Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego

6 Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego Hugo Corbella1,2 and Luis E. Lara3 1

Museo Argentino de Ciencias Naturales, Buenos Aires, Argentina 2 CONICET 3 ´ Servicio Nacional de Geologıa y Minerı´a, Av. Santa Marı´a 104, Santiago, Chile

Quaternary volcanism. Thus, Andean volcanic provinces, as defined by Lo´pez-Escobar et al. (1995a and references therein), respond to these main driving factors. In addition, the architecture of the Late Cenozoic volcanic arc is related to the long-term geological evolution of Patagonia, which has built a heterogeneous continental crust as a substrate for volcanism. Tectonic evolution in Patagonia has been characterized by subsequent episodes of compression and transpression– transtension along the volcanic front with localized extension in the backarc region. Since the Middle Tertiary the convergence vector between the Nazca and South American plates has been oblique to the continental margin when a noncontractional orogen started to build in a transpressional (or transtensional) setting (Cembrano et al., 1996, 2000, 2002). After the main Andean orogenesis, whose strongest pulse occurred at the end of the Miocene (e.g. Thompson, 2002), an episode of east–west contraction has been reported in the Andean Cordillera between 39 and 46 S while the extra-Andean region remained undeformed (Lavenu and Cembrano, 1999a). Because of the oblique subduction, the northeast dextral transpression was resumed along the entire Quaternary arc in the southern Andes where the Liquin˜e–Ofqui fault system occurs (Lavenu and Cembrano, 1999a). A transtensional regime has been described at the northern end of this regional fault system (Folguera et al., 2004) and a similar setting was proposed for the southernmost portion (Forsythe and Diemer, 2006). In the extra-Andean area, the eastern foothills of the northern Patagonian Andes are characterized by structural blocks bounded by NNW- and NW-trending high-angle normal and reverse faults. These structures would have been acquired during the Paleozoic (Coira et al., 1975a) or the Late Triassic–Jurassic extensional events that generated grabens and half-grabens in Patagonia (Uliana et al., 1985; Barcat et al., 1989). The regional distribution of these faults, mostly oblique to the N–S direction of the Andes, can be seen in echelon arrangement along 1000 km throughout the cordilleran eastern foothills (Fig. 1). A large number of the Plio–Pleistocene volcanic emissions are spatially related to these NNW–NW fractures or other secondary faults that surround them. South of the Chile triple junction (46 S), arc tectonics is dominated by the Chile ridge subduction and the southernmost segment is influenced by the sinistral Magallanes fault system (Klepeis, 1994).

1. Introduction Patagonia, the southern region of the South American continent, extends from the Huincul Arch, which crosses the continent at ca. 39 S, to Cape Horn (56 S) in Tierra del Fuego (Baldis and Febrer, 1983; Ghidella et al., 1995; Chernicoff and Zapettini, 2004; Ramos et al., 2004a). In terms of volcanism, the northern boundary of the Patagonian Late Cenozoic domain could be established further north at the Cortaderas alignment near 37 S (Kay, 2005; Kay et al., 2006). Moreover, the Pleistocene–Holocene volcanic front shows a regional border at this latitude. Considering this, the geographical boundaries of Patagonia have been partially extended in this chapter to provide a synoptic view of magmatic provinces and volcanic episodes. Patagonia has both a vast and significant volcanic history, which started in the Triassic and developed mainly during the Jurassic with the emplacement of extended ignimbritic–rhyolitic plateaus covering the major part of the present area and of the Atlantic continental platform. With the break-up of Gondwana and the beginning of the migration of South America towards the west, during most of the Cretaceous, arc magmatism developed at the western margin of South America. To describe and better understand the evolution of the Late Cenozoic volcanic processes and their relationship with the geodynamic setting, a more extended period from the Late Miocene to the Holocene has been considered. However, mostly Pliocene to Holocene volcanic centers and sequences are described in detail as part of major volcanic provinces.

2. Late Cenozoic Tectonic Setting of Volcanism Quaternary volcanism in Patagonia has been strongly influenced by tectonic processes involved in both arc and backarc domains. The South American continental margin is in front of the Nazca and Antarctic plates, which are subducting underneath South America (Fig. 1). The Nazca–South America convergence occurs at ca. 8 cm/yr (De Mets et al., 1994), with a subduction angle of ca. 25 at this latitude (Cahill and Isacks, 1992; Bohm et al., 2002). Likewise, the Antarctic–South America and the Antarctic–Scotia convergence take place at ca. 2 cm/yr (Tebbens and Cande, 1997; Lagabrielle et al., 2004). These first-order features imprint geochemical signatures and exert a regional tectonic control on

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DEVELOPMENTS IN QUATERNARY SCIENCES VOLUME 11 ISSN 1571-0866 95

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Fig. 1. Generalized map of Patagonia showing the present location of the principal geological and tectonic features; i.e. Pacific oceanic crust fracture zones, Chile seismic oceanic ridge, continental faults and lineaments, and the North Patagonian and Deseado Massifs. Data from De Barrio et al., 1994; Nullo et al., 1994; Delpino and Deza, 1995; Lizuaı´n et al., 1995; Caminos and Gonza´lez, 1996; Vivallo et al., 1999.

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego 0.5133

Chile Ridge

Nd / 144Nd

0.5131

AVZ Cook Island Transitional Plateau Lavas

0.5129

TSVZ

Pali-Aike

SSVZ

143

Since the Miocene, the plate convergence has caused the collision of the Chile ridge with the Chile–Peru Trench. The initial ridge collision started ca. 14 Ma at the southwestern tip of Tierra del Fuego and, given the angle of oblique convergence, the triple junction migrated northward to its present position at the Taitao Peninsula at ca. 46 S (Cande and Leslie, 1986; Forsythe et al., 1986). Ridge–trench interactions along continental destructive plate margins cause the development of slab windows or volcanic gaps during ridge subduction (Dickinson and Snyder, 1979; Forsythe and Nelson, 1985; Thorkelson and Taylor, 1989). Between 46300 and 49 S, above a subducted transform segment, arc magmatism ceases, thus creating the Patagonian magmatic gap (Ramos et al., 1982; Stern et al., 1984). In turn, the pass of an active ridge segment favors a mixing of Nazca and continental subarc mantle. In the backarc or foreland region, mafic, tholeiitic to alkaline volcanism with intraplate signatures seems to be also temporally and spatially related to slab windows (Johnson and O’Neil, 1984; Forsythe and Nelson, 1985; Hole et al., 1991). Adakitic magmas (Kay, 1978; Defant and Drummond, 1990) can be generated near the triple junction by partial melting of the young and buoyant oceanic plate.

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0.5127

0.5127 0.702

AVZ

Mio-Pleistocene Patagonian Plateau Lavas

0.703

0.704

0.705

0.706

87Sr / 86Sr

Fig. 2. Geochemical signatures of Neogene plateau and arc volcanic rocks of Patagonia in a 143Nd /144Nd versus 87 Sr/ 86Sr isotope plot. Data from Kay and Gorring, 1999; Kay et al., 2004b; D’Orazio et al., 2001, 2004, 2005; Stern, 2004. This diagram emphasizes the different geochemical signatures of Patagonian magmas, wich reflect heterogeneities in source, geodynamic settings and evolving process.

3. Late Cenozoic Magmatic Processes Along the central and southernmost Southern Volcanic Zone (CSVZ and SSVZ provinces after Lo´pez-Escobar et al., 1995a; Fig. 1), tholeiitic and high-alumina basalts and basaltic andesites are the dominant rock types from both Quaternary stratovolcanoes and monogenetic centers (Hickey-Vargas et al., 1984, 1986, 1989; Futa and Stern, 1988; Lo´pez-Escobar et al., 1993; Stern, 2004), although evolved compositions also occur (Gerlach et al., 1988; Stern, 2004). Geochemical signatures suggest mainly an asthenospheric mantle source, partially mixed with a small amount of Nazca subducted sediments and slab-derived fluids (Morris et al., 1990; Hickey-Vargas et al., 2002; Sigmarsson et al., 2002; Stern, 2004, for a complete overview). With minor exceptions (McMillan et al., 1989; Hickey-Vargas et al., 1995), crustal assimilation decreases southward or is virtually nonexistent (Fig. 2). Input of the slab-derived fluids to the subarc mantle decreases eastward as does the degree of mantle melting (Hickey-Vargas et al., 1989; Lara et al., 2004b; Mella et al., 2005). In extra-Andean Patagonia, along the backarc region, alkali basalts derived by even lower degrees of partial mantle melting exhibit little or no evidence of slabderived components (Stern et al., 1990). Magmatism can result from ridge–trench interaction along continental plate margins, causing asthenospheric upwelling above slab windows (Ramos and Kay, 1992; Gorring et al., 1997, 2002, 2003; D’Orazio et al., 2001, 2004, 2005; Gorring and Kay, 2001; Kay, 2002a; Kay et al., 2004b). Also, in the absence of significant extensional tectonics, some large volumes of Tertiary and Quaternary plateau lavas have been considered to be of an asthenospheric

plume-like mantle source (Gorring et al., 1997; Ntaflos et al., 2000; Kay, 2002a; Kay et al., 2004a). Finally, transient regional extension could have also caused adiabatic decompression of the as the nospheric mantle and basaltic outpours. In the Austral Volcanic Zone (AVZ 49–55 S), south of the Patagonian gap, adakitic hornblende andesites and dacites widely predominate and the usual magma compositions of the Andes are absent (Stern et al., 1984; Guivel et al., 2002; Kay, 2002a; Stern, 2004). Adakitic magmas are formed by partial melting of young subducting oceanic crust mixed with variable amounts of mantle material. They have also interacted to a greater extent with the overlying continental lithosphere (Sigmarsson et al., 1998), but this interaction decreases to the south (Stern, 2004). In the backarc area south of 49 S, Late Miocene adakitic lavas with age decreasing to the north are also known (Ramos et al., 1991, 1994, 2004b; Kay et al., 1993a, b). They seem to be formed in coincidence with the collision of discrete segments of the Chile ridge against the oceanic trench during the migration of the triple junction to the north.

