Abstract

While plate motions of the Pacific Plate and the Australia-India Plate flanking Southeast Asia are well known from the Late Cretaceous onwards, the motion of the Southeast Asian part of Eurasia is somewhat problematical because of the deformation that has taken place in the eastern part of the Eurasia Plate. It is best to consider the plates separately in the hot spot reference frame and to try to resolve relative motions by geological and palaeomagnetlc arguments. There is some uncertainty in the distribution of spreading in the mid-Cretaceous. The reconstructions presented here assume that the Indian plate spreading from Late Cretaceous to mthe Indian plate spreading from Late Cretaceous to middle Eocene time did not extend east of Myanmar. The Southeast Asian Plate is thought to have been near stationary, apart from an easterly motion in the last 15 Ma.

Oblique subduction under the Sunda Arc, which commenced as Australia started to move north in the middle Eocene resulted in partial coupling of the margin to the subducting slab, and applied a dextral shear to the western margin of Southeast Asia and a sinistral shear to the southern margin. These shears have been a major driving force in basin genesis throughout the region and have maintained their influence during the development of the basins near the plate boundary.

The Philippine Plate has rotated clockwise from Eocene until the Pliocene about a pole located east of Guam. The Philippine Islands were on the Philippine Plate as a west facing arc on which fore-arc and intra-arc basins developed from the late Oligocene or early

Miocene onwards. The rotation resulted in sinistral shear on the eastern margin of the Palawan Block and finally collision in late Miocene time. This collision and the Pliocene change in the pole of clockwise rotation to a location near Honshu describes the deformation and partial transfer of the islands to the Southeast Asia Plate.

In Northern New Guinea and the Banda Sea, the motion of the Philippine Plate has been east-west. It has carried a collage of continental and oceanic terranes westwards into the Banda Sea region from the top of the Australian continental block as it moved north. A subduction zone developed under Western Sulawesi. The tectonic transport probably commenced during the early Miocene. Easterly moving continental blocks collided with Sulawesi at the end of middle to late Miocene time resulting in “molasse” deposits. Thrusting in the Lengguru Fold Belt produced the present configuration of the Bintuni Basin. The oblique collision of the Melanesian arc resulted in the formation of the North New Guinea Basin and the development of the New Guinea thrust belt.