Focused Magmatism Beneath Uturuncu Volcano, Bolivia: Insights from Seismic Tomography and Deformation Modeling GEOSPHERE; V

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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 geology 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

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 deformation 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 this study complements these previous efforts. by Jay et al. (2012) who investigated the distribution of local seismicity and the Travel-time tomography, based on local earthquakes, provides three-dimen- volcano response to major regional and global earthquakes. sional (3-D) images of the P- and S-wave velocity (Vp and Vs) in a volume Based on receiver function analysis, Chmielowski et al. (1999) suggested of crust, around and beneath Uturuncu. By including data from earthquakes an extensive magma body beneath the Altiplano-Puna volcanic complex. Us- occurring on the underlying subducting slab, this tomography is able to pro- ing the same method, but newer station networks, Zandt et al. (2003) provided vide resolution in the mid- and lower crust. When evaluated in the context of more arguments for the presence of the magma body and determined that it is existing studies, the tomography presented here provides a glimpse of the associated with a very low-velocity layer. They suggested that this anomalous deep crustal roots of the Uturuncu system.

DATA AND ALGORITHM ing criteria. Automated magnitudes were assigned to each event based on

a local magnitude (ML) algorithm. Readers are referred to Thompson and The seismic data were recorded as a part of the PLUTONS project from West (2010) for a complete treatment of the automated earthquake detec- April 2010 to October 2012. The network consisted of 33 stations surrounding tion process. the volcano with an aperture of ~100 km (Fig. 2). The network was operated in This automated catalog of earthquakes was the starting point for creating two parts, each for roughly 18 mo, such that the full network was in place for a manually reviewed earthquake catalog. Because the tomography relies on one year. All stations comprised three-component Guralp CMG-3T (120s) sen- high-quality arrival times, all phase arrivals were reviewed, and adjusted or de- sors digitized on Reftek RT-130 dataloggers at 100 samples per second. leted manually. P and S phases were added, where appropriate, for additional An initial catalog of earthquakes for this time period was compiled us- stations. New hypocenters were calculated using the GENLOC algorithm of ing the automated approach described by Thompson and West (2010). In- Pavlis et al. (2004). Shallow earthquakes >80 km from Uturuncu are excluded clusion in the catalog required an earthquake to have detections consistent from the catalog because they fall outside the aperture of the network and are with P-wave phase arrivals on the vertical component seismograms at five poorly constrained. The crustal earthquakes cluster primarily in the top 15 km or more stations. To emphasize local earthquakes and minimize the inclu- below sea level (Hutchinson, 2015). We augmented this catalog with earth- sion of teleseisms, detections were carried out on traces bandpass filtered quakes occurring on the subducting slab. The Nazca slab is a prolific source on 3–20 Hz. Each set of arrivals was compared, via grid search, to the travel of seismicity across the study area. To integrate these earthquakes, we used times predicted from a grid of hypocenters extended 0.5° in each direction the hypocenters assigned by the Observatorio San Calixto (OSC; Bolivia) in from Uturuncu and to a depth of 200 km. If the detections matched predicted their national earthquake catalog for Bolivia. We then added P- and S-phase arrivals within a tolerance of 1 s, then the event was assigned a hypocenter picks for the PLUTONS stations. While the Uturuncu stations do not constrain corresponding to the best-fit grid point. Events were declared based solely the slab events well (especially in depth), the steep and deep ray paths from on the presence of consistent P-wave arrivals. However, S waves detected these earthquakes hold the potential to image the crust much deeper than the on horizontal component seismograms using the same filters and detection local earthquakes. The locations assigned are based on the regional seismic criteria were associated with earthquakes if their arrival times fit the match- network and are generally well constrained in location and depth. We included

