Magnetotelluric images of distribution beneath Volcán , Bolivia: Implications for magma dynamics

Matthew J. Comeau1, Martyn J. Unsworth1*, Faustino Ticona2, and Mayel Sunagua3 1Department of Physics, University of Alberta, Edmonton, Alberta T6E 2E1, Canada 2Universidad Mayor de San Andrés, Casilla 10077 Correo Central, La Paz, Bolivia 3Empresa Kawsaqi, Correo Central, La Paz, Bolivia

ABSTRACT and therefore electromagnetic geophysical The -Puna volcanic complex in the central records a history of major cal- methods are useful for investigations of subsur- dera-forming eruptions over the past 10 m.y. Geophysical and geodetic data indicate that face magma distribution. magma is still present, and perhaps mobile, in the . Broadband magnetotelluric data were used to generate two-dimensional and three-dimensional electrical resistivity models of MT DATA COLLECTION AND ANALYSIS the Altiplano-Puna magma body (APMB) with a focus on the zone of inflation around Volcán Previous long-period MT data from the Uturuncu in southern Bolivia. Low electrical resistivities (<3 Wm) at a depth of ~15 km below APVC showed a high conductivity zone in the sea level are interpreted as being due to the presence of andesitic melts of the APMB and mid-crust (Schilling et al., 2006), but the large require a melt fraction >20%. The upper surface of the APMB is shallowest beneath Uturuncu interstation spacing (~15 km) could not pro- and the geometry is consistent with geodynamic models that require the upward movement duce detailed images of crustal magma bodies. of a melt layer at this location. The shallower resistivity structure is characterized by discrete An array of 180 broadband MT stations was electrically conductive bodies, oriented east-west near sea level (depth of 5 km), which are collected from 2011 to 2013 to investigate the interpreted as a combination of partial melt and fluids. crustal magma distribution below the APVC (Fig. 1). The MT data were of high quality and INTRODUCTION mation, geophysical data are needed to image indicate the presence of a multilayer resistivity The Altiplano-Puna (South America) is the subsurface structure and understand the magma structure (see the GSA Data Repository1). only modern example of a high plateau that dynamics. The dimensionality of the MT data was inves- has formed above a zone, and con- Magnetotelluric (MT) data are useful because tigated, using the methods of McNeice and Jones vergence has produced a crustal thickness that they map subsurface electrical resistivity using (2001) and Caldwell et al. (2004), to determine reaches 75 km (e.g., Beck et al., 2014). In addi- natural electromagnetic signals (Chave and if a two-dimensional (2-D) or 3-D approach was tion to ongoing arc magmatism, the region Jones, 2012). The resistivity of a is sensi- required. The short-period MT data (0.003–1 s) underwent a major flare-up during the tive to the quantity and composition of magma, sample the near-surface structure and indicate past 10 m.y. that formed the Altiplano-Puna vol- canic complex (APVC) (de Silva, 1989). Geo- physical studies have shown that the APVC is 21.5˚ S underlain by anomalously low seismic velocities Bolivia N (Chmielowski et al., 1999; Zandt et al., 2003), high seismic attenuation (Haberland, 2003), and low electrical resistivities (e.g., Schilling et al., W E 2006), and is associated with a negative Bouguer anomaly (del Potro et al., 2013). Together, these 22.0˚ S anomalies are evidence of a major mid-crustal A S magma body, the Altiplano-Puna magma body U (APMB), and make this region a key location for studying potential granite emplacement and plu- Q

