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Journal of Volcanology and Geothermal Research 186 (2009) 169–185

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Journal of Volcanology and Geothermal Research

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Retroarc volcanism in the northern San Rafael Block (34°–35°30′S), southern Central Andes: Occurrence, age, and tectonic setting

Andrés Folguera a,⁎, José A. Naranjo b, Yuji Orihashi c, Hirochika Sumino d, Keisuke Nagao d, Edmundo Polanco b, Victor A. Ramos a a Laboratorio de Tectónica Andina, FCEyN, Universidad de Buenos Aires — CONICET, Argentina b Servicio Nacional de Geológía y Minería, Casilla 10465, Santiago, Chile c Earthquake Research Institute, the University of Tokyo, Bunkyo, Tokyo 113-0032, Japan d Laboratory for Earthquake Chemistry, Graduate School of Science, the University of Tokyo, Bunkyo, Tokyo 113-0033, Japan article info abstract

Article history: One of the major retroarc volcanic provinces in the southern Central Andes (34° and 37°S) is developed in the Received 16 October 2008 Andean foothills of the San Rafael region between the orogenic front and foreland basement uplifts of Late Accepted 30 June 2009 Miocene age. Here we present the ﬁrst comprehensive geochronological study of the Quaternary volcanism, Available online 7 July 2009 previously dated mainly on the basis of stratigraphy. The new unspiked K–Ar radiometric and two radiocarbon determinations encompass many volcanic centers, most of them monogenetic and of basaltic Keywords: composition exposed between 34° and 35°30′S. The data constrains the basaltic volcanism to between retroarc basalts back arc extension ~1.8 Ma and the Holocene. The spatiotemporal distribution of the ages indicates that eruption in the retroarc Mendoza was episodic with some distinct patterns. The orogenic front of the San Rafael Block is associated with 1.8– Payenia 0.7 Ma volcanic eruptions, while the Malargüe fold and thrust belt front in the Andean foothills is related to K–Ar dating younger eruptions produced at 0.1–0.01 Ma. Both areas are associated with Late Cenozoic normal faults that volcanoes dismembered an uplifted a Late Miocene peneplain as indicated by younger over older fault-relationships between Paleozoic rocks and Tertiary strata. This linkage indicates a major relationship between Pleistocene– Holocene retroarc eruptions of the basaltic centers, and extensional collapse of the foreland region, that shows a migration of the last volcanic activity towards the trench. © 2009 Elsevier B.V. All rights reserved.

1. Introduction related to strong asthenospheric influx due to the steepening of the subducted Nazca plate after a cycle of shallow subduction in the area (Kay Jurassic to Neogene magmatism along the western South American et al., 2006). Recently, seismic tomographies showed abnormal “heated” margin is the direct consequence of subduction of oceanic lithosphere. sublithosphere beneath this volcanic province that supports the previous While arc magmatism has been associated with a single phenomenon hypothesis (Gilbert et al., 2006). Poor radiometric covering has not related to the dehydration of the subducted oceanic crust at depth, allowed to reconstruct accurately eruptive evolution of the area, as well volcanism at retroarc positions (Fig. 1) has been explained by different as associated Quaternary tectonism. processes that encompass from development of asthenospheric win- Compositional variations and changes in volcanic and structural dows, back-arc extension, eastward arc migration due to shallowing of style through time along the Present south Andean arc (Fig. 1), as well thesubductedlithosphereandlowerlithosphere overheating due to slow as their related causes, have been discussed in numerous works (see plate displacements (see discussion in Kay et al., 1999, 2005, 2006, 2007; Jordan et al., 1983; Kay et al., 2005, among others). Regional studies Risse et al., 2008). The largest—less than 5 Ma retroarc volcanic plateau in have shown the segmented nature of the volcanic arc from 2° N to theentireSouthernAndes—corresponds to the Payenia volcanic field 55° S, where around 200 stratovolcanoes and 10 potentially active (Fig. 1; 34°30′–38°S) (Muñoz and Stern, 1988; Stern, 1989)thatcovers calderas are present (Stern, 2004; Stern et al., 2007). This segmenta- the Andean Late Miocene orogenic front. This has been explained as tion is a direct consequence of many variable tectonic factors along the western active margin of the South American plate, such as age of the subducted oceanic floor and thickness of the Andean crust, that ⁎ Corresponding author. determine distinctive geochemical patterns and consequent eruptive E-mail addresses: [email protected], [email protected] mechanisms and type of volcanic rocks. These segments also show (A. Folguera), [email protected] (J.A. Naranjo), [email protected] (Y. Orihashi), [email protected] (H. Sumino), remarkable variations regarding general ages of main volcanic [email protected] (K. Nagao). provinces and life-span of associated individual centers.

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Fig. 1. Southern Andean tectonic setting and Cenozoic retroarc plateau basalts in Patagonia. The Payenia plateau basalts constitute the largest retroarc volcanic province generated in the last 5 Ma in the entire Southern Central and Patagonian Andes (taken from Ramos et al., 1982; Kay et al., 2006, 2007).

In this context, the Southern Volcanic Zone (Fig. 1)(SVZ,33°–46° S) is the Main Andes (Figs. 2 and 3). Then, we present an evolutionary model of special interest due to the occurrence of most of the active volcanoes for the progression of Quaternary deformation in the area and related along the margin, and because of their relation to highly populated areas volcanism. on both slopes of the Andes. The northernmost section of this segment around 33°S is characterized by a west to east arc to retroarc zoning 2. Previous work in the region describing four discrete areas where eruptive styles, magmatic composition and volcanic types were highly variable during the Miocene to Several workers have studied partial aspects regarding the retroarc Holocene time-interval: (1) the Maipo and its associated Diamante associations that are present at the San Rafael Block and in the eastern caldera, Palomo, Tinguiririca, and Planchón volcanoes are the biggest Andean foothills. Since the ´70 these studies have intended to volcanic centers in this sector, and form the arc front located on the Main interpret these mafic fields from a tectonic point of view using very Andes next to the continental divide (Fig. 2); (2) major volcanic centers limited radiometric tools, as well as geochemical analyses. Valencio such as the Overo, Guanaquero and Sosneado volcanoes on the eastern et al. (1970) performed the first temporal determinations, using the side of the Main Andes, although smaller than the ones located at the arc K–Ar method and paleomagnetic analyses over Pleistocene volcanic front, defining the maximum heights of the eastern slope of the Andes sequences south of the latitude of the present study. Then Toubes and (Fig. 3); (3) Immediately to the east, over the orogenic front a series of Spikermann (1979) obtained K–Ar ages in Pliocene to Pleistocene monogenetic basaltic fields named the Hoyada, Lagunita, Loma Negra and volcanic successions through the San Rafael Block, and found the Hoyo Colorado (Fig. 3); (4) further to the east, emplaced around the San oldest ages for these retroarc associations. Araña Saavedra et al. Rafael Block (Figs. 2–4), a basement block uplifted in the foreland area. (1984) studied these retroarc volcanics between 34° and 37°S In this paper we focus on the last two groups, describing their age determining an alkaline signature and a magmatic source enriched and morphology, and finally their structural control. We present the first in K, Al and Ti contents. These authors discussed their potential unspiked K–Ar data set of the region to temporally define this retroarc linkage to the pre-Pliocene calc-alkaline volcanics outcropping in the province, hosted in the northern San Rafael Block (34°–35°15′S), east of same area. Author's personal copy

