Altiplano–Puna volcanic complex

A satellite photograph of the Central Andes, looking towards Argentina
The APVC lies in the lower part of the image, above the chain of volcanoes at the bottom.

The Altiplano–Puna volcanic complex (Spanish: Complejo volcánico Altiplano-Puna), also known as APVC, is a complex of volcanic systems in the Puna of the Andes. It is located in the Altiplano area, a highland bounded by the Bolivian Cordillera Real in the east and by the main chain of the Andes, the Western Cordillera, in the west. It results from the subduction of the Nazca Plate beneath the South American Plate. Melts from the subducting plate have generated the volcanoes of the Andean Volcanic Belt including the APVC. The volcanic province is located between 21° S–24° S latitude. The APVC spans the countries of Argentina, Bolivia and Chile.[1]

In the Miocene–Pliocene (10-1 mya), calderas erupted felsic ignimbrites[2] in four distinct pulses separated by periods of low levels of activity. At least three volcanic centres (Guacha caldera, La Pacana, Pastos Grandes, Vilama caldera) had eruptions of a Volcanic Exposivity Index (VEI) of 8, as well as smaller scale eruptive centres.[3] Activity waned after 2 mya, but present-day geothermal activity and volcanoes dated to the Holocene, as well as recent ground deformation at Uturunku volcano indicate still-extant present-day activity of the system.

Geography

The Andes mountain chain originated from the subduction of the Nazca Plate below the South American Plate and was accompanied by extensive volcanism. Between 14° S and 28° S lies one volcanic area with over fifty recently active systems, the Central Volcanic Zone (CVZ). Since the late Miocene between 21° S and 24° S a major ignimbrite province formed over 70 kilometres (43 mi) thick crust, the Altiplano–Puna volcanic complex, between the Atacama and the Altiplano. The Toba volcanic system in Indonesia and Taupo in New Zealand are analogous to the province.[4] The APVC is located in the southern Altiplano-Puna plateau, a surface plateau 300 kilometres (190 mi) wide and 2,000 kilometres (1,200 mi) long at an altitude of 4,000 metres (13,000 ft), and lies 50–150 kilometres (31–93 mi) east of the volcanic front of the Andes.[5] Deformational belts limit it in the east.[6] The Altiplano itself forms a block that has been geologically stable since the Eocene; below the Atacama area conversely recent extensional dynamics and a weakened crust exist.[7] The Puna has a higher average elevation than the Altiplano,[8] and some individual volcanic centres reach altitudes of more than 6,000 metres (20,000 ft).[9]

Geology

A photography of the Chao lava dome and flows
The lobate flows of the Cerro Chao lava dome

The APVC is generated by the subduction of the Nazca Plate beneath the South American Plate at an angle of nearly 30°. Delamination of the crust has occurred beneath the northern Puna and southern Altiplano. Below 20 kilometres (12 mi) depth, seismic data indicate the presence of melts in a layer called the Altiplano–Puna low velocity zone or Altiplano Puna magma body. Regional variations of activity north and south of 24°S have been attributed to the southwards moving subduction of the Juan Fernández Ridge. This southwards migration results in a steepening of the subducting plate behind the ridge, causing decompression melting.[6] Between 1:4 to 1:6 of the generated melts are erupted to the surface as ignimbrites.[6]

Mafic rocks are associated with strike-slip faults and normal faults and are found in the southern Puna and Altiplano. The southern Puna has calc-alkaline andesites erupted after 7 mya, with the least evolved magmas being the 6.7 mya Cerro Morado and 8-7 m Rachaite complex flows. Basaltic over shoshonitic (both 25 and 21 m) to andesitic (post-Miocene) lavas are found in the southern Altiplano.[6]