4. Volcanism in the Patagonian Cordillera The Patagonian Cordillera is located on the western and active margin of the South American plate. Since the Early Cenozoic, its geotectonic and magmatic evolution has been closely related to the subduction of the Nazca and Antarctic plates beneath South America. The remarkable fact is

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that the magmatic arc front has been static since the Early Miocene and has preserved the same location along the Andean Cordillera. Cenozoic volcanism in western Patagonia includes remnants of Oligocene–Miocene volcanic–sedimentary sequences like Curamallin or Trapa Trapa formations (Sua´rez and Empara´n, 1997) near 39 S, and Estratos de Lago Ranco (Campos et al., 1998) at 40 S. These volcanic successions appear as roof pendants intruded by Miocene plutonic rocks. Further south, the higher exhumation rates facilitate the exposition of the roots of the magmatic arc along the North Patagonian Batholith (Cembrano et al., 2000; Thompson, 2002; Thompson and Herve´, 2002; Adriasola et al., 2005). Structural blocks inside the Liquin˜e–Ofqui fault domain expose Middle–Late Miocene to Early Pliocene granitoids between 39 and 46 S (Campos et al., 1998; Lara and Moreno, 2004). The present volcanic Andean region includes the volcanic front and the eastern orogenic volcanoes that are part of oblique chains or appear as isolated vents. A first group of Plio–Quaternary stratovolcanoes and volcanic edifices is recognized between 38 and 42 S as the eroded remnants of ancient volcanoes whose ages range from ca. 2.5 to 0.7 Ma (Lara and Folguera, 2005). The more recent history of volcanism in the northern Patagonian Cordillera can be explained in two evolutionary stages (Mun˜oz and Stern, 1988, 1989; Lara et al., 2001; Lara and Folguera, 2006). Recently published 40 Ar/39Ar ages together with a complete data set of K/Ar ages support the hypothesis of Miocene–Pliocene broadening of the volcanic arc and subsequent Pliocene or Early Pleistocene narrowing maintaining the front (Lara et al., 2001; Lara and Folguera, 2006). Thus, mostly eroded Pliocene volcanoes and volcanic sequences can be recognized at the Andean domain, with the best-preserved examples in the eastern Andean foothills. Overlying them, the present arc and backarc volcanic centers are located.

4.1. Pliocene–Quaternary Volcanoes Early Pliocene volcanic rocks related to heavily eroded volcanic centers commonly form the base of the Plio– Quaternary stratovolcanoes. South of 37 S, a prominent volcanic and sedimentary sequence, the Malleco Formation (Sua´rez and Empara´n, 1997), is composed of basaltic to low-silica andesitic rocks that have yielded K/Ar ages from ca. 4.4 to 2.3 Ma (Lara and Folguera, 2006, and references therein). To the south near 40 S, another thick succession composed of basaltic lavas, breccias and coarse gravels, informally known as Estratos de Pitren˜o (Campos et al., 1998), has been dated at ca. 5.8–2.4 Ma (Campos et al., 1998; Lara and Moreno, 2004). A few kilometers east of the Andean Range in the Lonquimay area (38 S) subhorizontal, mainly basaltic lavas, Llanque´n– Ranquil and Tuetue´ sequences (Sua´rez and Empara´n, 1997), have Early Pliocene K/Ar ages from ca. 5.2 to 3.2 Ma. All of these thick, basaltic to andesitic, subhorizontal sequences present morphological features of effusive eruptions not related to compound volcanic structures but to shield volcanism. South of 40 S, Early Pliocene magmatic

rocks in the Andean Cordillera are granitoids bounded by sin-plutonic mylonites from the North Patagonian Batholith, although the Yeli Formation (Levi et al., 1966) at 43300 S could be an equivalent eroded extrusive sequence. Estimated depths of granites emplacement are near 2–3 km and together with cooling ages describe the high exhumation rates of the Andean Cordillera in southern Patagonia (Cembrano et al., 2002; Thompson, 2002; Adriasola et al., 2005). More preserved yet eroded, Late Pliocene to Early Pleistocene shield volcanoes formed a wide volcanic arc that partially overlies the Early Pliocene volcanic sequences from the main Andean Range to the uplifted blocks (Copahue–Pino Hachado) in the east. Near the modern arc front, these volcanic rocks occur in lava flows, volcaniclastic sequences and as deeply eroded stratovolcanoes. Remnant subhorizontal or gently dipping thin flows that are mostly basaltic in composition have mainly effusive volcanic features. The eastern belt is formed by partially preserved volcanic structures. The upper member of the Malleco Formation (38 S) that can be up to ca. 500 m thick consists of basaltic-andesitic lavas associated with poorly preserved necks (Sua´rez and Empara´n, 1997). Near 39 S, Cerro Trautre´n (ca. 0.8 Ma), Cerro Maichı´n (ca. 0.9 Ma), Laguna Los Patos and Carirrin˜e (Lara et al., 2001; Lara and Moreno, 2004) are small volcanic accumulations, which can have poorly preserved vent facies and cover granitoids of the North Patagonian Batholith. Further south, a thick sequence of basalts and laharic breccias constitutes the Estratos de Chapuco (ca. 1.0–0.4 Ma) at the base of the present Osorno volcano (Moreno et al., 1985; Lara et al., 2001). Volcanic rocks in the Estratos de Huen˜u–Huen˜u (ca. 1.43 Ma), at the base of the Calbuco volcano (41180 S), seem to be comparable remnants (Moreno et al., 1985). The extended volcanic Garganta del Diablo sequence (Mella et al., 2005) at the base of the Tronador volcano (41060 S) has a K/Ar age of ca. 1.3 Ma. Better-preserved central volcanoes are also part of this group. For example, Nevados de Caburgua (39 S) is a ring structure whose pyroclastic beds and lavas surround an andesitic laccolith (ca. 2.4–0.8 Ma). Huanquihue´ at 39480 S, Pirihueico at 39540 S (ca. 1.5–0.6 Ma) and Quelguenco and Chihuı´o at 39540 S (ca. 0.7 Ma) are stratocones with well-preserved necks or radial dyke swarms and are located at the present Andean water divide (Lara and Folguera, 2005). Huanquihue´ volcano has a Holocene pyroclastic cone over the northern flank showing the persistence of magmatic activity. Other central vents like Mencheca at 40300 S (ca. 0.53 Ma), Cordo´n de Alvarez (40360 S), Fiucha´ (40480 S) and Sarnoso at 40480 S (ca. 0.9 Ma) can be recognized at the base of the active Puyehue and Casablanca volcanoes. Near 41 S, the La Picada stratocone is located between Osorno and Puntiagudo volcanoes. The best-preserved stratovolcanoes in this group can have Middle Pleistocene lavas overlapping the basal parts of the active stratovolcanoes. Thus, Middle Pleistocene volcanoes are nearly indistinguishable from Late Pliocene to Early Pleistocene centers with respect to morphology and extent of erosion. Many of these volcanoes

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego are also partially collapsed structures whose upper units are older than ca. 200 ka, and consequently older than the basal units of the Pleistocene to Holocene centers. Among these Middle Pleistocene centers are the Sierra Velluda (37180 S), Quinquilil or Colmillo del Diablo (39300 S) and Pantoja (40060 S) volcanoes, which are prominent necks surrounded by basaltic lava flows, as well as the Hawaiian-type calderas of Sierra de Quinchilca at 40 S (ca. 1.4–0.3 Ma) and Cordillera Nevada at 40300 S (ca. 1.2–0.1 Ma).