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Fig. 2. Main morphotectonic units in the northern part of the Southern Andes and Payenia volcanic zone. Numbers indicate thickness in meters of Late Miocene accumulations related to the Río Grande foreland basin (Yrigoyen, 1994) that was covered by retroarc volcanic rocks corresponding to the Mendoza Basaltic Volcanic Field. Structure was compiled from Polanski (1954, 1963, 1964), Desanti, (1956), González Díaz (1964, 1972a,b,c, 1979), Holmberg (1964, 1973), Fidalgo (1973), Núñez (1976 a, b, 1979), Delpino and Bermúdez (1985), Cortés (2000).

Bermúdez and Delpino (1989) studied several aspects regarding In a regional analyses performed between 34° and 39°S, Muñoz et al. the volcanic associations cropping out between 35° and 37°S. First, (1989) recognized a series of N to NW-trending volcanic chains east of the they recognized mesosiliceous volcanic rocks forming part of the Late Pleistocene to Holocene arc front emplaced in the low lands of the basement of the Pliocene to Pleistocene–Holocene maﬁc associations Main Andes. Those form part of a Pliocene to Lower Pleistocene arc very more than 500 km away from the oceanic trench, similarly to Araña well developed between 37° and 39° S, which indicates a strong Saavedra et al. (1984). Second, they related maﬁc widespread and westward shifting of the arc front in the last 2 Ma. Those Pliocene to voluminous volcanic eruptions to an extensional retroarc setting, Lower Pleistocene centers are forming andesitic and basandesitic interpreting them as a product of combined arc and intraplate sources stratovolcanoes and rhyolitic and dacitic calderas. Both Late Pleistocene characterized by low melting percentages from the mantle. These to Holocene associations at the arc front, and the Pliocene to Lower authors compared this volcano–tectonic setting with others through- Pleistocene volcanoes in the western retroarc area are subalkaline (Fig. 2). out the world where either the crust extends behind the arc front or However, further to the east alkaline assemblages dominate the crust collapses in an intraplate setting controlling the eruption of eventually over the eastern sector of the fold and thrust belt with a tholeiitic to alkaline series, such as the Northern Island of New few exceptions such as the Tromen volcano in the Chos Malal fold and Zealand, the Japan Sea, Korea, eastern China, Basin and Range province thrust belt around 36° S (see Kay et al., 2006). Muñoz et al. (1989) also in the western United States, and Río Grande rift in New Mexico. proposed that most of these alkaline associations were hosted in Author's personal copy 172 .Flur ta./Junlo ocnlg n etemlRsac 8 20)169 (2009) 186 Research Geothermal and Volcanology of Journal / al. et Folguera A. – 185

Fig. 3. Radiometric ages obtained in the present study from retroarc monogenetic basaltic cones. Note two groups differentiated by age: one older of Early Pleistocene located at the San Rafael Block on the Andean orogenic front, and another younger at the eastern Malargüe fold and thrust belt of Late Pleistocene–Holocene (see details of the dated samples in Table 1). Author's personal copy

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Fig. 4. Morphotectonic map of the northern San Rafael Block and adjacent main cordillera, where the Mendoza Basaltic Volcanic Field is displayed in relation to neotectonic activity in the area. Note a general northwest structural trend associated with the retroarc eruptions as detected by Polanski (1963) and Cortés and Sruoga (1998). Structure is locally based on Bastías et al. (1993), Lucero (2002) and Costa et al. (2004). graben-like structures, which allowed to infer an extensional general- interval, and ﬁnally a fourth category in the Holocene, mainly based on ized tectonic setting between 34° and 39°S latitudes. Pliocene to morphological criteria. The ﬁrst three stages are based on radiometric Lower Pleistocene arc-related rocks east of the Present arc front and paleomagnetic studies (Valencio et al., 1970; Mendía and

(Fig. 2) have higher K2O ratios and higher amounts of incompatible Valencio, 1987; Muñoz et al., 1987; Linares and González, 1990) alkaline and light rare earth elements than the Late Pleistocene to complemented by lithostratigraphic and morphological analyses Holocene ones. 87Sr/86Sr isotopic ratios for the arc-related eastern (Bermúdez and Delpino, 1989). Bermúdez et al. (1993) characterized series are between 0.7038 and 0.7042 independently of the SiO2 on physical, petrological and geochemical grounds those time- content, with similar ranges than the arc front, inferring a common categories comparing them with the arc front at the same latitudes source in the arc zone for the Pliocene to Holocene lapse. represented by the Tatara–San Pedro complex and the Planchón Composition of the arc association is sensitive to fractional volcano (Fig. 2) (see Dungan et al., 2001). crystallization which leads to a progressive increase in K2O, light In relation to contemporary-to-volcanism structure, Cisneros and rare earth, and incompatible element contents. Other proposed Bastías (1993) studied neotectonic deformations at the eastern border process that constrained compositional variations through time is a of the San Rafael Block that affected retroarc monogenetic basaltic decrease in the degree of partial melting for the youngest associations. fields, detecting an important NNW-trending alignment, where Las South of 39°S Muñoz et al. (1989) recognized a clear difference, not Malvinas fault zone was recognized (Fig. 4)(Cisneros et al., 1989). This distinguishable north of this latitude, between arc, and retroarc- area was associated with the Villa Atuel-Las Malvinas earthquake in alkaline associations. Progressive increase in the age of the subducted May 30, 1861 that destroyed Las Malvinas and Villa Atuel villages oceanic floor to the north and subduction of the Valdivia fracture zone (Fig. 4), whose effects were interpreted as liquefaction in saturated at 39°S (Fig. 1) were invoked as the main mechanisms controlling sand soils. Other important earthquakes developed in this area those major changes at this latitude. correspond to San Carlos, (August 29, 1861), immediately to the Bermúdez et al. (1993) discriminated retroarc volcanic associa- north of the study area, and General Alvear (October 4, 1913) (Fig. 4). tions located between 35° and 37°S in two separate volcanic fields, They also recognized other fault systems located in the opposite west Llancanelo (10,700 km2) and Payún Matrú (5200 km2), that constitute border of the San Rafael Block as related to neotectonic deformations, the Payenia volcanic province (Fig. 1). Both developed from Pliocene particularly in the Carrizalito range (Fig. 4), where morphotectonic to Holocene times with four peaks of intensity nucleated in 3.6 Ma parameters indicated left lateral transtensional faults. These Quatern- corresponding to the 5.1–2.6 eruptive stage, 1.7 Ma, corresponding to ary faults were recognized as exerting a strong control in the eruption the 2–1.5 Ma interval, 450 ka, corresponding to the 650–100 ka of Neogene and Quaternary volcanic chains aligned along NNW and Author's personal copy