Ignimbrites deposited during eruptions of APVC volcanoes are formed by "boiling over" eruptions, where magma chambers containing viscous crystal-rich volatile-poor magmas partially empty in tranquil, non-explosive fashion. As a result, the deposits are massive and homogeneous and show few size segregation or fluidization features. Such eruptions have been argued to require external triggers to occur.[6] There is a volume-dependent relationship between homogeneity of the eruption products and their volume; large volume ignimbrites have uniform mineralogical and compositional heterogeneity. Small volume ignimbrites often show gradation in composition. This pattern has been observed in other volcanic centres such as the Fish Canyon Tuff in the United States and the Toba ignimbrites in Indonesia.[10]

Petrologically, ignimbrites are derived from dacitic–rhyodacitic magmas. Phenocrysts include biotite, Fe–Ti-oxides, plagioclase and quartz with minor apatite and titanite. Northern Puna ignimbrites also contain amphibole, and clinopyroxene and orthopyroxene occur in low-Si magmas, while higher Si magmas also contain sanidine. These magmas have temperatures of 700–850 °C (1,292–1,562 °F) and originate in depths of 4–8 kilometres (2.5–5.0 mi).[6] The ignimbrites are collectively referred to as San Bartolo and Silapeti Groups.[7]

Eruptions are affected by the local conditions, resulting in high altitude eruption columns that are sorted by westerly stratospheric winds. Coarse deposits are deposited close to the vents, while fine ash is carried to the Chaco and eastern cordillera. The highest volcanoes in the world are located here, including 6,887 metres (22,595 ft) high Ojos del Salado and 6,723 metres (22,057 ft) high Llullaillaco. Some volcanoes have undergone flank collapses covering as much as 200 square kilometres (77 sq mi).[8] Most calderas are associated with fault systems that may play a role in caldera formation.[11]

Scientific investigation

The area's calderas are poorly understood and some may yet be undiscovered. Some calderas were subject to comprehensive research.[12] Research in this area is physically and logistically difficult.[7] Neodym, lead and boron isotope analysis has been used to determine the origin of eruption products.[13][14]

The dry climate and high altitude of the Atacama Desert has protected the deposits of APVC volcanism from erosion,[7][13] but also reduces the exposure of buried layers and structures.[3]

Geologic history

The APVC area before the upper Miocene was largely formed from sedimentary layers of Ordovician to Miocene age and deformed during previous stages of Andean orogeny, with low volume volcanics.[12] Activity until the late Miocene was effusive with andesite as the major product.[4] After a volcanic pause related to flat-slab subduction, starting from 27 mya volcanism increased suddenly.[3]

Ignimbrites range in age from 25 mya to 1 mya.[5] In the late Miocene, more evolved andesite magmas were erupted and the crustal components increased. In the late Tertiary until the Quaternary, a sudden decrease of mafic volcanism coupled with a sudden appearance of rhyodacitic and dacitic ignimbrites occurred.[15] During this flare-up it erupted primarily dacites with subordinate amounts of rhyolites and andesites.[5] The area was uplifted during the flare-up and the crust thickened to 60–70 kilometres (37–43 mi).[12] This triggered the formation of evaporite basins containing halite, boron and sulfate[13] and may have generated the nitrate deposits of the Atacama Desert.[16] The sudden increase is explained by a sudden steepening of the subducting plate, similar to the Mid-Tertiary ignimbrite flare-up.[8] In the northern Puna, ignimbrite activity began 10 mya, with large-scale activity occurring 5 to 3.8 Ma in the arc front and 8.4 to 6.4 Ma in the back arc. In the southern Puna, backarc activity set in 14–12 Ma and the largest eruptions occurred after 4 Ma.[6] The start of ignimbritic activity is not contemporaneous in the entire APVC area; north of 21°S the Alto de Pica formation and Oxaya Formations formed 15–17 and 18–23 mya respectively, whereas south of 21°S large scale ignimbrite activity didn't begin until 10.6 mya.[7]

