POSTGLACIAL ERUPTIVE HISTORY OF BARRANCAS CENTER, LAGUNA DEL MAULE VOLCANIC COMPLEX (36° 10´S, 70° 30? W), CHILE

PATRICIA SRUOGA; MANUELA ELISSONDO

Sicilia e Islas eólicas

Workshop; Volcano Geology 3rd International Workshop; 2016

IAVCEI Comission on Volcano Geology

The Laguna del Maule Volcanic Complex (LMVC) is among the most active Pleistocene-Holocene rhyolitic centers globally. It covers ~500 km2 on the Argentina-Chile border in the Southern Volcanic Zone. At least 130 separate vents erupted >350 km3 of effusive and explosive products since 1.5 Ma, suggesting persistence of a large magma reservoir (Hildreth et al., 2010). These include 24 postglacial vents which encompasses rhyolite and rhyodacite lava flows and domes, accompanied by tephra falls and pyroclastic flows. This recent flare-up of silicic volcanism has yielded at least 50 rhyolitic eruptions in the last 26 ka (Singer et al., 2014). Soil 14C dating and chemical correlation suggest that at least 33 mostly silicic lavas and pyroclastic units from LMVC vents are younger than 14 ka with half of them younger than 3.5 ka (Fierstein et al., 2014). Several geophysical studies document ongoing volcanic unrest within the LMVC. Geodetic observations since 2007 by both continuous GPS and InSAR have recorded uplift at a rate in excess of 20 cm/yr (Fournier et al., 2010; Feigl et al., 2014; Le Mével et al., 2015). In addition, frequent seismic swarms have been occurring at shallow depths at south LMVC (SERNAGEOMIN-OVDAS, unpublished REPORTS 20112-2015). Preliminary gravity, magnetotelluric and conductivity results also suggest the presence of a shallow magma system beneath the same area of deformation (Singer et al., 2014). Barrancas center, located at the southeastern LMVC, represents the most highly-productive and longest-lived postglacial vent. Due to the geophysical-documented ongoing unrest, unraveling its eruptive history is a key task in hazard-oriented assessment. Since 2011, detailed mapping has been carried out in the framework of an international collaboration project (SEGEMAR, SERNAGEOMIN, USGS, University of Wisconsin). Our main goal is to obtain a high-quality hazard map, based on precise identification of the products and accurate reconstruction of the eruptive stratigraphy. The ~5.5 km3 postglacial volcanic record is mainly preserved in Argentina, at the headwaters of Barrancas river (Sruoga et al. 2015) (Fig. 1). Two stages may be distinguished: 1) an early episode of dome building is dated at ~14.5 ka (Andersen et al., unpublished) and is followed by an explosive dome partial-collapse event that produced widespread block & ash deposits, up to ~60 m thick, extending ~13 km from source along Curamilio, Puente de Tierra and La Parva creeks (Fig. 1) and 2) 8 obsidian flows (Hildreth et al., 2010) and 3 pumice cones which produced highly ?recurrent tephra falls and several pyroclastic flows. One of the lava flows has been dated at 6.4 ka (Singer et al. 2014). The pyroclastic sequence Pampa del Rayo includes not only the explosive record of Barrancas center, but also of other synchronic LMVC vents. Ongoing tephrocronology studies and Ar/Ar dating will undoubtedly improve our understanding of a volcano which in case of reactivation may affect a widespread area in Chile and Argentina, particularly in south Mendoza and North Neuquen.