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New features from the Romblon Island Group: Key to understanding the arc–continent collision in Central Philippines

Abstract : A complete ophiolite sequence and mélange are reported for the first time in the Romblon Island Group located at the central part of the Philippine archipelago. The study area comprises Tablas, Romblon and Sibuyan Islands built on amalgamated crust and mantle rocks that are typically fault-bounded and stratigraphically disrupted. The Sibuyan Ophiolite Complex (SOC) consists of harzburgite and dunite, layered clinopyroxenite, layered and isotropic gabbro, diabase dike swarm and basaltic–andesitic pillow lava and flow deposits. A Jurassic–Cretaceous age of formation and late Early Miocene age of emplacement are suggested for the SOC by available data. Emplacement of these crust–mantle units is believed to have been facilitated through displacements along east-verging thrust faults developed consequent to the Early Miocene arc–continent collision in Central Philippines. Along with recent K–Ar isotopic ages (12 Ma) obtained for some of the metamorphic rocks in the area, these lithologic units and structural features provide critical information regarding the arc–continent collision which is one of the significant events in the evolution of the Philippine island arc system. The usefulness of potential field geophysical methods to studying lithologic and structural relationships in an extremely disrupted terrane is evaluated in this study. Observed variations in gravity and magnetic values correlated with lithologic distributions and geologic structures. Higher relative Bouguer and magnetic anomalies are observed over members of the complete ophiolite sequence and younger igneous materials exposed in Tablas and Sibuyan Islands. Their geophysical characteristics contrast markedly with the less dense and magnetically inferior sedimentary rocks exposed only on Tablas Island. On the other hand, metamorphic units in Tablas, Romblon and Sibuyan Islands display variable magnetic and gravity signatures that reflect differences in composition and alteration or weathering. Thrust faults related to the emplacement of the ophiolite as a result of the collision are inferred from sudden changes in gravity and magnetic values
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Carla B. Dimalanta, E. G. Ramos, Graciano P. Yumul Jr., Hervé Bellon. New features from the Romblon Island Group: Key to understanding the arc–continent collision in Central Philippines. Tectonophysics, Elsevier, 2009, 479 (1-2), pp.120-129. ⟨10.1016/j.tecto.2009.02.015⟩. ⟨insu-00448181⟩

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