Activity waned after 2 mya,[17] and after 1 mya and during the Holocene, activity was mostly andesitic in nature with large ignimbrites absent. Activity with composition similar to ignimbrites was limited to the eruption of lava domes and flows, interpreted as escaping from a regional sill 1–4 kilometres (0.62–2.49 mi) high at 14–17 kilometres (8.7–10.6 mi) depth.[4][10]

The APVC is still active, with recent unrest and ground inflation detected by InSAR at Uturuncu volcano starting in 1996. Research indicates that this unrest results from the intrusion of dacitic magma at 17 kilometres (11 mi) or more depth and may be a prelude to caldera formation and large scale eruptive activity.[18] Other active centres include the El Tatio and Sol de Mañana geothermal fields and the fields within Cerro Guacha and Pastos Grandes calderas. The latter also contains <10 ka rhyolitic flows and domes.[7] The implications of recent lava domes for future activity in the APVC are controversial.[19]

Extent

The APVC erupted over an area of 70,000 square kilometres (27,000 sq mi)[20] from ten major systems, some active over millions of years and comparable to Yellowstone Caldera and Long Valley Caldera in the United States.[4] The APVC is the largest ignimbrite province of the Neogene[17] with a volume of at least 15,000 cubic kilometres (3,600 cu mi),[20] and the underlying magmatic body is considered to be the largest continental melt zone,[17] forming a batholith.[7] Alternatively, the body revealed by seismic studies is the remnant mush of the magma accumulation zone.[9] Deposits from the volcanoes cover a surface area of more than 500,000 square kilometres (190,000 sq mi).[8] La Pacana is the largest single complex in the APVC with dimensions 100 by 70 square kilometres (39 sq mi × 27 sq mi), including the 65 by 35 kilometres (40 mi × 22 mi) caldera.[7]

Magma generation rates during the pulses are about 0.001 cubic kilometres per year (7.6×10−6 cu mi/Ms), based on the assumption that for each 50–100 cubic kilometres (12–24 cu mi) of arc there is one caldera. These rates are substantially higher than the average for the Central Volcanic Zone, 0.00015–0.0003 cubic kilometres per year (1.1×10−6–2.3×10−6 cu mi/Ms). During the three strong pulses, extrusion was even higher at 0.004–0.012 cubic kilometres per year (3.0×10−5–9.1×10−5 cu mi/Ms). Intrusion rates range from 0.003–0.005 cubic kilometres per year (2.3×10−5–3.8×10−5 cu mi/Ms) and resulted in plutons of 30,000–50,000 cubic kilometres (7,200–12,000 cu mi) volume beneath the calderas.[9]

Source of magmas

Modelling indicates a system where andesitic melts coming from the mantle rise through the crust and generate a zone of mafic volcanism. Increases in the melt flux and thus heat and volatile input causes partial melting of the crust, forming a layer containing melts reaching down to the Moho that inhibits the ascent of mafic magmas because of its higher buoyancy. Instead, melts generated in this zone eventually reach the surface, generating felsic volcanism. Some mafic magmas escape sideward after stalling in the melt containing zone; these generate more mafic volcanic systems at the edge of the felsic volcanism.[15] The magmas are mixtures of crust derived and mafic mantle-derived melts with a consistent petrological and chemical signature.[17]

Another model requires the intrusion of basaltic melts into an amphibole crust, resulting in the formation of hybrid magmas. Partial melting of the crust and of hydrous basalt generates andesitic–dacitic melts that escape upwards. A residual forms composed from garnet pyroxenite at a depth of 50 kilometres (31 mi). This residual is denser than the mantle peridotite and can cause delamination of the lower crust containing the residual.[6]

Between 18 and 12 mya the Puna-Altiplano region was subject to an episode of flat subduction of the Nazca Plate. A steepening of the subduction after 12 mya resulted in the influx of hot asthenosphere.[21] Until that point, differentiation and crystallization of rising mafic magmas had mostly produced andesitic magmas. The change in plate movements and increased melt generation caused an overturn and anatexis of the melt generating zone, forming a density barrier for mafic melts which subsequently ponded below the melt generating zone. Dacitic melts escaped from this zone, forming diapirs and the magma chambers that generated APVC ignimbrite volcanism.[7]

