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Altiplano-Puna Magma Body

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

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]

Composition

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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

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The Altiplano-Puna region around the Uturuncu volcano is experiencing a type of deformation termed 'sombrero uplift,' which means 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 ~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 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 periods.[10][9]

Imaging Techniques

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Seismic

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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

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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

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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

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  1. ^ a b 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. ^ a b c d 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. ^ a b 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. hdl:10261/88258. ISSN 1944-8007.
  4. ^ a b c d e 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. ^ a b c d e f g h 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. ^ a b 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. ^ a b c d e 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. hdl:1983/cf804ce1-dcfa-4abf-b2e3-0f267f7feed1. ISSN 1553-040X.
  10. ^ a b c d e f g 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. hdl:1983/fbadc0f3-b31d-477f-8858-6f083091280c.
  11. ^ a b c d 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. ^ a b 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. hdl:1983/b23b8814-995e-4186-9355-a8d7f9a685ae.
  13. ^ a b c 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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