A NEW BRANCH OF THE PHILIPPINE FAULT SYSTEM AS OBSERVED FROM AEROMAGNETIC AND SEISMIC DATA

The Philippine fault is one of the major strike-slip faults of the world, extending over 1200 km from the Lingayen-La Union region of northwestern Luzon to the Davao Gulf south of Mindanao. Nevertheless, its total slip, history of displacement, and its exact location have been uncertain and even controversial. Significant new insight has been provided by aeromagnetic and seismic surveys made as part of a basin evaluation project of the Philippines Bureau of Energy Development, funded by the World Bank. This paper outlines evidence from these surveys for a previously unrecognized major branch of the Philippine fault system and tentatively discusses its paleogeographic implications. We have interpreted aeromagnetic data covering most of the length of the fault and seismic profiles crossing it at various locations. The Philippine fault is well displayed in the aeromagnetic data. Surprisingly, the data also show a major, previously unrecognized, branch of the Philippine fault, here named the Sibuyan Sea branch. This branch is almost entirely offshore or is covered by Quaternary or Recent volcanic rocks. It leaves the known Philippine fault northeast of Masbate Island, passes along the northeast edge of the Sibuyan Sea southwest of the Bondoc Peninsula, passes northeast of Marinduque Island, apparently passes through the Taal Volcano, and then offshore west of the Batangas Peninsula. The Sibuyan Sea branch is 350 km long and is presently active. It is more obvious on seismic and aeromagnetic data than the northern part of the known Philippine fault and thus may have accommodated more motion. We have made a preliminary evaluation of the total slip on the Sibuyan Sea branch by attempting to restore long-wavelength geophysical and geologic features, including forearc basins, ophiolite belts, volcanic belts, and regional Bouguer, free-air and magnetic anomalies, which suggests the possibility of 200-300 km of slip since about the early or middle Miocene. In this reconstruction the Central Luzon basin and Luzon volcanic arc are continuous with the forearc basin and volcanic belt on Panay, as a continuous arc and forearc extending from northern Ilocos to southern Panay. After collision with the Palawan-Mindoro continental block, continued motion along the Manila trench north of Mindoro has ruptured the central Philippines along the Sibuyan Sea branch of the Philippine fault. Thus the Sibuyan Sea branch could account for most of the post-early to middle Miocene relative motion between Luzon and the Visayas. © 1990.