Altiplano-Puna Magma Body

The Altiplano-Puna Magma Body (APMB), a magma body located within the Altiplano-Puna plateau approximately 10-20 km beneath the Altiplano-Puna Volcanic Complex (APVC)[1] in the Central Andes. High resolution tomography shows that this magma body has a diameter of ~200 km, a depth of 14-20 km, with a total volume of ~500,000 km3,[2][3] making it the largest known active magma body on Earth.[1][4][5] Thickness estimates for the APMB are varied, with some as low as 1 km,[4][6] others around 10-20 km,[7] and some extending as far down as the Moho.[8] The APMB is primarily composed of 7-10 wt% water andesitic melts and the upper portion may contain more dacitic melts[9][10] with partial melt percentages ranging from 10-40%.[2] Measurements indicate that the region around the Uturuncu volcano in Bolivia is uplifting at a rate of ~10 mm/year, surrounded by a large region of subsidence.[5] This movement is likely a result of the APMB interacting with the surrounding rock and causing deformation.[5][10] Recent research demonstrates that this uplift rate may fluctuate over months or years and that it has decreased over the past decade.[11] Various techniques, such as seismic, gravity, and electromagnetic measurements have been used to image the low-velocity zone in the mid to upper crust known as the APMB.[9]

Location (in red) of the Altiplano-Puna plateau in South America

Composition

The APMB is likely compositionally zoned with the lower 18-30 km containing andesitic melts and the upper 9-18 km containing dacitic melts.[9] Estimates for the percentage of andesitic melt vary from 8 vol% on the low end and up to 30 vol% on the high end.[10] These andesitic melts also have a high water content (~7-10 wt.% water[10]) indicated by the high electrical conductivity measured in the APMB.[12] Measurements for the partial melt percentage in the APMB also vary, with seismic imaging indicating that it is anywhere from 10-40% partial melt.[2] For a magma body with ~20% partial melt, the viscosity is estimated to be <1016 Pa s.[13]

Deformation

The Altiplano-Puna region around the Uturuncu volcano is experiencing a type of deformation termed 'sombrero uplift,' which means that there is a central zone of uplift surrounded by a region of subsidence.[5] One potential explanation for this sombrero uplift pattern is the formation and growth of a large diapir arising from the APMB.[5] Lower density magma than the surrounding rocks is produced during partial melting in the APMB, causing a plume of buoyant magma to rise from the center of the magma body.[5] This causes material to be removed from the APMB to feed the growing diapir, resulting in a region of subsidence surrounding the uplift zone[5].

Deformation model showing the extent of the sombrero uplift pattern as well as one explanation for what is occurring beneath the surface.[10]

Data collected between 1992 and 2010 demonstrates that the region is uplifting at a rate of ~10 mm/year and subsiding at a slower rate (only a few mm/year).[5][11] More recent InSAR data, collected between September 2014 and December 2017, shows that the uplift rate over this time period has decreased to 3-5 mm/year and may experience short-term velocity reversals.[11] Additionally, there is evidence that the uplift and subsidence rates have balanced out over the past 16,000 years to create no net deformation.[9] These aspects of the uplift and subsidence cannot be easily explained by the diapir model, so other possible mechanisms for driving the deformation are being investigated.[11] One such mechanism that might explain the deformation is the movement of volatiles in a column connected to the APMB.[10] Movement like this may explain the surface deformation rate that varies on monthly or yearly scales and appears to have resulted in no net deformation over longer time periods.[10][9]

Imaging Techniques

Seismic

Between 1996 and 1997, several broadband seismic stations were deployed over the Altiplano-Puna Volcanic Complex (APVC) in order to characterize the magmatic structures beneath the surface.[4] These stations found a low velocity region approximately 10-20 km beneath the surface that was interpreted to be a sill-like magma body associated with the APVC.[4] Seismic studies and modeling continues to take place in this area, further constraining the extent and characteristics of this magma body.[2][14][6][15]

Gravity

A 3D density model of the Central Andes was developed based on modeling of Bouguer anomalies and it provided a more detailed view of the region's lithospheric structure and an estimation of the amount of partial melt present in the APMB (~9%).[16] Continued investigation of Bouguer anomaly data led to the discovery of a column-like, low density structure extending from the top of the APMB with a diameter of approximately 15 km.[3]

Electromagnetic

Electromagnetic methods have also been used to investigate structures in the Andes as well as determine characteristics of the APMB. Magnetotelluric stations were deployed across the Central Andes and resolved a highly conductive region beneath the Altiplano-Puna plateau, which appeared to coincide with the low velocity zone associated with the APMB.[4][17] Further magnetotelluric studies showed that the region has low electrical resistivities of <3 Ωm.[13] Resistivity values in this range are interpreted to only occur with magma that contains a minimum of 15% andesitic melt.[13] Additionally, these resistivity values indicate that the melt has a water content up to 10 wt.% H2O, which makes up approximately 10% of the APMB.[12]

