Paleomagnetic Results from Luzon and the Central Philippines

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, suggesting a post-Jurassic southward migration of Mindoro

Controls on tidal sedimentation and preservation : Insights from numerical tidal modelling in the Late Oligocene–Miocene South China Sea, Southeast Asia

Numerical tidal modelling, when integrated with other geological datasets, can significantly inform the analysis of physical sedimentation processes and the depositional and preservational record of ancient tide-influenced shoreline–shelf systems. This is illustrated in the Oligo–Miocene of the South China Sea (SCS), which experienced significant changes in basin physiography and where tide-influenced, shoreline–shelf deposition is preserved in ca 10 sub-basins. Palaeogeographic reconstructions, palaeotidal modelling and regional sedimentary facies analysis have been integrated in order to evaluate the spatial–temporal evolution and physiographic controls on tidal sedimentation and preservation during the ca 25 Myr Oligo–Miocene record in the SCS. Palaeotidal modelling, using an astronomically forced and global tidal model (Fluidity) at a maximum 10 km resolution, indicates that spring tides along Late Oligocene–Middle Miocene coastlines were predominantly mesotidal– macrotidal and capable of transporting sand, which reflects two main conditions: (1) increased tidal inflow through wider ocean connections to the Pacific Ocean; and (2) tidal amplification resulting from constriction of the tidal wave in a ‘blind gulf’ type of basin morphology. Since the Middle–Late Miocene, a reduction in the amplitude and strength of tides in the SCS was mainly due to diminishing tidal inflow from the Pacific Ocean caused by the northward movement of the Philippines and Izu-Bonin-Mariana arc. Sensitivity tests to palaeogeographic and palaeobathymetric uncertainty indicate that regional–scale (100–1000s 29 km) palaeogeographic changes influencing tidal inflow versus outflow can override local30 scale (1–100s km) changes to tidal resonance and convergence effects (funnelling and shoaling), such as shelf width and shoreline geometry. Palaeotidal model results compare favourably to the distribution and sedimentary fabric of Oligo–Miocene, tide-influenced, shoreline–shelf successions in peripheral SCS basins. However, the preservation potential of tidal deposits is lower in open coastline environments, probably due to enhanced reworking during storms and river floods

Paleomagnetism of Palawan Island: Implications for the opening of the South China Sea and for Philippine geology.

Paleomagnetism of Palawan Island: Implications for the opening of the South China Sea and for Philippine geology