Figure 2. Seismic stations, earthquake hypocenters, and ray paths, Uturuncu volcano (Bolivia). Red polygon is the study area. Black (left) and blue (right) triangles are seismic stations. Colored dots are earthquakes; hypocenter depth is shown in by color. Black solid line outlines the Alti plano-Puna magma body (APMB); black dashed line is the border of the Altiplano- Puna volcanic complex (APVC); white lines are political boundaries. Depth is relative to sea level.

a selection of slab events within 150 km of Uturuncu. These earthquakes were TABLE 1. REFERENCE ONE-DIMENSIONAL VELOCITY STRUCTURE FOR UTURUNCU VOLCANO (BOLIVIA)* manually selected from the OSC catalog to produce a geographically distributed set of slab earthquakes. Because the Nazca slab dips ~35° to the east Depth Vp (km) (km/s) in this area, these earthquakes increase from 100 to 300 km depth eastward across the study area (Fig. 2). –5 5.45 For tomography, we further subset these earthquakes to include only those 05.52 55.60 with at least eight total P- and S-wave picks. After performing preliminary 75.69 earthquake locations, we rejected travel times with residuals >1 s and >1.5 s 10 4.85 for the P and S waves, respectively. These threshold values represent possible 15 4.85 residuals estimated using the expected values of the anomalies and maximum 20 5.70 length of the rays. The selection resulted in a total of 7186 P-wave and 4579 40 6.15 S-wave ray paths from 677 earthquakes. 60 6.30 Earthquake location and the tomographic inversion for the P- and S-wave 1007.25

velocities were performed using the LOTOS code (Koulakov, 2009), which is *This model has a P- to S-wave velocity ratio (VP/VS) of 1. 7. The depth is relative to the sea level. Depth 0 km is sea level. freely available at http://www .ivan -art .com /science /LOTOS. The main input data for the algorithm are the arrival times of P and S waves and station coordinates. Additional requirements include a one-dimensional (1-D) seismic velocity reference model and several user-defined parameters for different vertical direction, the node spacing was inversely dependent on the ray den- stages of calculations. sity, but nowhere <3 km. To minimize artifacts related to the grid configuration, The first step is the absolute location of earthquakes within an optimum the inversions were performed independently on four separate grids, rotated 1-D velocity model. We use the grid search method described by Koulakov in azimuth by 0°, 22°, 45°, and 67°. These four results were then averaged into and Sobolev (2006), with a fixed Vp/Vs ratio and P-wave velocities defined at a single model. a variety of depth nodes. Between these levels, velocity values are linearly The first-derivative matrix includes the elements responsible for the P- and interpolated. Using a grid search algorithm to locate the earthquakes is much S-wave velocity distributions, source coordinates, and origin times. The in- slower than inverting the travel times, but is also considerably more stable. version was performed using the LSQR method (Paige and Saunders, 1982; This helps retain as much of the original data as possible. We started search- Nolet, 1987). The flattening of the velocity model was controlled by an addi- ing for an optimal 1-D model from P-velocity distributions derived by Jay et al. tional matrix block damping the difference between velocity parameters in (2012) and Ward et al. (2014). Then we perturbed velocity and Vp/Vs values to neighboring nodes. Increasing the weight of this block makes the anomalies minimize the travel-time residuals and retain as many phase picks as possi- smoother. To balance the flattening and the number of iterations, we fixed the ble in the data set with residuals smaller than the predefined thresholds. The number of iterations at five and tuned the stability of the solution by changing best model obtained after several iterations is presented in Table 1. This model the flattening coefficients. The most important parameters used for inversion serves as the reference velocity structure for the 3-D tomography. Note that are presented in Table 2. These values were determined through testing on during this optimization procedure, we considered models with and without synthetic models. low-velocity layers mimicking the Altiplano-Puna magma body. We did not however detect any significant difference in residuals. This suggested that the body-wave tomography would be relatively insensitive to broad 1-D variations TABLE 2. KEY INVERSION PARAMETERS* FOR VELOCITY in mid-crustal structure. MODEL OF UTURUNCU VOLCANO (BOLIVIA) The main part of the nonlinear tomographic process consisted of iterative ParameterValue repetition of the source location and matrix inversion steps. The source lo- Minimum number of phases 8 cation was performed in an updated 3-D velocity model using the bending Grid spacing (horizontal) (km) 3 algorithm of ray tracing (Um and Thurber, 1987). Unlike the preliminary 1-D Minimum vertical grid spacing (km) 3 location procedure, at this stage we solved for hypocenter locations using an Smoothing of the P-wave velocity model 0.8 iterative gradient descent method. This inversion is faster and more precise, Smoothing of the S-wave velocity model 1.5 Weight for the source coordinates in inversion 2 but less stable, than the grid search method. The velocity structure was param- Weight for the source origin time in inversion 1 eterized on a grid of nodes with spacing corresponding to the density of rays. Number of iterations 5 In map view, the nodes were distributed regularly with a spacing of 3 km in *For the LOTOS code (Koulakov, 2009). areas with sufficient ray density (>10% of the average ray density value). In the

