Subsurface Characterization of the Collision Zone in central Philippines: Constraints from Seismicity and Mantle Tomography Data

N. T. Ramos¹, C. B. Dimalanta¹, G. M. Besana^1,2, R. A. Tamayo, Jr.¹, G. P. Yumul, Jr.^1,3 and H. Negishi⁴

¹ National Institute of Geological Sciences, College of Science,
University of the Philippines, Diliman, Quezon City, PHILIPPINES
² Also with: Research Center for Seismology, Volcanology and Disaster Mitigation, Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, JAPAN
³ 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
⁴ Solid Earth Science Division, International Seismic Network Laboratory, National Disasters Information Center, National Research Institute for Earth Science and Disaster Prevention, Tenno-dai, Tsukuba, Ibaraki, JAPAN

Abstract

Despite its importance in understanding the growth of continents and island arcs, the convergence of the North Palawan microcontinental block (NPB) and the oceanic related Philippine Mobile Belt (PMB) in central Philippines remains poorly understood. Among the unresolved issues are the number of collision events, their timing, and the actual distributions of lithologic units associated with particular suturing episodes. Previous workers used petrologic and structural data to delineate suture zones in southwest Mindoro, Buruanga Peninsula in Panay, and the Romblon Island Group (RIG). Unlike the abundance of surficial evidence suggesting possible subduction episodes, there is a paucity of subsurface information (crust and upper mantle levels) on the region around the NPB-PMB suture. Evaluation of modeled 3D mantle images from the inversion of Philippine seismic intensity data implies the presence of a generally high attenuation structure (i.e., hot region) underneath central Philippines. Such structure might represent a region of either unusually high magmatic production or asthenospheric injection. These could be due to the partial melting of the NPB's leading oceanic edge, which has subducted beneath the western margin of the PMB. Moreover, because of the relatively hot character of materials underlying the area, low attenuation structures or cold regions (i.e., subducted slabs) are difficult to resolve at the imaged depths of 0-171 km. Alternatively, the absence of subducted slabs might be related to the difficulty of consuming a buoyant microcontinental block along the trench, the consequence of which would be the cessation of subduction. This scenario is consistent with the low seismicity distribution in west central Philippines compared with the rest of the archipelago. In addition, resolving relatively thin and short packets of cold regions in the vicinity of a large hot region might be difficult at scales and data density being used. By contrast, oceanic slabs subducting along the Philippine and Manila trenches are clearly imaged. Seismicity based subsurface images of the NPB-PMB suture zone provide independent constraints to surficial petrologic and structural data on arc-continent collision processes.