Abstract
Some characteristic magmatism on the structural highs of continental slopes indicates initiation of subduction of a new, relatively warm slab. An instance is the plagio-granitic and tholeiitic magmatism that occurred on the Middle Miocene topographic and structural highs of the continental slope of the outer zone of Southwest Japan. It was simultaneous with the initiation of subduction of the new Shikoku Basin slab, just following the off-ridge volcanism in the basin. The magmas intruded upward through the remnant fissure of the transform fault which had separated the Southwest Japan arc from the Philippine Sea plate until the cessation of the Shikoku Basin opening.
Application of the idea of "continental slope structural high magmatism" to the igneous rocks of the Daito Ridge Group provides a new hypothetical scenario for the tectonic history of the Northern Philippine Sea. Late Cretaceous collision of the "Kuroshio Paleo-island" with the Japan arc resulted in suspension of north-ward subduction of the Paleo-Philippine Sea plate, temporal southward subduction of a part of the plate under the Paleo-Daito arc, and the structural high magmatism on the Amami PIateau, the present northernmost ridge in the Daito Ridge Group.
