Robert McCabe

Pb-isotopic compositions of volcanic rocks in the West and East Philippine island arcs: presence of the Dupal isotopic anomaly

Abstract The Philippine islands are situated between two oppositely dipping zones of seismicity. With the exception of a few areas, such as in the west central Philippines where the North Palawan continental terrane (NPCT) has collided with the archipelago, these seismic zones are well defined to depths of 200 km. Active volcanic chains overlay segments in each of these zones, suggesting that subduction is presently taking place both east and west of the islands. Lavas we have studied are thus divided between what has been termed the West Philippine arc and the East Philippine arc. West Philippine arc volcanic rocks which were extruded before the Philippine archipelago collided with the NPCT, or which are younger than the collision but crop out hundreds of kilometers from the collision zone, and all but one of the rocks from the East Philippine arc fall in the MORB field on207Pb/204Pb versus206Pb/204Pb covariation diagrams. This is surprising considering the frequency with which arc materials have207Pb/204Pb ratios higher than those of MORB, the highBa/REE and Sr/REE ratios in the lavas and the possibility of sediment subduction given the small accretionary prisms. All of these rocks have high208Pb/204Pb ratios with respect to Pacific and Atlantic Ocean MORB, but are similar to Indian Ocean MORB and IOB. Thus the Philippines consist of island arcs with the peculiar Dupal isotopic anomaly documented between 0° and 60°S in the southern hemisphere and particularly in the Indian Ocean region. This demonstrates that the Dupal isotopic anomaly is not restricted to the southern hemisphere, or to MORB and OIB. Post-collision rocks cropping out near the NPCT, in the West Philippine arc, have elevated208Pb/204Pb and 207Pb/204Pb ratios that could be attributed to assimilation of the newly introduced continental crust (NPCT) by mantle-derived magmas or to the addition of a sedimentary component to mantle-derived magmas.

Kinematic model for the opening of the Sea of Japan and the bending of the Japanese islands

We have analyzed paleomagnetic data from Japan reported over the past 25 years in combination with marine geophysical and on-land geologic data to reconstruct the opening of the Sea of Japan and the bending of Honshu. From these data, we propose that the Sea of Japan opened by a two-stage back-arc spreading event. (1) In the early to middle Tertiary, Japan began to rift off the Asian coast as a single rigid block, rotated clockwise approximately 25°, and formed the Japan Basin. (2) In the Miocene, the Yamato and Tsushima basins opened, and shortly thereafter (19 Ma) the Japan arc collided with another island arc in central Hokkaido. At this stage, Honshu began to behave as two rigid blocks—southwest Honshu and northeast Honshu. Southwest Honshu continued to rotate clockwise approximately 45° as the Yamato and Tsushima basins continued to open. Northeastern Honshu rotated counterclockwise approximately 40° as Hokkaido was held fixed by the arc collision while the Yamato Basin continued to open. These two events—the opening of the Tsushima and Yamato basins and the arc collision in Hokkaido—resulted in the early to middle Miocene bending of the Japan arc observed in the paleomagnetic data. After 12 Ma, back-arc spreading ceased.

Borneo: a stable portion of the Eurasian margin since the Eocene

Samples collected from sixteen paleomagnetic sites of Plio-Pleistocene basaltic rocks from southeastern Borneo yield mean direction of Dm = 180.4°, Im = −13.8° (α95 7.8° k = 23.6) and a paleomagnetic pole of λ = 82.7° N, σ = 120.5° E. Samples collected from nine sites, Eocene-Early Miocene basalt, andesite, and shale, yield a mean direction of Dm = 182.0°, Im = −2.2° (α95 = 11.0° k = 22.8) and a mean Eocene-Early Miocene pole of λ = 87.8° N, σ = 173.5° E. These Eocene-Early Miocene directions pass a fold test. Rock magnetic studies suggest that the carrier of remanence in the samples is magnetite. These Eocene to Pleistocene paleomagnetic results from southeastern Borneo suggest the island has been at its present position with respect to Eurasia since Eocene. Our results are consistent with other studies showing no evidence for latitudinal displacement; however, these results do not concur with the previous reports of a large Tertiary counterclockwise rotation. Our data are in general agreement with the rotations predicted by the extrusion tectonic model for southeast Asia and argue against the numerous Cenozoic tectonic reconstructions of the region which require a large counterclockwise rotation of Borneo.