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NW trends, located mainly at the eastern border of the San Rafael onset of a shallow subduction zone from 34°30′ to 37°S (Fig. 2) Block. Recurrence in the activity during Quaternary times is also (Ramos and Folguera, 2005; Kay et al., 2006). Land mammal bearing inferred by younger faulting affecting lava flows that had been synorogenic sequences were exposed in the Middle to Late Miocene, emplaced in transtensional structures. Theoretical estimations sug- which constrained the last phase of orogenic uplift at these latitudes gest the occurrence of M 5.14 and maximum accelerations in the order (Soria, 1983). At this time, Middle to Late Miocene arc-derived of 88.96 cm/seg2 for Colonia Las Malvinas and 57.68 cm/seg2 for Villa volcanic rocks were emplaced over the eastern flank of the Main Atuel (Fig. 4). Andes as well as on the San Rafael Block, more than 500 km away from Cortés and Sruoga (1998) identified a structural control for the trench (Giambiagi et al., 2005, 2008; Kay et al., 2006). Widespread monogenetic basaltic cones erupted at the Andean foothills east of intraplate volcanic rocks were erupted mainly around the San Rafael the Carrizalito range (34°–34°30′S) between the Papagayos and Block and Río Grande Basin during Pliocene to Pleistocene times Diamante rivers (Fig. 4). They have interpreted their links to (Bermúdez et al., 1993), while small amounts were concentrated in Pleistocene and Holocene faults with extensional components. the eastern Malargüe fold and thrust belt as described by Cortés and Naranjo et al. (1999a) characterized compositionally a monogenetic Sruoga (1998) and Saavedra (in press) (Fig. 2). Those are uncom- basaltic–andesitic field in the eastern fold and thrust belt at the Salado formably covering the Late Miocene compressive structures and are river (around 35°30′S), 70 km east of the arc front, known as the mainly hosted in extensional troughs that are displacing previous Infiernillo volcanic field (Fig. 4). These centers show a strong extensional structures. Main Pliocene to Pleistocene troughs in the region are the control as depicted by Dajczgewand (in press), who described a normal Llancanelo Basin (Fig. 2), located in the Río Grande foreland basin fault with basaltic feeders along the Salado river. Their chemistry is also south of the study area, where monogenetic basalts are forming an significantly different from the arc front at these latitudes when almost continuous volcanic field covering folded synorogenic and compared with the Planchón center (Naranjo et al., 1999b), which is modern piedmont deposits. Another important trough, the Nihuil characterized by a higher differentiated source potentially connected to Basin, has a series of NW extensional faults which are affecting the a contrasting compressive regime at the western Andes. backlimb of the San Rafael Block producing a series of half-grabens In relation to physical volcanology, south of the San Rafael Block where basaltic volcanic fields were erupted (Figs. 2 and 4). and north of Río Colorado (Fig. 2), 40 monogenetic volcanic centers of During the last million years, minor transpressional deformation eastern volcanic field were studied by Bertotto et al. (2006), who affected the eastern front of the San Rafael Block (Bastías et al., 1993; described variations between Strombolian and Hawaiian activity for Costa et al., 2004, 2006; Lucero, 2002) deforming Pleistocene volcanic the formation of successions of weakly welded lapilli and bomb beds sequences erupted during the aforementioned phase of extension in and agglutinated spatter beds. Part of these eruptions comprises one the area. These deformations in the orogenic front are associated with of the longest pahoehoe inflated basaltic lava flows in the Payenia important evidence of crustal seismicity. volcanic province (Pasquaré et al., 2008). Recently, Risso et al. (2008) described the Llancanelo volcanic field south of the Malargüe town 4. Mendoza retroarc volcanic field with similar characteristics (Fig. 2). The extensive Mendoza Basaltic Volcanic Field comprises more 3. Tectonic setting than 400 (Bermúdez et al., 1993), or even around 800 (Risso et al., 2008) monogenetic small volcanoes in addition to a number of The analyzed volcanic field is within the Andean Southern Volcanic stratovolcanoes and shield volcanoes, distributed in an area of 10,000 Zone (SVZ) (Fig. 1)asdefined by López Escobar et al. (1977,and to 20,000 km2, between 34° and 38°S (Fig. 2). In this paper we focus contemporary works cited herein) and popularized in subsequent works the attention on the northern part of this area, around the San Rafael (Stern, 2004, among others). On the tectonic point of view it is part of the tectonic block, from 34° to 35°10′S(Figs. 3 and 4). southern Central Andes (27°–38°S) that are formed by two broad Monogenetic volcanoes and composite stratocones within the segments differentiated by structural styles and degree of shortening Mendoza Basaltic Volcanic Field are clustered, but they also constitute absorbed in the Neogene. The northern limit of the study area roughly linear chains that follow tectonic structures or are distributed on the coincides with the southern end of the Pampean flat slab subduction flanks of large shield volcanoes such as Payun Matrú volcano to the segment of the Nazca plate beneath the South American plate (27°–33°S; south of the area. Barazangi and Isacks, 1976). This boundary is the northern end of a Due to the low erosion rates in a fairly dry climate, relative dating normal subduction segment developed to the south of 33°S (Jordan et al., based on their geomorphology is difficult. Thus, we have employed 1983; Ramos et al., 2002). Morphotectonic changes along strike are unspiked K–Ar geochronological techniques to describe the activity and gradual and strongly influenced by pre-existing heterogeneities previous longevity (Fig. 3 and Table 1), as well as related neotectonic setting of to the Andean deformation, such as rift systems and thick sedimentary this volcanic field. At this stage, the main physical features of the prisms represented by the Meso-Cenozoic basins. Mendoza Basaltic Volcanic Field include a number of individual vents, The Andes, south of the present flat slab zone, between 34° and their distribution and relationship to modern faults. In the Table 2,itis 35°30′S are formed by two distinct mountain systems separated by a summarized some individual physical characteristics and longevity, of Neogene foreland basin: the Main Andes encompassing the arc and the northern part of the Mendoza Basaltic Volcanic Field. This can be western retroarc areas, and the San Rafael Block in a foreland position considered as a large but low density volcanic field compared for (Figs. 2 and 4). The Main Andes constitutes the drainage divide example with the Springerville volcanic field, Arizona, USA (Condit and between the Pacific and Atlantic oceans that has been shaped from 19 Connor, 1996). The study area has approximately 84 vents formed over to 17 Ma with the tectonic inversion of the Eocene–Late Oligocene the last 1.7 Ma, although co-genetic origin for multiple vents could also Abanico Basin (Godoy et al., 1999; Charrier et al., 2002, 2005). This have occurred. Thus, cases of alignment of different cones might be basin fill is presently exhumed at the western side of the Andes. The considered to represent single volcanic events. eastern Andean sector has been constructed by the stacking of Late In terms of spatial distribution and relative chronology among Triassic half-grabens and locally by thin-skinned structures that vents, short local alignments show no shift in their locus, but large- deformed late Mesozoic and Cenozoic successions from 15 to 8 Ma scale shifts have been observed from east to west along parallel (Giambiagi et al., 2008). Andean uplift at these latitudes was recorded structures trending NW. Vent clusters consisting of a 1 to 10 individual in the Río Grande Basin, a more than 2500 m thick foreland basin vents also show this general pattern within the northern Mendoza developed between 34° and 37°S that has been partially cannibalized Basaltic Volcanic Field. It can be estimated that the distribution of because of the uplift of the San Rafael Block to the east, during the clusters can be directly correlated with the distribution of main faults. Author's personal copy

