Existence of New Independent Regional Metamorphic Belt in the Sanbagawa Metamorphic Belt: Orogenic Evolution of Japan from Cretaceous to Tertiary

Abstract

The Sanbagawa metamorphic belt in SW Japan was previously considered to extend in the E-W direction from the Kanto Mountains to Kyushu Island, a distance ＞ 800 km. However, Aoki et al. （2007） recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after ca. 90-80 Ma. Furthermore, Aoki et al. （2008） showed that these rocks suffered blueschist metamorphism at 66-61 Ma, which differs from the timing of the Sanbagawa metamorphism. Thus, these results show that the Sanbagawa belt in Shikoku is a composite metamorphic belt. We, therefore, redefine the traditional San-bagawa belt; the structurally upper part is the Sanbagawa metamorphic belt （sensu stricto） . It formed as an accretionary complex at ca. 140-130 Ma and subsequently experienced BS-EC facies metamorphism at ca. 120-110 Ma （Okamoto et al., 2004） . By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary com-plex after ca. 90-80 Ma and experienced peak metamorphism at ca. 60 Ma. Our observations have important implications for the lateral extension of these two metamorphic belts in SW Japan. The accretionary ages of the traditional Sanbagawa belt in the Kanto Mountains are younger than the Sanbagawa peak metamorphic age （Tsutsumi et al., 2009） , clearly indicating that the entire region of Kanto Mountains Sanbagawa must belong to the Shimanto metamor-phic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsu-la （Otoh et al., 2010） . These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt （ss） is less than half of its previous extent. The metamorphic grade of the Kanto Mountains in the Shiman-to metamorphic belt ranges from pumpellyite-actinolite facies to epidote-amphibolite facies. Therefore, the higher-grade rocks of the Shimanto metamorphic rocks are exposed in the Kanto Mountains in comparison with Shikoku and Kii Peninsula. Hence, these two distinct BS-EA-EC 313