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° ( α 95 = 6.6°, k = 30.1) and a paleomagnetic pole λ = 88.9° N, φ =12.5° E ( A 95 = 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° ( α 95 = 9.3°, k = 36.2) and a paleomagnetic pole λ = 66.5° N, φ = 220.5° E ( A 95 = 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.