鈴鹿山脈東麓地域の第四紀における変形速度

Abstract

A seismic reflection profile implies that foothills and terrace surfaces in the eastern margin of the Suzuka Range are underlain by an east-facing monocline. Based on the stratigraphy of the late Pliocene-middle Pleistocene Tokai Group and a drill core of 500m deep, we correlated distinct seismic reflectors to the boundaries between the Komeno Formation and the Oizumi Formation of the Tokai Group. Analysis of growth strata demonstrates that monoclinal uplift of the Tokai Group and Paleozoic rocks began since the beginning of the deposition of the lowermost part of the Tokai Group (ca. 3.0Ma). Total vertical growth and shortening of the bottom of the Tokai Group during the deposition of the growth strata are estimated to more than 1, 800m and 1, 000m, respectively. Using an age of 2.9±0.2Ma for the Ichinohara volcanic ash, which overlies near the bottom of the Tokai Group, based on fission track method, uplift rate since late Pliocene is >0.58-0.68mm/yr, and horizontal shortening rate is >0.30-0.37mm/yr. The resulting long-term structural growth rates are comparable to those of the Fumotomura fault during late Quaternary based on previous geomorphic studies, which suggests that the Fumotomura fault is responsible for the growth of the monocline. If so, the fact that terraces are deformed more narrowly than the Tokai Group implies that the monocline and the warping of terrace surfaces is fault-propagation fold.