三波川変成帯中の新たな独立した広域変成帯の存在―白亜紀から第三紀の日本における造山運動―

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 Sanbagawa 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 complex 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 metamorphic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsula (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 Shimanto 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 (?) metamorphic belts are virtually equivalent in terms of spatial distribution, metamorphic range of grade, and facies series. Pacific-type orogenic belts typically comprise accretionary complex, high-P/T metamorphic belt, fore-arc sediments, and batholith belt landward from the trench (Maruyama et al., 1996). In SW Japan, the Sanbagawa belt (ss) is paired with the Ryoke low-P/T metamorphic belt and with the ca. 120-70 Ma Sanyo TTG batholith belt. Furthermore the related fore-arc basin may have developed penecontemporaneously with the Shimanto BS-EA orogeny, which is paired with the late Cretaceous to early Tertiary San-in TTG belt, which extending along the Japan Sea coast. In-between the intervening Izumi Group, a fore-arc basin deposit formed during the Campanian to Maastrichtian. Thus, these two groups of orogenic units, which formed during independent orogenies were both extensively modified during the opening of the Japan Sea ca. 20 Ma. The southward thrusting of the Ryoke and Cretaceous TTG belts over the Sanbagawa extended beyond the southern limit of the Sanbagawa, leading the up-down relationship of the Sanbagawa (ss) and the Ryoke belts.