ton formation, geological processes that are not B N c fully understood (e.g., Brown, 2013). 22.5˚ S Geodetic data have shown that a region of the APVC in southern Bolivia is being uplifted C at 10–15 mm/yr (e.g., Pritchard and Simons, ar olcanic 2004), while a surrounding ring is subsiding V Chile 20 km (Fig. 1) (Fialko and Pearse, 2012; Henderson 23.0˚ S and Pritchard, 2013). The deformation is cen- 68.0˚ W 67.5˚ W 67.0˚ W 66.5˚ W 66.0˚ W 65.5˚ W tered on Volcán Uturuncu, a composite volcano last active 270 k.y. ago (Sparks et al., 2008), and Figure 1. Study area. Magnetotelluric stations are denoted by small circles. Those in black gives strong evidence for magma motion in the were used in the regional two-dimensional inversion. U—Volcán Uturuncu; Q—Volcán Quetena. Large gray circles denote the limits of the inflation and subsidence (Henderson crust. It is significant that Uturuncu is located and Pritchard, 2013). White box shows the extent of the three-dimensional inversion. Rose close to the center of the APVC and above the diagram shows the geoelectric strike direction for the period range 10–3000 s for all stations. APMB, but there is no consensus on the mecha- Points A, B, and C define the profile shown in Figure 2A. nisms responsible for the uplift. Given the nonu- niqueness in the interpretation of surface defor- 1GSA Data Repository item 2015087, supporting documentation, related figures, and an explanations of meth- ods, is available online at www.geosociety​.org/pubs​ /ft2015.htm,​ or on request from [email protected] or *E-mail: [email protected] Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

GEOLOGY, March 2015; v. 43; no. 3; p. 243–246; Data Repository item 2015087 | doi:10.1130/G36258.1 | Published online 5 February 2015 GEOLOGY© 2015 Geological | Volume Society 43 | ofNumber America. 3 For| www.gsapubs.org permission to copy, contact [email protected]. 243 a 1-D resistivity structure. Longer period data sions. The 3-D inversion of Siripunvaraporn et Shallow Conductor (1–3000 s) sample to mid-crustal depths and al. (2005) was applied to a set of 73 stations Conductor C1 is <1 km thick and has highly detected a geoelectric strike direction of N30°E surrounding Uturuncu. variable properties. It is likely caused by hydro- on profile A-B that changes to N0°E on profile thermal alteration of the regionally extensive B-C (Fig. 1; Fig. DR3 in the Data Repository). RESULTS AND DISCUSSION ignimbrite layer. undergo low-tem- Dimensionality analysis showed that a 2-D The 2-D and 3-D resistivity models (Fig. 2) perature alteration to form highly conductive analysis was valid on a regional scale, but that show three distinct layers: (1) a shallow surface clay within a time scale of 1 m.y. (Bibby et al., 3-D resistivity structure was locally present. conductor (C1) <1 km thick; (2) a deeper, spa- 2005). Both 2-D and 3-D inversions were applied to tially uniform conductor (C2) that begins at a the MT data. The 2-D inversion used the algo- depth of 15–20 km below sea level (BSL); and Deeper Conductor rithm of Rodi and Mackie (2001), with data (3) an intermediate layer with spatially variable The prominent conductor C2 is shown in both rotated to N30°E and N0°E, for the profiles resistivity that contains a number of discrete the 2-D and 3-D models. The western margin A-B and B-C, respectively. Many combina- low-resistivity zones (C3–C7), particularly the of C2 is located beneath the and it tions of inversion parameters were investigated area near Uturuncu. Low resistivity in a volcanic extends ~170 km to the east. If the top is defined and the main resistivity features of the models environment can be caused by aqueous fluids, as the 3 Wm contour, then this conductor is did not depend on any specific parameters. No partial melt, or hydrothermal alteration, so inter- located at 15–20 km BSL in both models. This significant changes in the resistivity model pretation must use additional geophysical data contour was chosen based on the synthetic inver- were observed when strike angles of N20°E or and laboratory studies of fluids and melts to dis- sions, as it corresponds to the resistivity change N40°E were used on profile A-B. Model reso- tinguish between these possibilities (Chave and at the top of C2 (see Fig. DR12). Complications lution was investigated using synthetic inver- Jones, 2012). arise from the fact that the 2-D inversions cannot