Magma generation in the APVC is periodical, with pulses recognized 10, 8, 6, and 4 mya. The first stage included the Artola, Granada, Lower Rio San Pedro and Mucar ignimbrites. The second pulse involved the Panizos, Sifon and Vilama ignimbrites and the third was the largest, with a number of ignimbrites. The fourth pulse was weaker than the preceding ones and involved the Patao and Talabre ignimbrites among others.[9]

Tomographic studies

Seismic tomography is a technique that uses seismic waves produced by earthquakes to gather information on the composition of the crust and mantle below a volcanic system. Different layers and structures in the Earth have different propagation speeds of seismic waves and attenuate them differently, resulting in different arrival times and strengths of waves travelling in a certain direction. From various measurements 3D models of the geological structures can be inferred. Results of such research indicate that a highly hydrated slab derived from the Nazca Plate – a major source of melts in a collisional volcanism system – underlies the Western Cordillera. Below the Altiplano, low-velocity zones indicate the presence of large amounts of partial melts that correlate with volcanic zones south of 21° S, whereas north of 21° S thicker lithospheric layers may prevent the formation of melts. Next to the Eastern Cordillera, low-velocity zones extend farther north to 18.5° S.[22] A thermally weakened zone, evidenced by strong attenuation, in the crust is associated with the APVC. This indicates the presence of melts in the crust.[23] A layer of low velocity (shear speed of 1 kilometre per second (0.62 mi/s)) 17–19 kilometres (11–12 mi) thick is assumed to host the APVC magma body.[9] Other seismological data indicate a partial delamination of the crust under the Puna, resulting in increased volcanic activity and terrain height.[24]

