OFF-SCRAPING ACCRETIONARY PROCESS UNDER THE SUBDUCTION OF YOUNG OCEANIC-CRUST - THE SHIMANTO BELT OF OKINAWA ISLAND, RYUKYU ARC

The Eocene Kayo Formation exposed at the southwestern end of the Shima nto Belt in the Ryukyu Are records the progressive development of off- scraping accretion accompanied by increase in lithification and burial as well as synaccretionary thermal imprint. The formation consists al most wholly of coherent trench turbidites supplied by axial currents. Detailed structural analysis has revealed that two successive deformat ion stages are recognized in the Formation. The early deformation (sta ge 1) occurred by horizontal shortening related to the initial stage o f off-scraping accretion. In this stage, sands were water-rich and nea rly unlithified, and the competence contrast between sandstone and sha re was low. The late deformation (stage 2) is marked by the formation of seaward verging imbricate fold-and-thrust system within the accreti onary wedge. Compared with stage 1, sands in stage 2 were more dewater ed and lithified, and the competence contrast between sandstone and sh ale increased. In accordance with this deformation history, structures caused by high pore fluid pressure indicate increased cohesion of san ds during off-scraping accretion. Studies of the vitrinite reflectance and the illite crystallinity clearly suggest that the Formation becam e buried and suffered thermal imprint; pressure solution cleavage deve loped as an axial planar to the stage 2 folds. Thermal imprint on the Formation is characterized by high paleotemperatures at approximately 250 degrees C-300 degrees C in a region of the prehnite-pumpellyite fa cies metamorphism. This thermal event took place during and slightly a fter the stage 2 probably associated with subduction of a young and ho t oceanic plate.