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 30m 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.