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98-632: 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

196-512: A caldera floor due to movement in the magma chamber beneath it. Unlike a lava dome , a resurgent dome is not formed by the extrusion of highly viscous lava onto the surface, but rather by the uplift and deformation of the surface itself by magma movement underground. Resurgent domes are typically found near the center of very large open calderas such as Yellowstone Caldera or Valles Caldera , and in turn such calderas are often referred to as "resurgent-type" calderas to distinguish them from

294-455: 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. The collapse of the caldera cut through older volcanic centres, exposing

392-526: A broader aridification trend. The EMCI ended 18 million years ago, giving way to the Middle Miocene Warm Interval (MMWI), the warmest part of which was the MMCO that began 16 million years ago. As the world transitioned into the MMCO, carbon dioxide concentrations varied between 300 and 500 ppm. Global annual mean surface temperature during the MMCO was about 18.4 °C. MMCO warmth was driven by

490-519: A clade of large terrestrial predatory crocodyliformes distantly related to modern crocodilians, from which they likely diverged over 180 million years ago, are known from the Miocene of South America. The last Desmostylians thrived during this period before becoming the only extinct marine mammal order. The pinnipeds , which appeared near the end of the Oligocene, became more aquatic. A prominent genus

588-561: A cooler, drier climate. C 4 grasses, which are able to assimilate carbon dioxide and water more efficiently than C 3 grasses, expanded to become ecologically significant near the end of the Miocene between 6 and 7 million years ago, although they did not expand northward during the Late Miocene. The expansion of grasslands and radiations among terrestrial herbivores correlates to fluctuations in CO 2 . One study, however, has attributed

686-473: A good analogue for future warmer climates caused by anthropogenic global warming , with this being especially true of the global climate during the Middle Miocene Climatic Optimum (MMCO), because the last time carbon dioxide levels were comparable to projected future atmospheric carbon dioxide levels resulting from anthropogenic climate change was during the MMCO. The Ross Sea margin of

784-435: A good model for a "living fossil". Eucalyptus fossil leaves occur in the Miocene of New Zealand , where the genus is not native today, but have been introduced from Australia . Both marine and continental fauna were fairly modern, although marine mammals were less numerous. Only in isolated South America and Australia did widely divergent fauna exist. In Eurasia, genus richness shifted southward to lower latitudes from

882-579: A major expansion of Antarctic glaciers. This severed the connection between the Indian Ocean and the Mediterranean Sea and formed the present land connection between Afro-Arabia and Eurasia. The subsequent uplift of mountains in the western Mediterranean region and a global fall in sea levels combined to cause a temporary drying up of the Mediterranean Sea (known as the Messinian salinity crisis ) near

980-480: A major expansion of grass-grazer ecosystems . Herds of large, swift grazers were hunted by predators across broad sweeps of open grasslands , displacing desert, woodland, and browsers . The higher organic content and water retention of the deeper and richer grassland soils , with long-term burial of carbon in sediments, produced a carbon and water vapor sink. This, combined with higher surface albedo and lower evapotranspiration of grassland, contributed to

1078-488: 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),

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1176-578: A significant drop in atmospheric carbon dioxide levels. Both continental and oceanic thermal gradients in mid-latitudes during the Early Miocene were very similar to those in the present. Global cooling caused the East Asian Summer Monsoon (EASM) to begin to take on its modern form during the Early Miocene. From 22.1 to 19.7 Ma, the Xining Basin experienced relative warmth and humidity amidst

1274-695: A significant local decline along the northeastern coast of Australia during the Tortonian, most likely due to warming seawater. Cetaceans attained their greatest diversity during the Miocene, with over 20 recognized genera of baleen whales in comparison to only six living genera. This diversification correlates with emergence of gigantic macro-predators such as megatoothed sharks and raptorial sperm whales . Prominent examples are O. megalodon and L. melvillei . Other notable large sharks were O. chubutensis , Isurus hastalis , and Hemipristis serra . Crocodilians also showed signs of diversification during

1372-541: A surface area of about 7,700 square kilometres (3,000 sq mi), part of which was later eroded away. The Atana ignimbrite is fairly welded, rich in crystals and poor in lithics . It is underlaid by pumice and ash deposits. Pumice is also found as fragments within the ignimbrite, ranging from white rhyolite to gray andesite. After its eruption, wind and water driven erosion occurred on the Atana ignimbrite, carving valleys and yardangs into it. Some differences exist between

