Paleomagnetism of the Zambales ophiolite, Luzon, northern Philippines

Abstract

Paleomagnetic results are reported from 52 sites in the Zambales ophiolite. The sites provide vertical sampling from the overlying sediments, in the north and south, to layered gabbros and dunites in the Acoje and Coto mines, in the west. They extend over an area of approximately 100 by 50 km. A Late Miocene-Pliocene intrusion in the Tarlac region gives a direction marginally distinguishable from the present field indicating a small clockwise rotation. This is consistent with other indications of clockwise rotation since Late Miocene in Luzon. Early and Mid-Miocene sediments from Barlo in the north and Tarlac in the south indicate a period of rapid counterclockwise rotation during the Miocene, again consistent with other results from Luzon. Eocene-Oligocene sediments overlying the ophiolite in the north and south indicate further counterclockwise rotation with northerly translation. The interpretation of the directions in the ophiolite itself is more problematical. Directions similar to those in the Eocene-Oligocene and in Mid-Miocene sediments probably reflect secondary magnetizations and magnetization associated with the unroofing of the ophiolite. We interpret the most strongly counterclockwise rotated magnetizations represented by a 44 Ma sill in the Barlo area and of the 46 Ma dykes at Coto as the primary direction of the ophiolite. The counterclockwise rotation and northward translation interpreted from our results is consistent with the Holloway model of the origin of the Philippines. However, this model neglects the role of the Philippine Sea Plate in the tectonic history of the Philippines. The counterclockwise rotation is also consistent with rotation of Luzon in the left-lateral shear between the Philippine Sea Plate and Asia, as suggested by Sarewitz and Karig. Our data do not distinguish between counterclockwise rotation of a single block, or of a number of smaller blocks. The data suggest post-Late Miocene clockwise rotation for Luzon, when it was coupled to the Philippine Sea Plate. Our data are not consistent with a Philippine Sea Plate origin for Zambales.