Understanding the Geology of the Philippines through Gravity Anomalies Mel Anthony Asis Casulla ( [email protected] ) Kyushu University - Ito Campus: Kyushu Daigaku https://orcid.org/0000-0002-4608-214X Hideki Mizunaga Kyushu University - Ito Campus: Kyushu Daigaku Toshiaki Tanaka Kyushu University - Ito Campus: Kyushu Daigaku Carla Dimalanta UP-NIGS: University of the Philippines Diliman National Institute of Geological Sciences Research Article Keywords: World Gravity Map (WGM), Philippines, geology, basement, basin, subsurface structure Posted Date: February 10th, 2021 DOI: https://doi.org/10.21203/rs.3.rs-191156/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License 1 Understanding the Geology of the Philippines through 2 Gravity Anomalies 3 Mel Anthony A. Casulla1 4 Corresponding author 5 Email: [email protected] 6 7 Hideki Mizunaga2 8 Email: [email protected] 9 10 Toshiaki Tanaka3 11 Email: [email protected] 12 13 Carla B. Dimalanta4 14 Email: [email protected] 15 16 (Institutional addresses) 17 1 Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu 18 University, Fukuoka 819-3095, Japan 19 2 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu 20 University, Fukuoka 819-3095, Japan 21 3 Department of Earth Resources Engineering, Faculty of Engineering, Kyushu 22 University, Fukuoka 819-3095, Japan 23 4 Rushurgent Working Group, National Institute of Geological Sciences, College of 24 Science, University of the Philippines, Diliman, Quezon City, Philippines 25 26 Abstract 27 The Philippine Archipelago is a complex island arc system, where many 28 regions still lack geopotential studies. This study aims to present a general discussion of 29 the Philippine gravity anomaly distribution. The high-resolution isostatic anomaly digital 30 grid from the World Gravity Map (WGM) was processed and correlated with the 31 Philippines’ established geology and tectonics. This study also investigated the gravity 32 signatures that correspond to the regional features, e.g., geology, structures, sedimentary 33 basins, and basement rocks of the study area. Upward continuation, high-pass, and 34 gradient filters (i.e., first vertical derivative, horizontal gradient) were applied using the 35 Geosoft Oasis Montaj software. The interpreted gravity maps’ results highlighted the 36 known geologic features (e.g., trench manifestation, ophiolite distribution, basin 37 thickness). They revealed new gravity anomalies with tectonic significance (e.g., 38 basement characterization). The isostatic gravity anomaly map delineates the negative 39 zones. These zones represent the thick sedimentary accumulations along the trenches 40 surrounding the Philippine Mobile Belt (PMB). The Philippine island arc system is 41 characterized by different gravity anomaly signatures, which signify the density contrast 42 of subsurface geology. The negative anomalies (< 0 mGal) represent the thick 43 sedimentary basins, and the moderate signatures (0 to 80 mGal) correspond to the 44 metamorphic belts. The distinct very high gravity anomalies (> 80 mGal) typify the 45 ophiolitic basement rocks. The gravity data’s upward continuation revealed contrasting 46 deep gravity signatures; the central Philippines of continental affinity (20 – 35 mGal) was 47 distinguished from the remaining regions of oceanic affinity (45 – 200 mGal). Local 48 geologic features (e.g., limestone, ophiolitic rocks) and structures (e.g., North Bohol Fault, 49 East Bohol Fault) were also delineated downward continuation and gravity gradient maps 50 of Bohol Island. The WGM dataset’s effectiveness for geologic investigation was 51 achieved by comparing the established geologic features and interpreted gravity 52 anomalies. The processed gravity digital grids provided an efficient and innovative way 53 of investigating the Philippines’ regional geology and tectonics. 54 55 Keywords 56 World Gravity Map (WGM), Philippines, geology, basement, basin, subsurface structure 57 58 59 60 61 62 63 64 65 66 67 1 Introduction 68 Gravity data is fundamental in understanding and modeling Earth’s interior, 69 e.g., subsurface, crust, especially in studying its relationship to geology and structures. 70 With the advancement of technology, high-resolution satellite gravity data are being 71 utilized for geologic exploration and tectonic studies. Satellite gravity data were 72 processed and interpreted for bathymetry prediction (Majumdar and Bhattacharyya 2005), 73 lineament investigation (Braitenberg et al. 2011), crust-mantle boundary study (Steffen et 74 al. 2011), sediment basin survey (Vaish and Pal 2015), and geologic mapping (Pal et al. 75 2016). This emerging area of research was made possible by acquiring a more precise 76 Earth gravitational model. The Earth Gravitational Model 2008 (EGM2008) is an Earth’s 77 geopotential model. This model integrates satellite gravimetry, satellite altimetry, and 78 surface gravity measurements (Pavlis et al. 2008). Several studies already assessed and 79 validated the accuracy of EGM2008 (Arabelos and Tscherning 2010; Pavlis et al. 2012). 