Subsurface Characterization Using Gravity and Magnetic Field Data: 
Implications for the Post-Emplacement History of the ZambalesOphiolite Complex, Philippines

Ricky C. Salapare1, Carla B. Dimalanta1, Decibel V. Faustino-Eslava2, Karlo L. Queaño3, Noelynna T. Ramos1, Pearlyn C. Manalo1, Jenielyn T. Padrones4,Edanjarlo J. Marquez5and Graciano P. Yumul Jr.3

1Rushurgent Working Group, National Institute of Geological Sciences, College of Science, 
University of the Philippines, Diliman, Quezon City, Philippines
2School of Environmental Science and Management, University of the Philippines, 
Los Baños, Laguna, Philippines
3 Monte Oro Resources and Energy Inc., Makati City, Philippines
4Department of Earth Science and Technology, Akita University, Akita, Japan
5Department of Physical Sciences and Mathematics, University of the Philippines, Manila, Philippines


The Zambales Ophiolite Complex (ZOC) is one of the best exposed complete sequences of ancient lithospheric fragments in the world.Its evolution is marked by the amalgamation of two contrasting terranes: the Acoje and the Coto Blocks. The distinction between these two blocks is highlighted by their geochemical and petrographic signatures: the Acoje Block exhibits an island arc tholeiiticchemistry whereas the Coto Block is of transitional mid-oceanic-ridge-basalt/island arc characteristic. Recent studies also revealed different ages for these two blocks. An Eocene age has been adopted for the Coto Block based on 1) the paleontological dating of the overlying sedimentary carapace, Aksitero Formation, and 2) on the radiometric dating of diabasedikes that intrude the gabbro units in theCoto Mine area. On the other hand, a Middle Jurassic to Early Cretaceous age has been assigned to Acoje Block on the basis of radiolarian assemblages extracted from chertolistoliths, presumably derived from the ophiolite's sedimentary cover,incorporated as blocks in the Cabaluan Formation.

Because of the contrasting features of these two blocks, this study looked into the subsurface characterization of the western portion of the ZOC using gravity and magnetic information to determine whether their differences extend to other attributes. A prominent steep gradient in both gravity and magnetic anomaly maps marks the contact between the Acoje and the Coto Blocks that coincides with the trace of the Lawis Fault. Although gravity anomalies are indistinguishable over the two blocks, long wavelength magnetic anomalies characterize the Coto Block. Such magnetic signature indicates that the magnetic sources in the Coto Block are deeper than those in the Acoje Block. These, combined with inverse modeling, anomaly separations and Euler deconvolution solutions, collectively suggest a deeper ophiolitic basement in the Coto area and a thicker overlying sedimentary package. The delineation of the Lawis fault reaffirms the existence of the westward directed thrusting of the Acoje Block over the Coto Block. This implies the difference in the timing of emplacement of the two blocks. Results of this study are used to decipher the possible emplacement history of these two units of the ZOC.

Financial support for this study was provided by the Department of Science and Technology – Grants-In-Aid Program (DOST-GIA) - Philippine Council for Industry, Energy, and Emerging Technology Research and Development (PCIEERD) and logistical support was provided by the University of the Philippines - National Institute of Geological Sciences (UP-NIGS).


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