Pali-Aike volcanic field
Coordinates:  Pali-Aike volcanic field is a volcanic field in Argentina which straddles the border with Chile. It is part of a province of back-arc volcanoes in Patagonia, which formed from processes involving the collision of the Chile Rise with the Peru–Chile Trench. It lies farther east than the Austral Volcanic Zone, the volcanic arc which forms the Andean Volcanic Belt at this latitude.
Pali-Aike formed over a Jurassic basin starting from the late Miocene as a consequence of regional tectonic events and local extension. It consists of an older plateau basalt formation and younger volcanic centres in the form of pyroclastic cones, scoria cones, maars and associated lava flows. These vents often form local alignments along lineaments or faults. The volcanic field is noteworthy for the presence of large amounts of xenoliths in its rocks and because the maar Laguna Potrok Aike is located here. The field was active starting from 3.78 million years ago. The latest eruptions occurred during the Holocene, as indicated by the burial of archeological artifacts; Laguna Azul maar formed about 3,400 years before present.
Geology and structure
The Pali-Aike volcanic field spans the border between Argentina and Chile, northwest of the Magellanes Strait; most of the field lies in Argentina, the Chilean part is part of the commune of San Gregorio, Chile. The cities of Rio Gallegos and Punta Arenas lie northeast and southwest of Pali-Aike respectively. Rio Gallegos is about 80 kilometres (50 mi) away from the field, and the closest vent is only 23 kilometres (14 mi) away from the town.
Pali-Aike is part of the Patagonian back-arc, a province of plateau lavas of Cenozoic age. These plateau lavas are of alkaline to tholeiitic composition; hawaiite, trachyandesite and trachyte are present in smaller amounts. From south to north these plateau lavas include Pali-Aike itself, Meseta Vizcachas, Meseta de la Muerte, Gran Meseta Central, Meseta Buenos Aires, Cerro Pedrero, Meseta de Somuncura, Pino Hachado and Buta Ranquil. Their activity began 16 million years ago, when the Chile Rise collided with the Peru-Chile Trench and thus caused a tear in the subducting slab and the formation of a slab window beneath Patagonia; later it was suggested that slab rollback might instead be the mechanism by which volcanism is triggered in the Pali-Aike region. These plateau lavas become younger the farther northeast they are, following the movement of the triple junction to the north, with the exception of Pali-Aike whose activity commenced later probably due to local tectonic effects. However, some older plateau lavas in the north formed in response to an earlier ridge subduction event in the Eocene and Paleocene.
300 kilometres (190 mi) farther west from Pali-Aike lies the actual Andean volcanic arc in the form of the Austral Volcanic Zone, a chain of stratovolcanoes and one volcanic field; this volcanic field (Fueguino) is South America's southernmost volcano. The 2.9–2.5 million years old Camusu Aike volcanic field is 200 kilometres (120 mi) northwest and Morro Chico volcano about 50 kilometres (31 mi) west of Pali-Aike.
The Pali-Aike volcanic field covers a surface area of 4,500 square kilometres (1,700 sq mi), and extends over 150 kilometres (93 mi) from northwest to southeast. It is formed by a plateau of lava flows that is up to 120 metres (390 ft) thick in its northwestern reach and includes remnants of individual volcanic centres, with an average relief of 20–100 metres (66–328 ft). This plateau is formed by tables containing depressions and lakes, and whose margins are steep-dipping slopes that accumulate blocks at their feet. The volcanic rocks are emplaced atop Tertiary sediments, which are smoothened by glacial action. Some volcanic necks situated in the west-central part of the field may be the underground components of now-eroded volcanic edifices. Among these volcanic necks are the Cuadrado, Domeyko, Gay and Philippi hills, which conspicuously stick out of the surrounding plains.
