Pali-Aike volcanic field

The Pali-Aike volcanic field spans the border between Argentina and Chile, northwest of the Magellanes Strait.[2] Most of the field lies in Argentina[3] within the Santa Cruz Province,[4] the Chilean part is part of the commune of San Gregorio, Chile.[5] The cities of Rio Gallegos and Punta Arenas lie northeast and southwest of Pali-Aike respectively.[6] Rio Gallegos is about 80 kilometres (50 mi) away from the field,[7] and the closest vent is only 23 kilometres (14 mi)[8]-30 kilometres (19 mi) away from the town; the vents are easily observed from Rio Gallegos.[9] In addition, National Route 3 passes through the Pali-Aike volcanic field.[10]

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;[11] hawaiite, trachyandesite and trachyte are present in smaller amounts.[12] 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.[13] 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;[14] later it was suggested that slab rollback might instead be the mechanism by which volcanism is triggered in the Pali-Aike region.[15] The age trends of volcanism has been interpreted as indicating either a southward migration[4] or a northeastward one in the case of the plateau lavas, following the movement of the triple junction to the north;[16] in that case Pali-Aike would be an exception, probably due to local tectonic effects.[17] However, some older plateau lavas in the north formed in response to an earlier ridge subduction event in the Eocene and Paleocene.[18]

300 kilometres (190 mi) farther west from Pali-Aike lies the actual Andean volcanic arc[7] in the form of the Austral Volcanic Zone, a chain of stratovolcanoes and one volcanic field (Fueguino) which is South America's southernmost volcano.[19] 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.[17]

The Pali-Aike volcanic field covers a surface area of 4,500 square kilometres (1,700 sq mi),[11] and extends over 150 kilometres (93 mi) from northwest to southeast.[7] It is formed by a up to 120 metres (390 ft) (in its northwestern reach) thick plateau of lava flows,[20] with an average relief of 20–100 metres (66–328 ft).[21] This plateau is formed by tables containing depressions and lakes, and whose margins are steep-dipping slopes that accumulate blocks at their feet.[22] It includes remnants of individual volcanic centres,[20] some volcanic necks situated in the west–central part of the field may be the underground components of now-eroded volcanic edifices.[23] Among these volcanic necks are the Cuadrado, Domeyko, Gay and Philippi hills, which conspicuously stick out of the surrounding plains.[24] The volcanic rocks are emplaced atop Tertiary sediments,[25] which are smoothened by glacial action.[23]

A vent close to Laguna Azul

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.[20] These various centres rise between 20–160 metres (66–525 ft) above the surrounding terrain.[8] Nested craters, breached craters and fissure vents are common among the various vents,[26] as well as lava flows but there has been little research on the scoria cones.[27] Pyroclastic cones in Pali-Aike include Aymond, Colorado, Dinero, Fell and Negro.[1] The vent Cerro del Diablo, a pyroclastic cone, is the youngest volcano in the field and emitted both aa lava and pahoehoe lava,[28] which have a fresh appearance and no soil cover.[1] The vents are origins of lava flows, which sometimes breach the vents.[4] Some flows are older and covered with soil while younger ones are not.[1] Such young lava flows also have surface features including lava tunnels, hornitos, tumuli and a rugose surface.[8] Some of these are heavily eroded while the southeastern part of the field features fresh-looking centres,[20] where they form the "Basaltos del Diablo".[15] 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".[27]

Laguna Azul lake

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.[8] 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).[7] The periglacial ground is rich in ice and water, which might explain why there are so many maars in Pali-Aike.[8] 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.[29] 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.[30] Additional maars in the southwestern part of the field are the so-called "West Maar" and "East Maar",[31] which contain the lakes Laguna Salsa and Laguna del Ruido respectively,[27] Bismarck,[21] Carlota, Los Flamencos,[8] Laguna Salida/Laguna Ana and Timone Lake.[16]

A number of vents form various alignments, usually along northwest–southeast and east–northeast–west–southwest lines;[20] some older centres show a north–south pattern.[32] 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.[14] These lines match the strike of the Magallanes-Fagnano fault zone and the older Patagonian Austral Rift.[33] Faults within the field have been active in the Tertiary, and a graben in the southwestern part of the field has diverted lava flows.[34]

The Rio Gallegos and its tributary Rio Ci Aike cross the volcanic field from west to east and from southwest to northeast, respectively.[2] 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.[35] 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.[29] 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.[36]

The Pali-Aike volcanic field is mainly made up of basalt and basanite, which form a sodium-rich alkaline suite;[39] hawaiite is rare.[40] The most important phenocrystic phase is olivine which also appears as xenocrysts;[39] 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.[41] Pali-Aike rocks typically contain ultramafic xenoliths containing augite, dunite, eclogite, garnet, harzburgite, lherzolite, peridotite, phlogopite, pyroxenite, spinel and wehrlites.[12][39] The composition of these xenoliths indicates that they originated from both the crust and the mantle.[37] In addition, rocks from Pali-Aike contain inclusions of fluids consisting of carbon dioxide.[42]

Elemental composition is typical for alkaline intraplate basalts and has a strongly primitive magma signature.[43] 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.[44]

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.[45] 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.[46] An older oceanic lithosphere which was emplaced during the Proterozoic-Paleozoic in the area is also involved in magma genesis.[47] 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.[40]

The basement beneath Pali-Aike contains the Magallanes basin of Jurassic age,[11] which formed during the breakup of Gondwana and was later filled by volcanic and sedimentary rocks.[20] The partly Neoproterozoic Deseado Massif lies north of Pali-Aike and may extend beneath the field to Tierra del Fuego,[16] there is no evidence that a Precambrian basement exists in the Pali-Aike area.[37] During the Oligocene a marine transgression deposited the Patagonia Formation,[48] and during the Miocene fluvial sediments formed the Santa Cruz Formation.[49] 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.[50]

The Pali-Aike area was glaciated during the middle Pleistocene, and glaciers eroded contemporary lava flows. 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.[49]