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Engielle Panguican

Iriga, a volcano in southeastern Luzon, is well known for its southeast-elongated collapse scar, left in 1628 AD by a 1.5 km3 rockslide debris avalanche. The resulting rockslide-avalanche spread over 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. We have studied Iriga to investigate the causes and emplacement mechanisms of the rockslide debris avalanche. Using a combination of fieldwork and remote sensing we have identified an older debris avalanche deposit of similar size to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits spread out over wide areas of low, waterlogged plains, with large proportions of clays, epiclastics, and pyroclastic flow deposits that were incorporated by soft-sediment bulking and extensive bulldozing. Both events slid over the soft sedimentary, or ignimbrite surface. Structures mapped on the avalanche deposits show how the rockslide-avalanches were emplaced as they moved over the saturated plains. Their lithologies and structures indicate that the failures involved large initial volumes of substrata, but also significant bulking during motion. The collapse orientation and structure on both sectors, as well as the deposit constituents are consistent with predictions from analogue models of combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand on sedimentary basins with transtensional strike-slip faulting.

Hosts Bill Rose and Thomas Oommen