1470-414: A volume of 2,500–3,500 cubic kilometres (600–840 cu mi) and a volcanic explosivity index of 8. This makes the Atana eruption the fifth-largest explosive eruption known and La Pacana a supervolcano . 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. This flow sheet originally probably covered

1568-627: A zone of low rainfall in the Late Miocene. The Indian Plate continued to collide with the Eurasian Plate , creating new mountain ranges and uplifting the Tibetan Plateau , resulting in the rain shadowing and aridification of the Asian interior. The Tian Shan experienced significant uplift in the Late Miocene, blocking westerlies from coming into the Tarim Basin and drying it as a result. At

1666-410: 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

1764-480: 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. The formation of the caldera coincided with the eruption of the Atana ignimbrite; the eruption was still underway when the terrain subsided to a depth of 2–3 kilometres (1.2–1.9 mi) beneath

1862-415: 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 . 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. The Atana ignimbrite

1960-490: 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. Large scale ignimbritic activity continued until 2 million years ago. The Central Andes are the site of extensive ignimbrites that were erupted from large calderas usually located within the adjacent Altiplano , east of the principal volcanic arc . Many of these calderas are part of

2058-521: The Altiplano-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. The ignimbrites form a surface that lies at an average elevation of 4,000 metres (13,000 ft). Stratovolcanoes developed on top of these ignimbrite sheets and today form

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2156-656: The Cerro Guacha and Purico 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 the Holocene . Additional volcanic centres west and southwest of La Pacana are Acamarachi , Láscar , Colachi and Cordón de Puntas Negras . A number of faults transect

2254-448: 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; later these three domes were identified as post-caldera domes. Stratovolcanoes inside the caldera include

2352-539: The Middle Miocene Climate Transition (MMCT). Abrupt increases in opal deposition indicate this cooling was driven by enhanced drawdown of carbon dioxide via silicate weathering. The MMCT caused a sea surface temperature (SST) drop of approximately 6 °C in the North Atlantic. The drop in benthic foraminiferal δ O values was most noticeable in the waters around Antarctica, suggesting cooling

2450-664: 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), leading to volcanic activity at distances of 130–160 kilometres (81–99 mi) from the trench. Research indicates that subduction has been ongoing since the Jurassic 200 million years ago but accelerated 26 million years ago. After a phase of andesitic volcanism lasting from

2548-562: 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

2646-550: The carbon cycle occurred approximately 6 Ma, causing continental carbon reservoirs to no longer expand during cold spells, as they had done during cold periods in the Oligocene and most of the Miocene. At the end of the Miocene, global temperatures rose again as the amplitude of Earth's obliquity increased, which caused increased aridity in Central Asia. Around 5.5 Ma, the EAWM underwent a period of rapid intensification. Life during

2744-574: The geology of the Himalaya occurred during that epoch, affecting monsoonal patterns in Asia, which were interlinked with glacial periods in the northern hemisphere. The Miocene faunal stages from youngest to oldest are typically named according to the International Commission on Stratigraphy : Regionally, other systems are used, based on characteristic land mammals; some of them overlap with

2842-523: The " Zanclean flood ". Also during the early Miocene (specifically the Aquitanian and Burdigalian Stages), the apes first evolved, began diversifying, and became widespread throughout the Old World . Around the end of this epoch, the ancestors of humans had split away from the ancestors of the chimpanzees and had begun following their own evolutionary path during the final Messinian Stage (7.5–5.3 Ma) of

2940-466: The Atana ignimbrite and like the Toconao ignimbrite is mainly exposed on the western side of the caldera. The first large eruption, which took place between 4 ± 0.9 and 5.3 ± 1.1 million years ago, formed the Toconao ignimbrite. The Toconao ignimbrite crops out mainly west of the caldera; only later were units of the Toconao identified on the eastern side of La Pacana. This ignimbrite has a volume of approximately 180 cubic kilometres (43 cu mi) and

3038-410: The Atana ignimbrite. Phenocrysts within the ignimbrite are chiefly formed by plagioclase . Both the Atana and the Toconao ignimbrite include minerals like allanite , apatite , biotite , epidote , hornblende , ilmenite , magnetite , monazite , orthopyroxene , plagioclase , quartz , sanidine , titanite and zircon . Not all of these minerals are found in both ignimbrites, and not always in