4.2. Active Volcanoes Active Patagonian volcanoes, i.e. those formed since the Middle–Late Pleistocene having Holocene eruptions, some of them historical (during the last 450 yrs that followed the Spanish conquest), have a wide range of morphologies (calderas, stratovolcanoes, fissure systems and isolated monogenetic cones) and compositions (basalts to rhyolites, with tholeiitic-calc-alkaline to alkaline signatures), and their own volcanic evolution. At least 46 volcanic centers can be considered active along the Patagonian Cordillera, Villarrica and Llaima being among the most active volcanoes in South America. The SVZ is characterized by the presence of several oblique volcanic chains and volcanism located inside the structural domain of the Liquin˜e–Ofqui fault system. Along the CSVZ (37–42 S), some transverse chains were built on pre-Andean structures. In turn, a frontal arc formed by isolated stratovolcanoes forms the AVZ. South of 37 S, main stratovolcanoes and clusters of monogenetic cones are mentioned and briefly described below. At the northern end of the CSVZ, the Antuco volcano (37120 S) is a compound Late Pleistocene–Holocene volcanic complex, basaltic to basaltic andesitic in composition (Lohmar et al., 1999; Lohmar, 2000). Partial collapse at ca. 6.5 ka formed a horseshoe-shaped amphitheater open to the west as well as a huge avalanche deposit that extends to the Central valley (Thiele et al., 1998). A postcollapse cone was built with some basaltic flank vents. At least 20 historical eruptions have been recorded (Gonza´lez-Ferra´n, 1995). On the border between Chile and Argentina, the Copahue volcano (37480 S) is an active basaltic-andesitic to andesitic complex built since the Early Pleistocene inside the Caldera del Agrio, itself a huge Pliocene depression (Pesce, 1989; Linares et al., 1999, 2001; Folguera and Ramos, 2000; Melnick and Folguera, 2001; Melnick et al., 2005). A first shield stage was followed by a minor Middle Pleistocene caldera collapse (Polanco, 2003; Melnick et al., 2005). The present Late Pleistocene to Holocene cone, partially built beneath ice, has nine aligned craters, the easternmost hosting an acid lake. Twelve mostly phreatic eruptions have been reported over the last three centuries (Naranjo et al., 2000; Naranjo and Polanco, 2004). Callaqui volcano (37540 S) is a fissure system that has emitted basalts and basaltic andesites from the Late Pleistocene to Holocene (Moreno et al., 1986). Basaltic NE-trending fissures and cone alignments are the most

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recent Holocene emissions although no historical eruptions have been reported (Naranjo et al., 2000). Active fumaroles can still be observed on the flanks. Tolhuaca (38120 S) is a Late Pleistocene–Holocene stratovolcano, of which the upper part is partially eroded. Basaltic andesites have been emitted from the central crater and some flank vents together with Holocene pyroclastic flows and ash fallouts (Naranjo et al., 2000). Fresh lava flows can be observed, but no historical eruptions have been recorded. Lonquimay volcano (38180 S) is part of a NE-aligned cluster of cones, mainly active during the Holocene (Moreno and Gardeweg, 1989). Basalts to dacites have erupted from the eastern fissures, but andesites widely predominate. Holocene pyroclastic deposits are abundant, but only five historical eruptions have been reported, the last one causing serious damage in 1989 (Moreno and Gardeweg, 1989; Barrientos and Acevedo, 1992; Naranjo et al., 1992, 2000). Further south, Llaima (39360 S) is a Pleistocene– Holocene compound volcano that has emitted mainly basalts and basaltic andesites from the central crater or flank vents. A caldera collapse event formed a depression, which is partially filled by the postglacial cone (Naranjo and Moreno, 2005). A thick succession of postglacial pyroclastic deposits is recognized nearby, which includes the Curacautin Ignimbrite (ca. 13.5 ka) at the base (Naranjo and Moreno, 1991). At least 30 historical eruptions have been reported, the largest in 1640, and active fumaroles are visible in the central crater. Sollipulli caldera (39 S) is a Middle Pleistocene– Holocene ice-filled volcanic complex that erupted basalts to dacites (Naranjo et al., 1993b). On the ancient caldera wall, several subglacial domes were emplaced (Gilbert et al., 1996) and explosion craters formed. The most recent one would be related to the Alpehue´ Ignimbrite eruption in ca. 2.9 ka (Naranjo et al., 1993b). Further south, Caburgua (39180 S) is a cluster of five Holocene pyroclastic cones that have erupted between 8 and 11 ka BP. They emitted basaltic lavas followed by Strombolian and phreatic eruptions that built the cones just above the Liquin˜e–Ofqui fault. Villarrica (39240 S) is a Pleistocene–Holocene compound volcano that has emitted mainly basaltic andesites (Moreno, 2000; Lara and Clavero, 2004; Moreno and Clavero, in press). Two nested calderas form a depression, which was filled by the present cone. A thick sequence of postglacial pyroclastic deposits starts with the Lica´n Ignimbrite (ca. 12.5 ka). A recent small caldera was probably related to the eruption of Puco´n Ignimbrite in ca. 3.5 ka (Clavero, 1996). More than 30 historical eruptions have been reported, the more recent in 1949, 1963 and 1971 causing severe damage in Puco´n village. Small Strombolian eruptions frequently occur due to the dynamics of the crater lava lake. Quetrupilla´n (39300 S) is a Pleistocene–Holocene volcanic complex formed by two nested calderas and several dome complexes and pyroclastic cones (Pavez, 1997). It is lying on top of the Liquin˜e–Ofqui fault and has emitted basalts to rhyolites. A thick postglacial pyroclastic succession has been recognized (Lara and Moreno, 2004). Some historical eruptions were reported, the most recent in 1872.

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Lanı´n (39420 S) is a Pleistocene–Holocene stratovolcano that has erupted basalts and siliceous andesites to dacites (Hickey-Vargas et al., 1989; Lara and Moreno, 2004; Lara et al., 2004a). Mafic magmas were mainly emitted from lateral vents or from an ancient crater rim. Instead, evolved lavas were evacuated through the central conduit. No historical eruptions have been recorded (Lara, 2004). Mocho–Choshuenco (39540 S) is a volcanic complex formed by a juxtaposed caldera and a coalescent Pleistocene stratocone (Choshuenco). Inside the caldera, a Holocene cone was built. Mocho–Choshuenco emitted mainly basaltic andesites to dacites with remarkable postglacial explosive eruptions (McMillan et al., 1989; Etchegaray et al., 1994; Rodrı´guez et al., 1999; Lara and Moreno, 2004). Further south, Carra´n–Los Venados (40180 S) is a cluster of mainly Holocene pyroclastic cones and maars, mostly basaltic in composition, which form a NE-trending alignment (Moreno, 1977; Rodrı´guez, 1999; Lara et al., 2005). Remarkable eruptions have been observed during the twentieth century in 1907, 1955 and 1979. Puyehue–Cordo´n Caulle (40300 S) is a Middle Pleistocene–Holocene volcanic complex formed by the Cordillera Nevada caldera, Cordo´n Caulle fissure system and the Puyehue volcano in a NW-trending alignment (Moreno, 1977; Lara et al., 2005, 2006a, b). After a basaltic shield stage, basalts to rhyolites were emitted together with large volumes of Holocene pyroclastic ejecta. Evolved compositions predominate in the youngest units of both Cordo´n Caulle and Puyehue volcanoes. A remarkable fissure eruption of rhyodacites followed the great magnitude earthquake (Mw: 9.5) in 1960 (Lara et al., 2004b). Casablanca (40420 S) is a Holocene cluster formed by the stratovolcano and several basaltic pyroclastic cones, which form a N- to NE-trending alignment. Late Holocene Strombolian and phreatic eruptions formed a pyroclastic succession nearby. Puntiagudo (41 S; Fig. 3) is a Pleistocene stratovolcano, which formed a NE-trending alignment with the

Fig. 3. Rising in the Patagonian Cordillera, Volca´n Puntiagudo (latitude 41 S), a Pleistocene stratovolcano of basaltic and basaltic-andesitic composition, partially eroded by Holocene and present glaciers (Photo by H. Corbella).

Fig. 4. At the foreground, Cerro Tronador (latitude 41.1 S) is a basaltic to dacitic Pleistocene stratovolcano eroded and covered by glaciers. Behind, the Pleistocene–Holocene Osorno stratovolcano stands out on the horizon (Photo by H. Corbella).

