| Pacana Caldera | |
|---|---|
 Viewing from inside the caldera towards the western margin | |
| Highest point | |
| Elevation | 4,500 m (14,800 ft) |
| Listing | List of volcanoes in Chile |
| Coordinates | 23°13′11″S67°27′58″W / 23.21972°S 67.46611°W /-23.21972; -67.46611[1] |
| Geography | |
  Pacana Caldera NorthernChile | |
| Geology | |
| Mountain type | Caldera |
| Volcanic zone | Altiplano–Puna volcanic complex |
| Last eruption | 2 mya |
La Pacana is aMiocene agecaldera in northernChile'sAntofagasta Region. Part of theCentral Volcanic Zone of the Andes, it is part of theAltiplano-Puna volcanic complex, a major caldera andsilicicignimbritevolcanic field. This volcanic field is located in remote regions at theZapaleritripoint between Chile,Bolivia andArgentina.
La Pacana along with other regional volcanoes was formed by thesubduction of theNazca Plate beneath theSouth American Plate in thePeru-Chile Trench. La Pacana is situated in abasement formed by variousPaleozoicformations andTertiary ignimbrites and volcanoes. Several majorfaults cross the region at La Pacana and have influenced its volcanic activity.
La Pacana is asupervolcano 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 thefifth-largestexplosive 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.
La Pacana lies in theAntofagasta Region ofChile, in theAndes[1] just north of theTropic of Capricorn[2] and close to thePaso de Jama between Chile and Argentina.[3] The border between Chile andBolivia crosses the northern sector of the caldera.[4] The area of La Pacana is largely uninhabitated;[1] small settlements such asSocaire,Talabre andToconao[5] exist close to theSalar de Atacama, where streams descend the mountain slopes to thesalar.[1] The caldera was discovered during mapping efforts in the region between 1980 and 1985.[6]
La Pacana is part of theCentral Volcanic Zone,[2] one of the four volcanic zones that make up theAndean Volcanic Belt and which are separated from each other by gaps without ongoing volcanic activity.[7] A number ofstratovolcanoes andignimbrite-forming centres have erupted in the Central Volcanic Zone since theMiocene,[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 atHuaynaputina inPeru in the Central Volcanic Zone, and the most active volcano of the Central Volcanic Zone isLáscar 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 isToba inSumatra 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) highresurgent dome known as Cordón La Pacana.[16] This resurgent dome is cut by numerousfaults and features a poorly developedgraben 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 bysediments formed by erosion and by[20]alluvial,evaporite andlacustrine sediments left behind by lakes.[16]
The collapse of the caldera cut through older volcanic centres, exposing the Ceja Alta and Quilapanaporphyry 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, forminglava domes between 4.1 and at least 1.6 million years ago.[15] These volcanic centres include theCorral de Coquenacrater and thelava domes of Morro Negro east, Cerro Bola and Purifican west andCerros de Guayaques north of the resurgent dome. The Arenoso, Chamaca and Chivato Muertolava 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 extanthot springs within the caldera may indicate that there is still ageothermal 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 assalt pans such asSalar de Aguas Calientes Norte,Salar de Aguas Calientes Sur,Salar de Pujsa andSalar de Quisquiro have developed within the moat.[23] The waterbodies in the southern part of the caldera appear to be connected throughgroundwater, 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 beintrusions associated with individualvents.[13]
In thePeru-Chile Trench, theNazca Plate subducts beneath theSouth 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 thatsubduction has been ongoing since theJurassic 200 million years ago but accelerated 26 million years ago.[25] After a phase ofandesitic volcanism lasting from the lateTertiary to theMiocene,[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 oldOxaya formation and the 15–17 million years old Altos de Pica formation. Later the San Bartolo and Silapetigroups were generated, ending by the earlyPleistocene.[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]
The Central Andes are the site of extensiveignimbrites that were erupted from largecalderas usually located within the adjacentAltiplano, east of the principalvolcanic arc. Many of these calderas are part of theAltiplano-Puna volcanic complex, a large volcanic complex covering a surface area of 70,000 square kilometres (27,000 sq mi) with about 30,000 cubic kilometres (7,200 cu mi) of ignimbrites. La Pacana is the largest caldera of the Altiplano-Puna volcanic complex.[11][15] The ignimbrites form a surface that lies at an average elevation of 4,000 metres (13,000 ft).[25]Stratovolcanoes developed on top of these ignimbrite sheets and today form the most clear expression of volcanic activity in the region,[9] with some of them exceeding the height of 6,000 metres (20,000 ft) above sea level.[25] The long-lasting dry climate means that traces of volcanic activity can be recognizable over long timeframes.[26]
