Abstract: Tertiary Motion of the Philippine Sea Plate: Implications for Southeast Asia

Robert Hall, Jason R. Ali

Many of the ideas of island arc processes have been based on interpretations of the tectonics of the Philippine Sea plate. The history of plate motions in the western Pacific is also critical to understanding the growth of southeast Asia and the origin of marginal basins at the eastern Eurasia margin, both from the point of view of the kinematics and the forces acting. A key element in understanding the tectonic evolution of the western Pacific is the Philippine Sea plate. However, the plate is the one major plate whose Tertiary motion is poorly constrained and whose origin is problematical.

The southern part of the plate in eastern Indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate, which are separated from remnant arcs of the northern Philippine Sea by the central West Philippine basin. The east Indonesian islands of the Halmahera-Waigeo region contain a good Mesozoic and Tertiary stratigraphic record, indicating a long arc history. New paleomagnetic data from these islands record the Tertiary history of motion of the Philippine Sea plate. They indicate large total clockwise rotations similar to those suggested for other parts of the plate but record discontinuous rotation and a more complex motion history than previously recognized. For the southern part of the plate, there was 40° rotation with northward translat on between 0 and 25 Ma, no significant rotation between 25 and 40 Ma, and there was 50° rotation with southward translation between 40 and 50 Ma. The translation history of the southern part of the plate in eastern Indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate (15°N, 160°E for the interval 5-25 Ma and 10°N, 150°E for the interval 40-50 Ma). Reconstructions of the plate based on these results and including opening of marginal basins within the plate show that all paleomagnetic data from the plate are consistent with this rotation model. The calculated positions of the plate place firm limits on the eastern edge of southeast Asia during the Tertiary within which tectonic reconstructio s of the region can be made.

The present southern boundary of the plate is the Sorong fault system, which is part of a major left-lateral fault system at the northern margin of the Australian plate. The results of the new geological and paleomagnetic work show that the arc system which collided with the Australian margin at 25 Ma belonged to the Philippine Sea plate. The collision terminated convergence and may have been the cause of the major plate boundary reorganization of the region. Ophiolites and volcanic arc rocks were emplaced on the Australian margin at about the same time as ophiolites were emplaced on the Sunda margin in Sulawesi. Northward motion of Australia has continued since 25 Ma, but the Australia-Philippine Sea plate boundary has been a strike-slip fault zone throughout the Neogene. This has be n possible because rotation of the Philippine Sea plate has caused it to move northward at about the same rate as Australia.

We suggest that many of the arc fragments in the New Guinea orogenic belt originated in the southern Philippine Sea plate arc. Post-collision strike-slip faulting may have dismembered the arc fragments now preserved as "terranes" in northern New Guinea. Similar arc and ophiolitic rocks now found in eastern Indonesia remained part of the Philippine Sea plate and have rotated clockwise and moved westward approximately 1500 km at an average rate of about 6 cm/year. In contrast, there is little evidence for the widely quoted suggestion that large fragments have been sliced from the Australian margin and transported west within the fault system. Instead, the micro-continents of east Indonesia appear to have their origin as fragments, which separated much earlier from Australia by rifting.< P>

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994