The Altiplano–Puna volcanic complex (), also known as APVC, is a complex of volcanic systems at 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 caused by subduction have generated the volcanoes of the Andean Volcanic Belt including the APVC. The volcanic province is located between latitude. The APVC spans the countries of Argentina, Bolivia, and Chile.
In the Miocene–Pliocene (10–1 Mya), calderas erupted felsic ignimbrites in four distinct pulses separated by periods of low levels of activity. At least three volcanic centres (Guacha caldera, La Pacana, Pastos Grandes, Vilama) had eruptions of a Volcanic Explosivity Index (VEI) of 8, as well as smaller-scale eruptive centres. Activity waned after 2 Mya, but present-day geothermal activity and volcanoes dated to the Holocene, as well as recent ground deformation at Uturuncu 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 crust, the Altiplano–Puna volcanic complex, between the Atacama and the Altiplano. The Toba volcanic system in Indonesia and Taupō in New Zealand are analogous to the province. The APVC is located in the southern Altiplano–Puna plateau, a surface plateau wide and long at an altitude of , and lies east of the volcanic front of the Andes. Deformational belts limit it in the east. 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. The Puna has a higher average elevation than the Altiplano, and some individual volcanic centres reach altitudes of more than . The basement of the northern Puna is of Ordovician to Eocene age. The Pastos Grandes-Lipez-Coranzuli lineament forms the northern boundary of the Altiplano–Puna volcanic complex.
Geology
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 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 and causing the area of active volcanism to move southward in the APVC. Between 1/4 and 1/6 of the generated melts are erupted to the surface as ignimbrites.
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-metre Rachaite complex flows. Basaltic over shoshonitic (both 25 and 21 metres) to andesitic (post-Miocene) lavas are found in the southern Altiplano.
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. There is a volume-dependent relationship between homogeneity of the eruption products and their volume; large-volume ignimbrites have uniform mineralogical and compositional heterogeneity, while 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.
Petrologically, ignimbrites are derived from dacitic–rhyodacitic magmas. Phenocrysts include biotite, iron–titanium oxides, plagioclase, and quartz with minor apatite and titanite. Northern Puna ignimbrites also contain amphibole, and clinopyroxene and orthopyroxene occur in low-silicon magmas, while higher-silicon magmas also contain sanidine. These magmas have temperatures of and originate in depths of . The ignimbrites are collectively referred to as San Bartolo and Silapeti Groups.
Since the Miocene, less silicic magmas containing olivine, plagioclase, and clinopyroxene have been erupted by the APVC as well. These "mafic" magmas form various monogenetic volcanoes, inclusions in more silicic magmas and lava flows which sometimes occur in isolation and sometimes are linked to stratovolcanoes.
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 the Ojos del Salado and Llullaillaco. Some volcanoes have undergone flank collapses covering as much as . Most calderas are associated with fault systems that may play a role in caldera formation.
Scientific investigation The area's calderas are poorly understood and some may yet be undiscovered. Some calderas were subject to comprehensive research. Research in this area is physically and logistically difficult. Neodymium, lead, and boron isotope analysis has been used to determine the origin of eruption products.
The dry climate and high altitude of the Atacama Desert has protected the deposits of APVC volcanism from erosion, but limited erosion also reduces the exposure of buried layers and structures. Evidence of volcanic activity and cyclic variation has been obtained from remote fallout deposits as well.
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. Activity until the late Miocene was effusive with andesite as the major product. After a volcanic pause related to flat-slab subduction, starting from 27 Mya volcanism increased suddenly.
Ignimbrites range in age from 25 Mya to 1 Mya. 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. During this flare-up, it erupted primarily dacites with subordinate amounts of rhyolites and andesites. The area was uplifted during the flare-up, and the crust thickened to . This triggered the formation of evaporite basins containing halite, boron, and sulfate and may have generated the nitrate deposits of the Atacama Desert. The sudden increase is explained by a sudden steepening of the subducting plate, similar to the Mid-Tertiary ignimbrite flare-up. 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. The start of ignimbritic activity is not contemporaneous in the entire APVC area; north of 21°S the Alto de Pica and Oxaya Formations formed 15–17 and 18–23 Mya respectively, whereas south of 21°S large scale ignimbrite activity did not begin until 10.6 Mya, and the southernmost APVC segment has not undergone any widespread ignimbrite activity yet.
Activity waned after 2 Mya, 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 high at depth.
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 or more depth and may be a prelude to caldera formation and large-scale eruptive activity. 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. The implications of recent lava domes for future activity in the APVC are controversial, but the presence of mafic components in recently erupted volcanic rocks may indicate that the magma system is being recharged.
Extent
The APVC erupted over an area of from ten major systems, some active over millions of years and comparable to Yellowstone Caldera and Long Valley Caldera in the United States. The APVC is the largest ignimbrite province of the Neogene with a volume of at least , and the underlying magmatic body is considered to be the largest continental melt zone, forming a batholith. Alternatively, the body revealed by seismic studies is the remnant mush of the magma accumulation zone. Deposits from the volcanoes cover a surface area of more than . La Pacana is the largest single complex in the APVC with dimensions , including the caldera.
Magma generation rates during the pulses are about , based on the assumption that for each of arc there is one caldera. These rates are substantially higher than the average for the Central Volcanic Zone, . During the three strong pulses, extrusion was even higher at . Intrusion rates range from and resulted in plutons of volume beneath the calderas.
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, such as Cerro Bitiche. The magmas are mixtures of crust derived and mafic mantle-derived melts with a consistent petrological and chemical signature. The melt generation process may involve several different layers in the crust.
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 . This residual is denser than the mantle peridotite and can cause delamination of the lower crust containing the residual.