Upper Crustal Structure Beneath the Zambales Ophiolite Complex, Luzon, Philippines Inferred from Integrated Gravity, Magnetic and Geological Data

achmad munif;

The Zambales Ophiolite Complex (ZOC) in the western portion of Luzon Island, Philippines represents a typical exposure of an emplaced crust-upper mantle section of an ancient lithosphere. The ZOC is divided into the Acoje and Coto Blocks based on petrological, geochemical and age disparities, thus implying diverse and complex origins for the archipelago’s lithospheric sources.We used gravity and magnetic data to reveal differences in the subsurface characteristics of the two ophiolite blocks for the first time. Low Bouguer gravity (<135 mGal) and magnetic (<69 nT) anomalies characterize the ophiolitic units in the Acoje Block whereas high Bouguer gravity (>150 mGal) and magnetic (>110 nT) anomalies typify the Coto Block.
Such contrasting signatures further extend at greater depths which reflect the disparities in the crustal density, the basement structure, and the depth to Moho. Petrophysical characteristics such as density, magnetic susceptibility and natural remanent magnetization (NRM) intensities correlate well with the interpreted Bouguer gravity and magnetic anomalies. Densities of gabbros and peridotites from the Acoje and Coto Blocks reveal mean values ranging from 2640 to 2810 kg m–3 and 2570 to 2690 kg m–3, respectively. Magnetic susceptibility (>4.43 × 10−3 SI) and NRM (>0.69 A m–1) data are also generally higher over the Coto Block. Both Bouguer gravity and magnetic anomaly maps reveal a prominent steep gradient that potentially marks the structural contact between Acoje and Coto Blocks. We further infer that the steep anomaly gradient validates the presence of the Lawis Fault Zone which separates the two ophiolite blocks in the Masinloc Massif. Recent field evidence from rock exposures in Coto reveals both right lateral and vertical displacements along the fault zone.