Synthesis:PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes

The Central Andes is a key global location to study the enigmatic relation between volcanism and plutonism because it has been the site of large ignimbrite-forming eruptions during the past several million years and currently hosts the world’s largest zone of silicic partial melt in the form of the Altiplano-Puna Magma (or Mush) Body (APMB) and the Southern Puna Magma Body (SPMB). In this themed issue, results from the recently completed PLUTONS project are synthesized. This project focused an interdisciplinary study on two regions of large-scale surface uplift that have been found to represent ongoing movement of magmatic fluids in the middle to upper crust. The locations are Uturuncu in Bolivia near the center of the APMB and Lazufre on the Chile-Argentina border, on the edge of the SPMB. These studies use a suite of geological, geochemical, geophysical (seismology, gravity, surface deformation, and electromagnetic methods), petrological, and geomorphological techniques with numerical modeling to infer the subsurface distribution, quantity, and movements of magmatic fluids, as well as the past history of eruptions. Both Uturuncu and Lazufre show separate geophysical anomalies in the upper, middle, and lower crust (e.g., low seismic velocity, low resistivity, etc.) indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used—reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, connectivity, and volatile content and highlight the need for integrated, multidisciplinary studies. While the PLUTONS project has led to significant progress, many unresolved issues remain and new questions have been raised

Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion

Uturuncu volcano, SW Bolivia, is a dormant stratovolcano (similar to 85 km(3)) dominated by dacitic lava domes and flows. Ar-39/Ar-40 ages show that the volcano was active between 890 ka and 271 ka, with the lavas becoming younger and less extensive at higher elevations. There are current signs of unrest. Between 1992 and 2006 geodetic satellite measurements record an ongoing 70 kin deformation field with a central uplift rate of I to 2 cm/yr. Deformation indicates volume changes of 400 x 108 m(3) over 14 years, an average of similar to 1 m(3)/s (10(-2) km(3)/yr). The deformation is attributed to magma intrusion into the Altiplano-Puna regional crustal magma body. Deformation models indicate a source at depths of 17 to 30 kin beneath current local relief. In a reconnaissance survey, persistent seismic activity (mean of 2.6 earthquakes per hour with a maximum of 14 per hour) was recorded at about 4 kin depth below the center of the uplift, 4 km SW of the volcano's summit. The seismic events have a normal b value (similar to 1.04) and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion. The porphyritic dacite lavas (64-68% SiO2) have a plagioclase-orthopyroxene-biotite-magnetite-ilmenite assemblage and commonly contain juvenile silicic andesite inclusions, cognate norite nodules and crustal xenoliths. Temperature estimates are in the range 805 to 872 degrees C for the dacites and about 980 degrees C for the silicic andesites. The dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust. Compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite. Reversely zoned orthopyroxene phenocrysts in the andesitic end-member are explained by changing oxidation states during crystallization. Fe3+ /Fe2+ ratios from orthopyroxene crystals and Fe3+ in plagioclase provide evidence for a relatively reduced melt that subsequently ascended, degassed and became more oxidized as a consequence of degassing. The geophysical and petrological observations suggest that dacite magma is being intruded into the Altiplano-Puna regional crustal magma body at 17 kin or more depth, consistent with deformation models. In the Late Pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together. The current unrest, together with geophysical anomalies and 270 ka of dormancy, indicate that the magmatic system is in a prolonged period of intrusion. Such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation

Reconnaissance earthquake studies at nine volcanic areas of the central Andes with coincident satellite thermal and InSAR observations

