Off‐scraped Permian‐Jurassic bedded chert thrust on Jurassic‐early Cretaceous accretionary prism: Radiolarian evidence from le Island, central Ryukyu Island Arc

Abstract

Abstract The pre‐Neogene basement of the central Ryukyu Island Arc shows zonal structures analogous to those of the outer belt of southwest Japan. The innermost terrane (Iheya Zone) consists of isoclinally folded beds dipping northwestward; the anticlinal cores are composed mainly of Permian chert, whereas the synclinal parts are represented by Jurassic to Cretaceous sandstone‐rich alternating siliceous shale and chert, bearing appropriate radiolarian fossils. At the east‐central area of Ie Island, the basement rocks are exposed as a 172 m high peak, Tattyu. The flank area of Tattyu is composed of latest Jurassic to Berriasian siliceous shale and chert as part of an accretionary prism, while most of Tattyu is composed of a continuous and very compact sequence of Norian through Kimmeridgian (?) bedded chert which is rather gently inclined. Beyond an unexposed part below the Norian chert, Guadalupian chert is recognized. It is inferred that this pelagic chert (Tattyu sequence) was off‐scraped and thrust on to the accretionary prism which developed on its flank area in an accretion process after the Early Cretaceous.