La Pacana

La Pacana lies in the Antofagasta Region of Chile, in the Andes[1] just north of the Tropic of Capricorn[2] and close to the Paso de Jama between Chile and Argentina.[3] The border between Chile and Bolivia crosses the northern sector of the caldera.[4] The area of La Pacana is largely uninhabitated;[1] small settlements such as Socaire, Talabre and Toconao[5] exist close to the Salar de Atacama, where streams descend the mountain slopes to the salar.[1] The caldera was discovered during mapping efforts in the region between 1980 and 1985.[6]

La Pacana is part of the Central Volcanic Zone,[2] 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.[7] A number of stratovolcanoes and ignimbrite-forming centres have erupted in the Central Volcanic Zone since the Miocene,[8] about 50 of which are considered to be active.[9] 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.[7]

La Pacana has a diameter of 60 by 35 kilometres (37 mi × 22 mi) with a north–south elongation.[10] This is one of the best exposed and largest calderas in the world;[11] the largest caldera known is Toba in Sumatra with a maximum length of 100 kilometres (62 mi).[12] La Pacana might not be a single caldera; some reconstructions imply that the northern parts of the caldera are actually a separate collapse structure.[13] 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.[10] After the formation of the caldera, sediments and[14] tuffs within the caldera were uplifted[15] 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.[16] This resurgent dome is cut by numerous faults and features a poorly developed graben on its summit.[14] Originally it was believed that the present-day calder rim did not coincide with the caldera ring fault,[10] 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.[17] 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,[18] 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.[19] The moat is filled by sediments formed by erosion and by[20] alluvial, evaporite and lacustrine sediments left behind by lakes.[16]

The Guayaques volcanic group

The collapse of the caldera cut through older volcanic centres, exposing the Ceja Alta and Quilapana porphyry deposits. Other older volcanic centres exposed in the walls of the caldera are the Cerro Aguas Calientes stratovolcano in the eastern wall and the Cerro Gigantes in the western wall. [21] Volcanic activity resumed within the caldera and at the edge of the resurgent dome, forming lava domes between 4.1 and at least 1.6 million years ago.[15] These volcanic centres include the Corral de Coquena crater and the lava domes of Morro Negro east, Cerro Bola and Purifican west and Cerros de Guayaques north of the resurgent dome. The Arenoso, Chamaca and Chivato Muerto lava domes in the southern wall of the caldera were originally considered to be pre-caldera;[21] later these three domes were identified as post-caldera domes.[22] Stratovolcanoes inside the caldera include the cones associated with the Cerros de Guayaques lava domes and the volcanoes Cerro Incaguasi, Cerros de Pili, Cerros Negros and Huailitas.[21]

Some extant hot springs within the caldera may indicate that there is still a geothermal system associated with La Pacana, although not a very important one considering their low temperature (less than 25 °C (77 °F)).[12] A few lakes such as the spring-fed Laguna de Chivato Muerto, Laguna Trinchera and Ojos del Rió Salado,[12] as well as salt pans such as Salar de Aguas Calientes Norte, Salar de Aguas Calientes Sur, Salar de Pujsa and Salar de Quisquiro have developed within the moat.[23] The waterbodies in the southern part of the caldera appear to be connected through groundwater, as they have similar water levels. The western caldera rim itself impedes the groundwater from draining out of the caldera.[24] Streams such as Río de Pili and Río Salado complete the hydrology of the caldera.[12]

Gravimetric observations have been conducted on La Pacana. A large negative anomaly (an anomaly with less-than-expected mass crust) coincides with the surface of the La Pacana caldera and extends past its borders; it may be a consequence of the caldera being infilled with low-density material. Positive anomalies (anomalies with more-than-expected mass in the crust) are found in the areas surrounding the caldera and dot discrete zones within it; the former represent the dense basement and the latter may be intrusions associated with individual vents.[13]

In the Peru-Chile Trench, the Nazca Plate subducts beneath the South American Plate at a rate of about 7–9 centimetres per year (2.8–3.5 in/year),[7] leading to volcanic activity at distances of 130–160 kilometres (81–99 mi) from the trench.[8]

Research indicates that subduction has been ongoing since the Jurassic 200 million years ago but accelerated 26 million years ago.[25] After a phase of andesitic volcanism lasting from the late Tertiary to the Miocene,[26] large scale ignimbritic volcanism commenced about 23 million years ago and is still ongoing.[27] It began north of 21° southern latitude with the 23-18 million years old Oxaya formation and the 15–17 million years old Altos de Pica formation. Later the San Bartolo and Silapeti groups were generated, ending by the early Pleistocene.[26] Volcanic activity at La Pacana is more recent than elsewhere in the region, with the oldest volcanic rocks that crop out in La Pacana being between 11 and 7.5 million years old.[8] Large scale ignimbritic activity continued until 2 million years ago.[13]

Weather records are available for the Salar de Aguas Calientes. There, an average temperature of 1 °C (34 °F) and average precipitation of 150 millimetres per year (5.9 in/year) have been recorded.[33]

There is little vegetation in the dry Altiplano. Nevertheless, a number of animal species are found, such as rheas, vicuñas and vizcachas. Ducks, geese and flamingos frequent waterbodies and salars.[1]

