La Pacana is a Miocene age caldera in northern Chile's Antofagasta Region. Part of the Central Volcanic Zone of the Andes, it is part of the Altiplano-Puna volcanic complex, a major caldera and silicic ignimbrite volcanic field. This volcanic field is located in remote regions at the Zapaleri tripoint between Chile, Bolivia and Argentina. La Pacana along with other regional volcanoes was formed by the subduction of the Nazca Plate beneath the South American Plate in the Peru-Chile Trench. La Pacana is situated in a basement formed by various Paleozoic formations and Tertiary ignimbrites and volcanoes. Several major faults cross the region at La Pacana and have influenced its volcanic activity. La Pacana is a supervolcano and is responsible for the eruption of the giant Atana ignimbrite, which reaches a volume of 2,500–3,500 cubic kilometres (600–840 cu mi) and constitutes the fifth-largest explosive eruption known. The Atana ignimbrite was erupted 3.8 ± 0.1 and 4.2 ± 0.1 million years ago, almost simultaneously with the much smaller (volume of 180 cubic kilometres (43 cu mi)) Toconao ignimbrite. The Pujsa ignimbrite was erupted by La Pacana before the Atana/Toconao ignimbrites, and the Filo Delgado and Pampa Chamaca/Talabre ignimbrites afterwards.
Geography and structure La Pacana lies in the Antofagasta Region of Chile, in the Andes just north of the Tropic of Capricorn and close to the Paso de Jama between Chile and Argentina. The border between Chile and Bolivia crosses the northern sector of the caldera. The area of La Pacana is largely uninhabitated; small settlements such as Socaire, Talabre and Toconao exist close to the Salar de Atacama, where streams descend the mountain slopes to the salar. The caldera was discovered during mapping efforts in the region between 1980 and 1985. La Pacana is part of the Central Volcanic Zone, one of the four volcanic zones that make up the Andean Volcanic Belt and which are separated from each other by gaps without ongoing volcanic activity. A number of stratovolcanoes and ignimbrite-forming centres have erupted in the Central Volcanic Zone since the Miocene, about 50 of which are considered to be active. In addition, the Central Volcanic Zone features about 18 minor volcanic fields. The largest historical eruption of the Andes occurred in 1600 at Huaynaputina in Peru in the Central Volcanic Zone, and the most active volcano of the Central Volcanic Zone is Láscar in Chile. La Pacana has a diameter of 60 by 35 kilometres (37 mi × 22 mi) with a north–south elongation. This is one of the best exposed and largest calderas in the world; the largest caldera known is Toba in Sumatra with a maximum length of 100 kilometres (62 mi). La Pacana might not be a single caldera; some reconstructions imply that the northern parts of the caldera are actually a separate collapse structure. The floor of the caldera lies at an elevation of 4,200–4,500 metres (13,800–14,800 ft), the central uplift and the caldera rim are higher and reach 5,200 metres (17,100 ft). The caldera rim is well exposed except in the northern and western sides, where later volcanism has buried it. After the formation of the caldera, sediments and tuffs within the caldera were uplifted over an angular area of 350 square kilometres (140 sq mi), forming the 1 kilometre (0.62 mi) high resurgent dome known as Cordón La Pacana. This resurgent dome is cut by numerous faults and features a poorly developed graben on its summit. Originally it was believed that the present-day calder rim did not coincide with the caldera ring fault, which was instead identified to coincide with margins of the resurgent dome; later research however indicates the present-day topographic margin as the caldera edge. The resurgent dome is separated from the caldera rim by a 2–10 kilometres (1.2–6.2 mi) wide moat that makes up about two thirds of the entire surface of the caldera, but is interrupted on the northern side of the caldera by the "hinge" of the caldera collapse, which assumed the form of a trap-door. The moat is filled by sediments formed by erosion and by alluvial, evaporite and lacustrine sediments left behind by lakes.
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