Cordo´n Los Cenizos fissure system and the Osorno volcano. Puntiagudo emitted mainly basalts and basaltic andesites and it is partially eroded by Holocene flank glaciers. Osorno (41060 S; Fig. 4) is a Pleistocene–Holocene stratovolcano that has erupted mainly basalts and basaltic andesites (Moreno et al., 1985). Holocene isolated dacitic domes and basaltic pyroclastic cones lie on the flanks. At least 10 historical eruptions have been reported, the last in 1835 when a NE-trending systems of fissures and cones erupted (Lo´pez-Escobar and Parada, 1991; Moreno, 1999a; Petit-Breuilh, 1999). Tronador (41060 S; Fig. 4) is a partially eroded Pleistocene stratovolcano that erupted basalts to dacites. It is widely covered by glaciers and no clear evidence of Holocene activity exists (Mella et al., 2005). Cayutue–La Viguerı´a (41120 S) is a cluster of Holocene pyroclastic cones and lavas that lie along the Liquin˜e–Ofqui fault (Moreno et al., 1985). Calbuco (41180 S) is a Pleistocene–Holocene compound volcano that emitted mainly andesitic magmas (Lo´pez-Escobar et al., 1992; 1995b). A sector collapse occurred at the early postglacial period when an avalanche flowed to the north (Moreno et al., 1985; Moreno, 1999b). An andesitic dome grew inside the collapse amphitheater. Eleven historical eruptions have been recorded, the last in 1961 (Petit-Breuilh, 1999). The SSVZ (42–46 S) is characterized by the presence of several stratovolcanoes and monogenetic cones located inside the structural duplex of the Liquin˜e–Ofqui fault system. The Yate volcano (41480 S) is a partially eroded Pleistocene–Holocene stratovolcano that erupted basalts to andesites. It is covered by glaciers and exhibits several scars of sector collapses. Two siliceous lava domes inside an amphitheater in the southern flank record the late activity. Yate volcano forms a NE-trending alignment with Hualaihue´–Cordo´n Cabrera volcanoes and it is sitting on top of the Liquin˜e–Ofqui fault. Hualaihue´–Cordo´n Cabrera (41540 S) is a NE-trending alignment formed by the Hualaihue´ (or Apagado) volcano

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego and the Cordo´n Cabrera fissure system. Hualaihue´ is a Holocene basaltic pyroclastic cone built inside a glacial amphitheatre. Cordo´n Cabrera is a Pleistocene–Holocene cluster formed by aligned necks, basaltic pyroclastic cones and a siliceous dome (Lo´pez-Escobar et al., 1993). Hornopire´n (41540 S) is a Holocene basaltic stratovolcano whose summit has three probably historic pyroclastic cones. To the north, it is aligned with a set of fissures and basaltic pyroclastic cones. Huequi–Calle–Porcelana (42240 S) is a cluster formed by the Huequi volcano, a small dome complex with historic eruptions, the Calle postglacial lava dome and the Porcelana Pleistocene volcano, which exhibits Holocene parasitic cones. Michinmahuida (42420 S) is a larger volcanic complex formed by an ice-filled Pleistocene caldera, a central stratovolcano and several parasitic Holocene fissure vents. Mostly basalts and some dacites have been erupted (Kilian and Lo´pez-Escobar, 1991; Lo´pez-Escobar et al., 1993). A thick postglacial pyroclastic succession is recognized nearby. At least two historical eruptions have been reported. Compositionally different, Chaite´n volcano (42480 S) is a complex formed by a Holocene dome grown inside a caldera complex. The basaltic caldera sequence appears deeply eroded while the inner rhyolitic dome and some other parasitic domes are probably related to historical eruptions (Kilian and Lo´pez-Escobar, 1991; Lo´pez-Escobar et al., 1993). Corcovado volcano (43120 S) is a partially eroded Pleistocene stratovolcano, the central neck of which is exposed. Mostly basaltic lavas erupted from the central vent while Holocene andesitic domes appear as isolated or parasitic centers. Palvitad is a group of eroded basaltic lavas and Holocene pyroclastic cones, siliceous domes and maars (Lo´pez-Escobar et al., 1993). Avalanchas–Cordo´n Yelcho (43180 S) is a NWtrending volcanic alignment formed by heavily eroded lava sequences, necks and stratocones partially icecovered. Yanteles volcano (43300 S) is a Pleistocene ice-filled caldera with an inner Holocene pyroclastic cone and a peripheral NE-trending fissure system. Two ancient necks, Nevado and Yeli, take part of the main volcanic alignment. Basalts and basaltic andesites have been erupted. Melimoyu (44 S) is a Pleistocene–Holocene complex formed by an ice-filled caldera. Basalts and basaltic andesites have erupted from this vent. Puyuhuapi (44180 S) is a cluster of Holocene monogenetic cones that lie along the Liquin˜e–Ofqui fault. Alkaline basalts have been emitted from this volcano (Lo´pez-Escobar et al., 1995b). Mentolat volcano (44420 S) is a Pleistocene–Holocene stratovolcano with a small ice-filled summit caldera. Basaltic andesites and andesite lava flows have been erupted together with Holocene pyroclastic deposits (Lo´pez-Escobar et al., 1993; Naranjo and Stern, 2004). Cay (45060 S) is a partially eroded stratovolcano that has erupted basalts to dacites (Lo´pez-Escobar et al., 1993; D’Orazio et al., 2003). Several parasitic cones occur at the flanks in a NE-trending alignment.

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Maca´ (4506 S) is a large partially eroded stratovolcano that emitted basalts and basaltic andesites. Some parasitic cones occur at the flanks in a NE-trending alignment (Lo´pezEscobar et al., 1993; D’Orazio et al., 2003). Late Holocene pyroclastic deposits have been recognized nearby (Naranjo and Stern, 2004). Finally, Hudson volcano (45540 S) is a large Early Pleistocene–Holocene ice-filled caldera complex that emitted basalts to basaltic andesites and minor dacites (Orihashi et al., 2004). Two Holocene flank cones and large pyroclastic deposits have been described (Naranjo and Stern, 1998). The last historical eruption occurred in 1991 (Naranjo, 1991; Naranjo et al., 1993a), with severe ashfall impacts all around Patagonia and Tierra del Fuego (Banks and Iven, 1991; Corbella and Paz, 1991; Hildreth and Drake, 1992; Scasso et al., 1994; Bitschene and Ferna´ndez, 1995). South of the Patagonian gap caused by the Chilean Rise subduction, the AVZ (49–56 S) is composed of six Pleistocene–Holocene stratovolcanoes. Lautaro (49 S) is a partially ice-capped Pleistocene– Holocene stratovolcano that lies on the Southern Patagonian Ice Field. Andesites and dacites together with pyroclastic deposits have been erupted (Orihashi et al., 2004; Motoki et al., 2006). Six possible historical eruptions, the most recent in 1959–1960 (Martinic, 1988), make Lautaro the most active volcano of the AVZ. Viedma (49180 S) is an ice-covered stratovolcano that emerges from the Southern Patagonian Ice Field where mainly andesites have been erupted. Possible historical eruptions would have been fed from the four nested summit craters. Aguilera (50180 S) is a stratovolcano that has erupted mainly dacites and pyroclastic ejecta (Futa and Stern, 1988). Reclus (50540 S) is an ice-covered stratovolcano that has erupted mainly dacites and pyroclastic ejecta. The upper part may be postglacial and even possible historical eruptions have been reported (Harambour, 1988). Mount Burney (52180 S) is a volcanic complex formed by a somma and an inner stratocone. Andesites to dacites have been erupted and possible historical eruptions were also reported. Finally, Cook (54540 S) is a cluster of postglacial pyroclastic cones and domes located in southwestern Tierra del Fuego that has mainly erupted calc-alkaline andesites. Possible historical eruptions have been reported (Sua´rez et al., 1985).

5. Volcanism in Extra-Andean Patagonia In extra-Andean Patagonia, south of 39 S and east of the present Andean magmatic arc, plateau basalts cover over 120,000 km2, twice the size of Ireland. The basalts crop out as remnants of multiple lava-flow sequences that built plateaus or conspicuous isolated ‘‘mesetas’’ which unconformably overlie Mesozoic and Cenozoic volcanic rocks and sediments. These near-horizontal lava sequences, frequently with slight eastern dips, present almost flat surfaces and many of them have been controlled by structural plains developed before the lava

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emissions (Panza, 2002). Occasionally, it is possible to observe several structural plains that have been laid out in steps. The high front scarps of the ‘‘mesetas’’ are mainly the product of mass-wasting favored by the sharp lithological contrast between the more resistant basalts and a substratum of friable sedimentary rocks. Unlike the varied calc-alkaline lithology of the Andean magmatic arc, olivine basalts (s.l.) are predominant in the extra-Andean region. Nevertheless, the chemistry of these basic rocks shows an important compositional variety in the alkaline and subalkaline fields. Alkaline olivine basalts, hawaiites, basanites, nephelinites, leucitites, tholeiitic basalts, basaltic andesites and mugearites have been mentioned so far. The ‘‘mesetas’’ culminate in monogenetic volcanic centers such as cinder, spatter and scoria cones, maars, and necks that once were feeders of the volcanic features. Caused by Strombolian low explosive eruptive activity, the spatter and scoria cones, generally reach moderate heights, ca. 100–200 m. The flat surface of the basaltic ‘‘mesetas’’, especially in southern Patagonia, is pockmarked by near-circular closed depressions occupied by ponds or lacustrine sediments called ‘‘bajos sin salida’’. They vary in diameter, from tens to up to 3000 m, with shallow depths occasionally reaching 100 m and over. As to their origin, Methol (1967) considered that subwash and subsequent transportation and removal of the underlying rocks formed a cavity beneath the basalts that, without mechanical support, collapsed causing the depression. Volkheimer (1972) proposed a tectonic control and Panza (1982, 1995a, 2001) suggested inverted relief due to differential erosion on initially emerged parts of a step-toe. Lastly, in some cases, an explosive phreatomagmatic mechanism must be considered (Corbella, 2002). Plio–Pleistocene magmatism along extra-Andean Patagonia took place in two distinct tectonic scenarios, north and south of 46300 S. From 39 to 46300 S alkali basalts, basanites and hawaiites erupted forming reduced outcrops linked with normal faulting. South of 46 S, the extra-Andean basaltic outpourings, covering a much bigger area and expanding hundreds of kilometers eastward from the Andean Cordillera, were assigned to a slab window (Ramos and Kay, 1992; Gorring et al., 1997, 2002, 2003; D’Orazio et al., 2001, 2004, 2005; Gorring and Kay, 2001; Kay, 2002a; Kay et al., 2004b). 5.1. Northern Extra-Andean Volcanism (39–46300 S) From the Early Pliocene, the Patagonian extra-Andean volcanism between 39 and 46300 S took place further east of the present Andean volcanic arc and largely to the west of the Somuncura´ volcanic complex. Prior to the Plio–Pleistocene volcanism, Late Oligocene–Early Miocene intraplate eruptions of the Somuncura´ complex – the largest plateau magmatic event in Patagonia – occurred (Corbella, 1984; Ardolino and Franchi, 1993). The origin of these voluminous outcrops was associated with a hotspot or mantle thermal anomaly (Kay et al., 1993a), and with a hot asthenospheric corner flow channelized into the mantle wedge by rollback