RESULTS AND VERIFICATION real data, including the determination of an initial source location in the reference 1-D velocity model. Verification To assess spatial variations in horizontal resolution, we used checkerboard tests with alternating positive and negative anomalies. In the version presented We used a variety of synthetic tests to assess the reliability and resolution of in Figure 4, 15 × 15 km anomalies are separated with unperturbed areas 5 km in the data set and parameterization. We tested the impact of random noise using width. The amplitudes are ±7% and have opposite signs for the P- and S-wave a so-called odd-even test. We divided the entire experimental data set into two anomalies, creating contrasting patterns in the Vp/Vs ratio. These anomalies subsets (e.g., with odd- and even- numbered events) and performed indepen- extend in depth as columnar features. The features are relatively well resolved dent inversions for both subsets. To the extent that random noise is an influen- at 15 km depth, recovering the basic checkerboard geometry across much of tial factor in the data, it should cause discrepancies between the two versions the region with amplitudes diminished by roughly 30%. At a depth of 30 km, of the model. The results of this test for depth slices of 15 km and 30 km for the we observe a fair degree of lateral smearing in the northwest-southeast direc- P- and S-wave anomalies are shown in Figure 3. For the P-wave velocity model, tion. The Vp/Vs images, somewhat surprisingly, appear to be more robust than the resolved patterns are nearly identical. For the S-wave velocity model, we the P- and S-wave velocity images. can identify some modest differences. For example, a small positive anomaly It is common in tomographic studies to encounter poorer resolution with to the southwest of Uturuncu at 15 km depth is visible in one data set and is not depth as a result of tradeoffs between source depth and model velocity. For restored in the other one. This indicates that the anomaly might not be stable these data, we expect additional smearing in the vertical direction because and is quite possibly an artifact of noise in the travel-time data set. of the generally steep rays provided by the slab earthquakes. Though Fig- Another set of tests measures the ability to recover synthetic anomalies. ure 2 demonstrates considerable ray density beneath Uturuncu to depths of These tests are useful not only for checking spatial resolution but also for tun- ~100 km, these rays are generally steep and subparallel. As a result, we antici ing the parameters that control the inversion (Table 2). Synthetic travel times pate model features being potentially elongated in the vertical direction. To were computed for the original source-receiver pairs using the 3-D ray-tracing assess the impact of vertical smearing, we consider two tests with synthetic algorithm based on the bending method (Um and Thurber, 1987). The syn- models defined in the vertical section. In the first model, we consider two thetic travel times were then perturbed with random noise (in this case 0.1 s anomalies with the size of 15 × 15 km located in the depth interval between and 0.15 s for the P- and S-wave data). After computing the synthetic data, we 5 km and 20 km depth (Fig. 5). The recovered models show that the upper limit “forgot” all information related to the velocity model and source locations. of these anomalies at 5 km depth is fairly well recovered, whereas the lower The reconstruction was conducted using exactly the same workflow as for the limit at 20 km depth is strongly smeared. In the second case, we consider four

Figure 3. Odd-even test for horizontal slices at 15 and 30 km depths for P- and S-wave velocity anomalies. Crossed black lines indicate the summit of Uturuncu volcano. Black dots are earthquakes.