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3136-574: The Atana magma. 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. Its volume is about 0.1 cubic kilometres (0.024 cu mi). 2.4 ± 0.4 million years ago, the Pampa Chamaca ignimbrite filled the moat between the resurgent dome and the caldera rim. The Pampa Chamaca or Talabre ignimbrite

3234-793: The Atana/Toconao ignimbrites, and the Filo Delgado and Pampa Chamaca/Talabre ignimbrites afterwards. 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

3332-579: 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. 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. These volcanic centres include

3430-673: The Central Paratethys, cut off from sources of freshwater input by its separation from the Eastern Paratethys. From 13.36 to 12.65 Ma, the Central Paratethys was characterised by open marine conditions, before the reopening of the Bârlad Strait resulted in a shift to brackish-marine conditions in the Central Paratethys, causing the Badenian-Sarmatian Extinction Event. As a result of the Bârlad Strait's reopening,

3528-665: 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

3626-826: The Early to the Middle Miocene. Europe's large mammal diversity significantly declined during the Late Miocene. In the Early Miocene, several Oligocene groups were still diverse, including nimravids , entelodonts , and three-toed equids. As in the previous Oligocene Epoch, oreodonts were still diverse, only to disappear in the earliest Pliocene. During the later Miocene mammals were more modern, with easily recognizable canids , bears , red pandas , procyonids , equids , beavers , deer , camelids , and whales , along with now-extinct groups like borophagine canids , certain gomphotheres , three-toed horses , and hornless rhinos like Teleoceras and Aphelos . The late Miocene also marks

3724-677: The East Antarctic Ice Sheet (EAIS) was highly dynamic during the Early Miocene. The Miocene began with the Early Miocene Cool Event (Mi-1) around 23 million years ago, which marked the start of the Early Miocene Cool Interval (EMCI). This cool event occurred immediately after the Oligocene-Miocene Transition (OMT) during a major expansion of Antarctica's ice sheets, but was not associated with

3822-577: The East Asian Winter Monsoon (EAWM) became stronger synchronously with a southward shift of the subarctic front. Greenland may have begun to have large glaciers as early as 8 to 7 Ma, although the climate for the most part remained warm enough to support forests there well into the Pliocene. Zhejiang, China was noticeably more humid than today. In the Great Rift Valley of Kenya , there

3920-457: The LMC; extratropical sea surface temperatures dropped substantially by approximately 7–9 °C. 41 kyr obliquity cycles became the dominant orbital climatic control 7.7 Ma and this dominance strengthened 6.4 Ma. Benthic δ O values show significant glaciation occurred from 6.26 to 5.50 Ma, during which glacial-interglacial cycles were governed by the 41 kyr obliquity cycle. A major reorganisation of

4018-563: The Late Cretaceous, are known from the Miocene of Patagonia, represented by the mole-like Necrolestes . The youngest known representatives of metatherians (the broader grouping to which marsupials belong) in Europe, Asia and Africa are known from the Miocene, including the European herpetotheriid Amphiperatherium , the peradectids Siamoperadectes and Sinoperadectes from Asia, and

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4116-694: The Late Miocene, the Earth's climate began to display a high degree of similarity to that of the present day . The 173 kyr obliquity modulation cycle governed by Earth's interactions with Saturn became detectable in the Late Miocene. By 12 Ma, Oregon was a savanna akin to that of the western margins of the Sierra Nevada of northern California . Central Australia became progressively drier, although southwestern Australia experienced significant wettening from around 12 to 8 Ma. The South Asian Winter Monsoon (SAWM) underwent strengthening ~9.2–8.5 Ma. From 7.9 to 5.8 Ma,

4214-567: The Miocene Epoch was mostly supported by the two newly formed biomes , kelp forests and grasslands . Grasslands allow for more grazers, such as horses , rhinoceroses , and hippos . Ninety-five percent of modern plants existed by the end of this epoch . Modern bony fish genera were established. A modern-style latitudinal biodiversity gradient appeared ~15 Ma. The coevolution of gritty , fibrous, fire-tolerant grasses and long-legged gregarious ungulates with high-crowned teeth , led to