of the subducting plate (de Ignacio et al., 2001). The last eruptive manifestations of this notable magmatic event are scarce and occur as isolated Plio–Pleistocene outcrops: Cerro Trayen Niyeo, Meseta del Cuy, and postplateau cones, most of them located on the center and western side of the Somuncura´ complex. Northern extra-Andean Patagonia is crossed by NNWand NW-trending faults, which bound grabens along Bio ˜ irihuau and Mamil-Choique Bio-Alumine´; Collo´n Cura, N alignments; or host linear valleys of Cushamen, Arroyo ˜ orquinco, Chico-Genoa-Senguerr and Chubut Medio rivN ers (Gonza´lez Dı´az, 1978; Ramos, 1978; Ramos and Corte´s, 1984; Nullo et al., 1994; Panza and Nullo, 1994; Delpino and Deza, 1995; Lizuain et al., 1995). Comallo and Gastre are other oblique long-lived structural systems (Coira et al., 1975a) that intersect the Andean Cordillera (Fig. 1). Many Plio–Pleistocene volcanic outcrops are spatially related to these NNW–NW fractures or with secondary faults developed in proximity to the former. Most of Plio–Pleistocene extra-Andean lavas (39–46300 S) are silica-poor alkaline rocks, lack highly differentiated lithologies, and bear peridotitic xenoliths. The presence of these inclusions and the scarce geochemical evidence of crustal signatures have been interpreted as evidence of fast ascent through the crust, without ponding in intermediate magmatic chambers (Stern et al., 1990; Ntaflos et al., 2000; Stern, 2004; Kay et al., 2004b). The northern Pliocene extra-Andean basaltic eruptions are represented by the Coyocho, La Caban˜a and Epulef formations and Trayen Niyeu, Pereyra, Rumay and Huala´ Basalts. Basalts from the Coyocho Formation (Leanza and Leanza, 1979) crop out within a wide NNW-trending belt between 38400 and 40300 S at 70 W. They form extensive subhorizontal outcrops up to 70–100 m thick. Also known as Basalto II (Groeber, 1946a, b, 1947; Galli, 1969), Tipilihuque Formation (Turner, 1973, 1976) and Coyocho Formation (Rolleri et al., 1984; Cucchi, 1998), these basalts unconformably cover the Miocene Collo´n Cura Formation. K/Ar determinations indicate ages of ca. 4.9–4.6 Ma (Cortelezzi and Dirac, 1969). La Caban˜a Formation (Nullo, 1978) is composed of volcanic and pyroclastic rocks exposed mainly west of the Northern Patagonian Massif ca. 41 S–70 W. La Caban˜a Formation considered Pliocene in age (Cucchi, 1998, 1999) includes olivine basalts from the lower Loma Alta member, trachytic and trachyandesitic vitroclastic tuffs and lapillites from the Ojos de Agua member, and basalts from the upper Atraico´ member (Coira, 1979). In Sierra de Mesaniyeu this formation was also called Mesaniyeu Basalt (Cucchi, 1998, 1999), where it forms a large plateau. The Trayen Niyeu Basalt (Remesal et al., 2001) crops out in the northwest boundary of Meseta de Somuncura´ (41050 S–67500 W) far from the Andean Cordillera. The Cerro Trayen Niyeu is composed of alkaline olivine basalts with thick pyroclastic intercalations. These basalts have xenocrysts and peridotitic xenoliths. Radiometric dating indicates a Late Pliocene age of ca. 2 Ma (Cortelezzi and Dirac, 1969). The Pereyra Basalt (Getino, 1995) in the Meseta de Colitoro (41 S–69 W) and the basaltic lava flows south

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego 0

of Sierra de Pire Mahuida (4210 S) are also assigned to this epoch (Salani and Pa´rica, 1990). The Rumay Basalt (Nullo, 1978) crops out at the Meseta Escorial de Lipetre´n (41500 S–69400 W). It is composed of Pliocene olivine basalts in a pile up to 40 m thick at the edges. The Epulef Formation (Turner, 1983) crops out at Sierra de Languineo (43120 S–70000 W), where it forms small ‘‘mesetas’’. It is composed of basaltic lavas with interbedded pyroclastic deposits and has a maximum thickness of 20 m. The Huala´ Basalt (Ploszkiewicz and Ramos, 1977) is located in the foothills of the Andean Cordillera next to Lago Fontana (45450 S). Lava piles of olivine basalts form ‘‘mesetas’’ with the feeder cones jutting out. The largest outcrop is observed at Cerro Huala´ (44520 S–71130 W). These basalts also appear at the headwaters of Rı´o Apeleg Chico and Arroyo Seco, at the northern edge of Sierra de Payaniyeu and upper Rı´o Apeleg (Ploszkiewicz, 1987; Lapido and Ma´rquez, 1999). K/Ar determinations yielded ages between ca. 6 and 4 Ma (Sinito, 1980; Ramos, 1981). Pleistocene volcanic rocks were recognized in Chenqueniyeu, Genoa-Sengerr, Pichi Huala´ Basalts and Mojo´n Formation. The Chenqueniyeu Basalt or Campana Formation (Gonza´lez Bonorino, 1944) crops out in a NNW-trending belt, between the Limay (40300 S–70450 W) and Chico rivers (41450 S). Composed of olivine basalts, it crowns the Chenqueniyeu, Las Bayas and Cerro Campana ‘‘mesetas’’, with maximum thickness of ca. 30 m at the rims (Feruglio, 1941, 1947; Volkheimer, 1964; Dessanti, 1972; Turner, 1973; Volkheimer, 1973; Rabassa, 1975; Ravazzoli and Sesana, 1977; Nullo, 1978; Gonza´lez, 1998; Giacosa and Heredia, 2002). On the Chenqueniyeu ‘‘meseta’’, the effusive center is a shield volcano. On the other plateaus, vents as scoria and spatter cones up to 300 m high have been reported. The chemical composition indicates a transitional nature, between cratonic and arc-like lithologies (Stern et al., 1990; Kay and Gorring, 1999; Ntaflos et al., 2000; Kay et al., 2004b). The Mojo´n Formation (Ravazzoli and Sesana, 1977) crops out in the same area as the Cra´ter Formation (ca. 41400 S–70120 W) lying on top of Quaternary sediments. The lavas fill a NS-trending valley, giving origin to the Escorial Mamil Choique 80–100 m thick. The Genoa-Senguerr basalts crop out along these rivers between 44300 and 46 S. Basaltic and basanitic centers appear in a NNW alignment at the Cerros Saiquen, de los Chenques, Pedrero, Grande, Mirador and Manantiales Grandes. In most of them, lava flows reach the Quaternary terraces of the Senguerr and Genoa rivers 70 W (Quartino, 1957; Ferello, 1969). The Pichi Huala´ Basalt (Ploszkiewicz and Ramos, 1977) crops out in Lago Fontana area (45 S). It is a Late Pleistocene and Holocene sequence of lavas lying on glaciofluvial terraces (Ramos, 1981; Ploszkiewicz, 1987). Between 39 and 46 S, the extra-Andean most recent Holocene volcanics belong to the Cra´ter Formation and the Aneco´n Chico Basalt. Basalts from the Cra´ter Formation (Ravazzoli and Sesana, 1977) appear as large and discontinuous outcrops