80 The gravity field data used in generating the high-resolution World Gravity Map 2012 81 (WGM) is derived from the EGM2008. 82 In the Philippines, regional gravity exploration began in the twentieth century 83 when Teodoro (1970) compiled Luzon Island gravity surveys. Only a simple Bouguer 84 anomaly map could have been generated in those years due to a lack of detailed 85 topographic maps (Teodoro 1970). In 1982, the Philippines’ first regional gravity 86 anomaly map was presented, and different gravity anomalies were discussed relative to 87 various geologic factors (Sonido 1981). Gravity surveys have undergone continuous 88 development during the past twenty years. Ground and marine gravity surveys were 89 employed by several studies focusing on specific regions, e.g., the crustal structure and 90 tectonic evolution along Manila Trench (Hayes and Lewis 1984), the emplacement of 91 Bohol ophiolite (Barretto et al. 2000); the regional tectonics of northern Luzon (Milsom 92 et al. 2006), the arc-continent collision in the central Philippines (Dimalanta et al. 2009), 93 the crustal thickness of Central Philippines (Manalo et al. 2015), the upper crustal 94 structure beneath Zambales Ophiolite Complex (Salapare et al. 2015), and the terrane 95 boundary in northwest Panay (Gabo et al. 2015). 96 The historical overview of gravity surveys in the Philippines presents a wide 97 range of gravity survey scales and applicability. Earlier studies generated and presented 98 gravity maps based on limited point data from local to regional surveys (e.g., ground, 99 marine). With the advent of satellite-derived gravity data and global gravity data sets, 100 geologic studies’ scope is no longer limited to the previously available point data. The 101 recent isostatic anomalies from WGM were utilized to comprehensively investigate the 102 gravity anomalies around the Philippine Islands’ arc system. These may reveal regional 103 features, e.g., geology, structures, sedimentary basins, and basement rocks. This work 104 offers an innovative means of understanding the Philippines’ geology and tectonics 105 through the gravity signatures. 106 107 2 Tectonic and Geologic Setting 108 The Philippine Island arc system is a complex and tectonically active region. It 109 was characterized by ophiolite accretion, arc magmatism, ocean basin closure, and other 110 tectonic processes (Mitchell et al. 1986; Rangin 1991; Yumul et al. 2008a; Aurelio et al. 111 2013). The Philippine Archipelago consists of two general terranes: the Palawan-Mindoro 112 Microcontinental Block and the Philippine Mobile Belt (PMB). The Palawan-Mindoro 113 microcontinental block was once part of mainland Asia while the PMB originated from 114 the sub-equatorial regions (MGB, 2010; Rangin et al., 1990). The PMB is an actively 115 deforming zone between two oppositely-dipping subduction systems (Fig. 1). The 116 eastern side of the PMB is bounded by the west-dipping East Luzon Trough and the 117 Philippine Trench. The Archipelago’s western side is marked by east-dipping subduction 118 zones: Manila Trench, Negros Trench, Sulu Trench, and Cotabato Trench. The left- 119 lateral strike-slip Philippine Fault, which traverses the entire island arc system, 120 accommodates the oblique convergence between the Philippine Sea Plate and Eurasian 121 Plate (Barrier et al. 1991; Aurelio 2000). The amalgamation of different terranes paved 122 the way to forming tectonic collage with diverse lithologic characteristics categorized 123 into ophiolitic rocks, metamorphic rocks, magmatic arcs, and sediment basins (MGB, 124 2010). Ophiolitic and metamorphic basement rocks overprinted by relatively younger 125 volcanic series and thick sedimentary basins define the Philippines’ present geology. 126 127 3 Methodology 128 3.1 Data Acquisition 129 The Philippines’ isostatic anomaly digital grid was acquired from the World 130 Gravity Map (WGM) of the Bureau Gravimetrique International (BGI). The BGI 131 produced global gravity anomaly maps and digital grids considering an Earth model that 132 accounts for the influence of most surface masses, e.g., atmosphere, land, oceans, lakes 133 (Balmino et al. 2012). Different corrections were applied to the gravity data to remove 134 the non-geologic effects; three WGM anomaly maps were produced (i.e.,