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Volcano-tectonic controls and emplacement kinematics of the Iriga debris avalanches (Philippines)


Mt Iriga in southeastern Luzon is known for its spectacular collapse scar that was possibly created in 1628 ad by a 1.5-km3 debris avalanche. The debris avalanche deposit (DAD) covered 70 km2 and dammed the Barit River to form Lake Buhi. The collapse has been ascribed to a non-volcanic trigger related to a major strike-slip fault under the volcano. Using a combination of fieldwork and remote sensing, we have identified a similar size, older DAD to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits cover wide areas of low, waterlogged plains, to a distance of about 16 km for the oldest and 12 km for the youngest. Hundreds of metre-wide and up to 50-m-high hummocks of intact conglomerate, sand and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata. In addition, large proportions of both DAD consist of ring-plain sediments that were incorporated by soft-sediment bulking and extensive bulldozing. An ignimbrite unit incorporated into the younger DAD forms small (less than 5 m high) discrete hummocks between the larger ones. Both debris avalanches slid over water-saturated soft sediment or ignimbrite and spread out on a basal shear zone, accommodated by horst and graben formation and strike-slip faults in the main mass. The faults are listric and flatten into a well-developed basal shear zone. This shear zone contains components from the substrate and has a diffuse contact with the intact substrata. Long, transport-normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing. The collapse orientation and structure on both sectors and DAD constituents are consistent with collapse being a response to combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand in sedimentary basins undergoing transtensional strike-slip faulting.

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This study is a result of the co-tutelle PhD programme between the Université Blaise Pascal and the University of the Philippines. Funding was provided by the French Embassy in Manila and the EIFFEL Excellence Scholarship. Gerard ‘Titan’ Quina, Cathy Abon, Caloy Arcilla (NIGS Director), Meryl Calibo, Analiza and Jerry Morada and the family of Buhi, Camarines Sur, and Grace Bato provided invaluable field assistance and logistics support. Comments and reviews by Lee Siebert and an anonymous reviewer improved the manuscript. Editorial review by Professor Jocelyn McPhie was especially valuable and appreciated.

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Correspondence to E. M. R. Paguican.

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Editorial responsibility: J. McPhie

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Paguican, E.M.R., van Wyk de Vries, B. & Lagmay, A.M.F. Volcano-tectonic controls and emplacement kinematics of the Iriga debris avalanches (Philippines). Bull Volcanol 74, 2067–2081 (2012).

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  • Mt Iriga
  • Debris avalanche deposit
  • Volcano-tectonics
  • Transtensional faulting
  • Emplacement kinematics