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emplaced in a NNW belt, from Arroyo Comallo (40580 S–70130 W) to Cerro Ventana (42150 S). They form ‘‘mesetas’’, small cones and intracanyon channeled lava flows that fill Quaternary valleys and cover postglacial deposits (Methol, 1968; Nullo, 1978, 1979; Proserpio, 1978; Coira, 1979; Volkheimer and Lage, 1981; Nun˜ez and Cuchi, 1997; Cucchi, 1998). The Cra´ter Formation is composed of alkaline olivine basalts and hawaiites, bearing xenocrysts, and peridotitic xenoliths (Gelos and Hayase, 1979; Massaferro et al., 2002). These lavas were originally assigned to the Holocene (Ravazzoli and Sesana, 1977) but dated ca. 0.8 and 1.9 Ma by Mena et al. (2005). The Aneco´n Chico Basalt (Cucchi, 1998), which lies over the Collo´n Cura Formation and the Mesaniyeu Basalt, forms small cones, such as the Cerro Aneco´n Chico (40560 S–69470 W) and small ‘‘mesetas’’ that cap the Sierra de Mesaniyeu. These basalts have been correlated with the Cra´ter Formation (Cucchi, 1998). 5.2. Southern Extra-Andean Volcanism (46300 –52 S) South of 46300 S the extra-Andean basaltic outpours extend hundreds of kilometers eastward from the Andean Cordillera forming huge lava fields. Alkaline and subalkaline volcanics were erupted following the end of subduction due to the Chile ridge–trench collision and the formation of a slab-free window (Ramos and Kay, 1992; Gorring et al., 1997). In this area, available geochronological data suggest that the eruptive activity started with Late Miocene to Early Pliocene (12–5 Ma) subalkaline or moderately alkaline voluminous effusions, named the ‘‘main-plateau sequence’’. It was followed by the Plio–Pleistocene (7–2 Ma), less voluminous, more alkaline ‘‘post-plateau sequence’’. Main-plateau lavas are considered to represent large magma volumes related to a high degree of partial melting, whereas post-plateau lavas can represent small volumes of low-degree partial melts within the garnet stability field in the asthenospheric mantle (Baker et al., 1981; Gorring et al., 1997, 2002, 2003; D’Orazio et al., 2004, 2005). The fissure systems feeding the extensive lava fields are frequently hard to distinguish because they are covered by large lava piles. In some cases, the alignment of vents allows to infer the fault planes that controlled the effusions. In the southernmost outcrops, active strike-slip faulting controls the onset of the extra-Andean basaltic flows. During the Pleistocene, extensional conditions gave place to the formation of grabens in Pali-Aike and Camusu-Aike volcanic fields (Corbella, 2002; Haller et al., 2002; Corbella, 2004). The Meseta Lago Buenos Aires is a basaltic plateau of ca. 5000 km2 that extends from 46 to 47 S and 70 to 72 W. The basaltic lava flows of the plateau sequence (Lago Buenos Aires Formation; Lapido, 1979) erupted between 11 and 7 Ma, unconformably covering subhorizontal Miocene sediments (Ugarte, 1956) with a thickness of up to 30 m. The best represented lithologies are quartz tholeiites, olivine tholeiites and, to a lesser extent, alkali basalts (Hashimoto et al., 1977; Niemeyer, 1978, 1979; Baker et al., 1981; Brown et al., 2004).

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Most of the post-plateau sequence lavas, for example, the El Sello Formation (Busteros and Lapido, 1983), are basanites and highly potassic leucite basanites, alkali basalts and trachybasalts of Early Pleistocene age (Baker et al., 1981; Gorring et al., 2002). The last erupted lavas, the so-called Cerro Volca´n Basalt (Escosteguy et al., 2003), were considered Middle Pleistocene in age. Several of the younger lava flows overlie moraines or till deposits and could be Late Pleistocene (Mercer and Sutter, 1982; Ton That et al., 1999; Singer et al., 2004a, b). Radiometric ages for the basaltic sequences were given by Charrier et al. (1977, 1979), Sinito (1980), Baker et al. (1981), Mercer and Sutter (1982), Guillou and Singer (1997) and Ton That et al. (1999). As pointed out by Brown et al. (2004), data covering the last 11 Myr show that basaltic volcanism was episodic rather than continuous. The ages define at least 10 volcanic pulses at ca. 11–10 Ma, 7.3–7.8 Ma, 3.2–3.0 Ma, 2.4 Ma, 1.7 Ma, 1.35 Ma, 1.0 Ma, 750 ka, 430–330 ka and 109–66 ka. At the southwestern end of the ‘‘meseta’’, Pliocene subvolcanic bodies, dismantled volcanoes and eroded lava flows of trachytes and trachyandesites have been surveyed. Collectively named as the Cerro Lapiz Trachyte (Giacosa and Franchi, 2001), they intrude or lie over the Meseta Lago Buenos Aires Formation and in turn they are covered by lavas of the El Sello Formation. In turn, the Meseta Chile Chico, immediately west of Meseta Lago Buenos Aires, comprises alkali basalts dated at 8–4 Ma, with two interbedded acid rhyolitic flows (Espinoza et al., 2003; Guivel et al., 2005). Spinel harzburgite xenoliths were described in some basaltic necks (Niemeyer, 1978), and subvolcanic rhyolitic bodies were dated at 3.6 Ma (Charrier et al., 1977, 1979). The Deseado Massif, within 46300 –49 S and 66– 70300 W, has been exposed to multiple basaltic spills since the Upper Cretaceous. After a substantial diminution of eruptivity, the volcanism was reactivated in the Late Miocene–Early Pliocene, with the emission of the Cerro Tejedor Basalt (Sacomani, 1984a, b; Panza and Marı´n, 1996) and the Cerro Mojo´n Basalt (Panza, 2001) dated at 4.8 and 5.6 Ma, respectively (Gorring et al., 1997; Panza and Franchi, 2002). During the Late Pliocene–Early Pleistocene, the basaltic eruptions once more reached another peak of activity. Large areas were covered by basaltic outpours (Marı´n, 1982, 1984; Sacomani, 1984a, b), which are named La Angelita Basalt, dated at 2.8 and 1.9 Ma (Panza, 1982, 1984, 1986, 1995a, b) and characterized by the great extent and thickness of the lava flows and lava fields. Peridotite xenoliths collected in Plio–Pleistocene ash cones and lavas of Gobernador Gregores area were described by Bjerg et al. (2002) and Aliani et al. (2004). South of Lago Posadas (47300 S–71460 W), the Belgrano, Aguila, Guitarra, Del Pobre and Del Olnie ‘‘mesetas’’ are crowned by the Belgrano Basalt (Riggi, 1957; Ramos, 1979). The Belgrano Basalt, mostly tholeiitic, with an age of ca. 10.1 Ma (Gorring et al., 1997) belongs to the main-plateau sequence, and precedes the Plio–Pleistocene glaciations. Between Lago Azul and Laguna Olnie (47450 S– 71300 W) over a deeply eroded relief lie the lava flows of the Olnie Basalt (Ramos, 1982a). Its alkaline

chemistry and Early Pliocene absolute ages of 4.0 and 3.8 Ma (Ramos, 1982a; Gorring et al., 1997) allow it to be correlated to other post-plateau basalts. North and northwest of Lago Cardiel (49 S–71140 W), the Del Strobel and De La Muerte ‘‘mesetas’’ are covered by 20–50 m thick basaltic lava flows (the Strobel Basalt; Ramos, 1982b) with radiometric ages of ca. 8.6 and 6.0 Ma, which are correlated with the main-plateau sequence. Between the Cascajosa and De La Muerte ‘‘mesetas’’, the Strobel Basalt is covered by olivine basaltic andesites – the Las Tunas Basalt – with radiometric ages of ca. 5–4 Ma (Ramos, 1982b; Gorring et al., 1997), which belong to the post-plateau sequence. The volcanic sequence culminates at Rı´o Cardiel with the La Cueva Basalt (Ramos, 1982b), which name refers to the lava caves up to 100 m long and 4 m high found there. These lavas lie unconformably above the Las Tunas Basalt and are ca. 4 Ma old. Another ‘‘meseta’’ between the Santa Cruz and Chalı´a rivers (50 S), extending over 2500 km2, is formed by sediments of the Miocene Monte Leo´n and Santa Cruz formations, capped unconformably by Pliocene lavas from the La Siberia Basalt and by Level I gravels, that were later covered by the Laguna Barrosa Basalt (Strelin et al., 1996, 1999; Cobos and Panza, 2003). The main vents of these younger lavas are Cerro Bi-Aike and other unnamed cones. The lavas flowed southeastward up to the Rı´o Santa Cruz valley, where they cover the Level II terraces (Panza, 2002) and the Early–Middle Miocene Condor Cliff Basalt. K/Ar data for the Laguna Barrosa Basalt indicate ages between 3.52 and 2.25 Ma (Schellmann, 1999; Wenzens, 2000). The Camusu-Aike Volcanic Field, with a surface of ca. 200 km2, crops out between 50170 –50370 S and 71000 – 71190 W. The volcanic rocks lie atop the Meseta Pampa Alta, a high proglacial plain located south of the upper Rı´o Santa Cruz valley (Strelin et al., 1999). It is composed by a sequence of lava flows emitted from several cones and eruptive fissures of two main NW and NE fracture systems. The area was affected by extensional tectonics, which generated a NW–SE graben in the northeastern sector (Haller et al., 2002; D’Orazio et al., 2005). The lithology includes tholeiitic basalts, hawaiites and quartz-normative basaltic andesites. Two absolute 40Ar/39Ar determinations yield ages of ca. 2.98 and 3.02 Ma (Mejia et al., 2004). Southeast of Lago Argentino and northeast of the Cordillera del Paine, the Meseta Las Vizcachas (50300 – 51000 S) rises up to 1500 m a.s.l. over 1400 km2. The ‘‘meseta’’ is topped by plateau and post-plateau basaltic sequences. The oldest unit is a thick and extended (ca. 200 km2) subhorizontal sequence of lavas, breccias, tuffs of tholeiitic basalt and hawaiitic composition with interbedded glacial and fluvial sediments and considered Late Pliocene–Early Pleistocene in age, deeply carved by glacial erosion (Mun˜oz, 1982; Strelin et al., 1996; D’Orazio et al., 2005). The youngest unit, ca. 10 km2 large and less than 100 m thick, consists of lava flows that partially fill glacial valleys, necks and basaltic dikes, sometimes bearing peridotitic xenoliths (Mun˜oz, 1981). These lavas, also affected by deep glacial erosion, were considered Late Pleistocene in age. By the Rı´o Turbio, the Glencross outcrops (51500 S– 71420 W) include three volcanic necks, Mt. Phillipi,

