The Median Tectonic Line (MTL) runs from SW Japan through Shimonita in western Gunma prefecture, dividing the Cretaceous granitoids of the Ryoke belt and the Jurassic accretionary complex of the Mino–Tanba–Ashio belt to the north from the Cretaceous blueschists of the Sanbagawa belt to the south. Immediately to the south of the MTL, a peculiar set of rocks occurs as a klippe above the Sanbagawa schists, separated by a low-angle fault. The Atokura klippe comprises various rock types, such as Permian granitoids, hornfels, and Cretaceous–Paleogene conglomerates/sandstones, although the amount of each component is small. The klippe has been studied since the 1950s, nonetheless, its origin has long remained a mystery. The photo taken along the Nanmoku River in southern Shimonita shows bedded sandstone/mudstone of the Lower Cretaceous Atokura Formation, which is totally overturned. Recent U–Pb dating of detrital zircons from the Cretaceous sandstones in the Kanto Mtn. identified abundant Proterozoic (ca. 2500-1500 Ma) grains in the Atokura Fm (Nakahata et al., 2015, 2016). This unique age spectrum is significantly distinct from that of coeval sandstones in the Chichibu belt, ca. 10 km to the south, indicating a unique provenance for the Atokura Fm. The associated Permian granitoids are also unique, because they are extremely rare in SW Japan, except in the Hida and Maizuru belts along the Japan Sea side. These indicate that the Atokura klippe represents an allochthonous unit, primarily derived from the continental side of Mesozoic Japan, and secondarily transported over a long distance of up to 100 km. Shimonita is well-known for its local delicacies, such as Konjac (jelly made from special yam roots) and endemic green onion, which are both suitable additions for Sukiyaki; nonetheless, its geology conceals more valuable secrets on the origin of Japanese Islands. It is fun to see a local train with “Atokura klippe” painted in large letters on its side, not only for train-spotters but also for many geoscientists.
(Photograph & Explanation: Yukio ISOZAKI)