4312-484: The Miocene. As in the Oligocene before it, grasslands continued to expand, and forests to dwindle. In the seas of the Miocene, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems. The plants and animals of the Miocene were recognizably modern. Mammals and birds were well established. Whales , pinnipeds , and kelp spread. The Miocene is of particular interest to geologists and palaeoclimatologists because major phases of

4410-449: The Miocene. The largest form among them was a gigantic caiman Purussaurus which inhabited South America. Another gigantic form was a false gharial Rhamphosuchus , which inhabited modern age India . A strange form, Mourasuchus also thrived alongside Purussaurus . This species developed a specialized filter-feeding mechanism, and it likely preyed upon small fauna despite its gigantic size. The youngest members of Sebecidae ,

4508-534: The North American Great Plains and in Argentina . The global trend was towards increasing aridity caused primarily by global cooling reducing the ability of the atmosphere to absorb moisture, particularly after 7 to 8 million years ago. Uplift of East Africa in the late Miocene was partly responsible for the shrinking of tropical rain forests in that region, and Australia got drier as it entered

4606-648: The Oligocene–Miocene transgression. As the southern Andes rose in the Middle Miocene (14–12 million years ago) the resulting rain shadow originated the Patagonian Desert to the east. Far northern Australia was monsoonal during the Miocene. Although northern Australia is often believed to have been much wetter during the Miocene, this interpretation may be an artefact of preservation bias of riparian and lacustrine plants; this finding has itself been challenged by other papers. Western Australia, like today,

4704-598: The Qiongdongnan Basin in the northern South China Sea indicates the Pearl River was a major source of sediment flux into the sea during the Early Miocene and was a major fluvial system as in the present. During the Oligocene and Early Miocene, the coast of northern Brazil, Colombia, south-central Peru , central Chile and large swathes of inland Patagonia were subject to a marine transgression . The transgressions in

4802-487: The activity of the Columbia River Basalts and enhanced by decreased albedo from the reduction of deserts and expansion of forests. Climate modelling suggests additional, currently unknown, factors also worked to create the warm conditions of the MMCO. The MMCO saw the expansion of the tropical climatic zone to much larger than its current size. The July ITCZ, the zone of maximal monsoonal rainfall, moved to

4900-472: The aridity of the former. Unequivocally-recognizable dabbling ducks , plovers , typical owls , cockatoos and crows appear during the Miocene. By the epoch's end, all or almost all modern bird groups are believed to have been present; the few post-Miocene bird fossils which cannot be placed in the evolutionary tree with full confidence are simply too badly preserved, rather than too equivocal in character. Marine birds reached their highest diversity ever in

4998-619: The beginning of the Miocene, the northern margin of the Arabian plate, then part of the African landmass, collided with Eurasia; as a result, the Tethys seaway continued to shrink and then disappeared as Africa collided with Eurasia in the Turkish – Arabian region. The first step of this closure occurred 20 Ma, reducing water mass exchange by 90%, while the second step occurred around 13.8 Ma, coincident with

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5096-470: 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

5194-481: The climate slowly cooled towards a series of ice ages . The Miocene boundaries are not marked by distinct global events but by regionally defined transitions from the warmer Oligocene to the cooler Pliocene Epoch. During the Early Miocene, Afro-Arabia collided with Eurasia, severing the connection between the Mediterranean and Indian Oceans, and allowing the interchange of fauna between Eurasia and Africa, including

5292-687: The cones associated with the Cerros de Guayaques lava domes and the volcanoes Cerro Incaguasi, Cerros de Pili, Cerros Negros and Huailitas. 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)). A few lakes such as the spring-fed Laguna de Chivato Muerto, Laguna Trinchera and Ojos del Rió Salado, 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

5390-566: The course of this epoch . The youngest representatives of Choristodera , an extinct order of aquatic reptiles that first appeared in the Middle Jurassic , are known from the Miocene of Europe, belonging to the genus Lazarussuchus , which had been the only known surviving genus of the group since the beginning of the Eocene. The last known representatives of the archaic primitive mammal order Meridiolestida , which dominated South America during

5488-503: 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 as Fish Canyon , Long Valley and Yellowstone . 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. There

5586-570: The dispersal of proboscideans and hominoids into Eurasia. During the late Miocene, the connections between the Atlantic and Mediterranean closed, causing the Mediterranean Sea to almost completely evaporate. This event is referred to as the " Messinian salinity crisis ". Then, at the Miocene–Pliocene boundary, the Strait of Gibraltar opened, and the Mediterranean refilled. That event is referred to as

