Neptunian dykes in Late Pliocene basaltic rocks, Kumejima Island, Ryukyus, southwest Japan

Abstract

This note presents mesoscopic and microscopic features of neptunian dykes of calcareous sediments hosted by Late Pliocene basaltic lavas of the Uegusukudake Formation of Kumejima Island, the Ryukyus. The dykes, 1 to 20cm wide, consist mainly of micritic limestone rich in varied skeletal debris of a shallow marine affinity, with a subhorizontal depositional surface. Two dominant strikes of the dykes, NE and NW, are parallel and perpendicular to the modern central axis of the Okinawa Trough, respectively. The youngest foraminifera in the dykes were identified as Globorotalia inflata, of which first appearance is in Late Pliocene. Available K-Ar age data of the basaltic lava and Sr age data of the overlying Ryukyu Limestones indicate that infilling rapidly occurred during a period from 2.1 Ma to 1.3 Ma. These date are synthesized as follows. Open fractures formed due to rifting in an extensional regime, and were filled with shallow-marine calcareous sediments to form neptunian dykes under the sea. This rifting is postulated to have been related to the reactivation of rifting at the Okinawa Trough.