Understanding the Geology of the Philippines through Gravity Anomalies


 The Philippine Archipelago is a complex island arc system, where many regions still lack geopotential studies. This study aims to present a general discussion of the Philippine gravity anomaly distribution. The high-resolution isostatic anomaly digital grid from the World Gravity Map (WGM) was processed and correlated with the Philippines’ established geology and tectonics. This study also investigated the gravity signatures that correspond to the regional features, e.g., geology, structures, sedimentary basins, and basement rocks of the study area. Upward continuation, high-pass, and gradient filters (i.e., first vertical derivative, horizontal gradient) were applied using the Geosoft Oasis Montaj software. The interpreted gravity maps’ results highlighted the known geologic features (e.g., trench manifestation, ophiolite distribution, basin thickness). They revealed new gravity anomalies with tectonic significance (e.g., basement characterization). The isostatic gravity anomaly map delineates the negative zones. These zones represent the thick sedimentary accumulations along the trenches surrounding the Philippine Mobile Belt (PMB). The Philippine island arc system is characterized by different gravity anomaly signatures, which signify the density contrast of subsurface geology. The negative anomalies (< 0 mGal) represent the thick sedimentary basins, and 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 of continental affinity (20 – 35 mGal) was distinguished from the remaining regions of oceanic affinity (45 – 200 mGal). Local geologic features (e.g., limestone, ophiolitic rocks) and structures (e.g., North Bohol Fault, East Bohol Fault) were also delineated downward continuation and gravity gradient maps of Bohol Island. The WGM dataset’s effectiveness for geologic investigation was achieved by comparing the established geologic features and interpreted gravity anomalies. The processed gravity digital grids provided an efficient and innovative way of investigating the Philippines’ regional geology and tectonics.


Introduction
stretch also indicate a heterogeneous subduction zone morphology, similar to Manila 234

Trench. 235
The east-dipping Manila trench shows a non-uniform negative gravity anomaly 236 that generally corresponds to sedimentary deposits' thickness overlying basement rocks. 237 Hayes and Lewis (1984) reported that the Manila trench's forearc basins have a maximum 238 sediment thickness of 4.5 km. They also suggested that the thickness variation in the 239 forearc basin is due to sediment accumulation and the accretionary prism's local uplift Taiwan-Eurasia (north) and Mindoro-PMB (south ) (Hayes and Lewis 1984). The very 249 high and contiguous gravity anomaly along the offshore western Luzon Island was 250 interpreted as the extension of Zambales Ophiolite (ZOE) (Hayes and Lewis 1984). 251 The isostatic gravity anomalies, which characterize the Negros, Sulu, and 252 Cotabato Trenches, have a similar prominent gravity low associated with thick low-253 density sediments (e.g., Lowrie and Fichtner 2019). Based on the previously defined 254 correlation between the processed isostatic gravity anomaly map and detailed ground 255 surveys, these three trenches' complex forearc basin system (i.e., Negros, Sulu, Cotabato) 256 can be understood. The peculiar, very low gravity zones were noted at the intersection of Negros and Sulu Trenches (NS) and the southern side part of the Cotabato Trench (C) 258 (Fig. 2a). Since there are no detailed studies about these three trenches, we can deduce 259 the gravity anomalies based on the signatures of Manila and East Luzon Trough. The very 260 low gravity zones suggest a very thick accumulation of sediments; these may indicate 261 active local tectonics along the negative zones. 262 The isostatic anomaly map also revealed the subsurface geology, sedimentary 263 basins, and basement rocks of the Philippines. The map reflects the variations of gravity 264 fields caused by density differences of materials in the upper crust. Based on the gravity 265 anomaly map, different regional lithologic units were also delineated according to the 266 classification of MGB (2010) (Fig.3). The summary of the regional lithologic geologic 267 groupings concerning the gravity anomaly map is presented in Table 1. Generally, 268 negative gravity signatures represent the sediment basins (< 0 mGal), moderate gravity 269 anomalies correspond to the metamorphic rocks (0 to 80 mGal), and very high gravity 270 anomalies typify ophiolitic basement rocks (> 80 mGal).  Circular gravity lows were also delineated across the Bohol Sea (BS), signifying a very thick sediment accumulation. This feature was previously interpreted as proto-Southeast 281 Bohol Trench that bound the Western Visayan Block (Yumul et al. 2008b). 282 The distribution of metamorphic rocks generally coincides with moderate 283 gravity anomaly values (0 to 80 mGal) (Fig. 3b). MGB (2010) classified metamorphic 284 rocks into Pre-cretaceous (continental) and cretaceous (island arc) metamorphic zones.   Pubellier et al. 1991;Quebral 1994) supported this concept. 296 The regional groupings of ophiolitic rocks, delineated by MGB (2010), exactly 297 coincide with areas having very high gravity anomalies (> 70mGal). The occurrence of 298 ophiolitic rocks, which serve as basement rocks of most islands, is extensive within the 299 Philippines. Lower gravity anomalies are due to metamorphism in some ophiolitic zones 300