5684-425: The early to mid Miocene (23–15 Ma). Oceans cooled partly due to the formation of the Antarctic Circumpolar Current , and about 15 million years ago the ice cap in the southern hemisphere started to grow to its present form. The Greenland ice cap developed later, in the Middle Pliocene time, about 3 million years ago. Resurgent dome In geology , a resurgent dome is a dome formed by swelling or rising of

5782-451: The end of the Miocene due to increased habitat uniformity. The expansion of grasslands in North America also led to an explosive radiation among snakes. Previously, snakes were a minor component of the North American fauna, but during the Miocene, the number of species and their prevalence increased dramatically with the first appearances of vipers and elapids in North America and the significant diversification of Colubridae (including

5880-422: The end of the Miocene. The Paratethys underwent a significant transgression during the early Middle Miocene. Around 13.8 Ma, during a global sea level drop, the Eastern Paratethys was cut off from the global ocean by the closure of the Bârlad Strait, effectively turning it into a saltwater lake. From 13.8 to 13.36 Ma, an evaporite period similar to the later Messinian salinity crisis in the Mediterranean ensued in

5978-429: The enigmatic Saint Bathans Mammal . Microbial life in the igneous crust of the Fennoscandian Shield shifted from being dominated by methanogens to being primarily composed of sulphate-reducing prokaryotes . The change resulted from fracture reactivation during the Pyrenean-Alpine orogeny, enabling sulphate-reducing microbes to permeate into the Fennoscandian Shield via descending surficial waters. Diatom diversity

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6076-461: The evolution of both groups into modern representatives. The early Miocene Saint Bathans Fauna is the only Cenozoic terrestrial fossil record of the landmass, showcasing a wide variety of not only bird species, including early representatives of clades such as moa , kiwi and adzebills , but also a diverse herpetofauna of sphenodontians , crocodiles and turtles as well as a rich terrestrial mammal fauna composed of various species of bats and

6174-499: The expansion of grasslands not to a CO 2 drop but to the increasing seasonality and aridity, coupled with a monsoon climate, which made wildfires highly prevalent compared to before. The Late Miocene expansion of grasslands had cascading effects on the global carbon cycle, evidenced by the imprint it left in carbon isotope records. Cycads between 11.5 and 5 million years ago began to rediversify after previous declines in variety due to climatic changes, and thus modern cycads are not

6272-653: The extinction of the last-surviving members of the hyaenodonts . Islands began to form between South and North America in the Late Miocene, allowing ground sloths like Thinobadistes to island-hop to North America. The expansion of silica-rich C 4 grasses led to worldwide extinctions of herbivorous species without high-crowned teeth . Mustelids diversified into their largest forms as terrestrial predators like Ekorus , Eomellivora , and Megalictis and bunodont otters like Enhydriodon and Sivaonyx appeared. Eulipotyphlans were widespread in Europe, being less diverse in Southern Europe than farther north due to

6370-456: 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. 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. This separate ignimbrite

6468-421: The findings of marine invertebrate fossils of both Atlantic and Pacific affinity in La Cascada Formation . Connection would have occurred through narrow epicontinental seaways that formed channels in a dissected topography . The Antarctic Plate started to subduct beneath South America 14 million years ago in the Miocene, forming the Chile Triple Junction . At first the Antarctic Plate subducted only in

6566-419: 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

6664-411: The human lineage) appeared in Africa at the very end of the Miocene, including Sahelanthropus , Orrorin , and an early form of Ardipithecus ( A. kadabba ). The chimpanzee–human divergence is thought to have occurred at this time. The evolution of bipedalism in apes at the end of the Miocene instigated an increased rate of faunal turnover in Africa. In contrast, European apes met their end at

6762-408: The ignimbrite-forming secondary magmas. Presently, the inferred margin of this partially molten zone coincides fairly well with a negative gravimetric anomaly that clusters around the tripoint between Argentina, Bolivia and Chile and with the extent of the Altiplano-Puna volcanic complex. The basement beneath La Pacana is formed by sediments of Ordovician age, Devonian - Permian quartzites ,

6860-422: The lake levels of the Eastern Paratethys dropped as it once again became a sea. The Fram Strait opened during the Miocene and acted as the only throughflow for Atlantic Water into the Arctic Ocean until the Quaternary period. Due to regional uplift of the continental shelf, this water could not move through the Barents Seaway in the Miocene. The modern day Mekong Delta took shape after 8 Ma. Geochemistry of