La Pacana has erupted two ignimbrites which differ in composition and were emplaced one shortly after the other: The dacitic Atana ignimbrite and the rhyolitic Toconao ignimbrite.[15] The Atana ignimbrite was once considered part of the Guaitiquina ignimbrite, which was later split off,[2] while the Puripicar ignimbrite may be correlated with the Atana instead.[14] Also, some of the ignimbrites erupted by La Pacana originally were attributed to Cerro Guacha.[11] Both ignimbrites originated from different parts of the same magma chamber and their origin in the La Pacana caldera is established by isotope ratios of the rocks and the geographical distribution of their outcrops.[15] Pyroclastic deposits in the Eastern Cordillera of Argentina may have their origin at La Pacana.[34]

Before the eruption of the Toconao and Atana ignimbrites, early activity generated the Pujsa ignimbrite[8] between 5.8 ± 0.1 and 5.7 ± 0.4 million years ago and some stratovolcanoes and porphyries that are cut by the caldera walls.[21] The Pujsa ignimbrite resembles the Atana ignimbrite and like the Toconao ignimbrite is mainly exposed on the western side of the caldera.[8]

The first large eruption, which took place between 4 ± 0.9 and 5.3 ± 1.1 million years ago, formed the Toconao ignimbrite.[15] The Toconao ignimbrite crops out mainly west of the caldera;[8] only later were units of the Toconao identified on the eastern side of La Pacana.[35] This ignimbrite has a volume of approximately 180 cubic kilometres (43 cu mi) and is formed by a lower un-indurated and an upper indurated subunit. Tube pumices are contained in the lower subunit and in a less than 10 centimetres (3.9 in) Plinian deposit that was emplaced beneath the Toconao ignimbrite.[15]

The formation of the caldera coincided with the eruption of the Atana ignimbrite; the eruption was still underway when the terrain subsided[10] to a depth of 2–3 kilometres (1.2–1.9 mi) beneath the previous surface in the northwestern segment of La Pacana.[13] Dates obtained on the Atana ignimbrite are between 3.8 ± 0.1 and 4.2 ± 0.1 million years ago, which is not clearly distinguishable from the dates of the Toconao ignimbrite seeing as there is no indication that a pause occurred between the eruption of the two ignimbrites. This ignimbrite is considerably larger than the Toconao ignimbrite,[15] reaching a volume of 2,500–3,500 cubic kilometres (600–840 cu mi)[36] and a volcanic explosivity index of 8. This makes the Atana eruption the fifth-largest explosive eruption known and La Pacana a supervolcano.[13] The Atana ignimbrite forms a flow sheet that extends from within the caldera to the outside in the form of a 30–40 metres (98–131 ft) thick structure.[15] This flow sheet originally probably covered a surface area of about 7,700 square kilometres (3,000 sq mi), part of which was later eroded away.[14] The Atana ignimbrite is fairly welded, rich in crystals and poor in lithics. It is underlaid by pumice and ash deposits.[15] Pumice is also found as fragments within the ignimbrite, ranging from white rhyolite to gray andesite.[37] After its eruption, wind and water driven erosion occurred on the Atana ignimbrite, carving valleys and yardangs into it.[4]

Some differences exist between the facies of the ignimbrite inside and outside of the caldera, as well as between the western and eastern outcrops. Such differences concern the degree of welding of the ignimbrite, the occurrence or absence of devitrification and the jointing patterns.[38] In fact, a segment of the northern Atana ignimbrite was later considered to not be actually part of the Atana ignimbrite at all because of the different facies and petrology.[39] This separate ignimbrite was christened to be an upper and a lower Tara ignimbrite, possibly erupted by the Cerro Guacha caldera.[40] The Tara ignimbrite fills part of the La Pacana caldera.[41] The total volume of the La Pacana ignimbrites proper is estimated to be about 3,400–3,500 cubic kilometres (820–840 cu mi), on the basis of gravimetric information about the volume of the caldera and the infill ignimbrites.[13]

The most likely theory for the origin of both Atana and Toconao ignimbrites is that they formed by crystal fractionation within a magma chamber, where the Toconao magma was extracted from convecting dacitic magma that was undergoing crystallization. This volatile-rich and crystal-poor extracted magma erupted first as a Plinian eruption. Then a tectonic event, most likely a movement along a fault cutting through the caldera, prompted the rise and eruption of the Atana ignimbrite.[15] Two potential vents have been found at the northern and western margins of the caldera, where breccia deposits occur within the Atana ignimbrite.[42] Some of the magma that gave rise to the Atana ignimbrite was erupted after the ignimbrite; the lava domes formed after the caldera collapse were generated by this magma.[15] This category of dependent postcaldera volcanism includes Corral de Coquena and Morro Negro; other postcaldera volcanic centres have different compositions and thus probably formed from different sources than the Atana magma.[43]

Ignimbrite eruptions continued after the formation of the caldera. The Filo Delgado ignimbrite was erupted at some time during the Pliocene from the Huailitas volcano.[21] Its volume is about 0.1 cubic kilometres (0.024 cu mi).[18] 2.4 ± 0.4 million years ago, the Pampa Chamaca ignimbrite filled the moat between the resurgent dome and the caldera rim.[21] The Pampa Chamaca or Talabre[29] ignimbrite was erupted from a vent probably buried beneath the present-day Cordon de Puntas Negras[18] or the Salar de Aguas Calientes[44] and reached a volume of about 0.5 cubic kilometres (0.12 cu mi).[18] The 3.49 million years old Tara Ignimbrite from the Cerro Guacha caldera reached the northern margin of La Pacana.[45]