North Luzon and the Philippine Sea Plate motion model: Insights following paleomagnetic, structural, and age-dating investigations

Results of one of the most comprehensive paleomagnetic and supporting geological programs ever carried out in offshore SE Asia on North Luzon, northern Philippines, are reported. Six new results, based on 66 sites, are reported from a total collection of 243 individual sites. Declinations in the data subset are sometimes scattered, likely reflecting combinations of major plate and local rotations in both clockwise and counterclockwise directions, and thus have a somewhat limited value for tectonic modeling. The inclination data are, however, much more valuable and can be best explained if North Luzon traveled as part of the Philippine Sea Plate for most of its history, a scenario which is compatible with the known geology of the eastern Philippines and broader region. In the proposed model, for all of its Eocene-Pliocene history, North Luzon is placed on the western edge of the Philippine Sea Plate, effectively always just to the west of the site where the Benham Plateau formed ~40 Ma. The paleomagnetic data indicate a substantial northward migration of the area since the start of the Neogene, with an earlier interval stretching back to at least the mid-Early Cretaceous when this part of the plate occupied equatorial latitudes. Post-15 Ma motion of the plate has involved the indentation of the Palawan microcontinental block into the western side of the Philippine Archipelago. Deformations induced by this process offer the most likely explanation for the scattered declinations observed in North Luzon and areas a short distance to the south. Copyright 2007 by the American Geophysical Union.published_or_final_versio

Peculiar geometry of northern Luzon, Philippines: Implications for regional tectonics of new gravity and paleomagnetic data

[1] The northern termination of the Philippine archipelago is remarkably abrupt. The distinctive, almost rectangular, shape of northern Luzon is a consequence of the almost simultaneous disappearance near 18°N of the Central Cordillera, the much narrower Sierra Madre, and the broad floodplain of the Cagayan River. The floodplain is underlain by a deep sedimentary basin that is closed off just inland of the coast by an ENE-WSW structural high known as the Sicalao Ridge. Gravity surveys show that the ridge has steep flanks that are almost certainly defined by faulting. In the same region, there is also a dramatic change in the Luzon Arc, which is the volcanic expression of subduction of the South China Sea. The volcanic centers in the Central Cordillera (the North Luzon Segment of the arc) are all now inactive, but further north eruptions are frequent on the small eastern islands of the Bashi Segment. The differences between the North Luzon and Bashi segments suggest that they had very different geological histories until the late Neogene, but the evolution of northern Luzon is so poorly understood that it is not even known on which plate it originated. New paleomagnetic data and the existence of the Sicalao Ridge provide important constraints on reconstructions and, when combined with geological data, favor the possibility that in the Paleogene the Central Cordillera and Sierra Madre were combined as parts of an arc at the southern margin of the Philippine Sea. The Sicalao Ridge can be interpreted as a rift margin feature created during the detachment of Luzon from continental Sundaland. Copyright 2006 by the American Geophysical Union.link_to_subscribed_fulltex