Over 450 different monogenetic volcanoes are emplaced on the lava plateau at elevations of 110–180 metres (360–590 ft) above sea level and include maars, tuff rings and scoria cones. Some of these are heavily eroded while the southeastern part of the field features fresh-looking centres, where they form the "Basaltos del Diablo". These various centres rise between 20–160 metres (66–525 ft) above the surrounding terrain. Nested craters, breached craters and fissure vents are common among the various vents, as well as lava flows but there has been little research on the scoria cones. Pyroclastic cones in Pali-Aike include Aymond, Colorado, Dinero, Fell and Negro. The vent Cerro del Diablo, a pyroclastic cone, is the youngest volcano in the field and emitted both aa lava and pahoehoe lava, which have a fresh appearance and no soil cover. The vents are origins of lava flows, some of which are older and covered with soil while younger ones are not. Such young lava flows also have surface features including lava tunnels, hornitos, tumuli and a rugose surface. The individual volcanoes are subdivided into three groups which are referred to as "U2" (the older centres) and "U3" (for the more recent vents); the plateau lavas are hence called "U1".
Maars are depressions in the ground which are encircled by a ring of sediment that rises above the surrounding terrain; they typically form where frozen or liquid water interacts with rising magma. In Pali-Aike there are about 100 of them, with diameters ranging from 500 metres (1,600 ft) to about 4,000 metres (13,000 ft). The periglacial ground is rich in ice and water, which might explain why there are so many maars in Pali-Aike. Notable among these lakes is Laguna Azul, a crater lake which is located within a pyroclastic ring at the side of a scoria cone. This maar formed during three stages in three separate craters and is also the source of a lava flow. Laguna Potrok Aike in comparison is much larger (crater diameter of 5 kilometres (3.1 mi)), its rim is barely recognizable and appears to be more akin to a maar. Additional maars in the southwestern part of the field are the so-called "West Maar" and "East Maar", which contain the lakes Laguna Salsa and Laguna del Ruido respectively, Bismarck, Carlota, Los Flamencos, Laguna Salida/Laguna Ana and Timone Lake.
A number of vents form various alignments, usually along northwest-southeast and east-northeast-west-southwest lines; some older centres show a north-south pattern. Such alignments occur when local lineations act as a pathway for magma to ascend to the crust and control not only the position of the vents, but also the shape of the volcanoes forming on top of the vents. These lines match the strike of the Magallanes-Fagnano fault zone and the older Patagonian Austral Rift. Faults within the field have been active in the Tertiary, and a graben in the southwestern part of the field has diverted lava flows.
The Rio Gallegos and its tributary Rio Ci Aike cross the volcanic field from west to east and from southwest to northeast, respectively. The terrain of the field is highly permeable to water, which later forms wetlands that attract a number of birds and springs that are used as a source of water. Maars are not the only water bodies within the field; glacial lakes and lakes formed by wind deflation also exist. Some of these water bodies dry up late in summer, allowing wind to remove sediments from their lakebeds which thus become the origin of long dune fields. Active growth of such windstreaks has been observed in Pali-Aike. Windstreaks are an uncommon occurrence on Earth; they are much more common on Mars.
At the southern end of South America, the Antarctic Plate subducts beneath South America at a rate of 2 centimetres per year (0.79 in/year) in the Peru-Chile Trench. This subduction process has caused adakitic volcanism on the western margin of southernmost South America, forming the Austral Volcanic Zone.
Patagonia is a region where four tectonic plates, the Antarctic Plate, the Nazca Plate, the Scotia Plate and the South America Plate, interact. Starting 4 million years ago the Chile Rise collided with the Peru-Chile Trench. This collision originally occurred west of Tierra del Fuego, but has since moved northward towards the Taitao Peninsula. Farther south the interaction between the Scotia and South America plates gave rise to the Deseado and Magallanes-Fagnano faults. This fault system has since about 6–8 million years ago been subject to lateral spreading; most likely volcanism was possible at Pali-Aike only after this tectonic change.
The Pali-Aike volcanic field is mainly made up of basalt and basanite, which form a sodium-rich alkaline suite; hawaiite is rare. The most important phenocrystic phase is olivine which also appears as xenocrysts; other minerals include clinopyroxene, diopside and plagioclase. The groundmass has a similar composition with the addition of augite, feldspar and magnetite and occasionally ilmenite and nepheline. Pali-Aike rocks typically contain ultramafic xenoliths containing augite, dunite, eclogite, garnet, harzburgite, lherzolite, peridotite, phlogopite, pyroxenite, spinel and wehrlites. The composition of these xenoliths indicates that they originated in both the crust and the mantle. In addition, rocks from Pali-Aike contain inclusions of fluids consisting of carbon dioxide.
