Research Paper THEMED ISSUE: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes GEOSPHERE Focused magmatism beneath Uturuncu volcano, Bolivia: Insights from seismic tomography and deformation modeling GEOSPHERE; v. 13, no. 6 Ekaterina Kukarina1,2, Michael West3, Laura Hutchinson Keyson3, Ivan Koulakov1,2, Leonid Tsibizov1,2, and Sergey Smirnov4,5 1A.A. Trofimuk Institute of Petroleum Geology and Geophysics, Siberian Branch of the Russian Academy of Sciences, Prospekt Koptyuga, 3, Novosibirsk, 630090, Russia doi:10.1130/GES01403.1 2Novosibirsk State University, Pirogova Street, 2, Novosibirsk, 630090, Russia 3Geophysical Institute, University of Alaska Fairbanks, 903 N Koyukuk Dr., Fairbanks, Alaska 99709, USA 9 figures; 2 tables 4V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Prospekt Koptyuga, 3, Novosibirsk, 630090, Russia 5Tomsk State University, 36 Lenin Ave, Tomsk, 634050, Russia CORRESPONDENCE: ekaterina _kukarina@mail .ru CITATION: Kukarina, E., West, M., Hutchinson ABSTRACT modest, the scale of these eruptions makes them significant. They imprint Keyson, L., Koulakov, I., Tsibizov, L., and Smirnov, S., 2017, Focused magmatism beneath Uturuncu geol ogy around the world, capture the imagination of the public, and repre- volcano, Bolivia: Insights from seismic tomography We have carried out a tomographic inversion for seismic velocity in the sent a formative process in continental earth science. and deformation modeling: Geosphere, v. 13, no. 6, vicinity of Uturuncu volcano (Bolivia) based on a 33-station temporary seis- Perhaps more importantly, the scale of these eruptions indicates the ex- p. 1855–1866, doi:10.1130/GES01403.1. mic network deployment. We combine travel times from earthquakes in the istence of prodigious magmatic systems in the crust beneath them. Decades shallow crust with those from earthquakes on the subducting Nazca plate of field studies have demonstrated that, indeed, surface volcanism is but one Received 29 July 2016 Revision received 11 August 2017 to broadly constrain velocities throughout the crust using the LOTOS tomog- manifestation of a larger subsurface process that is ultimately responsible for Accepted 19 September 2017 raphy algorithm. The reliability and resolution of the tomography is verified constructing much of the Earth’s continental crust. Crustal magmatism consists Published online 19 October 2017 using a series of tests on real and synthetic data. The resulting three-dimen- of both intrusive and extrusive processes. While there is little debate that both sional distributions of Vp, Vs, and Vp/Vs reveal a large tooth-shaped anomaly intrusive and extrusive magmatism contribute to the formation of continental rooted in the deep crust and stopping abruptly 6 km below the surface. This crust, the relationship between the two is less clear (e.g., Bachmann et al., 2007). feature exhibits very high values of Vp/Vs (up to 2.0) extending to ~80 km Specifically, it is unclear whether large ignimbrite eruptions are necessarily depth. To explain the relationship of this anomaly with the surface uplift ob- paired with the formation of new plutons (e.g., Glazner et al., 2004). Alterna- served in interferometric synthetic aperture radar (InSAR) data, we propose tively, perhaps large intrusions are less prone to eruption because they ther- two scenarios. In the first, the feature is a pathway for liquid volatiles that con- mally weaken the crust, making it more accommodating to in situ magmas (e.g., vert to gas, due to decompression, at ~6 km depth, causing a volume increase. Jellinek and DePaolo, 2003). All of these reasons provide relevance for the study This expansion drives seismicity in the overlying crust. In the second model, of volcanic systems capable of hosting large caldera-forming eruptions. this anomaly is a buoyant pulse of magma within the batholith, ascending Uturuncu volcano (Bolivia) is one such location. It is part of the Altiplano- due to gravitational instability. We propose a simplified numerical simula- Puna volcanic complex (white dashed line in Fig. 1), above the central Andes tion to demonstrate how this second model generally supports many of the subduction zone and the downgoing Nazca plate. It sits near the border junc- observations. We conclude that both of these scenarios might be valid and ture between Chile, Argentina, and Bolivia. In the late Miocene, a “flare-up” of complement each other for the Uturuncu case. Based on joint analysis of the ignimbrite volcanism is recorded throughout the Altiplano-Puna volcanic com- tomography results and available geochemical and petrological information, plex. Since 10 Ma, >30 caldera forming eruptions have occurred here. At least we have