Altiplano–Puna volcanic complex
Melts caused by subduction have generated the volcanoes of the Andean Volcanic Belt including the APVC.
[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.
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.
[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).
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.
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.
[15] This triggered the formation of evaporite basins containing halite, boron and sulfate[16] and may have generated the nitrate deposits of the Atacama Desert.
[4][11] 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.
These rates are substantially higher than the average for the Central Volcanic Zone, 0.00015–0.0003 cubic kilometres per year (0.0048–0.0095 m3/s).
[9] Modelling indicates a system where andesitic melts coming from the mantle rise through the crust and generate a zone of mafic volcanism.
[10] The magmas are mixtures of crust derived and mafic mantle-derived melts with a consistent petrological and chemical signature.
[27] Another model requires the intrusion of basaltic melts into an amphibole crust, resulting in the formation of hybrid magmas.
Dacitic melts escaped from this zone, forming diapirs and the magma chambers that generated APVC ignimbrite volcanism.
The first stage included the Artola, Granada, Lower Rio San Pedro and Mucar ignimbrites.
A volume ratio of about 10-20% of water has been invoked to explain the pattern of electrical conductivity at a depth of 15–30 kilometres (9.3–18.6 mi).
The total amount of water has been estimated to be c. 14,000,000,000,000,000 kilograms (3.1×1016 lb), comparable to large lakes on Earth.
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.
[12] Other seismological data indicate a partial delamination of the crust under the Puna, resulting in increased volcanic activity and terrain height.