Subsystems

Ignimbrites

References

  1. ↑ Schnurr, W. B. W.; Trumbull, R. B.; Clavero, J.; Hahne, K.; Siebel, W.; Gardeweg, M. (2007). "Twenty-five million years of silicic volcanism in the southern central volcanic zone of the Andes: Geochemistry and magma genesis of ignimbrites from 25 to 27 °S, 67 to 72 °W". Journal of Volcanology and Geothermal Research 166 (1): 17–46. doi:10.1016/j.jvolgeores.2007.06.005.
  2. ↑ Ramelow, Juliane; Riller, Ulrich; Romer, Rolf L.; Oncken, Onno (2005). "Kinematic link between episodic trapdoor collapse of the Negra Muerta Caldera and motion on the Olacapato-El Toro Fault Zone, southern central Andes". International Journal of Earth Sciences 95 (3): 529–541. doi:10.1007/s00531-005-0042-x.
  3. 1 2 3 4 5 6 Salisbury, M. J.; Jicha, B. R.; de Silva, S. L.; Singer, B. S.; Jimenez, N. C.; Ort, M. H. (2010). "40Ar/39Ar chronostratigraphy of Altiplano-Puna volcanic complex ignimbrites reveals the development of a major magmatic province". Geological Society of America Bulletin 123 (5–6): 821–840. doi:10.1130/B30280.1.
  4. 1 2 3 4 5 6 Fernandez-Turiel, J. L.; Garcia-Valles, M.; Gimeno-Torrente, D.; Saavedra-Alonso, J.; Martinez-Manent, S. (2005). "The hot spring and geyser sinters of El Tatio, Northern Chile". Sedimentary Geology 180 (3–4): 125–147. doi:10.1016/j.sedgeo.2005.07.005.
  5. 1 2 3 Ort, Michael H. (1993). "Eruptive processes and caldera formation in a nested downsagcollapse caldera: Cerro Panizos, central Andes Mountains". Journal of Volcanology and Geothermal Research 56 (3): 221–252. doi:10.1016/0377-0273(93)90018-M.
  6. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 Kay, Suzanne Mahlburg; Coira, Beatriz L.; Caffe, Pablo J.; Chen, Chang-Hwa (2010). "Regional chemical diversity, crustal and mantle sources and evolution of central Andean Puna plateau ignimbrites". Journal of Volcanology and Geothermal Research 198 (1–2): 81–111. doi:10.1016/j.jvolgeores.2010.08.013.
  7. 1 2 3 4 5 6 7 8 9 10 de Silva, S. L. (1989). "Altiplano-Puna volcanic complex of the central Andes". Geology 17 (12): 1102. doi:10.1130/0091-7613(1989)017<1102:APVCOT>2.3.CO;2.
  8. 1 2 3 4 Allmendinger, Richard W.; Jordan, Teresa E.; Kay, Suzanne M.; Isacks, Bryan L. (1997). "The Evolution of the Altiplano-Puna Plateau of the Central Andes". Annual Review of Earth and Planetary Sciences 25 (1): 139–174. Bibcode:1997AREPS..25..139A. doi:10.1146/annurev.earth.25.1.139.
  9. 1 2 3 4 5 de Silva, Shanaka L.; Gosnold, William D. (2007). "Episodic construction of batholiths: Insights from the spatiotemporal development of an ignimbrite flare-up". Journal of Volcanology and Geothermal Research 167 (1–4): 320–335. doi:10.1016/j.jvolgeores.2007.07.015.
  10. 1 2 3 de Silva, S. L. (1991). "Styles of zoning in central Andean ignimbrites; Insights into magma chamber processes". Andean Magmatism and Its Tectonic Setting. Geological Society of America Special Papers 265. pp. 217–232. doi:10.1130/SPE265-p217. ISBN 0-8137-2265-9.
  11. ↑ Riller, Ulrich; Petrinovic, Ivan; Ramelow, Juliane; Strecker, Manfred; Oncken, Onno (2001). "Late Cenozoic tectonism, collapse caldera and plateau formation in the central Andes". Earth and Planetary Science Letters 188 (3–4): 299–311. doi:10.1016/S0012-821X(01)00333-8.
  12. 1 2 3 4 Caffe, P. J.; Soler, M. M.; Coira, B. L.; Onoe, A. T.; Cordani, U. G. (2008). "The Granada ignimbrite: A compound pyroclastic unit and its relationship with Upper Miocene caldera volcanism in the northern Puna". Journal of South American Earth Sciences 25 (4): 464–484. doi:10.1016/j.jsames.2007.10.004.
  13. 1 2 3 Schmitt, Axel K.; Kasemann, Simone; Meixner, Anette; Rhede, Dieter (2002). "Boron in central Andean ignimbrites: implications for crustal boron cycles in an active continental margin". Chemical Geology 183 (1–4): 333–347. doi:10.1016/S0009-2541(01)00382-5.
  14. ↑ Mamani, Mirian; Tassara, Andrés; Wörner, Gerhard (2008). "Composition and structural control of crustal domains in the central Andes". Geochemistry, Geophysics, Geosystems 9 (3): n/a–n/a. doi:10.1029/2007GC001925.