constructed a model of the Uturuncu magma system that illustrates ten of these eruptions left a mark on the global scale (Salisbury et al., 2010). the main stages of phase transitions and melting. These ignimbrite strata cover most of the area around Uturuncu providing a clear, if ominous, reminder of eruptions in the geologic past. Uturuncu lavas consist of andesites and dacites. Rock compositions show INTRODUCTION that dacites are likely formed during andesite fractional crystallization with segregation of noritic cumulates, and zonal phenocrysts of orthopyroxene and Massive caldera-forming eruptions represent rare but catastrophic events. plagioclase indicate the subsequent mixing of dacitic and andesitic magmas Strong explosive eruptions in recent human history—including Mount Tam- (Sparks et al., 2008). bora in Indonesia (A.D. 1815) (Stothers, 1984) and Huaynaputina in Peru (A.D. Argon-argon (39Ar/40Ar) dating show that the volcano was active 890–270 For permission to copy, contact Copyright 1601) (de Silva and Zielinski, 1998)—were locally devastating and impacted k.y. ago, and the eruption activity included lava doming and flows (Sparks Permissions, GSA, or [email protected]. lives in other countries. Though the actual risk from eruptions of this type is et al., 2008; Salisbury et al., 2010). The influence of glaciers is seen on the © 2017 Geological Society of America GEOSPHERE | Volume 13 | Number 6 Kukarina et al. | Uturuncu tomography Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/6/1855/3990626/1855.pdf 1855 by guest on 01 October 2021 Research Paper Figure 1. Shaded-relief map of Uturuncu volcano (Bolivia) and surroundings. White dashed line represents the border of the Altiplano-Puna volcanic complex (APVC). Black solid line shows the Altiplano-Puna magma body (APMB) boundaries. Grey lines are political boundaries. Red square is the study region. White area on the right panel shows the inflating area; dashed lines are approximate isolines of defor- mation velocity (after Fialko and Pearse, 2012). The contour interval between the lines is ~2 mm/year. summit lavas, confirming the absence of Holocene effusive activity. The lavas layer is a sill-structured magma body beneath the entire Altiplano-Puna volcanic contain various xenoliths: gabbroids, cumulates, and basement rocks (horn- complex, and proposed a special term for it—the Altiplano-Puna magma body. fels, sandstones, limestones). Two fumarole fields are active at the Uturuncu Based on gravity inversion, integrated with the available geophysical, geologi- summit. They produce significant amounts of sulfur, with gas temperatures cal, and petrological observations, del Potro et al. (2013) proposed that partially <80 °C (Sparks et al., 2008). With the absence of modern cooling lava domes, molten felsic bodies ascend as diapirs through the hot ductile middle to upper the fumaroles are a primary indicator for an active magmatic reservoir. crust. They claimed specifically that these features are likely rooted in, and ex- Uturuncu has received considerable attention in recent years following the tend up from, the Altiplano-Puna magma body near Uturuncu. A joint inversion recognition that it was inflating at 1–2 cm/yr during the late 1990s in a manner of surface wave data and receiver functions (Ward et al., 2014) suggests a more consistent with magma accumulation at depths of 10–20 km (Pritchard and distributed zone on the order of 10 km thick and an estimated potential volume Simons, 2002). The radially symmetric inflation, surrounded by a ring of slight of ~500,000 km3. Ambient noise tomography (Jay et al., 2012) suggested the deflation, was later interpreted by Fialko and Pearse (2012) as evidence for a potential for magma above sea level just north of the Uturuncu summit. diapir sourced by the mid-crustal magma layer. The authors posited that this Each of these techniques provides one view of the magmatic system, diapir accumulates magma beneath Uturuncu by drawing it out of the sur- shaped by the selective resolution of that particular approach. For example, rounding region, thus creating the long-wavelength subsidence as well as the ambient noise tomography images structure in the upper crust, but cannot re- narrower uplift feature. trieve features at deeper levels where the major magma sources are expected. This combination of subsidence and uplift changes strain patterns on ex- Receiver function methods provide information on the interface geometry, but isting fractures and in hydrothermal systems near sea level, generating vol- suffer from depth tradeoffs with velocity parameters. cano-tectonic earthquakes that often manifest as swarms. This was supported The analysis presented in