Evolution of the western pacific and its margin

Abstract An evolutionary history of the western Pacific is presented by reconstructing the spreading systems of the region since Middle Mesozoic time on the basis of marine magnetic lineations, paleomagnetic data, DSDP results, and selected other data. Subduction along the Asian plate margin throughout this time has resulted in general northward movement of the plates surrounding Asia. An E-W spreading ridge system, offset by N-S transform faults of great length, extended from the Pacific into the Tethys Sea and migrated north as the oceanic plates to the north were being subducted along Asia. Segments of this ridge system were subducted at different times along the Asian margin, in some cases initiating marginal-sea formation. As the plates south of these ridge segments started to subduct at the Asian margin, new spreading ridges formed far to the south, rifting India from Antarctica at about −100 m.y. and Australia from Antarctica at about −52 m.y. Subduction of the Pacific ridge system in the North and Southwest Pacific resulted in a change of direction in Pacific plate motion from NNW to WNW at about −45 m.y. The long N-S transform faults of the western Pacific provided zones of weakness along which new subduction was initiated. This occurred along the present N-S trend of the Philippine arc, along the Kyushu—Palau ridge, in an original position farther east, and along the approximate present position of the Lau—Havre ridge. Oceanic crust in the West Philippine Sea and possibly the South Fiji basin was entrapped by development of the new subduction zones. Subduction of the Pacific plate accelerated as the system became well established by about −24 m.y. Back-arc spreading was initiated as a result of the faster subduction and formed the Miocene and younger eastern portions of the western Pacific marginal seas. The marginal seas at both ends of the Sunda arc have more complex histories, being formed by a combination of processes including entrapment and rifting of the plates along transform plate boundaries.