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Table 1 Analytical values of the unspiked K–Ar ages.

Sample no. K 40Ar rad 38Ar/36Ar (40Ar/36Ar) initiala Age Air fraction (wt.%) 10− 8 cm3 STP/g Fractionated Ar assumed (Ma) (%) a) Río Salado Group 1 Las Hoyadas 090499-1C 1.35±0.04 −0.26±0.36 0.18758±0.00049 294.7±1.5 −0.049±0.068 100.4 2 Las Hoyadas 090499-2 1.29±0.04 0.08±0.38 0.18785±0.00047 295.5±1.5 0.015±0.076 99.90 3 Las Hoyadas 090499-3A 1.28±0.04 0.02±0.58 0.18797±0.00054 295.9±1.7 0.005±0.116 99.98 4 Lagunilla1 111199-2 1.18±0.04 0.15±0.30 0.18746±0.00040 294.3±1.3 0.034±0.066 99.78 5 Hoyada 111199-5 1.09±0.03 0.29±0.21 0.18723±0.00035 293.6±1.1 0.069±0.050 99.48 6 Lagunilla 2 111199-6 1.20±0.04 0.51±0.47 0.18714±0.00052 293.3±1.7 0.10±0.10 99.39 b) Papagayos Group 7 Los Leones W 081199-1 1.09±0.03 2.56±0.15 0.18835±0.00045 0.607±0.039 95.88 8 Pozo 081199-8A 0.80±0.02 0.796±0.048 0.18824±0.00046 0.257±0.017 96.70 9 Pozo 081199-9 0.99±0.03 0.408±0.039 0.18832±0.00040 0.106±0.011 98.68 10 Pozo 081199-10 0.98±0.03 0.350±0.039 0.18805±0.00059 0.092±0.011 98.75 c) Los Tolditos Group 11 Chato 091199-8 1.16±0.03 2.83±0.15 0.18873±0.00049 0.631±0.039 89.93 12 Rodeo 091199-9 1.10±0.03 3.17±0.17 0.18866±0.00051 0.739±0.045 86.54 13 Rodeo 091199-10B 1.04±0.03 2.77±0.15 0.18800±0.00054 0.684±0.042 91.45 d) Diamante Volcano Group 14 Diamante 091199-13 0.70±0.02 1.349±0.084 0.18785±0.00044 0.495±0.034 97.42 15 Diamante 091199-14 1.26±0.04 0.268±0.021 0.18752±0.00047 0.0546±0.0046 98.72 16 Diamante 091199-15B 1.51±0.05 0.478±0.028 0.18791±0.00044 0.0817±0.0053 96.66 17 Diamante Basement 091199-16 1.49±0.04 42.8±2.2 0.18801±0.00058 7.38±0.43 39.35 18 Diamante 091199-17 1.14±0.03 2.24±0.12 0.18793±0.00077 0.505±0.031 95.19 19 Diamante 101199-10A 0.86±0.03 1.616±0.086 0.18785±0.00064 0.484±0.030 94.48 20 Diamante 101199-10B 1.06±0.03 4.77±0.26 0.18793±0.00056 1.164±0.073 77.05 21 Diamante 101199-12 0.98±0.03 0.950±0.060 0.18851±0.00047 0.251±0.018 97.05 22 Diamante 101199-13 1.79±0.05 0.375±0.039 0.18783±0.00056 0.0540±0.0059 99.12 e) Medio Group 23 Ao Hondo NW 101199-2 1.09±0.03 1.89±0.10 0.18769±0.00049 0.449±0.028 91.33 24 Ao Hondo 101199-4 1.08±0.03 1.82±0.10 0.18800±0.00040 0.434±0.027 93.06 25 Loma del Medio 101199-6A 1.12±0.03 0.590±0.049 0.18829±0.00045 0.136±0.012 98.63 f) Las Malvinas Group 26 Negro 111202-1A 0.92±0.03 2.87±0.15 0.18842±0.00043 0.801±0.049 84.20 27 El Puntudo 111202-2 0.93±0.03 3.35±0.18 0.18781±0.00045 0.932±0.056 86.20 28 Guadal 111202-3D 0.78±0.02 2.45±0.14 0.18773±0.00068 0.805±0.051 85.93 29 Puntano 111202-4 0.83±0.02 5.74±0.30 0.18823±0.00050 1.78±0.11 93.53 30 Solo 111202-5 0.27±0.01 0.801±0.049 0.18781±0.00047 0.750±0.051 97.66 g) Guadal Volcano 31 La Carbonilla 121202-1A 1.30±0.04 2.67±0.14 0.18843±0.00047 0.530±0.033 95.76 32 La Carbonilla 121202-1B 1.34±0.04 3.26±0.17 0.18823±0.00050 0.629±0.038 95.49 h) Aisol Volcano 33 Nihuil 121202-2 0.88±0.03 5.02±0.26 0.18849±0.00048 1.474±0.089 84.25