Figure 4. Checkerboard test for horizontal slices at 15 and 30 km depths for P- and S-wave velocity (Vp and Vs) anomalies. Anomaly size is 15 × 15 km (black squares), with a gap of 5 km between anomalies. Values shown across the center of the upper left panel mark distance along Profile A1-A2 (km), which is used for performing vertical checkerboard test (Fig. 5).

anomalies located in two depth rows. The transition between the rows occurs The P- and S-wave velocity anomalies, as well as Vp/Vs ratio, are presented between 10 and 15 km depth. We see that the anomalies are better recovered in two horizontal sections at 15 and 30 km depth corresponding to the upper for the P-wave model; in the case of the S-wave model, the high-velocity anom- and mid-crust (Fig. 6) and in one vertical section cutting west-east through the aly is smeared with a dip to the west, and we observe the same for Vp/Vs ratio. Uturuncu edifice (Fig. 7). These images are relative to the starting 1-D model. These tests indicate a fair ability to resolve features with length scales In most areas, we observe that the Vp and Vs features generally correlate with of 15 km or more, with the caveat that smearing may be a factor in some each other, though with different amplitudes. For example, at 15 km depth, to directions. the north of Uturuncu, we observe a positive (fast) velocity anomaly, which is stronger in the Vs model and less prominent for the Vp model. At 15 km depth, the Uturuncu complex seems to be associated with a circular low-veloc- Results and Discussion ity anomaly surrounding a local higher-velocity anomaly, which is especially clear for the Vp model. The final model reduced P-wave travel-time residuals from 0.143 to 0.089 s In the vertical section, starting from the depth of ~2 km below sea level (38%) and the S-wave residuals from 0.282 to 0.155 s (45%). The higher reduc- (bsl), a local high-Vp pattern coexists with a strong low-Vs feature. This creates tion in S-wave travel-time residuals stands out considering the higher noise a very strong high in the Vp/Vs ratio reaching 2.0 at 15–20 km bsl. This feature levels for S-wave phase picks. This can be explained by the higher sensitivity is roughly shaped like a tooth, having a flat upper boundary and roots down of the S waves to crustal properties. The travel-time residuals are larger, and to ~80 km depth. As we see in synthetic tests in Figure 5, the limited vertical the resulting anomalies are larger. resolution might be responsible for downward smearing of crustal anomalies;

Figure 6. Horizontal slices at 15 and 30 km depths for P- and S-wave velocity anomalies and Vp/Vs ratio, based on the whole Figure 5. Vertical checkerboard test along profile A1-A2 (see location in the upper left panel of Fig. 4) for two dataset inversion. Dashed lines indicate the approximate isolines of inflating velocity; the contour interval between the lines is different models, showing P-wave and S-wave velocity (Vp, Vs) anomalies and Vp/Vs. Black squares indicate approximately 2 mm/year (after Fialko and Pearse, 2012) The profile A1-A2 position for vertical section is shown (see Fig. 7); dots the input anomalies. Depth is relative to sea level. and diagonal numbers indicate distance (km) along the profile.