A preliminary Neogene paleomagnetic data set from Leyte and its relation to motion on the Philippine Fault

Abstract Leyte Island is bisected by the left-lateral Philippine fault. Paleomagnetic samples were collected from nineteen lava flows in Leyte and nearby islands to the north. The data collected from these sites were combined with previously reported results from northeastern Mindanao. The paleomagnetic data from Neogene rocks within or very near the Philippine fault zone on Leyte, Mindanao, Biliran, Maripipi, and Genuruan Islands fall into two populations. Seventeen late Neogene sites (fourteen normal polarity and three reversed polarity) yield a mean direction D = 358.9°, I = 20.1° (α95 = 6.6°, k = 30.1) and a paleomagnetic pole λ = 88.9°N, φ =12.5°E (A95 = 4.8°, K=56.1). Eight early Neogene sites (three normal polarity and five reversed polarity) give a mean direction D = 23.2°, I = 13.9° (α95 = 9.3°, k = 36.2) and a paleomagnetic pole λ = 66.5°N, φ = 220.5°E (A95 = 7.1°, K= 62.0). The late Neogene pole is indistinguishable from Plio-Pleistocene poles from the Philippines, Taiwan, and Vietnam, and the late Miocene pole of the central Philippines suggesting there have been no significant rotations or translations of this region with respect to other Southeast Asian regions during the past 5 M.y. The early Neogene paleomagnetic pole is statistically indistinguishable from the middle to early Miocene pole of the central Philippines. The early Neogene pole is statistically distinguishable from early Miocene pole of Marinduque Island (Philippines) as well as the late Miocene pole of the central Philippines and the Plio-Pleistocene pole of the entire Philippines. This preliminary paleomagnetic data set suggests that Leyte and adjacent islands to the north have been a portion of the same tectonic block as northeastern Mindanao, Cebu, and western Panay since early Neogene. There are also no paleomagnetically detectable fault-related rotations associated with the Philippine fault given the resolution of this data set. No consensus exists as to the amount of offset or the age of inception of the Philippine fault. The 110 km offset between ophiolitic basement in northeast Leyte and that in southwest Leyte suggests this amount of left-lateral displacement has occurred along the Philippine fault since the middle Tertiary. In contrast to data from continental regions, no shear-related rotations are found associated with the Philippine fault. This observation suggests that the hot, immature, transitional island arc/continental crust of the Philippine region deforms differently in response to shear than well developed continental crust.

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.

Miocene paleomagnetic results from southeastern Korea

Paleomagnetic samples were collected from 25 sites of Late Oligocene to Middle Miocene age from southeastern South Korea. The mean direction obtained is D = 39.4°, I = 46.2° with α95 = 9.5°, which is statistically distinguishable from the present axial geocentric dipole field direction. When combined with the previous reported results of eight sites from this region, we get a mean direction (D = 40.1°, I = 47.4°, α95= 7.9°) for 33 sites of Late Oligocene to Middle Miocene age. This direction yields an early Neogene virtual geomagnetic pole of 56.5°N, 215.2°E with an α95 of 7.9°, discordant from the Cretaceous reference pole for Korea. The paleomagnetic data suggest that portions of southeastern Korea underwent a large clockwise rotation of about 40° during the Neogene. Two large NE-trending right-lateral faults (the Yangsan Fault and the Dongnae Fault, Fig. 1) cut the study areas. Both these faults show evidence for recent activity. In addition, minor faults with similar trends to these major faults occur. The observed deflections in paleomagnetic declinations were probably caused by rigid block rotations of crustal blocks in a right lateral shear system during the Neogene. The paleomagnetic directions for Miocene aged rocks from southeastern Korea and southwestern Japan are similar in direction and magnitude. This suggests that the cause of the rotations in both areas may be related to a similar tectonic event in this area during the Neogene.