(40Ar/36Ar) initial=296. 0 is assumed. Error: 1s. a(40Ar/36Ar) initial was estimated from the measured 38Ar/36Ar ratio, which was fractionated from the atmospheric value of 0. 1880.

Moreover, in some cases a strong correlation has been recognized general morphological descriptions were available (Desanti, 1956; between structural trends and vent alignments which may indicate González Díaz, 1972a; Bermúdez et al., 1993). Scoria cone is the most contemporaneous cone building associated with single episodes of common type and few examples of tuff rings and tuff cones also occur. dike injection (Fig. 4). These commonly occur in groups, although isolated cones have been In the northern part of the Mendoza Basaltic Volcanic Field, distinguished. More complex structures such as Diamante stratovol- Diamante volcano composite cone (Figs. 3 and 4) is the unique case cano and Negro small shield volcano are exceptional cases, as well as where magma supply has been sufficient to maintain a thermal lava flows with open craters as Hoyo Colorado, Hoyada and Lagunillas anomaly around a central vent. In the monogenetic cones magma among others (Naranjo et al., 1999a). supply rates have been so low that new ascending magma batches Cinder cones are truncated, conic or horseshoe-shaped, with have found their own fault conduits to the surface, with no typical bowl-shaped craters in the younger examples, but no crater at opportunity to accumulate at shallow crustal magma chambers. the top of the older ones. Elongate cones are scarce even in those built above fissures, where more complex aligned vent systems occur. The 4.1. Eruptive centers in the northern part of the Mendoza retroarc deposits of scoria cones typically consist of bombs, scoriaceous lapilli volcanic field and minor ash. Spatter cones and scoria-agglutinate cones consisted largely of welded lava spatter are also common. We have distinguished ~84 volcanic centers in the northern part of Conspicuous examples of maars are Pozo and Arroyo Hondo NW the Mendoza Basaltic Volcanic Field (Fig. 3, Tables 1 and 2), where volcanoes. The former consists of a NW oriented twin explosion craters Author's personal copy

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Table 2 Main morphologic characteristics of the retroarc vents in the study area.

Volcano Morphology Elevation Diam (apron included) Emission-centre (H, m) (km) La Carbonilla Cone crater 120 m diameter 40 1.2 La Carbonilla SE No crater cone 80 3.5 Los Leones No crater cone 160 1.75×1 Los Leones W No crater cone b15 0.4 Sepultura No crater cone and SE explosion crater 30+10 deep 0.5–0.5 Del Medio No crater cone 65 0.5 Gaspar No crater cone 75 0.75 Zorro No crater cone 40 0.9 Guadaloso No crater cone 65 2 Pozo NW oriented twin explosion craters-the Nested dome 50 m height–0.65 1.3 (2) Twin tuff ring craters western-youngest diameter–58 m and 75 deep Ao Hondo NW Explosion crater ring tuff 20 m deep 35 0.60 Twin tuff ring crater Ao Hondo Pyroclastic cone with crater 185 1 0.2 km crater 2449 Pyroclastic cone with crater 176 0.9 0.25 km crater, b12 km lava flow Loma del Medio 1 km diameter crater, 0.6 km nested cone 220 1.3 Multi-crater and nested cone with a crater of 35×0.40 composed cone 2247 Pyroclastic cone with lava flows 200 1.3 Deformed crater Bs Blancas 1 Pyroclastic cone 80 1 350 m flat crater Bs Blancas 2 Pyroclastic cone 100 0.85 250 m flat crater Bs Blancas 3 Pyroclastic cone 121 1×0,7 0.2 km crater Bs Blancas 4 0.4 Bs Blancas 5 Explosion crater 10 m 0,4 Tuff ring crater 6 m deep Bs Blancas 6 Uncratered cone 45 0.4 Uncratered cone 1.25 km lava flow Bs Blancas 7 Uncratered cone 50 m 0.35 Bs Blancas 8 Pyroclastic cone 35 m 0.70×0.50 Open crater 0.24 km Chato Asymmetric cone 40 2×1.6 Uncratered cone Rodeo Cone 50 2.3 Uncratered cone 1784 del Medio Cone 40 1.5 0.17 m diameter crater, open to N La Chilena Cone 45 2.3 Uncratered cone Las Bolas 1 Cone 110 1.5 Uncratered cone, 1 km NE lava flow Las Bolas 2 Eroded cone 108 1.5 Uncratered cone, NE lava flow Diamantito (parasitic cone) 0.8 Diamante Stratovolcano 830 6.2 0.7 km diameter crater, 3 km lava flows, 15–20 m thick, parasitic cones Diamante Basement Morado Stratocone 140 3×2 0.25 km crater, 2.6 km lava flows, 25 m thick Chico Stratocone 210 4 Uncratered stratocone Chato W Extrusive dome 300 3.7×2.3 Torta dome Negro WW Coulee 260 2.9×1.8 Torta dome La Leña W1 Pyroclastic cone 70 1.3 Uncratered cone La Leña W2 Torta dome coulee 60 2.9×2.2 Flowing from W La Leña W3 Torta dome coulee 120 2×1.25 Flowing from W La Leña Cone 200 2.9×2.3 Eroded crater cone El Nihuil NW Eroded cone 50 3 Eroded crater cone Nihuil Deeply eroded cone 270 6 Eroded crater cone El Nihuil NE Lava field 100 4×2 Eroded emission centre Nihuil S Deeply eroded cone 210 6.7 Eroded crater cone Nihuil SW 1 Cone 20 2.5 Uncratered cone Nihuil SW 2 Cone 20 2 Uncratered cone Nihuil SW 3 1 Negro E Eroded cone 185 4.9 Uncratered cone Negro Small shield volcanoes 275 10×7.5 Uncratered summit El Puntudo Eroded shield 240 4×1.7 Not determinable Solo Cone 21 0.5×0.3 Uncratered cone Guadal Double crater cone 290 3.2 0.23 m craters Morado N Eroded cone 190 2.4 0.24 km eroded crater Morado S Elongated cone 220 2.7×1 Eroded summit craters Aguirre Deeply eroded double cone 190 2.3 Eroded summit craters El Chenque N 5–6 cone cluster b70 1.45 total b0.1 km craters El Chenque Elongated cone 60 2.5×0.8 Eroded summit craters El Chenque S Undetermined lava 70 3 No evidence Aguirre W Eroded cone 90 1 Eroded crater Aguirre SW Eroded cone 100 1.1 Eroded crater Chihuido N Small cones aligned 55 1.5×0.4 Eroded summit craters Ancho Elongated cone 305 5.1×3 2 summit crters 0.65 km, 100 m deep Ancho S Deeply eroded cone cluster 265 3.5 Not determinable Guadal E Compound cone 215 5 Multiple emission centres and crater 150×250 and eroded Mesa Small stratovolcano cone and lavas 232 5.2×3 Partially eroded summit crater and 2.75 km lavas La Parva Eroded cone 130 2.5 Eroded summit emission crentre Punón Trehue Partually collapsed? Cone 440 6 Irregualar 0.3×0.2 crater Author's personal copy

