Altiplano–Puna volcanic complex


The Altiplano–Puna volcanic complex, 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 caused by subduction 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.
In the Miocene–Pliocene, calderas erupted felsic ignimbrites in four distinct pulses separated by periods of low levels of activity. At least three volcanic centres had eruptions of a Volcanic Exposivity Index 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 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. Since the late Miocene between 21° S and 24° S a major ignimbrite province formed over 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. 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.

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. Between 1:4 to 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 m Rachaite complex flows. Basaltic over shoshonitic to andesitic 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. 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, 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 and originate in depths of. The ignimbrites are collectively referred to as San Bartolo and Silapeti Groups.
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 high Ojos del Salado and high 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. Neodym, 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.

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 didn't begin until 10.6 mya.
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.

Tomographic studies

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. 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. A layer of low velocity thick is assumed to host the APVC magma body. This body has a volume of about. Other seismological data indicate a partial delamination of the crust under the Puna, resulting in increased volcanic activity and terrain height.

Subsystems