Elemental composition is typical for alkaline intraplate basalts and has a strongly primitive magma signature. The various isotope ratios are typical for so-called "cratonic" Patagonian back-arc basalts which are remote from the Andean Volcanic Belt and resemble ocean island basalts.
The geochemistry of Pali-Aike rocks has been interpreted as originating from the melting of mantle peridotite along with fractionation of olivine and with residual garnet; there is no trace of geochemical influence of the adjacent Andean Volcanic Belt and the associated subduction zone. The slab window generated by the Chile Rise's subduction passed at the latitudes of Pali-Aike about 4.5 million years ago; volcanic activity commenced soon afterwards but the time difference was enough for any subduction-influenced mantle to be displaced by fresher mantle moving through the window. An older oceanic lithosphere which was emplaced during the Proterozoic-Paleozoic in the area is also involved in magma genesis. The large amounts of xenoliths and primitiveness of the magmas suggest that once they had formed, they very quickly rose through the crust to the surface.
The basement beneath Pali-Aike contains the Magallanes basin of Jurassic age, which formed during the breakup of Gondwana and was later filled by volcanic and sedimentary rocks. The partly Neoproterozoic Deseado Massif lies north of Pali-Aike and may extend beneath the field to Tierra del Fuego, although there is no evidence that a Precambrian basement exists in the Pali-Aike area. During the Oligocene a marine transgression deposited the Patagonia Formation, and during the Miocene fluvial sediments formed the Santa Cruz Formation. Sedimentation ceased in the region 14 million years ago, probably because by that time the rain shadow of the Andes was effective in the area.
The Pali-Aike area was glaciated during the middle Pleistocene, and glaciers eroded lava flows from that time. In part on the basis of the dates of these lava flows, it was established that the older and larger glaciation (Bella Vista Glaciation) occurred between 1.17 and 1.02 million years ago. The last glaciation (Cabo Vírgenes, Río Ciaike and Telken VI-I) was less extensive but reached the Atlantic Ocean at times. This glaciation ended before 760,000 years ago; there is no evidence of last glacial maximum/Llanquihue glaciation glaciers in the area.
Climate and vegetation
The climate in the region is windy and cold, with mild winters owing to the oceanic influence, and dry, bordering on semi-desert with precipitation ranging 300–150 millimetres per year (11.8–5.9 in/year). This pattern is owing to the closeness of Antarctica and the cold Humboldt current and Falklands current ocean currents as well as the rain shadow of the Andes. Some maars and craters in Pali-Aike have been the site of paleoclimatological research in the form of sediment core analysis, such as Laguna Azul, Laguna Potrok Aike and Magallanes Maar.
The regional vegetation is a grassland vegetation which also features shrubs. Dominant species are Festuca gracillima in the drier east and Festuca pallescens in the wetter west. The highly permeable basalts intercept precipitation, forming active aquifers that feed into wetlands. The vegetation is not free of human influences, in the form of invasive European weeds and sheep farming.
Potassium-argon dating has been used to constrain volcanic activity to between 3.78 and 0.17 million years ago, spanning the late Pliocene to the Holocene; a Miocene volcanic stage ("Basaltos Bella Vista") crops out at the northwestern end of the volcanic field and is heavily eroded. The age of Laguna Potrok Aike is not known with certainty but its minimum age on the basis of sediment core data is 240,000 years before present.
Volcanic deposits have covered archeological artifacts at the Pali-Aike Cave, indicating volcanic activity between 10,000–5,000 years before present; the Global Volcanism Program mentions an eruption 5,550 ± 2,500 BCE. Sediment cores from Laguna Azul give an approximate age of 3,400 years before present, suggesting that this vent formed during the late Holocene. Unusually for Argentine volcanoes, the Pali-Aike vents are fairly close to modern urban areas; Rio Gallegos is only 23 kilometres (14 mi) away.
Archeology and human history
Early humans inhabited the Pali-Aike region, especially various caves such as Fell Cave and Pali-Aike cave. The volcanic landscape has a reliable supply of water and offered refuge to these people. They left petroglyphs and rock carvings behind; even some ancient burials have been found.
Sheep farming occurs today in the volcanic field. The Pali-Aike volcanic field today is part of the Pali Aike National Park on the Chilean side, and a few volcanic centres have been investigated as possible geosites.
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