Basement Rocks and Basins 319
In understanding deeper large-scale crustal features, gravity anomalies due to 320 smaller local small structures are less important than the regional anomalies. The deeper 321 and regional signals can be enhanced (Lowrie and Fichtner 2019). The upward 322 continuation was implemented to further investigate the high-density ophiolitic basement 323 rocks and low gravity sediment basins at depth. The 5, 10, and 20 km continuation depths 324 represent a minimum depth of 2.5, 5, and 10 km, respectively (Fig. 4). 325 The Philippines' upward continuation maps show that the very high gravity Miocene magmatic belts along Central Cordillera (MGB, 2010) (Fig. 5b). The main 378 north-trending negative anomalies (-15 to -37 mGal) are still present until the 20 km 379 upward continued depth (Fig. 5c)

Local Geology and Structures 394
The gravity anomalies that correspond to shallow features and structures were 395 characterized by suppressing regional gravity signals, using the high-pass filtering (e.g., 396 Lowrie and Fichtner 2019). Bohol Island was chosen as the representative area for 397 correlating the high-pass filtered gravity map and local geology because it has diverse 398 geology and lithology that reflects a density contrast. Maps are shown as illuminated from 399 the northwest to emphasize the significant areas that manifest gravity lows and highs. The high-pass filtered gravity map of Bohol Island shows values that range from (-4 to 145 401 mGal) (Fig. 6); it helps delineate geologic formations and lithological units concerning 402 their inherent physical characteristics (e.g., density). The summary of the correlation 403 between the high-pass filtered gravity map and the geologic map of BMG (1987) was 404 presented in Table 3. analysis of gravity data is also very advantageous in exploring mineral deposits with very 433 distinct density characteristics, e.g., chromite in ophiolitic rocks. 434 The first vertical derivative (1VD) and horizontal gradient (HG) maps were 435 prepared to delineate geologic features, e.g., fault, lithologic contact (Fig. 7). south-eastern side (Fig. 7a). The long but minor gravity anomaly (F2) adjacent and concordant to the EBF may also be a structure-related lineament. On the western side of 449 the map, a strong gradient anomaly (F3) was correlated to the northeast-trending NBF, 450 which generated the 2013 Bohol Earthquake (e.g., Kobayashi 2014; Lagmay and Eco 451 2014). Linear parallel gravity anomalies (F4, F5), with a similar orientation of the NBF, 452 were traced on the northeast and southwest of NBF; these anomalies may suggest major 453 NBF-related structure. Some minor lineaments were also identified in the horizontal 454 gradient map, particularly in south-eastern Bohol Island (Fig. 8). These minor anomalies 455 correspond to the lineaments and features delineated by the (MGB 1987). High amplitude 456 linear feature (F6), which may represent a geologic contact or a major submarine structure, 457 was also recognized along the offshore of southern Bohol. 458 The First Vertical Derivative map was also prepared to validate and supplement 459 Bohol Island structures delineated on the horizontal gradient map (Fig. 7b). Vertical Additional geologic structures, e.g., geologic contacts, lineaments were delineated on the 469 first vertical derivative map (e.g., F10). Table 2 summarizes the delineated geophysical 470 lineaments based on the horizontal gradient and first vertical derivative maps. Other 471 lineaments, which were revealed by the gravity gradient maps, can be considered starting points for future detailed structural and geologic surveys. Geological features, revealed 473 by the gravity gradient maps, are generally parallel to known structures, e.g., major fault 474 and geologic contact. 475 The prominent gravity highs and lows (delineated on the high-pass filtered 476 gravity map) and the lineaments (defined on the gradient maps) were overlaid on the 477 established geologic map of Bohol (Fig. 8) (BMG, 1987). The map shows a positive 478 correlation between the high gravity anomaly value and the distinctive density of the 479 subsurface lithology (e.g., high anomaly corresponds to ultramafic rock). It highlights 480 parallel gravity anomaly lineaments and known structures, e.g., fault, geologic contact. in reconnaissance surveys and very useful in regional mineral exploration (e.g., chromite). 504 The gravity gradient analysis of the WGM data provides a promising scientific 505 supplement in delineating subsurface structures (e.g., fault, geologic contact). With the 506 availability and proved efficiency of the WGM data, these techniques are applicable and 507 valuable in future structural and geologic explorations in the Philippines.

Availability of data and materials 530
The gravity digital grids and data used in this study are available online at https://bgi.obs-531 mip.fr/data-products/grids-and-models/wgm2012-global-model/. The DEM from the 532 SRTM can be downloaded at https://www2.jpl.nasa.gov/srtm/cbanddataproducts.html.