  15. 1 2 Laube, Norbert; Springer, Jörn (1998). "Crustal melting by ponding of mafic magmas: A numerical model". Journal of Volcanology and Geothermal Research 81 (1–2): 19–35. doi:10.1016/S0377-0273(97)00072-3.
  16. ↑ Oyarzun, Jorge; Oyarzun, Roberto (2007). "Massive Volcanism in the Altiplano-Puna Volcanic Plateau and Formation of the Huge Atacama Desert Nitrate Deposits: A Case for Thermal and Electric Fixation of Atmospheric Nitrogen". International Geology Review 49 (10): 962–968. doi:10.2747/0020-6814.49.10.962.
  17. 1 2 3 4 del Potro, Rodrigo; 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.
  18. 1 2 Sparks, R. S. J.; Folkes, C. B.; Humphreys, M. C. S.; Barfod, D. N.; Clavero, J.; Sunagua, M. C.; McNutt, S. R.; Pritchard, M. E. (2008). "Uturuncu volcano, Bolivia: Volcanic unrest due to mid-crustal magma intrusion". American Journal of Science 308 (6): 727–769. doi:10.2475/06.2008.01.
  19. 1 2 3 4 5 6 de Silva, S. L.; Self, S.; Francis, P. W.; Drake, R. E.; Carlos, Ramirez R. (1994). "Effusive silicic volcanism in the Central Andes: The Chao dacite and other young lavas of the Altiplano-Puna Volcanic Complex". Journal of Geophysical Research 99 (B9): 17805. doi:10.1029/94JB00652.
  20. 1 2 3 Hickey, James; Gottsmann, Joachim; del Potro, Rodrigo (2013). "The large-scale surface uplift in the Altiplano-Puna region of Bolivia: A parametric study of source characteristics and crustal rheology using finite element analysis". Geochemistry, Geophysics, Geosystems 14 (3): 540–555. doi:10.1002/ggge.20057.
  21. ↑ Ramos, V. A.; Folguera, A. (2009). "Andean flat-slab subduction through time". Geological Society, London, Special Publications 327 (1): 31–54. doi:10.1144/SP327.3.
  22. ↑ Myers, Stephen C.; Beck, Susan; Zandt, George; Wallace, Terry (1998). "Lithospheric-scale structure across the Bolivian Andes from tomographic images of velocity and attenuation forPandSwaves". Journal of Geophysical Research 103 (B9): 21233. doi:10.1029/98JB00956.
  23. ↑ Haberland, Christian; Rietbrock, Andreas (2001). "Attenuation tomography in the western central Andes: A detailed insight into the structure of a magmatic arc". Journal of Geophysical Research 106 (B6): 11151. doi:10.1029/2000JB900472.
  24. ↑ Schurr, B.; Rietbrock, A.; Asch, G.; Kind, R.; Oncken, O. (2006). "Evidence for lithospheric detachment in the central Andes from local earthquake tomography". Tectonophysics 415 (1–4): 203–223. doi:10.1016/j.tecto.2005.12.007.
  25. ↑ Petrinovic, I. A.; Martí, J.; Aguirre-Díaz, G. J.; Guzmán, S.; Geyer, A.; Paz, N. Salado (2010). "The Cerro Aguas Calientes caldera, NW Argentina: An example of a tectonically controlled, polygenetic collapse caldera, and its regional significance". Journal of Volcanology and Geothermal Research 194 (1–3): 15–26. doi:10.1016/j.jvolgeores.2010.04.012.
  26. ↑ Cabrera, A.P.; Caffe, P.J. (2009). "The Cerro Morado Andesites: Volcanic history and eruptive styles of a mafic volcanic field from northern Puna, Argentina". Journal of South American Earth Sciences 28 (2): 113–131. doi:10.1016/j.jsames.2009.03.007.
  27. 1 2 3 4 5 6 Matteini, M.; Mazzuoli, R.; Omarini, R.; Cas, R.; Maas, R. (2002). "The geochemical variations of the upper cenozoic volcanism along the Calama–Olacapato–El Toro transversal fault system in central Andes (~24°S): petrogenetic and geodynamic implications". Tectonophysics 345 (1–4): 211–227. doi:10.1016/S0040-1951(01)00214-1.
  28. 1 2 Schmitt, Axel K; Lindsay, Jan M; de Silva, Shan; Trumbull, Robert B (2003). "U–Pb zircon chronostratigraphy of early-Pliocene ignimbrites from La Pacana, north Chile: implications for the formation of stratified magma chambers". Journal of Volcanology and Geothermal Research 120 (1–2): 43–53. doi:10.1016/S0377-0273(02)00359-1.

Bibliography

External links

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