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Table 2 (continued) Volcano Morphology Elevation Diam (apron included) Emission-centre (H, m) (km) Chato 667 Cinder cone 45 1.3 0.25 km diameter semi-circular crater Los Mojados Eroded cone 95 1.7 0.2 km diameter semi-circular crater Los Embanques Deeply eroded cone cluster 140 2.7×2 Multiple emission centres and destroyed crater de los Chanchos Deeply eroded cone 115 2.8 Eroded summit emission centre Castrino Deeply eroded cone 80 2.3 Eroded summit emission centre Mal Barco Lava flow 120 0.9 (7.5 km long lava) Uncratered dome Hoyada Open crater and lava flow 50 0.5 0.27 diameter and 50 m deep crater, 2.8 km long lava flow Lagunilla Open crater and lava flow 50 0.6 0.34 diameter and 50 m deep crater, 2.4 km long lava flow Lagunilla 1 Flat lava cone–coalescence craters 100 0.65 70 m deep 0.43×0.3 km double crater Lagunilla 2 Double lava cone 36 0.65×0.46 Doubles crater NW–SE 0.12–0.16 diam (16–40 mdeep, respectively) Loma Negra Open crater lava cone and flow 40 0.6 30 m deep SE open crater, 1.6 km long lava Mesillas Pyroclastic cone and lava flow 125 1.2 0.35 km diam open S crater; 2.3 km long lava Laguna Blanca Flat lava? Cone 70 1.4 0.3 km diam 5 m deep crater Hoyo Colorado Lava field 40 0.6 2.2 to 4 km long and b45 m thick lavas already interpreted as maars by Cortés and Sruoga (1998).A650m wholerocksample(0.3–0.6 g) was fused at 1700 °C and evaporated gas diameter and 50 m high dome is nested within the western and was purifiedandanalyzed.Arisotopeanalysesweremadeonarelatively youngest crater. The eastern maar is clearly excavated in the piedmont small amount of sample Ar gas (b2×10−7 cm3 STP). If the amount of substrate, where ignimbrite deposits dated in 0.450 Ma (Stern et al., Ar gas extracted from the sample exceeded this limit, the amount of 1984) are exposed about 10 m under the present surface (Fig. 5A–E). Ar gas was reduced using the purification line. Errors on 40Ar Their deposits are composed of juvenile pyroclastic (lapilli and bombs) sensitivity and 40Ar/36Ar ratio are estimated to be 5% and 0.2%, and accidental clasts derived from the subvolcanic basement including respectively, based on repeated measurements of the atmospheric gravels from the piedmont, where magma–water interaction occurred standard containing 1.5×10− 7 cm3 STP of 40Ar. K concentration was in the shallow water-table (Fig. 5D). determined for an aliquot of the crushed and sieved whole rock Negro volcano consists of a small 10×7.5 km uncratered shield fraction used for Ar analysis by the X-ray fluorescence (XRF) method volcano with a 275 m high summit (Fig. 6). On the other hand, (Phillips PW 2400) at Earthquake Research Institute, the University of Diamante is a 6.2 km diameter stratovolcano, with a 0.7 km diameter Tokyo. Details of procedures applied for dating are described in Nagao summit (830 m high) crater, 3 km lobed blocky lava flows of 15–20 m et al. (1991) and Orihashi et al. (2004). thick and a couple parasitic cones. Other pyroclastic cones are aligned Geochronological results and analytical values are indicated in and fault controlled (Fig. 7). Table 1.TheerrorsshownintheTable 1 are 1σ of single analysis of each Four examples of lapilli and ash dispersion lobes from different vents sample, including statistical errors associated with ion collection of Ar are conspicuously distinguishable on the piedmont surface, demonstrat- isotopes and errors in blank correction (less than 1% of the sample gases) ing the Pleistocene arid prevailing climatic conditions of the area. In fact, and in the sensitivity and discrimination factors of the mass spectro- western Pozo tuff ring explosion generated a 20 km long by 6 km wide meter. Most 38Ar/36Ar ratios for the samples were in agreement with dispersion lobe directed to the east (Fig. 5D), which clearly overly the the modern atmospheric value of 0.1880 within the range of analytical dispersion lobe of Arroyo Hondo cone, a 33 km long by 10 km wide lobe error by 2σ. Six samples have either lower 38Ar/36Ar ratio (0.18723; dispersed to the northeast. The same direction took the tephra dispersion #11199-5) than the atmospheric value beyond the range of the analyti- of the easternmost vent of Loma del Medio, producing a 20 by 10 km lobe. cal error or negative values (#090488-1C, 2 and 3A, #111199-2 and 6) Finally, one of the last summit eruptions of Diamante volcano originated in K–Ar age calculated by the conventional method. In these cases, the an elongated ash dispersion lobe of 46 km long and a maximum of 15 km mass fractionation effect was corrected using the measured 38Ar/36Ar wide, also directed to the northeast. They consist of veneer deposits of ratios of these samples and then K–Ar ages were recalculated. lapilli size of dense juvenile scorias and accidental lithics. 6. Discussion 5. K–Ar dates and analytical procedure 6.1. Structure of the San Rafael Block The first thirty three K–Ar ages for the Mendoza Basaltic Volcanic Field were determined using the unspiked sensitivity method, in which The San Rafael Block is an east-verging asymmetric basement block the radiogenic 40Ar concentration is determined by a direct comparison (Figs. 2, 4 and 7). Its eastern steeper flank is associated with a series of between the 40Ar/36Ar ratio and 40Ar signal intensity of the samples and high angle faults that uplift Late Triassic clastic sequences over their those of volumetrically calibrated amount of atmospheric Ar at the same basement constituted by Paleozoic sequences highly deformed in Early condition of the mass spectrometer. The technique can precisely date Permian times (Figs. 6 and 8)(González Díaz, 1964). younger rocks than 0.1 Ma since it permits measurement of small These faults are mainly the result of inverted normal faults that amounts of radiogenic 40Ar and determines the isotopic composition of constituted the western edge of the Triassic Alvear Basin located to the the initial Ar in the sample by measuring 38Ar/36Ar without assuming east and buried beneath thick piles of Tertiary synorogenic sequences that the 40Ar/36Ar ratio in sample is equal to the modern atmospheric (Figs. 2 and 8). The Alvear Basin constitutes the southern end of a value of 296 (e.g., Nagao et al., 1991; Matsumoto and Kobayashi, 1995; series of extensional troughs of Late Triassic age that were partially Orihashi et al., 2004; Scaillet and Guillou, 2004). incorporated in the fold and thrust belt (Ramos and Kay, 1991)(Figs. 7 Ar analyses were performed using anoblegasmassspectrometerMS- and 8). The structural sections of the Andean orogen, where vergence III (modified-VG5400) in the Laboratory for Earthquake Chemistry, is mainly controlled by the polarity of previous normal faults, are Graduate School of Science, University of Tokyo. The crushed and sieved defined by thick-skinned deformation, such as in the San Rafael Block Author's personal copy