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego Mt. Domeyko and Mt. Cuadrado, rising ca. 200 m above the surrounding moraines in an EW-trending alignment. The necks are composed by subalkaline basalts and basaltic andesites (D’Orazio et al., 2001) that yield radiometric ages of ca. 8.5 and 8.0 Ma (Meglioli et al., 1990; Meglioli, 1992). In turn, the Pali-Aike volcanic field crops out north of the Magellan Strait (51400 –52200 S and 69100 –70500 W). Flanked to the south and west by various moraine systems, most of the emissions – scoria and spatter cones, maars and subvolcanic bodies – were controlled by two fracture systems: one with NW direction, the result of the rejuvenation of an ancient rift system, and the other with E–W direction (Chelotti and Trinchero, 1990, 1991; Corbella et al., 1991; Agostini et al., 1999; Corbella, 2004). The lithology comprises alkaline basalts and basanites (Skewes, 1978; Skewes and Stern, 1979; Stern, 1990; Corbella et al., 1991, 1996; D’Orazio et al., 2000) bearing a rich variety of peridotitic and granulitic xenoliths (Skewes and Stern, 1979; Selverstone, 1982; Selverstone and Stern, 1983; Stern et al., 1985, 1986a, b, c, 1989, 1999; Stern, 1989; Kempton et al., 1999a, b; Kilian et al., 2002; Vannucci et al., 2002). Most of the lava outpours took place between the Late Pliocene (3.82 Ma) and the Holocene, but there is also evidence of former Late Miocene basalt layers (Mercer, 1976; Meglioli, 1992; Corbella, 1999, 2002; Ton-That et al., 1999, ; Mejı´a et al., 2004). Finally, Cerro Pampa (47550 S), Puesto Nuevo (48590 S) and Chalte´n (49250 S) Miocene outcrops of hornblende-bearing dacites with adakite geochemical signatures are located east of the Andean axis (Ramos et al., 1991, 1994, 2004b; Kay et al., 1994; Kay, 2002b; Ramos, 2002a, b). Recent age determinations show that these adakites decrease in age northward. This is consistent with the melting of the trailing edge of the Nazca plate as the Chilean ridge–trench collision progressed northward (Kay et al., 2004a; Ramos et al., 2004b).

6. Quaternary Tephras Tephra is the name given to the solid materials ejected from the crater in an explosive volcanic eruption. The fine fraction of tephras, transported and dispersed by the predominant winds, can travel great distances and form widespread ash layers on different depositional environments. Because of the main direction of tropospheric winds, most of the fine-grained pyroclasts are dispersed to the east and southeast reaching extraAndean Patagonia, although proximal facies of both pyroclastic density currents and ash fallouts are also placed in the Andean Cordillera. In general, ash fallouts provide significant stratigraphic markers allowing correlations over huge distances and they also permit the study of the eruptive dynamics and frequency of volcanic eruptions confining a wide range of natural and human events. In southern South America, during the two major Plinian volcanic eruptions recorded in historical times, Quizapu in 1932 and Hudson in 1991, the fine-grained ashes reached places thousands of kilometers away from their sources (Banks and Iven, 1991; Corbella and Paz,

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1991; Hildreth and Drake, 1992; Scasso et al., 1994; Bitschene and Ferna´ndez, 1995). Proof of this are the Patagonian volcanic glass shards identified from dust deposited in east Antarctica during the last glacial periods (Basile et al., 1997). Auer (1946, 1948a, b, 1950, 1965), pioneer in tephrochronological studies in South America, was the first to emphasize the importance of tephras in lacustrine sediments, soils and peat bog to correlate glacial, climatic and flora events during the Holocene. His work, together with those of Sahlstein (1932) and Salmi (1942), outlined the first Patagonian tephrochronology studies. Later studies on tephras from Tierra del Fuego and the AVZ enlarged and improved the knowledge of nature, age and dispersion area of several tephra layers coming from the Hudson, Lautaro, Aguilera, Reclus and Burney volcanoes (Heusser et al., 1990; Stern, 1990, 1991, 1992, 2000; Naranjo et al., 2001; Kilian et al., 2003; Markgraf et al., 2003; Orihashi et al., 2004). Near the Magellan Strait, Paleoindian cremation burials were found in Pali-Aike cave (52 S) lying on white volcanic ashes (Bird, 1938, 1983) that were assigned to the ‘‘first cycle of Postglacial volcanism’’, the so-called Tephra I, and dated according to Auer at ca. 9000 yrs BP (Massone, 1981). In the Pali-Aike volcanic field, several tephra layers were recovered in lacustrine sediment cores in Laguna Azul and Laguna Potrok-Aike (Haberzettl et al., 2006; Zolitschka et al., 2006). Hudson volcano (45540 S) is the most active volcanic center in SSVZ. Tephra layers preserved in soil and sedimentary deposits record at least 12 explosive Holocene eruptions (Naranjo and Stern, 1998), including the 1971 and 1991 Plinian eruptions, the last one ejecting more than 4 km3 of pyroclastic material. Early Holocene Hudson tephras (ca. 11,910 and 9960 yrs BP) were found as thin layers in lake sediments on Taitao Peninsula, 150 km southwest of the volcano (Lumley and Switsur, 1993). Other young tephra layer (<10,000 yrs) occurs in Las Guanacas cave, Aise´n, 100 km southeast of Hudson volcano (Mena, 1983; Stern, 1990, 1991; Stern and Naranjo, 1995). In extra-Andean Patagonia, lacustrine cores and sediments of Lago Cardiel area (48500 S–71200 W) show two levels of tephras (Markgraf et al., 2003), one of them coming from Hudson Volcano. The major explosive events of Hudson volcano occurred in ca. 6700 yrs BP, 3600 yrs BP and 1991. The 6700 yrs BP deposit is the largest among them. It crops out as a gray-green andesitic tephra layer with a thickness decreasing southeastward, although a secondary maximum thickness occurs 900 km to the south in Tierra del Fuego, covering ca. 40,000 km2 (Stern, 1991, 1992). This eruption, which is considered to be the largest for any volcano in the southern Andes during the Holocene, may have created the 10 km diameter summit caldera (Naranjo and Stern, 1998). To the north, in the Andean Cordillera between latitude 42300 and 45 S, tephras from 11 Holocene explosive eruptions from 7 stratovolcanoes have been recognized (Naranjo and Stern, 1998, 2004). These eruptions of Chaite´n, Michimahuida, Corcovado, Yanteles, Melimoyu, Mentolat and Maca´ volcanoes occurred

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between 9370 and 1540 yrs BP. This implies a rate of one explosive eruption every 725 yrs in this segment of the Andean SVZ. Further north, in the Lago Nahuel Huapi area (41 S), along the road between Portezuelo Puyehue and Nahuel Huapi, seven Holocene tephra layers have been pointed out (Collini, 1943; Laya, 1977). The tephras recovered by piston coring from Lago Mascardi, Mallı´n Grande, Brazo Huemul, Villa La Angostura and Laguna de Carilaufquen were analyzed and described by Mazzoni (1983), del Valle et al. (1996) and Tatur et al. (2002). In Lago Mascardi (41200 S), tephra layers were deposited during postglacial times. More than 60 pyroclastic layers have been identified in an 11 m core of lacustrine sediments from the last 15,000 yrs. Rhyodacitic, dacitic, andesitic, basandesitic and basaltic tephras were dated and geochemically correlated with young rocks from Osorno, Puyehue and Calbuco volcanoes (Villarosa et al., 1999, 2001). At Villarrica volcano (39250 S), one of the historically most active volcanoes of the Patagonian arc, two ignimbritic events occurred at ca. 13,800 and 3635 yrs BP, leaving large pyroclastic deposits (Lohmar et al., 2005; Silva et al., 2005). Over the flank of the eroded Huanquihue´ volcano (39480 S), El Escorial cone emitted black tephras that underlie Holocene white lapilli tephras (Risso, 1977; Corbella and Alonso, 1989; Mazzoni and Stura, 1993) belonging to Rı´o Pireco Formation, which was dated at ca. 1400 yrs BP. Finally, on the Atlantic coast of Buenos Aires (38–39 S), distal deposits of rhyolitic cinerites have been described and dated. The 20,900 BP ashes could be correlated with present north Patagonian volcanoes distant 1000 km to the southwest (Osterrieth and Martı´nez, 1992; Corbella et al., 2000).