thus, the observed tooth-shaped anomaly might be partially caused by this high degree of partial melting and volatiles (e.g., Takei, 2002). P-wave velocity effect. At the same time, when comparing the results in Figures 5 and 7, we is quite sensitive to composition, tending to manifest mafic magmas as high see that the downward propagation of synthetic anomalies is less important velocity (Mechie et al., 1994; Trampert et al., 2001; Behn and Kelemen, 2003). In than that of the real ones, which indicates that this anomaly does really exist contrast, the S-wave velocity is quite sensitive to the percentage and connect- in the mantle. A flat upper boundary appears to be consistent with the flat- edness of any liquid phases (Karato, 1995; Hammond and Humphreys, 2000; topped magma body suggested by Walter and Motagh (2014). The width of Schutt and Lesher, 2006). As a result, magmas tend to manifest as negative Vs this feature is ~20 km. The depth of the most-pronounced velocity anomalies anomalies, regardless of composition. corresponds to the location of the magma body responsible for the ground On top of the large tooth-like feature, at ~2–6 km bsl, there is a cluster of deformation as estimated by Fialko and Pearse (2012). earthquakes. We observe this cluster on the resulting velocity anomaly model; This combination of high Vp and very low Vs anomaly has been observed earthquakes were registered during the seismic network was operated, in years beneath several active volcanoes, including Mount Spurr (Alaska, USA; 2010–2012. Above this cluster, the tomographic results show a decrease in both Koulakov et al., 2013b), Mount Redoubt (Alaska; Kasatkina et al., 2014), and Vp and Vs, though the amplitude of the Vp anomaly is stronger. This results in Klyuchevskoy volcano (Kamchatka Peninsula, Russia; Koulakov et al., 2013a). a low Vp/Vs ratio. Note that a very similar observation exists at Mount Spurr This relationship might be explained by the intrusion of mafic magmas with a corresponding to a period of unrest in A.D. 2004–2005 (Koulakov et al., 2013b).

Figure 7. P-wave and S-wave velocity (Vp, Vs) anomalies and Vp/Vs in vertical section for the final model results (see location in Fig. 6). Black dots represent earthquakes. Depth is relative to sea level.

Beneath Mount Spurr, a deep columnar feature with high Vp/Vs ratio was over- an axisymmetric linear elastic model with a radius of 100 km and a depth of lain by a shallow zone of low Vp/Vs ratio. The transition between these two 104 km. We define zero normal stresses on the top surface and zero normal features occurred at ~2 km bsl, coincident with a vigorous cluster of earth- displacements on the side and bottom surfaces. Density is set according to quakes. At Mount Spurr, this Vp/Vs change was explained by the decompres- the geologic information on the Altiplano-Puna region (Prezzi et al., 2009). sion transition of liquid volatiles (high Vp/Vs) to gases (low Vp/Vs). As shown The elastic Lamé parameters are defined according to the P- and S-wave by experimental studies (e.g., Takei, 2002) and some field experiments (e.g., velocities (Table 1). The shape of the magmatic chamber is modeled as a Chatterjee et al., 1985, De Siena et al., 2010), the Vp/Vs ratio is strongly sensi- smoothed cylinder with a radius of 10 km, height of 30 km, and density of tive to the pore content. Gas phases create low Vp/Vs ratio (e.g., Julian et al., 2650 kg/m3. 1996). The shallow seismicity at a few kilometers below sea level beneath both The displacement suggested by the model is qualitatively similar to ob- Mount Spurr and Uturuncu may reflect the considerable volume change as served and modeled surface uplift patterns (Fialko and Pearse, 2012). Polyansky volatiles flash into a gas phase. In both the Mount Spurr and Uturuncu cases, et al. (2010) considered a thermo-mechanical viscoplastic dynamic model for the presence of the shallow low Vp/Vs anomaly and seismicity coincides with a similarly sized magmatic diapir to obtain a cumulative surface uplift of hun- fumarole activity. dreds of meters. The fracture areas (suggested by von Mises stress; Figs. 8, 9) A second hypothesis explaining the relationship between the Vp/Vs and are located not only in the uplift center but also in the peripheral ring-shaped seismicity patterns might be associated with the growth of the magma body zone. A similar ring-shaped distribution of the third principal stress was ob- beneath Uturuncu. In this case, the deep high-Vp/Vs feature may represent tained by Walter and Motagh (2014) for a deflating source. Our simple elas- very hot crust that contains a significant percentage of volatile-rich melts. Verti tic-only model neglects changes in crustal properties with depth. The high heat cal ascent of such magmas would cause the overlying rock to deform me- (Jay et al., 2012) in this region would quickly increase viscous and creep effects chanically, leading to increased seismicity. This hypothesis is supported by the with depth, leading to a decrease in von Mises stresses. The resulting decrease observed surface inflation in the Uturuncu area measured with interferometric in seismicity that would be expected with depth is consistent with the observed synthetic aperture radar (InSAR) data (Pritchard and Simons, 2002; Fialko and seismicity at Uturuncu. Pearse, 2012) and by the observed earthquake focal mechanisms (Hutchinson, The low Vs, high Vp/Vs, and lack of seismicity strongly suggest the pres- 2015; Alvizuri and Tape, 2016). ence of partial melt. Based on this, we suggest that the deformation source To assess the reliability of these scenarios, we present a simplified me- does, in fact, represent some type of localized magmatic reservoir. At the same chanical model simulating the stress and deformation effect of a gravita- time, the opposite signs of the Vp and Vs anomalies strongly suggest compo- tionally unstable magma body. The calculations were performed using the sitional differences between magma body and the surrounding rocks. Comeau COMSOL Multiphysics 4.4 software (https://www .comsol.com). We consider et al. (2015) made the same conclusion based on magnetoteluric models.

