The use of paleomagnetic studies in understanding the complex Tertiary tectonic history of East and Southeast Asia

Abstract

Many published paleomagnetic studies in East and Southeast Asia focus on Mesozoic and Paleozoic sequences. Many of these studies ignore Cenozoic rocks which, in many places, overlie these older rocks. Examination of paleomagnetic results from some of these Cenozoic sequences shows that many of the older rocks contain magnetizations which have the same direction as the Cenozoic rocks. This suggests that the directions of magnetization in the older rocks were reset by Cenozoic events. This fact argues that until paleomagnetic studies are completed on the Tertiary sections in the region, results published from the older rocks should be regarded as suspect. Paleomagnetic studies on Cenozoic rocks from Japan, the Philippines, Indo-China and Korea have helped better constrain some of the active Tertiary tectonic events in this region. For example, paleoinclination studies from the Philippines and the Philippine Sea Plate show that rocks of the same age from both the Philippines and the Philippine Sea Plate exhibit similar inclinations, suggesting that the Philippine Arc has been on the western edge of the Philippine Sea Plate throughout much of the Tertiary and has undergone a similar northward drift. Paleodeclination data from the Philippine Arc shows a differential bending. This bending occurred during the Miocene. The timing of this deformation and geological studies in the central Philippines help us constrain the age of docking of the Palawan Microcontinent with the Philippine Arc. Paleodeclination studies in Japan have been used to support a Middle Miocene opening of the Japan Sea and to suggest that Japan rotated, as a simple rigid block, eastward away from the Asian continent. Paleodeclination studies in Thailand show there have been some Late Neogene translations in the region; however, the mechanism of the rotations is still a subject of controversy. Paleomagnetic studies in strike-slip fault regimes have yielded two different results. Paleodeclinations from southeastern Korea are consistent with Neogene fault-related block rotations in the region. In contrast, reported paleodeclinations from close to the left-lateral Philippine Fault show no evidence for fault-related rotations occurring in this region.