North Palawan Block, Philippines--Its Relation to Asian Mainland and Role in Evolution of South China Sea

Abstract

The Island of Mindoro, the northern part of Palawan Island, and the Reed Bank area (southwestern Philippines) together constitute a continental fragment, the North Palawan block, lying within an island arc-oceanic setting. The Permian to Paleogene rocks of these areas indicate a geologic origin and history for the block contrasting with that of the rest of the Philippine Archipelago. These rocks also suggest that the North Palawan block once occupied a pre-drift position contiguous with the south China mainland. Four prominent pre-Neogene regional unconformities are recognized both onshore and offshore the China mainland, in Taiwan, and in the North Palawan block. The synchrony of these unconformities and the facies relation of the unconformity-bounded sedimentary units strongly suggest a common pre-Neogene history for all these areas. By contrast, an important regional unconformity, occurring at the top of the middle Miocene throughout the Palawan area, is absent from the Asian mainland. Extension in the South China Sea basin since the Mesozoic, which has separated the North Palawan block from the Asian mainland, has been approximately uniform from west to east. However, there is strong evidence to suggest that this extension has been achieved by temporally separated phases of continental crustal attenuation and more recent sea-floor spreading. From the foregoing observations and using the most recent magnetic spreading anomaly data for the South China Sea, a suite of palinspastic reconstructions has been prepared, which shows the evolution of the South China Sea area from the Late Triassic to the Pliocene. The reconstructions illustrate (1) the convergent continental margin setting of the North Palawan block during much of the Jurassic and Cretaceous; (2) the Late Cretaceous inception of the Philippine island-arc system; (3) the subsequent counter-clockwise rotation of the arc system from the late Eocene onward; (4) the Paleocene to middle Miocene opening of the South China Sea; and (5) the early to middle Miocene collision between the North Palawan block and the Palawan subduction system.