178 A. Folguera et al. / Journal of Volcanology and Geothermal Research 186 (2009) 169–185 and the Malargüe fold and thrust belt at the same latitudes (Fig. 8) deformation and the Andean inverted thrusts that have exhumed a (Kozlowski et al., 1993; Giambiagi et al., 2008). Therefore, the Paleozoic peneplain which deﬁnes its morphology (Figs. 4 and 8). This northern half of the San Rafael Block (34°–35°30′S) is characterized foreland system joins at its northern edge the Main Andes front by a marked NW trend deﬁned by the strike of Triassic extensional through the incorporation of the Río Grande foreland basin in the Author's personal copy

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Fig. 6. Eastern neotectonic front of the San Rafael Block. Late Triassic half-grabens are defining the eastern edge of this system controlling the emplacement of Early–Middle Pleistocene monogenetic basaltic fields. (A) Vertical displacements in Pleistocene lavas associated with reverse faulting at the eastern San Rafael Block. (B) 3D digital elevation model superimposed to TM and interpretation of basement structure, vertically exaggerated X4. Note the easternmost inverted halfgraben producing an eastward facing scarp affecting Pleistocene Cerro Negro lavas dated in 0.801±0.049 Ma (see Fig. 3 for location). tectonic wedge constituting a series of low hills (Yrigoyen, 1993), minor isolated patches of basaltic cover allow visualizing the relation while to the south the two systems are highly differentiated by the with their basement in comparison with the southern sector. maximum longitudinal development of the Río Grande Basin in between (Figs. 2 and 3). 6.2. Structural control on basaltic eruptions in the northern San Rafael Contrastingly, the western edge of the San Rafael Block is defined Block by a series of normal faults that are affecting up to Middle–Late Miocene strata of the Aisol Formation. The northwest-trending Valle The northern part of the San Rafael Block has widespread Grande and Carrizalito faults have down-thrown blocks to the west, neotectonic activity (Polanski, 1963) and has been affected by and can be interpreted as normal faults with minor transtensional extensional deformation after the deposition and uplift of the components (Fig. 7). Both have recorded normal displacements synorogenic sequences in Late Miocene times. However, Pleistocene affecting previously folded Paleozoic sequences. Their displacements volcanic sequences have also been affected by those fault systems together with other minor faults in the area produce the gradual indicating a much younger tectonic activity (Figs. 6 and 7). Four NW- sinking of the Paleozoic peneplain and exhumed Neogene sequences trending fault systems can be individualized in relation to Quaternary beneath alluvial fan deposits that are flanking the San Rafael Block eruptions in the area, that partially complement those described even (Figs. 7 and 8). Most of these faults are spatially associated with further north by Cortés and Sruoga (1998). The Carrizalito fault Pleistocene monogenetic centers in the area (Fig. 7). Their vents are (Fig. 7)defines the western border of the San Rafael Block at 35°S usually aligned through fault scarps, and occasionally lava emissions (González Díaz, 1964). This structure is well developed at surface are faulted as revealed by satellite images and radar topography defining a tectonic contact between Paleozoic basement and Cenozoic (Fig. 7). However, stronger indicators of post-Pleistocene deformation strata, but it is neither associated with young indicators of tectonic are present at the eastern edge of the San Rafael Block, where lavas activity, nor important volumes of erupted volcanic material. On the associated with monogenetic activity are vertically and laterally contrary, the eastern fault systems of the San Rafael Block show robust displaced and even folded with an east-vergence (Fig. 6)(Costa morphological evidence of more recent activity deforming Pleistocene et al., 2006). lava flows (Figs. 6 and 7). The Valle Grande fault is a conspicuous west Structure associated with Pleistocene eruptions is particularly facing scarp determining a normal relationship between Permian revealed at the northernmost half of the San Rafael Block, where rocks to the east and Late Miocene sediments to the west and controls

