The Romblon Island Group:
Evidence for Arc – Continent Collision in Central Philippines

G.P. Yumul,Jr.^1,2, C.B. Dimalanta¹, R.A. Tamayo Jr.¹, E.G.L. Ramos¹, N.T. Ramos¹, T.A. Tam III^1,3 and E.J. Marquez⁴

¹Tectonics and Geodynamics Group, National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, PHILIPPINES
²Philippine Council for Industry and Energy Research and Development and Philippine Atmospheric, Geophysical and Astronomical Services Administration, Department of Science and Technology, Metro Manila, PHILIPPINES
³Faculty of Science, Okayama University, Okayama, JAPAN
⁴College of Arts and Sciences, University of the Philippines-Manila, Padre Faura, Manila, PHILIPPINES

The subduction and indentation of a plate carrying an oceanic bathymetric high (OBH) with another plate is known to result into distinct tectonic and geologic features. Cusping, ophiolite emplacement, uplift of the forearc region, microblock rotation, subduction polarity reversal and formation of extensional regimes in the back arc region are some of these features (e.g. Vogt 1973; Collot et al. 1985; Mann et al. 1998; Dominguez et al. 1998). Some of these features are also recognized in the Central Philippines where the North Palawan Block is colliding with the Philippine Mobile Belt (McCabe et al 1985; Yumul et al. 2005). Our recent geological, geochemical and geophysical study in the region led to the conclusion that the collisional boundary is located farther east, in the Romblon Island Group, as compared to previously indicated collision boundaries (e.g. Yumul et al. 2003; Dimalanta and Yumul 2004). The Romblon Island Group is underlain by a complete ophiolite complex (Sibuyan Ophiolite Complex), metamorphic rock complexes and sedimentary formations that bear witness to the rapid uplift of the area, among other results, as a consequence of the collision (e.g. Tamayo et al. 2005; Ramos et al. 2005a). The Sibuyan Ophiolite Complex is made up of harzburgites, layered ultramafic and mafic rocks, isotropic gabbro, diabase dike swarms, and basaltic pillow and lava flow deposits. Available K-Ar isotopic age dating of metamorphic rocks from this group of islands suggests a Middle Miocene (12 + 0.3 Ma) age of metamorphism. This, we believe, is consistent with the inferred Early Miocene collision event involving the North Palawan Block and the oceanic arc Philippine Mobile Belt. The ophiolitic rocks in the island group manifest supra-subduction zone geochemical signatures (e.g. negative Ti and Nb anomalies). Tectonic mélange is associated in one of the ophiolitic suites exposed. Gravity and magnetic anomalies from the Romblon Island Group correlate with the distribution of the different lithologic units as well as delineate major structural features. Tomographic images and seismicity plots of the west-central Philippine region reveal a distinct low velocity anomaly and pronounced decrease in intermediate to deep earthquakes in the region. This points to the aseismic nature of the subducting slabs in this region (Ramos et al. 2005b). All of the field and laboratory observations point to the continuous complex interaction in the Central Philippines, up to the present, of terranes with varying affinities.

Analyses done at the University of Bretagne Occidentale (ICP-AES, K-Ar) and Okayama University (XRF) are acknowledged with thanks. R. Maury, H. Bellon, J. Cotten, S. Suzuki and our colleagues at the UP-NIGS, among others, are thanked for their help and discussions. Financial and technical support came from the Department of Science and Technology and Philippine Council for Industry and Energy Research and Development.