APVC Subsidence Uplift Subsidence ABQVU C C

A C1 C1 C1 Figure 2. A: Regional two-dimensional (2-D) 0 C5 R1 C4 resistivity model obtained from the inver- R2 10 C3 C7 sion of transverse electric and transverse C3 C6 20 magnetic mode magnetotelluric (MT) data at 73 stations. A root mean square (r.m.s.) 30 C2 misfit of 1.5 was achieved. Red triangle in- C2 40 dicates the volcanic arc. White box shows the extent of the 3-D model. U—Volcán Utu- 50 runcu; Q—Volcán Quetena; VC—Vilama Depth below sea level (km) 20 40 60 80 100 120 20 40 60 80 100 120 140 Distance along profile (km) ; APVC—Altiplano-Puna Volcanic Complex. Hachure pattern indicates minimal U U -5 300 resolution by showing approximate limit of B C penetration of electromagnetic signals with 0 C4 C4 a period of 1000 s. B: East-west (EW) vertical 100 section through the 3-D resistivity model. 5 Misfit error (r.m.s.) of 0.95 was achieved.

m) Black dots are earthquake hypocenters from 10 30 Ω Jay et al. (2012). The locations of each sec- tion or slice are indicated by white lines. C: 15 North-south (NS) vertical sections through the 3-D resistivity model. Misfit error (r.m.s.) 20 C2 10 C2 of 0.95 was achieved. Black dots, white lines 25 Resistivity ( as in B. D: A summary of geodynamic and 3 seismic models. APMB-RF—Altiplano-Puna Depth below sea level (km) 30 magma body as defined by seismic receiver EW slice (22.267°S) NS slice (67.192°W) functions (Zandt et al., 2003). APMB-ANT is 1 the low-velocity layer determined from am-

bient noise tomography (Ward et al., 2014). The inflation center is located ~3 km west D U E 22.1° S -5 of the summit (Pritchard and Simons, 2004). ) N 1—Spherical inflation source 17 km be- 0 low sea level (BSL) (Pritchard and Simons, 2004). 2—Horizontal prolate source 19 km 5 4 22.2° S BSL (Pritchard and Simons, 2004). 3—Diapir 5 extending vertically 5 km from APMB (Fialko 10 APMB-ANT and Pearse, 2012). 4—Prolate ellipsoid 16 3 km BSL (Hickey et al., 2013). 5—Flat-topped U 15 1 APMB-RF body rising 7 km from the APMB (Walter and 22.3° S Motagh, 2014). E: Horizontal slice of the 3-D 20 2 model. The 73 MT stations used in the inver- 25 5 km sion are shown as gray circles. Black dots, Depth below sea level (km white lines as described in B. 0.9 km BSL

