Paleomagnetic results from Luzon and the central Philippines

Abstract

Samples were collected from 86 paleomagnetic sites from the islands of Luzon, Marinduque, Mindoro, Panay, Negros, Cebu, and Mindanao in the Philippine Arc. The sampling sites range in age from Pleistocene to Jurassic. Characteristic directions of magnetization of the samples were determined by the use of vector plots. Curie temperature determinations, thin section studies, and hysteresis studies showed that remanence of these samples is carried by fine‐grained (pseudo‐single domain) magnetite. Positive fold tests from Miocene data from Panay, Jurassic data from Mindoro, and Cretaceous data from Cebu suggest that the magnetization of these regions was acquired prior to folding. Rotations reported below are measured with respect to the axial goecentric dipole field. The Plio‐Pleistocene data set shows no resolvable rotation for the 22 sites. This data set suggests that the various terranes that make up the Philippine Arc have behaved as a single unit during the past 5 m.y. or that deformation has been below the limits of resolution. The inclination data from the Plio‐Pleistocene sites have anomalously shallow inclination and are consistent with other Plio‐Pleistocene data from Vietnam, Taiwan, and the Marianas. These data support earlier suggestions for a late Neogene offset dipole effect. The late Miocene sites fall into two separate groups. Ten sites from western Luzon show evidence for around 20° of clockwise rotation. In contrast to this, late Miocene samples from the Bicol region, Negros, Marinduque, and Mindanao are not rotated. The cause of the postlate Miocene clockwise rotation of Luzon is unknown, but a Pliocene collision of the North Luzon Arc with Taiwan is suggested. Early Neogene results also separate into two different populations. The population from Marinduque shows evidence for a large counterclockwise rotation. The second early Neogene population comes from Panay, Cebu, and Mindanao and clearly shows evidence for a clockwise rotation. The validity of this rotation is further supported by a fold test and a reversal test. These early Neogene data sets are consistent with a middle to late Miocene collision of the Palawan Continental Terrane and the Central Philippine Arc. Data from six dikes of possible Oligocene age from the Zambales Ophiolite are highly discordant from the present field, being rotated approximately 60° clockwise. The directions from these dikes are similar to a direction reported earlier from late Oligocene sediments also from the Zambales region. These two data sets support the interpretation that the Eocene direction from Zambales is recording a large clockwise rotation of the region. Data from the Mesozoic sites are from two regions. Data from the Cretaceous Pandan formation of Cebu are discordant with data from the Upper Jurassic from Mindoro. The presence of a fold test from each region and a reversal test from Mindoro supports the interpretation that each of these data sets is reliable. The VGP of Mindoro is displaced southward from the Late Jurassic VGP of South China, sugg...

Indenter-tectonics in the Philippines: Example from the Palawan Microcontinental Block - Philippine Mobile Belt Collision

“…Continued movement resulted into the evolution of the thrust fault into a subduction zone corresponding to the late Early Miocene initiation of the present-day Manila Trench (e.g., Bellon and Yumul, 2000;Yumul et al, 2003). This model is consistent with a) the reported paleomagnetic data of 20°to 30°counterclock-wise rotation which occurred during post Oligocene and pre-Upper Miocene times in Northern Luzon (e.g., McCabe et al, 1987); b) the explanation on how the South China Sea plate could subduct eastward beneath Luzon when its opening direction was generally north-south; and c) the geology of Northern Luzon, specifically in the Baguio District, showing the formation of the batholithic Central Cordillera plutonic and volcanic complexes during the Middle Miocene (e.g., Yumul et al, 1995). The generation of the Middle Miocene igneous rock complexes is attributed to the Early Miocene subduction of the South China Sea crust along the Manila Trench.…”

“…2a). Immediately to the south, paleomagnetic data and the present dispositions of the islands in Central Philippines suggest microblock rotation after the initiation of the PCB-PMB collision (e.g., McCabe et al, 1982;1987;Yumul et al, 2000). The whole Western Visayan block, made up of Panay, Negros, Cebu and Bohol rotated clockwise while north of the PCB indenter, the islands of Luzon, Mindoro and Marinduque rotated counter-clockwise (Fig.…”

“…Applying the shear partitioning concept of Fitch (1972), the westward strain component of the subduction must be primarily accommodated by the Philippine Trench, with the excess strain transferred to the left-lateral Philippine Fault Zone to the west (e.g., Quebral et al, 1996). Some workers have suggested that the initiation of the Philippine Trench was a response to the collision of the PCB with the PMB (e.g., McCabe et al, 1987), that is, the Philippine Trench originally accommodated the excess strain resulting from the docking of the OBH with the Central Philippines. Proposed initiation ages of the Philippine Trench range from Pliocene (~5 Ma) to Present (Cardwell et al, 1980;Rangin et al, 1999aRangin et al, , 1999bSajona et al, 1997;Aurelio, 2000).…”

Introduction

“…However, other studies have interpreted easterly declinations along the plate margin in terms of local rotations that arose from oblique subduction of the Pacific Plate or from the collisions of island arcs, rifted continental fragments, and aseismic ridges with the margin (McCabe, 1984). If the deflected declinations represent plate-wide motion, much of the clockwise rotations endemic to at least the southern parts of the Philippine Archipelago (McCabe et al, 1987) might serve as evidence for their attachment to the Philippine Sea Plate earlier in the Tertiary.…”

Philippine Island Arc System Tectonic Features Inferred from Magnetic Data Analysis

“…The archipelago consists mainly of accreted volcanic arcs, marginal basins, ophiolites, and continental fragments constituting the Philippine Mobile Belt (PMB) (Karig 1983;Mitchell et al 1986;McCabe et al 1987), on the western border of the Philippine Sea plate. This region is a complex portion of the tectonic boundary between the Eurasian Plate (Sunda Block), Indo-Australian and Philippine Sea Plate, comprising most of the Philippines.…”