Research Paper )

Figure 8. (Left) Results of numerical model- 12 m

vertical velocity ( 21.8°S t ing of an ascending buoyant cylinder-shaped )

r 8 n

y 10 surface uplift (model) e

magma body. Upper panel presents modeled / m

surface vertical displacements compared m 21.9°S

e m

8 6 c

with the observed ground velocities derived (

t p

from interferometric synthetic aperture radar i 22°S s

c 6 i

o 4 d

(InSAR) measurements (Fialko and Pearse, l

e a

2012). Lower panel shows the von Mises v 4 22.1°S

i t

stresses in a vertical section (MPa). Depth a

2 r

c i 2 e

is relative to sea level. Note that the area of t v

r 22.2°S Uturuncu e

highest stress above the buoyant body may e

V v

0 0 i

explain the seismicity occurrence in the up- t a

l 22.3°S

per crust beneath Uturuncu volcano (Bolivia). e

(Right) Model result: von Mises stress lateral –100 –80 –60 –40 –20 020 40 60 80 100 R distribution at a depth of 0.4 km below the sur- 22.4°S face (3.4 km above sea level) reveals the most Reduced von Mises stress MРa

) 20

probable fracture zones at Uturuncu; a volcano 0 22.5°S

m k

chain follows the ring-shaped fracture zone. ( 10 h

Gray dashed lines indicate the approximate t 20 0 22.6°S p isolines of inflating velocity; the contour in- e 30 terval between the lines is ~2 mm/year (Fialko D 40 22.7°S and Pearse, 2012). Gray solid line is the politi- –20 –100 –50 0 50 100 67.6°W 67.4°W67.2°W67°W 66.8°W cal boundary between Bolivia and Argentina. Distance along the profile (km)

surface uplift

20.5°S fractured wall rocks 21°S

21.5°S Cerro Juvina excessive Pastos Grandes pressure Figure 9. (Left) Uturuncu volcano (Bolivia) 22°S and surroundings. Gray area indicates UTURUNCU Panizos mountain regions with a height of ~4000 m above sea level. Red line outlines the Alti 22.5°S A1 Guacha Vilama A2 plano-Puna magma body (APMB). Red triangles indicate volcanoes. Black lines 0 23°S W E outline main calderas. (Right) Geological scheme of the magma chamber inversion. Dots indicate earthquakes. See Figures 23.5°S hot mixed crystallized part 4 and 6 for the location of profile A1-A2. 20 of reservoir La Pacana rejuvenated melts Depth is relative to the sea level. Distance non-mobile is shown along profile A1-A2 (see Figs. 4 24°S mush and 6). Depth (km) 24.5°S 40

