Subsurface Investigation of Buruanga Peninsula and Vicinity: Characterizing the Collision Zone Using the Gravity Method

J.A.S. Gabo¹, C.B. Dimalanta¹, E.G.L. Ramos¹, K.L. Queaño², L.R. Zamoras³, E.J. Marquez⁴, M.G.S. Asio¹, L.T. Armada¹, G.P. Yumul Jr. ¹,⁵, and R.A. Tamayo Jr. ¹

¹Tectonics and Geodynamics Group – Rushurgent Working Group,
National Institute of Geological Sciences, College of Science,
University of the Philippines, Diliman, Quezon City
²Mapua Institute and Technology, Intramuros, Manila;
Mines and Geosciences Bureau, North Avenue, Quezon City
³Department of Physical Sciences and Mathematics,
University of the Philippines-Manila, Padre Faura, Manila
⁴Philex Mining Corporation, 27 Brixton St., Pasig City, Philippines
⁵Department of Science and Technology, Bicutan, Taguig City

Features which may be related to the collision between the North Palawan Block and the Philippine Mobile Belt are believed to be preserved in Northwest Panay. Gravity surveys were conducted in northwest Panay to gather subsurface evidence for the presence of structures which may be related to the collision . Results show a sudden decrease in Bouguer anomaly values in the area between Buruanga Peninsula and the Antique Range . The steep gradient observed in the gravity data coincides with the location of faults identified in the field and interpreted from satellite images.

The gravity profile indicates higher Bouguer anomaly values for the Buruanga Peninsula compared to the Antique Range, which may be explained by several possibilities. It can be attributed to the older age (Jurassic to Cretaceous) age of the Buruanga Peninsula compared to the Antique Range (Middle Miocene), since density of sedimentary rock units increases with age. Another possibility would be the difference in crustal thickness of the two terranes. Buruanga Peninsula may have a significantly thinner crust than the Antique Range, and therefore higher Bouguer anomaly values since a greater part of the dense mantle material is detected. The presence of a subducted slab underneath the Buruanga Peninsula can also account for the higher gravity anomaly values.

The lower Bouguer anomaly values that characterize the area between the Buruanga Peninsula and the Antique Range may indicate the presence of a fault. This may be related to the collision between the North Palawan Block and the Philippine Mobile Belt. A significant difference in Bouguer anomaly values between the two terranes can also support the suggestion that this part of northwest Panay is the southeastern boundary of the collision.