22.4° S −20 −10 01020 W W W

Distance (km) .1° W 67.3° 67.2° 67 67.0°

244 www.gsapubs.org | Volume 43 | Number 3 | GEOLOGY account for small-scale structures that can cause the APMB and give evidence for the heteroge- in C4, or perhaps the APMB. Aranovich et al. static shifts in the data (Chave and Jones, 2012), neity of the lid (del Potro et al., 2013). Leidig (2013) suggested that such fluids can move ver- but the 3-D inversions are less sensitive to this and Zandt (2003) interpreted seismic anisot- tically over significant distances and accumulate effect. The depth of C2 in the 2-D and 3-D mod- ropy in this layer as being due to the presence in this location. els was reconciled with minimal change in the of fluid-filled cracks, but did not have sufficient Tomographic analysis from a local seismic data fit by carefully correcting for the static shifts resolution to detect heterogeneity. Conductor array detected a low-velocity zone at sea level in the 2-D inversions. MT signals diffuse in the C5 is spatially associated with the vent that pro- that is broadly coincident with C4 (Jay et al., Earth and their depth of investigation increases duced the Laguna Colorada ignimbrite 1.98 Ma 2012). The shallow magma body C4 is <10 km with period. Thus MT data can give a reliable (Salisbury et al., 2011). Here the low resistivity across, and combined with the shallow depth, estimate of the depth to a conductor, but it is not could be due to hydrothermal alteration from its inflation alone cannot explain the pattern of always possible to record long enough periods to that eruption, since any original magma would surface uplift, which is driven by inflation of a detect the base of a conductor. Therefore, there have likely long since solidified, but it could deeper source (Fialko and Pearse, 2012). Sparks is minimal resolution within the conductor, since represent a new batch of magma or hydrother- et al. (2008) hypothesized that such a shallow the signals diffuse in the conductor, causing the mal fluid circulation. A resistive region (R1) magma body existed, but it was undetected, pos- resistivities to smear downward. located beneath Volcán Quetena may be associ- sibly because the deformation signal of the shal- Based on the horizontal extent and the resis- ated with the high resistivities of a crystallized low body was hidden by the larger signal from tivity values observed, C2 can be associated with intrusion. Vilama Caldera is also underlain by a the APMB. Other researchers have detected the APMB (Chmielowski et al., 1999; Schilling relatively resistive zone (R2), perhaps related to preeruptive shallow magma storage that ascends et al., 2006). Seismic receiver functions showed solidification of magma from eruptions 8.4 Ma vertically from a deeper source below (Blundy that the top of the low-velocity layer was at ~15 (Salisbury et al., 2011). Recently crystallized and Cashman, 2005). km BSL (Zandt et al., 2003), which agrees with rock lacks volatiles and porosity and thus does the depth of C2 in the MT model. However, not effectively conduct electricity. Implications for Magma Dynamics ambient noise tomography (ANT) data gave a The conductor near sea level beneath Utu- The resistivity models give new constraints shallower estimate and required a low S-wave runcu (C4) is imaged in both the 2-D and 3-D on the distribution of magma beneath Uturuncu. velocity anomaly from 10 to 25 km BSL (Ward models, and can be identified as the shallow The APMB is a regional-scale magma body, et al., 2014). Reconciling these varying depth dacitic inferred by Muir et containing 20% or greater andesitic melt. The estimates requires careful consideration of the al. (2014) to be 2–6 km below the surface. The top of this layer appears to shallow beneath Utu- various data sets and their respective resolu- 3-D model shows resistivity variations beneath runcu. Seismic imaging results show a similar tions. It is striking that C2 has a spatially uni- Uturuncu in the range 1–300 Wm while the 2-D geometry below Uturuncu, but at a shallower form resistivity, in contrast to the overlying model shows an average in the range 3–10 Wm, depth (Ward et al., 2014). Shallow zones of layer, which is relatively heterogeneous. This reflecting the fact that the 2-D inversion is not low resistivity connect the APMB to the surface heterogeneity is well resolved by the MT sta- able to reproduce structure that is located off across the entire extent of the APMB and repre- tion spacing of 1–5 km used in this study. The profile. Thus for subvolcanic structures, the sent both active magmatic systems (magma and ANT analysis used a broader station spacing 3-D model gives the most robust description aqueous fluids) and the hydrothermal alteration and, combined with the long wavelength of the of conductor geometry. In the 3-D model, C4 and intrusions produced by past magmatic epi- surface waves, this means that the ANT model consists of a series of dike-shaped conductors sodes. Regions where past eruptions took place cannot recover small-scale features. Both saline striking ~N70°W that extend vertically down- must have contained shallow magma chambers, fluids and partial melt will lower the S-wave ward toward the APMB. The 3-D model has and these have long since solidified (e.g., beneath velocity (Watanabe and Kurita, 1993). Thus one been overlaid with earthquake hypocenters from Volcán Quetena, Vilama Caldera) and become explanation for the difference in depth is that the Jay et al. (2012) that show that the seismicity resistive because of the lack of volatiles. All that seismically defined APMB includes the effect of is confined to the edges of the conductors. This is left are zones of hydrothermal alteration and discrete magma bodies above the more continu- gives additional evidence for the correct depth perhaps saline fluids, such as the conductors ous APMB as imaged by MT data. These zones and geometry of this shallow body. Structural beneath northwestern Argentina (C6, C7). These are observed as distinct features by the MT data, lineament analysis of the area surrounding Utu- conductors could also be new batches of magma owing to the smaller station spacing. runcu showed a direction of N72°W (Walter and moving upward from the APMB. The low resistivity of C2 can be interpreted Motagh, 2014), which gives additional evidence What does the resistivity model say about the as a zone of partial melt. The bulk resistivity for a crustal fabric orientation in this direction. cause of the observed surface uplift? It is signifi- depends on the amount of melt, geometrical The melt fraction of C4 can be estimated cant that the 2-D model reveals that Uturuncu is distribution, and composition, mainly H2O and using the observed resistivity and composition located above the highest point of a long-wave-