25°S 75 km lower-crustal 69°W 68°W 67°W 66°W 65°W 60 basaltic or basaltic-andesitic melts

60 80 100 120 140 Distance (km)

The main question then is the relationship between this local accumulation the edifice is estimated at 50–89 km3 (Muir et al., 2015; Michelfelder et al., 2014). of magma and the broad regional feature of the Altiplano-Puna magma body. The presence of biotite and amphibole indicates that volatiles were present in Some research (Lipman, 2007; de Silva and Gosnold, 2007; Annen, 2009) sug- magma, but the melts were probably not fluid rich. Although the parameters gests a model of batholith formation as an amalgamation of magmatic cham- discussed and the scale of processes indicated are very large (the area of sur- bers in volcanic provinces. According to this model, the Uturuncu magma face deformation, the volume of the magma and volatiles needed to support body is part of the batholith. Any magmatic system beneath Uturuncu is dy- this deformation, the size and shape of the reservoir from the tomographic namic and interacting continuously with the surrounding crust and other mag- images), we cannot state unequivocally that these amount to evidence for a matic components. Geochemical and geochronological constrains by Muir so-called super-eruption similar to those that took place in the late Miocene et al. (2015) show that late-stage (<485 ka) erupted lavas were derived from (Gregg et al., 2015). However, these parameters do suggest that a more modest a source that experienced extensive magma mixing or contamination. These eruption is at least plausible at some point in the future, and a better estimate authors envisaged mixing of a dacite magma with two different andesites. One of possible eruption volume would entail an analysis of all structural complex- of the andesitic components is characterized by high MgO and Cr contents, ity, including time variations and approximation of the future system devel- suggesting an influx of more-mafic magma. The isotopic composition of the opment (e.g., Gottsmann et al. [2017]). The numerical model proposed here Uturuncu lava domes and flows also implies that they originate from mingling demonstrates how such an eruption might lead to caldera formation (Fig. 8). between very different, but coeval, magmas. These magmas could be created The lateral distribution of von Mises stress shows a ring-like zone where in- by differentiating primitive island-arc basalts and through strong contamina- creased fracturing should be anticipated. This feature is roughly 40–80 km in tion by upper-crustal material (Michelfelder et al., 2014). This is evidence for diameter, roughly consistent with calderas observed throughout the region. long-lived replenishment of the Uturuncu magma reservoir. The interaction of The shallow crust in this zone is where fractures should be most anticipated high-temperature lower-crust magmas with dacitic and andesitic mush or with during an eruption. These ring fractures may facilitate caldera collapse and are already solidified parts of the reservoir may lead to a decrease of the mush a likely zone of melt penetration prior to eruptions. crystallinity or even partial melting of solidified parts of the reservoir (Fig. 9). The hot mix of intruding magma and siliceous partial melts at the base of the magmatic column become buoyant and penetrates upward into the magmatic CONCLUSION mush (Huppert and Sparks, 1988; Couch et al., 2001; Burgisser and Bergantz, 2011; Muir et al., 2015). These remobilized buoyant magmas generate pres- We have cataloged earthquake activity and constructed a seismic velocity sure on the reservoir roof. This combination of buoyant force and accompa- model with 3-D distributions of P- and S-wave velocities and Vp/Vs ratio be- nying degassing offer an explanation for recent unrest at Uturuncu volcano neath the Uturuncu area in the crust and uppermost mantle. The earthquake (Sparks et al., 2008). The transient pressure associated with these remobilized distributions reveal two independent features. The deep earthquakes associ- buoyant magmas would create temporary uplift. If this is the case, then it is ated with the subducting Nazca plate are located at depths from 100 km to possible that subsidence might later be expected as these magmas solidify 300 km beneath the study area. Travel times from these earthquakes provide and lose buoyancy. In this model, the shallow layer of seismicity is caused by constraints on seismic velocity structures in the mid- and lower crust. A sec- (1) transient stresses above the magma body caused by the arrival of buoyant ond group of earthquakes is located in the upper crust and adds resolution magmas, and (2) pore-pressure increases caused by the degassing of these in the uppermost part of the model. Based on a series of tests of real and magmas. It is interesting to note that these processes may occur on different synthetic travel-time data, we can postulate that the derived seismic models time scales. Seismicity related to degassing might occur long after magmas are reliable and horizontal resolution is good, whereas in the vertical direction, arrive and as they begin to cool. If so, then there is no reason to expect that up- anomalies are smeared. lift and seismicity should always be concurrent. As an example, though defor The most prominent feature is a large tooth-shaped anomaly with Vp/Vs mation observed in the 1990s had ceased by the time of this experiment, the >2.0 and with roots extending down to a depth of ~80 km. The upper limit of observed seismicity was much more vigorous than in surrounding areas. It is this anomaly is flat and is located at ~2 km bsl (~6 km below the surrounding possible that this seismicity is an after-effect, tied to degassing, of the inflation area, and ~8 km below the Uturuncu summit). Above this feature, we observe over the previous decade or more. low Vp/Vs and a vigorous cluster of laterally distributed earthquakes. The lo- The mid-crustal feature observed here, which is suggestive of a magma cation of this anomaly coincides with the surface uplift identified in InSAR ob- feeding zone, appears very localized to Uturuncu. Uturuncu lies between large servations. We propose two possible scenarios explaining the link between the Miocene caldera complexes (Fig. 9), implying that the voluminous caldera tooth-like body and surface deformations. In the first scenario, the columnar eruptions in its vicinity have been common in the past (Salisbury et al., 2010). anomaly of high Vp/Vs represents a pathway of ascending partial melts. At a This kind of eruption is associated with fluid-rich magmas. At the same time, depth of ~6 km below the surface, volatiles are released from the melt as a the eruptions of Uturuncu have never been voluminous. The total volume of result of decompression. This causes considerable volume increase, surface