6958-437: The largest structure consisting of near-molten (10–20%) rock on Earth. This partial melt zone was formed by the injection of mafic magmas into the lower crust; a major episode of overturning before 10.6 million years ago caused crustal anatexis and started the onset of ignimbritic volcanism. 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

7056-458: The late Tertiary to the Miocene , large scale ignimbritic volcanism commenced about 23 million years ago and is still ongoing. 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 . Volcanic activity at La Pacana

7154-554: The mixed Salta formation also of Permian age and sediments of Cretaceous - Tertiary age. At the eastern margin of La Pacana in Argentina, they overlay an even older Precambrian basement. Most of this original basement however is covered by Miocene ignimbrites from centres that may coincide with the La Pacana caldera. Two of these older ignimbrites are known as the Pampa Múcar and Antigua Chacaliri ignimbrites. La Pacana together with

7252-464: The moat. 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. Streams such as Río de Pili and Río Salado complete the hydrology of the caldera. Gravimetric observations have been conducted on La Pacana. A large negative anomaly (an anomaly with less-than-expected mass crust) coincides with

7350-457: The more common (but much smaller) calderas found on shield volcanoes and stratovolcanoes . The structure that makes a resurgent dome possible is a fracture zone made up of ring faults surrounded by concentric normal faults around the outside of the rings. During initial formation of the caldera these ring faults provide vents for ash-flow eruptions and are the point at which subsidence of the cauldron block occurs. Subsequent magma flows then push

7448-406: The most clear expression of volcanic activity in the region, with some of them exceeding the height of 6,000 metres (20,000 ft) above sea level. The long-lasting dry climate means that traces of volcanic activity can be recognizable over long timeframes. The Altiplano-Puna volcanic complex is underpinned by a large seismic velocity anomaly at a depth of 20 kilometres (12 mi), which may be

7546-543: The north, increasing precipitation over southern China whilst simultaneously decreasing it over Indochina during the EASM. Western Australia was at this time characterised by exceptional aridity. In Antarctica, average summer temperatures on land reached 10 °C. In the oceans, the lysocline shoaled by approximately half of a kilometre during warm phases that corresponded to orbital eccentricity maxima. The MMCO ended around 14 million years ago, when global temperatures fell in

7644-415: 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

7742-523: The origin of many modern genera such as Nerodia , Lampropeltis , Pituophis and Pantherophis ). Arthropods were abundant, including in areas such as Tibet where they have traditionally been thought to be undiverse. Neoisopterans diversified and expanded into areas they previously were absent from, such as Madagascar and Australia. In the oceans, brown algae , called kelp , proliferated, supporting new species of sea life, including otters , fish and various invertebrates . Corals suffered

7840-463: The possible herpetotheriid Morotodon from the late Early Miocene of Uganda. Approximately 100 species of apes lived during this time , ranging throughout Africa, Asia and Europe and varying widely in size, diet, and anatomy. Due to scanty fossil evidence it is unclear which ape or apes contributed to the modern hominid clade, but molecular evidence indicates this ape lived between 18 and 13 million years ago. The first hominins ( bipedal apes of

7938-781: The preceding Oligocene and following Pliocene Epochs: Continents continued to drift toward their present positions. Of the modern geologic features, only the land bridge between South America and North America was absent, although South America was approaching the western subduction zone in the Pacific Ocean , causing both the rise of the Andes and a southward extension of the Meso-American peninsula. Mountain building took place in western North America , Europe , and East Asia . Both continental and marine Miocene deposits are common worldwide with marine outcrops common near modern shorelines. Well studied continental exposures occur in

8036-411: The previous surface in the northwestern segment of La Pacana. 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, reaching

8134-528: 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. The Toconao and Atana ignimbrites are formed by rhyolite and dacite - rhyodacite , respectively. They form a potassium -rich calc-alkaline suite. Both contain pumices , three different types of which are found in

8232-573: The rise and eruption of the Atana ignimbrite. Two potential vents have been found at the northern and western margins of the caldera, where breccia deposits occur within the Atana ignimbrite. 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. 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