Na2O content (Pommier and Le-Trong, 2011). of lavas from Uturuncu that equilibrated length upwelling of the APMB. Geodynamic An andesite sample from Uturuncu was taken close to sea level and are inferred to have origi- models have proposed both lateral and vertical to be typical of the APMB, based on an equili- nated in this body (Muir et al., 2014). The con- motion of magma, and their respective geom- bration temperature of 870 °C (Sparks et al., centration of Na2O and water from the etries are shown in Figure 2. The 3-D model 2008). Assuming a high degree of interconnec- predict that the melt has a resistivity of 5 Wm shows that, at a local scale, the MT results are tion, this requires at least 20% melt (see Fig. (Fig. DR13; Laumonier et al., 2014). Thus to consistent with a diapir, as proposed by Fialko DR14), which is consistent with prior MT stud- explain a bulk resistivity of 1 Wm, as observed and Pearse (2012). However, the MT model ies (Schilling et al., 2006). in the 3-D model, even a zone of pure melt is shows that the resistivity of the diapir is greater insufficient. Saline aqueous fluids, in addition than that of the APMB; this could imply that it Lid Above APMB to magma or alone, are needed to explain the has a lower melt fraction, a different melt con- The conductive features above the APMB observed resistivity values. Saline fluids would nectivity, or different composition (e.g., more are significant because they represent pathways be more conductive, helping to explain the exsolved volatiles) than the APMB. Hickey et al. of past or present fluid motion. Gravity data observed resistivity value of 5 Wm. The fluids (2013) proposed that the broad surface deforma- detected a set of low-density bodies rising from could be exsolved from magma crystallizing tion pattern can be fit with a vertically elongated