uplift, and elevated seismicity in the uppermost crust. The second scenario Gottsmann, J., Blundy, J., Henderson, S., Pritchard, M.E., and Sparks, R.S.J., 2017, Thermo posits a magma body ascending due to gravitational instability. We use a sim- mechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization: Geosphere, v. 13, p. 1042–1065, https://doi.org/10.1130 ple model to simulate the stress field caused by a buoyant body of realistic /GES01420.1. shape to derive a qualitative fit with the observed ground deformations. It is Gregg, P.M., Grosfils, E.B., and de Silva, S.L., 2015, Catastrophic caldera-forming eruptions II: likely that components of both of these models exist at Uturuncu. The subordinate role of magma buoyancy as an eruption trigger: Journal of Volcanology and Geothermal Research, v. 305, p. 100–113, https://doi.org/10.1016/j.jvolgeores.2015.09 .022. Hammond, W.C., and Humphreys, E.D., 2000, Upper mantle seismic wave velocity: Effects of ACKNOWLEDGMENTS realistic partial melt geometries: Journal of Geophysical Research, v. 105, p. 10,975–10,986, This study is supported by the U.S. National Science Foundation (grant 0909254) and the Russian https://doi.org/10.1029/2000JB900041. Science Foundation (grant 14-17-00430). The authors are grateful to O.P. Polyansky for the consul- Huppert, H.E., and Sparks, R.S.J., 1988, Melting the roof of a chamber containing a hot, turbu- tation in numerical modeling of the observed body. We thank N.L. Dobretsov for suggestions and lently convecting fluid: Journal of Fluid Mechanics, v. 188, p. 107–131, https://doi .org/10.1017 improvements. We are grateful to the Supercomputing Center of Novosibirsk State University /S0022112088000655. for computational resources. 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