Geology and Geohistory of the Northwestern Philippine Sea, with Special Reference to the Results of the Recent Japanese Research Cruises

Abstract

1) Collection of acidic intrusive rocks, schists and other basement rocks, and Eocene Nummulites is one of the most remarkable results of the recent research cruises in the Philippine Sea. 2) Pre-Cenozoic island arc activity, such as regional low pressure metamorphism, volcanism, and acidic intrusion occurred and formed non-oceanic crust of the Daito Ridge Group Region. These rocks suffered upheaval and subsequent erosion, and then overlain by the shallow sea sediments containing Nummulites and other foraminifers. Differential subsidence occurred in the Oligocene and/or the Miocene. After then, whole Daito Ridge Group Region sunk 1000-2000 m. Activities of the Kyushu-Palau Ridge, such as volcanism, acidic intrusion, and upheaval occurred in the Eocene and the Oligocene. This ridge also sunk at least 600 m, since Late Miocene. It may be assumed that the Kinan Seamount Chain has not appeared before the development of the Kyushu-Palau Ridge. 3) Based upon the results of the recent studies, maturity of the island arcs and ridges is suggested.