GEOLOGY | Volume 43 | Number 3 | www.gsapubs.org 245 inflation source driven by the lateral motion of Bibby, H.M., Risk, G.F., Caldwell, T.G., and Bennie, netotelluric data: Geophysics, v. 66, p. 158–173, magma. This model predicts a shallowing in the S.L., 2005, Misinterpretation of electrical resis- doi:10.1190/1.1444891. upper surface of C2 that is consistent with the tivity data in geothermal prospecting: A case Muir, D.D., Blundy, J.D., Rust, A.C., and Hickey, J., study from the Taupo Volcanic Zone, in Geolog- 2014, Experimental constraints on dacite pre- MT results. The resistivity model does not allow ical and Nuclear Sciences: World Geothermal eruptive magma storage conditions beneath us to distinguish between all of the existing geo- Congress, Antalya, Turkey p. 1–8. 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Pritchard, M., and Simons, M., 2004, An InSAR-based However, above 10 km BSL the magma dis- Caldwell, T.G., Bibby, H.M., and Brown, C., 2004, survey of volcanic deformation in the Central The magnetotelluric phase tensor: Geophysical Andes: , Geophysics, Geosystems, tribution is not radially symmetric. Here the Journal International, v. 158, p. 457–469, doi:​ v. 5, Q02002, doi:10.1029/2003GC000610. 3-D model shows the magma and/or fluids 10.1111​/j​.1365​-246X.2004.02281.x. Rodi, W., and Mackie, R.L., 2001, Nonlinear con- moving upward as a series of discrete narrow Chave, A.D., and Jones, A.G., eds., 2012, The mag- jugate gradients algorithm for 2-D magnetotel- zones striking roughly east-west, as opposed netotelluric method: Theory and practice: Cam- luric inversion: Geophysics, v. 66, p. 174–187, to a broad homogeneous upwelling. This depth bridge, Cambridge University Press, 552 p. doi:​10.1190​/1.1444893. Chmielowski, J., Zandt, G., and Haberland, C., 1999, Salisbury, M.J., Jicha, B.R., de Silva, S.L., Singer, coincides with the brittle-ductile transition The Central Andean Altiplano-Puna magma B.S., Jiménez, N.C., and Ort, M.H., 2011, depth determined by Jay et al. (2012). body: Geophysical Research Letters, v. 26, 40Ar/ 39Ar chronostratigraphy of Altiplano-Puna p. 783–786, doi:10.1029/1999GL900078. volcanic complex ignimbrites reveals the devel- CONCLUSIONS de Silva, S.L., 1989, Altiplano-Puna volcanic complex opment of a major magmatic province: Geologi- A deep, horizontally continuous, low-resistiv- of the Central Andes: Geology, v. 17, p. 1102– cal Society of America Bulletin, v. 123, p. 821– 1106, doi:10.1130/0091-7613(1989)017​<1102​:​ 840, doi:10.1130/B30280.1. ity layer beneath the APVC can be identified as APVCOT​>2.3.CO;2. Schilling, F.R., et al., 2006, in the cen- the APMB, and is overlain with discrete zones del Potro, R., Díez, M., Blundy, J., Camacho, A., and tral Andean crust: A review of geophysical, pet- of low resistivity that correspond to both past Gottsmann, J., 2013, Diapiric ascent of silicic rophysical, and petrologic evidence, in Oncken, and present episodes of magmatism. Beneath magma beneath the Bolivian Altiplano: Geo- O., et al., eds., The Andes: Active subduction physical Research Letters, v. 40, p. 2044–2048, : Springer Verlag Frontiers in Earth Sci- the region of surface deformation the magma doi:10.1002/grl.50493. ences Volume 1, p. 459–474. extends above the APMB, and the distribution is Fialko, Y., and Pearse, J., 2012, Sombrero uplift above Siripunvaraporn, W., Egbert, G.D., Lenbury, Y., and inconsistent with some inflation source geome- the Altiplano-Puna magma body: Evidence of a Uyeshima, M., 2005, Three dimensional mag- tries. In the upper crust, melt is confined to a set ballooning mid-crustal diapir: Science, v. 338, netotelluric inversion: Data subspace method: of dike-shaped zones, and aqueous fluids may p. 250–252, doi:10.1126/science.1226358. Physics of the Earth and Planetary Interiors, Haberland, C., Rietbrock, A., Schurr, B., and Brasse, v. 150, p. 3–14, doi:10.1016/j.pepi.2004.08.023. be required in addition to partial melt to explain H., 2003, Coincident anomalies of seismic Sparks, R.S.J., Folkes, C.B., Humphreys, M.C.S., Bar- the observed resistivity values. attenuation and electrical resistivity beneath the fod, D.N., Clavero, J., Sunagua, M.C., McNutt, southern Bolivian Altiplano plateau: Geophysi- S.R., and Pritchard, M.E., 2008, Uturuncu vol- ACKNOWLEDGMENTS cal Research Letters, v. 30, 1923, doi:10.1029​ cano, Bolivia: Volcanic unrest due to mid-crustal This research was supported by a Natural Sci- /2003GL017492. magma intrusion: American Journal of Science, ences and Engineering Research Council of Canada Henderson, S.T., and Pritchard, M.E., 2013, Decadal v. 308, p. 727–769, doi:10.2475/06.2008.01. (NSERC) Discovery grant and National Science volcanic deformation in the Central Andes Walter, T.R., and Motagh, M., 2014, Deflation and infla- Foundation grant EAR-0908281 to Cornell University. Volcanic Zone revealed by InSAR time series: tion of a large magma body beneath Uturuncu We thank Alan Jones and Gary McNeice for provid- Geochemistry, Geophysics, Geosystems, v. 14, volcano, Bolivia? Insights from InSAR data, sur- ing their decomposition program, Randy Mackie and p. 1358–1374, doi:10.1002/ggge.20074. face lineaments and stress modelling: Geophysi- Weerachai Siripunvaraporn for their inversion codes, Hickey, J., Gottsmann, J., and del Potro, R., 2013, The cal Journal International, v. 198, p. 462–473, doi:​ and Henri Brasse for sharing magnetotelluric data from large-scale surface uplift in the Altiplano-Puna 10.1093/gji/ggu080.​ Chile. 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