References
  1. Perkins, Jonathan P.; Ward, Kevin M.; de Silva, Shanaka L.; Zandt, George; Beck, Susan L.; Finnegan, Noah J. (2016). "Surface uplift in the Central Andes driven by growth of the Altiplano Puna Magma Body". Nature Communications. 7 (1): 13185. doi:10.1038/ncomms13185. ISSN 2041-1723. PMC 5093326. PMID 27779183.
  2. Ward, Kevin M.; Zandt, George; Beck, Susan L.; Christensen, Douglas H.; McFarlin, Heather (2014). "Seismic imaging of the magmatic underpinnings beneath the Altiplano-Puna volcanic complex from the joint inversion of surface wave dispersion and receiver functions". Earth and Planetary Science Letters. 404: 43–53. doi:10.1016/j.epsl.2014.07.022.
  3. Potro, Rodrigo del; Díez, Mikel; Blundy, Jon; Camacho, Antonio G.; Gottsmann, Joachim (2013). "Diapiric ascent of silicic magma beneath the Bolivian Altiplano". Geophysical Research Letters. 40 (10): 2044–2048. doi:10.1002/grl.50493. ISSN 1944-8007.
  4. Chmielowski, Josef; Zandt, George; Haberland, Christian (1999). "The Central Andean Altiplano-Puna magma body". Geophysical Research Letters. 26 (6): 783–786. doi:10.1029/1999GL900078. S2CID 129812369.
  5. Fialko, Y.; Pearse, J. (2012). "Sombrero Uplift Above the Altiplano-Puna Magma Body: Evidence of a Ballooning Mid-Crustal Diapir". Science. 338 (6104): 250–252. doi:10.1126/science.1226358. ISSN 0036-8075. PMID 23066078. S2CID 206543306.
  6. Leidig, Mark; Zandt, George (2003). "Modeling of highly anisotropic crust and application to the Altiplano-Puna volcanic complex of the central Andes: HIGHLY ANISOTROPIC CRUST IN THE APVC". Journal of Geophysical Research: Solid Earth. 108 (B1): ESE 5–1–ESE 5-15. doi:10.1029/2001JB000649.
  7. Yuan, X.; Sobolev, S. V.; Kind, R.; Oncken, O.; Bock, G.; Asch, G.; Schurr, B.; Graeber, F.; Rudloff, A.; Hanka, W.; Wylegalla, K. (2000). "Subduction and collision processes in the Central Andes constrained by converted seismic phases". Nature. 408 (6815): 958–961. doi:10.1038/35050073. ISSN 0028-0836. PMID 11140679. S2CID 4424146.
  8. Schurr, B.; Asch, G.; Rietbrock, A.; Trumbull, R.; Haberland, C. (2003). "Complex patterns of fluid and melt transport in the central Andean subduction zone revealed by attenuation tomography". Earth and Planetary Science Letters. 215 (1–2): 105–119. doi:10.1016/S0012-821X(03)00441-2.
  9. Pritchard, M.E.; de Silva, S.L.; Michelfelder, G.; Zandt, G.; McNutt, S.R.; Gottsmann, J.; West, M.E.; Blundy, J.; Christensen, D.H.; Finnegan, N.J.; Minaya, E. (2018). "Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes". Geosphere. 14 (3): 954–982. doi:10.1130/GES01578.1. ISSN 1553-040X.
  10. Gottsmann, J.; Blundy, J.; Henderson, S.; Pritchard, M. E.; Sparks, R. S. J. (2017). "Thermomechanical modeling of the Altiplano-Puna deformation anomaly: Multiparameter insights into magma mush reorganization". Geosphere. 13 (4): 1042–1065. doi:10.1130/GES01420.1.
  11. Lau, Nicholas; Tymofyeyeva, Ekaterina; Fialko, Yuri (2018). "Variations in the long-term uplift rate due to the Altiplano–Puna magma body observed with Sentinel-1 interferometry". Earth and Planetary Science Letters. 491: 43–47. doi:10.1016/j.epsl.2018.03.026.
  12. Laumonier, Mickael; Gaillard, Fabrice; Muir, Duncan; Blundy, Jon; Unsworth, Martyn (2017). "Giant magmatic water reservoirs at mid-crustal depth inferred from electrical conductivity and the growth of the continental crust". Earth and Planetary Science Letters. 457: 173–180. doi:10.1016/j.epsl.2016.10.023.
  13. Comeau, Matthew J.; Unsworth, Martyn J.; Cordell, Darcy (2016). "New constraints on the magma distribution and composition beneath Volcán Uturuncu and the southern Bolivian Altiplano from magnetotelluric data". Geosphere. 12 (5): 1391–1421. doi:10.1130/GES01277.1.
  14. Zandt, G.; Leidig, M.; Chmielowski, J.; Baumont, D.; Yuan, X. (2003). "Seismic Detection and Characterization of the Altiplano-Puna Magma Body, Central Andes". Pure and Applied Geophysics. 160 (3): 789–807. doi:10.1007/PL00012557. ISSN 0033-4553. S2CID 186228544.
  15. Jay, Jennifer A.; Pritchard, Matthew E.; West, Michael E.; Christensen, Douglas; Haney, Matthew; Minaya, Estela; Sunagua, Mayel; McNutt, Stephen R.; Zabala, Mario (2012). "Shallow seismicity, triggered seismicity, and ambient noise tomography at the long-dormant Uturuncu Volcano, Bolivia". Bulletin of Volcanology. 74 (4): 817–837. doi:10.1007/s00445-011-0568-7. ISSN 0258-8900. S2CID 54170163.
  16. Prezzi, Claudia B.; Götze, Hans-Jürgen; Schmidt, Sabine (2009). "3D density model of the Central Andes". Physics of the Earth and Planetary Interiors. 177 (3–4): 217–234. doi:10.1016/j.pepi.2009.09.004. hdl:11336/75381.
  17. Brasse, Heinrich (2002). "The Bolivian Altiplano conductivity anomaly". Journal of Geophysical Research. 107 (B5): 2096. doi:10.1029/2001JB000391. ISSN 0148-0227.
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