8330-534: The rocks and the geographical distribution of their outcrops. Pyroclastic deposits in the Eastern Cordillera of Argentina may have their origin at La Pacana. Before the eruption of the Toconao and Atana ignimbrites, early activity generated the Pujsa ignimbrite 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. The Pujsa ignimbrite resembles

8428-553: The same phase (crystals or matrix). Ultimately, the magmas at La Pacana are the products of mantle melts interacting with various crustal domains deep in the crust , within the partially molten zone that has been found at depths of c. 20 kilometres (12 mi) beneath the Altiplano-Puna volcanic complex. 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

8526-753: The southernmost tip of Patagonia, meaning that the Chile Triple Junction lay near the Strait of Magellan . As the southern part of Nazca Plate and the Chile Rise became consumed by subduction the more northerly regions of the Antarctic Plate begun to subduct beneath Patagonia so that the Chile Triple Junction advanced to the north over time. The asthenospheric window associated to the triple junction disturbed previous patterns of mantle convection beneath Patagonia inducing an uplift of ca. 1 km that reversed

8624-417: 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 . In

8722-542: The west coast of South America are thought to be caused by a regional phenomenon while the steadily rising central segment of the Andes represents an exception. While there are numerous registers of Oligocene–Miocene transgressions around the world it is doubtful that these correlate. It is thought that the Oligocene–Miocene transgression in Patagonia could have temporarily linked the Pacific and Atlantic Oceans, as inferred from

8820-423: Was Allodesmus . A ferocious walrus , Pelagiarctos may have preyed upon other species of pinnipeds including Allodesmus . Furthermore, South American waters witnessed the arrival of Megapiranha paranensis , which were considerably larger than modern age piranhas . New Zealand 's Miocene fossil record is particularly rich. Marine deposits showcase a variety of cetaceans and penguins , illustrating

8918-610: Was a gradual and progressive trend of increasing aridification, though it was not unidirectional, and wet humid episodes continued to occur. Between 7 and 5.3 Ma, temperatures dropped sharply again in the Late Miocene Cooling (LMC), most likely as a result of a decline in atmospheric carbon dioxide and a drop in the amplitude of Earth's obliquity, and the Antarctic ice sheet was approaching its present-day size and thickness. Ocean temperatures plummeted to near-modern values during

9016-458: Was arid, particularly so during the Middle Miocene. Climates remained moderately warm, although the slow global cooling that eventually led to the Pleistocene glaciations continued. Although a long-term cooling trend was well underway, there is evidence of a warm period during the Miocene when the global climate rivalled that of the Oligocene . The climate of the Miocene has been suggested as

9114-481: Was christened to be an upper and a lower Tara ignimbrite, possibly erupted by the Cerro Guacha caldera. The Tara ignimbrite fills part of the La Pacana caldera. 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. The most likely theory for

9212-411: Was erupted from a vent probably buried beneath the present-day Cordon de Puntas Negras or the Salar de Aguas Calientes and reached a volume of about 0.5 cubic kilometres (0.12 cu mi). The 3.49 million years old Tara Ignimbrite from the Cerro Guacha caldera reached the northern margin of La Pacana. Miocene As Earth went from the Oligocene through the Miocene and into the Pliocene,

9310-404: 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

9408-671: Was inversely correlated with carbon dioxide levels and global temperatures during the Miocene. Most modern lineages of diatoms appeared by the Late Miocene. There is evidence from oxygen isotopes at Deep Sea Drilling Program sites that ice began to build up in Antarctica about 36 Ma during the Eocene . Further marked decreases in temperature during the Middle Miocene at 15 Ma probably reflect increased ice growth in Antarctica. It can therefore be assumed that East Antarctica had some glaciers during

9506-627: Was most intense there. Around this time the Mi3b glacial event (a massive expansion of Antarctic glaciers) occurred. The East Antarctic Ice Sheet (EAIS) markedly stabilised following the MMCT. The intensification of glaciation caused a decoherence of sediment deposition from the 405 kyr eccentricity cycle. The MMWI ended about 11 Ma, when the Late Miocene Cool Interval (LMCI) started. A major but transient warming occurred around 10.8-10.7 Ma. During

9604-515: Was once considered part of the Guaitiquina ignimbrite, which was later split off, while the Puripicar ignimbrite may be correlated with the Atana instead. Also, some of the ignimbrites erupted by La Pacana originally were attributed to Cerro Guacha . 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

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