SOME FEATURES OF THE ARC-CONTINENT COLLISION ZONE IN THE RYUKYU SUBDUCTION SYSTEM, TAIWAN JUNCTION AREA

To the northeast of Taiwan, northwestward subduction of the Philippine Sea plate is occurring beneath the Eurasian plate along the Ryukyu Tr ench. The Ryukyu Trench, which is well defined along the northeastern part of the Ryukyu are, cannot be easily defined west of 123 degrees e ast. This is an area where the Gagua Ridge (whose origin is controvers ial) enters the trench from the south. On the basis of the marine geop hysical survey data the following results have been obtained. The stru ctural elements associated with the Ryukyu subduction system deform an d partially disappear west of 123 degrees east. Among other things the Ryukyu Trench terminates close to the western slope of the Gagua Ridg e. The Gagua Ridge is the result of tectonic heaping and is likely to be an uplifted sliver of oceanic crust. The interaction between the Ry ukyu subduction system and the Taiwan collision zone encompasses a wid e region from Taiwan to the longitude 124.5 degrees east. The Gagua Ri dge is a boundary between the active deformation zone related to the c ollision in Taiwan and the West Philippine Basin. It is proposed that there is a tectonic zone that can be traced from the Okinawa Trough on the north to the southern termination of the Gagua Ridge on the south .