Determination of the Exact Surface Trace of the Median Tectonic Line in the Ise Area, Southwest Japan

Abstract

The Miocene collision of the Izu-Bonin arc with the Honshu arc in Japan is thought to have initiated eastward bending of the Median Tectonic Line （MTL） from an approximate E-W strike to an ENE-WSW strike in the Ise area of the easternmost Kii Peninsula. However, the exact surface trace of the MTL in the Ise area remains poorly defined because of Quaternary sediment cover and the development of urban areas. Determination of the surface trace of the MTL is important to evaluate the characteristics of seismic motion upon the paired metamorphic belts divided by the MTL, which reflects the subsurface geology, and to better explain the curvature of the MTL in central Japan. This paper presents the results of petrographic observations from several localities in the Ise area, including Tamaki Town and Ise City : boring cores at （1） the factory building of Miwa Lock Co., Ltd., in Tamaki Town, which overlie the MTL, （2） Ise City Hall, （3） Ise City Tourism and Culture Hall, as well as additional boring data from Ise City. The depth of pre-Neogene basement increases eastward from the Gokatsura area, ranging from 6-12 m in the Sanbagawa belt near Ise City Hall to greater than 30 m in the Ryoke Belt. A geological survey of rare exposures, including the Ryoke rocks of the Ooike pond area in Tamaki Town, Miocene conglomerate along the Miyagawa River in the Ryoke belt, and Sanbagawa schists in Ise City, have helped to constrain the exact location of the MTL in Ise City. Through this study we have also signalized the potential of investigated sites in Ise city to be used as prospect geosites in future. After proper development and promotion these sites can become important points of attention to be seen by visitors and geologists and provide various opportunities for local economy growth.

Identification of prospective geosites that show features of the active continental margin in eastern Kii Peninsula, Southwest Japan

“…The precise location of this ENE-WSW striking fault has recently been confirmed in the region (Suzuki et al, 2015). The surface trace of the MTL starts to bend gradually eastward as it crosses the peninsula into the Chubu region, as a result of the middle Miocene (c. 15 Ma) to recent collision of the Izu-Bo nin arc with the Honshu arc.…”

“…3, and they must therefore have been rotated together with the MTL during the Miocene opening of the Sea of Japan, as mentioned above. The rock types of clasts include granite, sandstone, chert, gneiss, hornfels, granitic mylonite or cataclasite, greenstone, and greenschist, and the conglomerates are locally intercalated with sandstone layers (Suzuki et al, 2015). The granitic, gneissose and mylonitic, and hornfels clasts were derived from the Ryoke belt, while the schist and greenstone clasts are correlated with the Sanbagawa metamorphic rocks.…”