Quetrupillán volcanic complex, Chile: Holocene volcanism, magmatic plumbing system, and future hazards

Abstract

The Quetrupillán Volcanic Complex in the Southern Andes of Chile (39°30’ S, 71°43’ W) is a composite volcanic system that has been active since the Pleistocene (>15 ky). It consists of a truncated and eroded stratocone plus sixteen well-preserved satellite vents on its lower flanks. In this work I combine the results of field observations, petrological and geochemical analyses of erupted products, and structural and tectonic considerations, to determine the volcanic activity that has occurred at Quetrupillán during the Holocene, the nature and geometry of the magmatic plumbing system below the volcanic edifice, and what hazards may be produced by Quetrupillán when it next erupts. Intense scouring of the stratocone’s flanks by ice has removed much evidence of its Holocene eruptions, and thus the Holocene evolution of the stratocone cannot be determined. However, the sixteen flank vents lie outside the glacially scoured zone and so are well-preserved. They display a range of features, with some having constructed scoria cones, some having excavated craters, some having built tuff rings, and some having effused lava flows. The variation in vent morphology implies that a variety of eruption styles occurred from the flanks of Quetrupillán. Lava flows from flank vents range in size from less than 1 × 106 m3 to over 60 × 106 m3 . The majority (~97%) of this lava is trachytic in composition, with only minor trachyandesite and basaltic andesite compositions erupted from the flanks of Quetrupillán. Trachyte, trachyandesite, basaltic andesite and basalt lavas have been erupted from the summit region of the stratocone. The evolution of the magmatic plumbing system at Quetrupillán has been influenced by the structural control imposed by the surrounding Liquiñe-Ofqui Fault Zone. Basaltic melt is hindered on its ascent through the crust due to a compressional tectonic regime, causing it to stall and evolve through fractional crystallisation, resulting in a network of trachytic melt pockets within a transcrustal magmatic system. Extraction of melt from these pockets has generated the numerous trachytic eruptions which have occurred from the summit and flanks of Quetrupillán during the Holocene. Occasionally, some of the basaltic source melt has reached shallow levels within the plumbing system with minimal or no interaction with the trachytic melt, resulting in eruption of basalt, basaltic andesite and trachyandesite lavas. Quetrupillán’s diverse volcanism poses issues for hazard mitigation, as the range of eruption styles means that a variety of hazards may be produced, including lava flows, tephra fall and pyroclastic density currents, though they are unlikely to reach large centres of population. As Quetrupillán is snow-covered for much of the year, any eruption is also likely to lead to the formation of lahars, which may affect the isolated settlements in the valleys on the flanks of the volcano.