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    Imaging crustal features and Moho depths through enhancements and inversion of gravity data from the Philippine island arc system

    Mel Anthony A. Casulla, Hideki Mizunaga, Toshiaki Tanaka, Carla B. Dimalanta

    World Gravity Map (WGM), Philippines, Two-dimensional (2-D) radially averaged power spectrum, Three-dimensional (3-D) gravity inversion, Basement and Moho depths, Basin

    Isostatic anomaly map of the Philippines showing the general distribution of a sedimentary basins (polygons with hachures), metamorphic rocks (heavy line polygons), and b ophiolitic rocks. Numbers represent the sedimentary basins of PMB affinity: 1 = Ilocos-Central Luzon (ICL), 2 = Cagayan Valley (CV), 3 = Mindoro, 4 = Southern Luzon-Bicol, 5 = Iloilo, 6 = Visayan Sea, 7 = Samar, 8 = Agusan-Davao (AD), 9 = Cotabato (C). BS = Bohol Sea, NPM = Northern Palawan-Mindoro block (white polygon), AR = Antique Range. Circles symbolize the occurrences of nickel (black) and chromite (white) deposits in the Philippines.

    The Philippine archipelago is a complex island arc system, where many regions still lack geopotential field studies. The high-resolution isostatic anomaly and free-air anomaly digital grids from the World Gravity Map (WGM) were processed and analyzed to present a general discussion of the Philippines’ gravity signatures and contribute to understanding its regional geology and tectonics. The isostatic anomaly map was continued upward to investigate the high-density ophiolitic basement rocks and low-gravity sedimentary basins at depth. The first vertical derivative (1VD) filter was applied to the free-air anomaly grid map to locate regional structures represented by density contrast boundaries. The depth to the top of the Moho and basement rock over the Sulu Sea was computed using the two-dimensional (2-D) radially averaged power spectrum analysis. Three-dimensional (3-D) gravity inversion was applied to some major sedimentary basins in the Philippines to present 3-D subsurface density contrast models. The interpreted gravity maps highlighted prominent geologic features (e.g., trench manifestation, ophiolite distribution, basin thickness). The negative isostatic anomalies (< 0 mGal) represent the thick sedimentary basins, while the moderate signatures (0 to 80 mGal) correspond to the metamorphic belts. The distinct very high-gravity anomalies (> 80 mGal) typify the ophiolitic basement rocks. The gravity data’s upward continuation revealed contrasting deep gravity signatures; the central Philippines with continental affinity (with 20–35 mGal) was distinguished from the regions with oceanic affinity (with 45–200 mGal). The 1VD map over the Sulu Sea showed anomalies associated with shallow features dominantly related to the Cagayan Ridge. The 2-D radially averaged power spectrum analysis exposed gravity anomalies with tectonic significance (e.g., basement characterization, Moho depth estimation). The estimated average Moho depth in the Sulu Sea is from 12 to 22 km, while the average basement depth is within the range of 5 to 11 km. Lastly, the 3-D subsurface density contrast models characterized the very low-density zones representing the deep (> 7 km) sedimentary basins in the northern Cagayan Valley and southern Central Luzon basins. Furthermore, thin (~ 3.5 km) sedimentary formations are inferred for the low-density areas in northern Agusan-Davao and eastern Cotabato basins.