7. Volcanism and Glacial Processes The most extended Neogene glacial deposits in South America are found in the Patagonian Andes. Ice fields and mountain glaciers covered the land from 39 S to Cape Horn (55500 S). In the northern section the glaciers remained restricted to the mountains and piedmont. On the western slope of the Andean Cordillera, from Chiloe´ Island (42 S) to Cape Horn, the ice field outlet glaciers have repeatedly reached the Pacific Ocean. South of 46 S, toward the east, glaciers advanced over the extra-Andean plains reaching the present Atlantic shoreline from Rı´o Gallegos (51430 S) to the south (Rabassa, 1999). Because of the great extent of Late Cenozoic glacial processes in South America, which started during the Late Miocene and include several Plio–Pleistocene pulses (Mercer, 1969, 1976, 1983; Clapperton, 1993; Malagnino, 1995; Denton et al., 1999; Rabassa, 1999; Schellmann, 1999; Ton That et al., 1999; Rabassa and Coronato, 2002), the study of Neogene volcanism in Patagonia cannot be separated from the glacial history. Moreover, Pleistocene–Holocene volcanoes partially grew beneath the glaciers, which exerted also a physical influence on eruptive styles.

The first approach for a volcanic stratigraphy of active volcanoes is to determine the level of glacial erosion or the relation of volcanic units with some well-known glacial deposits. Present improvement of dating methods, such as 40 Ar/39Ar, allows an absolute chronostratigraphy of both glacial and volcanic processes. In the area of Copahue volcano (37450 S) two glacial periods were recognized, the principal one filling the caldera depression (Groeber, 1925). Subglacial eruptions cropping out as extrusive domes and flat-top outpourings linked to the glacial deposits were dated by 14C at 30,000 yrs BP (Bermu´dez and Delpino, 1999; Gonza´lez Dı´az, 2003). The Solipulli caldera (39 S) and the internal silicarich domes were described by Gilbert et al. (1996) as a constructional structure related to the influence of the thick ice-filling of an older depression. Eastward of Lanı´n volcano (39300 S), several drift deposits have been recognized whose minimum age is constrained by the Pino Santo andesite (Rabassa et al., 1990), dated at ca. 90 ka (Lara et al., 2004a). Both north and southern flanks of Lanı´n volcano present evidence of magma–ice interactions to a greater extent. Further south, in the Lago Nahuel Huapi area (41 S) two groups of moraines have been described (Feruglio, 1949–1950; Flint and Fidalgo, 1963; Schlieder, 1989; Rabassa et al., 1990; Rabassa and Evenson, 1996). The glacial deposits interbedded with volcanics at the foot of Cerro Tronador dated 1.32 and 1.39 Ma have been correlated with the oldest drift phases (Rabassa and Clapperton, 1990; Rabassa and Evenson, 1996). Geochronologic data suggest that volcanic activity in the Tronador Volcanic Complex started 1.0 Ma (Mella et al., 2005) and ended before the last two glacial cycles recognized in this region of the Andes, which occurred between ca. 262–132 and 70–14 Ka (Mercer, 1976; Porter, 1981; Clapperton, 1993). Phreatomagmatic deposits formed in a subglacial environment between 0.47 and 0.34 Ma are in part coincident with the last of these glacial cycles (Mella et al., 2003a, b, 2005). In the northwest corner of Meseta Lago Buenos Aires, a large till deposit 30–40 m thick lies between two basaltic lava flows. These flows, K/Ar dated at ca. 7.0–6.75 Ma and 5.0–4.43 Ma, respectively, indicate that a glacial advance took place in such early times as the latest Miocene. Three other basaltic lava flows overlying moraines or till deposits have been dated at 0.125, 0.764 and 1.016 Ma (Mercer and Sutter, 1982; Singer et al., 2004a, b). North of Lago Viedma, at Meseta Desocupada (49280 S–72250 W) and Meseta Chica (49320 S– 72150 W), basaltic lava flows are interbedded with till deposits. The ages of the lavas in these two ‘‘mesetas’’ are 3.48–3.55 and 3.50–3.68 Ma, respectively (Fleck et al., 1972; Mercer et al., 1975; Mercer, 1976). In an unnamed hill 1240 m a.s.l., (50270 S–72170 W), till deposits lie between two basaltic lava flows. Radiometric determinations indicate an age of 1.99 Ma (Fleck et al., 1972; Mercer et al., 1975; Mercer, 1976). Between Lago Argentino and Angostura Fortaleza Moraine sequences of at least five to six foreland glaciations have been noticed (Schellmann, 1999). In the upper Rı´o Santa Cruz valley (50150 S), the oldest Pleistocene glaciation reached the basaltic narrow

Late Cenozoic Quaternary Volcanism in Patagonia and Tierra del Fuego pass of Angostura Fortaleza, 75 km east of Lago Argentino. At Condor Cliff (50080 S–70480 W), basalts dated at ca. 2.66 and 2.95 Ma are interbedded with the Patagonian gravels (Mercer et al., 1975; Mercer, 1976). In the same valley, at Cerro Nunatak (50110 S–71150 W), glaciofluvial gravels interbedded with pyroclastic layers cover a thick glaciolacustrine deposit (Strelin and Malagnino, 1996; Strelin et al., 1999). South of Lago Argentino, Cerro del Fraile (50330 S– 72400 W) is composed of Cretaceous sediments unconformably capped on the western slope by ca. 180 m of interstratified till deposits and lava flows (Feruglio, 1944; Fleck et al., 1972; Mercer et al., 1975; Mercer, 1976; Rabassa et al., 1996). The sequence is composed of 10 lava flows interbedded with at least 7 till deposits. Absolute dating of the lava flows indicate that these eruptions occurred between 2.18 and 1.07 Ma (Singer et al., 2004a, b). Finally, the largest eastward-extending till deposits crop out in Pali-Aike, north of the Magellan Strait. The age of the Bella Vista Drift was estimated at ca. 1.2 Ma by dating the over- and underlying basaltic lava flows (Mercer, 1976; Meglioli, 1992; Mejia et al., 2004). Thus, the close relationship between glacial and volcanic processes shows a great extent in Patagonia with both genetic and stratigraphic consequences.

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Quaternary extra-Andean volcanism contrasts with the previous large backarc effusions from the Late Miocene and Early Pliocene because of its minor volume. During the Holocene, while the centers along the Quaternary volcanic front remained active, volcanic eruptions in the extra-Andean area were less frequent. Only the youngest outcrops of the Pichi Huala´ Basalt, the Cra´ter Formation and the Pali-Aike volcanic field – till now without radiometric age data – and the Cerro Volca´n Basalt have been considered Holocene. Nevertheless, along the Pacific but mainly in Atlantic Patagonia, tephras transported by the predominant western and northwestern winds are found as distal pyroclastic deposits of the Andean eruptions. Such is the case of several recognized tephra layers from the Hudson, Chaite´n, Michimahuida, Corcovado, Yanteles, Melimoyu, Mentolat, Maca´, Osorno, Puyehue and Calbuco volcanoes left there during the last 15,000 yrs. In southern South America, from Late Miocene to Holocene, extended glacial processes were coeval with volcanic eruptions. Modern dating methods applied on volcanic rocks interbedded with glacial deposits allow dating both glacial and volcanic processes along the Andean and extra-Andean Patagonia.

References 8. Concluding Remarks In Patagonia, Late Cenozoic volcanic activity is closely related to the subduction of the Nazca and Antarctic plates beneath South America. These plates form a triple junction at 46300 S, where the Chile ridge impinges on the Chile Trench. The asymmetric distribution of volcanoes north and south of the triple junction and their contrasting composition are related to different tectonic regimes of Nazca, Antarctic, and South American plates. Along the Andes, north of the triple junction, in the CSVZ and SSVZ, there are more than 34 active volcanic centers. Basalts and basaltic andesites and scarce andesites, dacites and rhyolites built them up. Oblique chains and volcanic clusters are controlled by both blind structures of the basement and first-order intraarc faults such as the Liquin˜e–Ofqui fault system. South of the Patagonian volcanic gap (49 S) in the AVZ, there are only six Quaternary volcanoes. They are formed by adakitic hornblende andesites and dacites, which are thought to be the result of partial melting of a young subducting oceanic crust mixed with variable amounts of a mantle source. In extra-Andean Patagonia, between 39 and 46300 S, volcanic outcrops of silica-poor alkaline rocks are spatially related to NNW–NW fractures. In the southern extraAndean area, Miocene to Early Pliocene voluminous volcanic fields of subalkaline and alkaline basalts cover thousands of square kilometers, followed during the Plio– Pleistocene by smaller and more alkaline spills. Some backarc plateau lavas have been considered to be of an asthenospheric plume-like mantle source. East of the Patagonian volcanic gap, plateau basalts seem to be linked to a slab window due to the Chile ridge–trench collision.

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