The Altiplano-Puna volcanic complex is underpinned by a largeseismic velocity anomaly at a depth of 20 kilometres (12 mi), which may be the largest structure consisting of near-molten (10–20%) rock on Earth.[15] This partial melt zone was formed by the injection ofmafic magmas into the lower crust; a major episode of overturning before 10.6 million years ago caused crustalanatexis and started the onset of ignimbritic volcanism.[28] Magmas formed within this melt zone rose into the upper crust and differentiated between depths of 8–4 kilometres (5.0–2.5 mi) to form the ignimbrite-forming secondary magmas.[29] Presently, the inferred margin of this partially molten zone coincides fairly well with a negativegravimetric anomaly that clusters around thetripoint between Argentina, Bolivia and Chile and with the extent of the Altiplano-Puna volcanic complex.[30]
Thebasement beneath La Pacana is formed by sediments ofOrdovician age,Devonian-Permianquartzites, the mixed Saltaformation also of Permian age and sediments ofCretaceous-Tertiary age.[31] At the eastern margin of La Pacana in Argentina, they overlay an even olderPrecambrian basement.[8] Most of this original basement however is covered byMiocene ignimbrites from centres that may coincide with the La Pacana caldera.[31] Two of these older ignimbrites are known as the Pampa Múcar and Antigua Chacaliri ignimbrites.[32]
La Pacana together with theCerro Guacha andPurico Complex calderas forms the La Pacana Complex. Guacha experienced two major eruptions, of which one occurred 4.1 million years ago. The Purico complex began erupting 1.3 million years ago; it is the youngest centre of the La Pacana Complex with the youngest eruptions occurring during theHolocene.[28] Additional volcanic centres west and southwest of La Pacana areAcamarachi,Láscar,Colachi andCordón de Puntas Negras.[5]
A number offaults transect the region at La Pacana, including the north-south Miscanti Lineament and the Socompa and Quisiquiro lineaments. These lineaments or faults have influenced volcanism and geomorphology in the region, with volcanoes and vents aligning along these lineaments.[8]
The Toconao and Atana ignimbrites are formed byrhyolite anddacite-rhyodacite, respectively. They form apotassium-richcalc-alkaline suite. Both containpumices, three different types of which are found in the Atana ignimbrite.Phenocrysts within the ignimbrite are chiefly formed byplagioclase.[15]
Both the Atana and the Toconao ignimbrite include minerals likeallanite,apatite,biotite,epidote,hornblende,ilmenite,magnetite,monazite,orthopyroxene,plagioclase,quartz,sanidine,titanite andzircon. Not all of these minerals are found in both ignimbrites, and not always in the same phase (crystals or matrix).[15]
Ultimately, the magmas at La Pacana are the products ofmantle melts interacting with various crustal domains deep in thecrust, within the partially molten zone that has been found at depths ofc. 20 kilometres (12 mi) beneath the Altiplano-Puna volcanic complex.[15]
Various geothermometers indicate that the Toconao ignimbrite was colder than the Atana ignimbrite; temperatures have been estimated at 730–750 °C (1,350–1,380 °F) and 750–790 °C (1,380–1,450 °F) respectively. While the depth at which the Toconao ignimbrite formed is unknown, the Atana ignimbrite formed at a depth of 7–8.5 kilometres (4.3–5.3 mi). Such a formation depth is comparable to depths estimated for other magmatic systems such asFish Canyon,Long Valley andYellowstone.[15]
Weather records are available for theSalar 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 dryAltiplano. Nevertheless, a number of animal species are found, such asrheas,vicuñas andvizcachas.Ducks,geese andflamingos frequent waterbodies andsalars.[1]
La Pacana has erupted two ignimbrites which differ in composition and were emplaced one shortly after the other: Thedacitic Atana ignimbrite and therhyolitic 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 toCerro Guacha.[11] Both ignimbrites originated from different parts of the samemagma chamber and their origin in the La Pacana caldera is established byisotope ratios of the rocks and the geographical distribution of their outcrops.[15]Pyroclastic deposits in theEastern 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 andporphyries 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. Tubepumices 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 avolcanic explosivity index of 8. This makes the Atana eruption the fifth-largestexplosive eruption known and La Pacana asupervolcano.[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 inlithics. 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 andyardangs 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 ofdevitrification 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 ofgravimetric 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 bycrystal 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 aPlinian eruption. Then a tectonic event, most likely a movement along afault 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, wherebreccia 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 thePliocene 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-dayCordon de Puntas Negras[18] or theSalar 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 theCerro Guacha caldera reached the northern margin of La Pacana.[45]
