Geophysical Investigations Within an Arc – Continent Collision Zone: The Romblon Island Group

E.G.L. Ramos¹, C.B. Dimalanta¹, N.T. Ramos¹, R. A. Tamayo, Jr.¹, G.P. Yumul, Jr.^1,2 and E.J. Marquez³

¹ Tectonics and Geodynamics Group,
National Institute of Geological Sciences,
College of Science, University of the Philippines, Diliman,
Quezon City, PHILIPPINES
² Also with: Philippine Council for Industry and Energy Research and Development,
Department of Science and Technology, Bicutan, Taguig, PHILIPPINES
and: Philippine Atmospheric, Geophysical and Astronomical Services Administration,
Department of Science and Technology, Science Garden, Quezon City, PHILIPPINES
³ University of the Philippines, Manila, PHILIPPINES

Abstract

The collision between the Palawan microcontinental block and the Philippine mobile belt in west central Philippines is a significant event in the tectonic evolution of the Philippine island arc system. Geological features ascribed to the collision event include an emplaced complete ophiolite complex and a suite of metamorphic rocks exposed in the Romblon Island Group (Faure et al., 1989; Yumul et al., 2003; Ramos et al., 2005c). Ground gravity and magnetic surveys were conducted in the islands of Tablas, Romblon and Sibuyan revealing the geophysical signatures of rocks that lie within the collision zone. The different varieties of metamorphic rocks are generally seen as low gravity and low magnetic anomalies whereas the units comprising the ophiolite complex exhibit high gravity anomalies with variable magnetic signatures (Ramos et al., 2005a). Tertiary to Quaternary sedimentary units, which make up the western part of Tablas Island, are clearly defined by low gravity and low magnetic anomalies. Large-scale structural boundaries (e.g. normal and thrust faults) separating the lithologic units are seen as linear features on the gravity and magnetic anomaly maps. Some of the fault contacts, particularly the thrust faults observed in Sibuyan Island, are deemed to result from the impingement of the Palawan microcontinental block against the Philippine mobile belt (Yumul et al., 2005).

With the recognition of surface features believed to be related to the collision event, seismic tomographic studies were also carried out to obtain an image of the subsurface configuration in the Central Philippine region. The inversion of Philippine seismic intensity data revealed a distinct low velocity seismic anomaly on the tomographic images. Seismicity plots also showed a significant decrease in intermediate to deep (>150 km) earthquakes in this region (Ramos et al, 2005b). These may be attributed to the slow convergence between the Palawan microcontinental block and Central Philippines, thus leading to a prolonged heating of the slab/s. However, these subsurface attenuation characteristics of the collision zone do not very well constrain the presence of collision and subduction-related features in the Romblon Island Group.