basin at about 0.6 Ma. The fluvial terrace surfaces along the Souzu and Matsuda Rivers are classified into four levels: H1, H2, M, and L in descending order. The Zentsubo and Kousenji II tephras are intercalated in the upper part of the H1 terrace deposits, indicating that the age of the H1 terrace surface is approximately 0.6 Ma. The occurrence of the Ogawa tephra in L terrace deposits or in tales deposits on L terrace deposits suggests that the L terrace surface was formed around or after 70-80 ka. The H1 terrace is a broad fill-top terrace formed by the paleo-Minamiuwa River, taking a different course from the present courses of the Souzu and Matsuda Rivers, whereas the lower terraces occur along the present river systems. The paleo-Minamiuwa River disappeared after the H1 terrace was formed by a large-scale river capture due to the beginning of upwarping in the middle reaches. The thickness of the H1 terrace deposits indicates that the tectonic setting in this area was stable or subsiding until 0.6 Ma. Subsequently, the upwarping of the Shikoku Mountains began at around 0.6 Ma and has continued up to the present, causing the deformation of the longitudinal profile of the H1 terrace surface at Uwaodo and Kamiari along the Souzu and Matsuda Rivers. This upwarping triggered a large-scale river capture by the Matsuda and Akagi Rivers at around 0.6 Ma.