We record non-eruptive background seismicity at eight potentially active volcanoes and one geothermal area in Chile and Bolivia for the first time in order to set a baseline for future episodes of unrest. We also compare seismicity to coincident new regional observations of ground deformation from InSAR and satellite observed thermal anomalies from the ASTER instrument. We deploy small temporary seismometer networks (1 to 5 stations each) of short and intermediate period instruments for 3–27 months at the nine areas between the years 2004 and 2012 at: Parinacota, Guallatiri, Isluga, Irruputuncu, Olca–Paruma, Ollagüe, Sol de Mañana, Putana, and Láscar. Despite the lack of shallow earthquakes in the global catalogs at these volcanoes, we find that all have volcano-tectonic events with at least 27 earthquake swarms — the most active are Putana, Guallatiri and Ollagüe. We find two examples where changes in seismicity are likely related to either deformation (in 2009 at Putana) or an increase in temperature (in 2012 at Isluga). Further, we document for the first time ground deformation at a Pliocene volcano called Sillajhuay, located in the Holocene volcano gap (i.e., 70 km from the nearest active volcano Isluga). We find that the four deforming volcanoes between 18 and 24°S are seismically active, but that seismic activity does not imply measurable ground deformation. Similarly, the seismically active volcanoes have satellite thermal hotspots, but there is no correlation between relative amounts of seismic activity and hotspot temperature. Because several of the volcanoes show variations in seismic activity, temperature, and deformation over the course of a few years unrelated to eruptions, decadal and longer observations are needed to constrain background activity in the central Andes.This work was supported by National Aeronautics and Space Administration grant NNX08AT02G, NASA Earth and Space Science Fellowships to J.A.J. and S. T. H., and IRIS (NSF 1156739) summer internship to D.K. V.S. was supported by: E.U. INTERREG-3B, VULMAC, 03/MAC/2.3/A4 program, and Spanish MEC programs: CGL2007-65110 and CGL2011-25494.Peer Reviewe

Volcanic hotspots of the central and southern Andes as seen from space by ASTER and MODVOLC between the years 2000 and 2010

<p>We examine 150 volcanoes and geothermal areas in the central, southern and austral Andes for thermal anomalies between the years 2000 and 2010 from two different spaceborne sensors: (1) those automatically detected by the MODVOLC algorithm from MODIS; and (2) manually identified hotspots in night-time images from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). Based on previous work, we expected to find eight volcanoes displaying thermal anomalies (Ubinas, Villarrica, Copahue, Láscar, Llaima, Chaitén, Lonquimay and Chiliques). We document 35 volcanic areas with pixel-integrated temperatures of 4 up to more than 100 K above background in at least two images, and another 16 areas that have questionable hotspots with either smaller anomalies or a hotspot identified in only one image. Most of the thermal anomalies are related to known activity (i.e. lava and pyroclastic flows, growing lava domes, fumaroles, and lakes) while others are of unknown origin or reflect activity at volcanoes that were not thought to have surface activity. A handful of volcanoes exhibit temporal variations in the magnitude and location of their temperature anomalies that can be related to both documented and undocumented pulses of activity. Our survey reveals that low-amplitude volcanic hotspots detectable from space are more common than expected, based on lower spatial resolution data, and that these features could be more widely used to monitor changes in the activity of remote volcanoes. We find no evidence from ASTER or MODVOLC that the thermal anomalies were affected by six earthquakes with <em>M</em>_w above 7 in our study area from 2000 to 2010, although the observations may not have been optimal to detect such anomalies. </p

Shallow Seismicity, Triggered Seismicity, and Ambient Noise Tomography at the Long-Dormant Uturuncu Volcano, Bolivia

Using a network of 15 seismometers around the inflating Uturuncu Volcano from April 2009 to 2010, we find an average rate of about three local volcano-tectonic earthquakes per day, and swarms of 5–60 events a few times per month with local magnitudes ranging from −1.2 to 3.7. The earthquake depths are near sea level, more than 10 km above the geodetically inferred inflation source and the Altiplano Puna Magma Body. The Mw 8.8 Maule earthquake on 27 February 2010 triggered hundreds of earthquakes at Uturuncu with the onset of the Love and Rayleigh waves and again with the passage of the X2/X3 overtone phases of Rayleigh waves. This is one of the first incidences in which triggering has been observed from multiple surface wave trains. The earthquakes are oriented NW–SE similar to the regional faults and lineaments. The b value of the catalog is 0.49, consistent with a tectonic origin of the earthquakes. We perform ambient noise tomography using Love wave cross-correlations to image a low-velocity zone at 1.9 to 3.9 km depth below the surface centered slightly north of the summit. The low velocities are perhaps related to the hydrothermal system and the low-velocity zone is spatially correlated with earthquake locations. The earthquake rate appears to vary with time—a seismic deployment from 1996 to 1997 reveals 1–5 earthquakes per day, whereas 60 events/day were seen during 5 days using one seismometer in 2003. However, differences in analysis methods and magnitudes of completeness do not allow direct comparison of these seismicity rates. The rate of seismic activity at Uturuncu is higher than at other well-monitored inflating volcanoes during periods of repose. The frequent swarms and triggered earthquakes suggest the hydrothermal system is metastable