Fig. 5. (A) View to the east of El Pozo volcanic ﬁeld (see Fig. 3 for location) corresponding to the youngest volcanic centers in the area with ages between 0.1 and 0.6 Ma. Note the proximal facies of the pyroclastics plume ejected to the east from the El Pozo volcanic center and the northwest-trending lineament that controlled the emplacement of the other cones (3D perspective). (B) Panoramic view to the southwest of the western El Pozo maar, unspiked concordant ages of 0.106±0.011 and 0.092 ±0.011 Ma were obtained for the nested lava dome (Table 1). (C) Panoramic view to the northeast of the eastern El Pozo maar. The excavated substrate clearly shows the ignimbrite interbedded in the piedmont. Remnants of a small lake deposits are exposed at the bottom. (D) Flat-bedded and surge layers exposed in the crater wall of the eastern El Pozo maar. Basaltic juvenile bombs (dated in 0.257±0.017 Ma) together with angular fragments of broken country rocks and palagonitised lapilli and ash form the constructional pyroclastic part of this maar. (E) Landsat image showing examples of tephra dispersion lobes from different volcanoes of the Mendoza Basaltic Volcanic Field. The dark colour of the veneer deposits is mainly given by coarse ash to medium lapilli size dense juvenile scoriaceous pyroclasts. Author's personal copy

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Fig. 7. Northern section of the San Rafael Block where the neotectonic eastern front is associated with monogenetic basaltic eruptions. The K–Ar radiometric ages obtained in this study are also indicated. the emplacement of minor monogenetic volcanoes (Fig. 7). El Jilguero Dajczgewand, in press) that truncated the Late Miocene contractional and Cerro Negro faults are associated with east facing scarps that set structure and the previous Cenozoic synorogenic deposits. the limits of the San Rafael Block through its eastern border (Figs. 6 Further to the east at these latitudes, Saavedra (in press) described and 7). These two fault systems are the ones that concentrate the north of the Río Diamante a series of monogenetic cones, as the Cerros clearest evidence of young deformation in the area and have Chato and Negro de las Mesillas, and other basaltic cones, which were constituted the most important paths for basaltic eruption. Cerro controlled by northeast-trending faults with clear evidence of neotec- Negro, Solo, Puntano, and Guadalito, with ages comprised between tonic activity (Figs. 3 and 4). The lavas and pyroclastic flows were 0.95 and 0.75 Ma (Figs. 4 and 7 and Tables 1 and 2), are the most separated in early and late Pleistocene groups by Saavedra (in press).The prominent volcanoes resting over the eastern edge of the San Rafael oldest ones were affected by the orogenic front, while the youngest Block, showing evidence of Pleistocene reactivation of those systems. cones were associated with neotectonic features that affected Los Cerro Negro volcano and neighbor basaltic cones are affected by east Mesones Formation of Middle Pleistocene age. The Cerro Negro and facing scarps that displace younger than 1 Ma rocks at the Cerro Chato were interpreted as younger than 0.450 Ma, because they prolongation of the mountain front (Figs. 6 and 7). are developed above the Los Mesones Formation, first aggradation level in the foothills. This contains the pyroclastic levels associated with the 6.3. Structural controls on basaltic–andesitic eruptions in the eastern Diamante Caldera deposits (Cortés and Sruoga, 1998)datedbyfission- Malargüe fold and thrust belt tracks in 0.47 Ma and 0.44 Ma of zircon separates by Stern et al. (1984). This caldera produced the catastrophic emplacement of ~350 km3 A series of minor monogenetic cones are located at the eastern of ignimbrites, which were widespread over most of the retroarc Malargüe fold and thrust belt next to the orogenic front (Figs. 3 and 4) plains of the region (Guerstein, 1990), as well as over the Andean among which Hoyada, Lagunitas and Puesto Pérez volcanoes are the forearc (Stern et al., 1984). The present Maipo Volcano built within most prominent vents (Naranjo et al., 1999a) with ages one order of the Diamante Caldera and had more than seven eruptive stages in magnitude younger than the previously described group (Fig. 3, Table the last ~100,000 years (Sruoga et al., 1998). 1). Those radiometric ages are in accordance with a radiocarbon age of 0.0123±0.00016 Ma age from organic sediments of dammed 6.4. Quaternary tectonic evolution of the northern San Rafael Block lacustrine deposits associated with volcanic activity in the area and eastern Malargüe fold and thrust belt, and its relation to (Fig. 3). Moreover, those ages are within the range of 0.0065±0.0005 retroarc volcanism to 0.0060±0.0008 Ma obtained by 3He dating determined from altered exposed material (Fig. 4; Marchetti et al., 2006). The previous results show that the Mendoza Basaltic Volcanic Field These centers are controlled by a pattern of north-trending faults is a considerably young retroarc volcanic field, less than 1 Ma in most corresponding to Infiernillo fault system (Fig. 4)(Kozlowski et al., 1993; cases and even younger than 100 ka in the westernmost analyzed area. Author's personal copy

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Fig. 8. Structural cross section at 34°30′S from the eastern Malargüe fold and thrust belt (based on Giambiagi et al., 2008) and the San Rafael Block to the east. Both systems where uplifted in Late Miocene times, and show extensional deformation during the last 2 Ma. These structures are spatially and temporally associated with retroarc volcanic eruptions. Attenuated lithosphere geometry beneath the San Rafael block and Payenia volcanic ﬁeld is taken from Gilbert et al. (2006).

These basaltic rocks were emplaced over old Neogene contractional as the progression of Quaternary deformation in the area to which structures systematically associated with Pleistocene–Holocene exten- volcanism is related. After a cycle of eastward displacement/ sionally reactivated faults between 34° and 35°30′S and even further expansion of the arc front between 19 and 4 Ma (Fig. 8)amagmatic south, beyond the scope of the present work. Their dispersion is retreat can be determined. Extensional collapse and related retroarc intimately related to the development of neotectonic activity in the area. volcanism has started at the eastern border of the San Rafael block at The eruptions of the El Pozo volcanic field with an age of 1.8 Ma, staying at this outer position at some 0.7 Ma (Figs. 8 and 9). ~92,000 years are covering a piedmont aggradation surface in the Then a jump in retroarc volcanic activity to the Rio Grande foreland western retroarc region (Figs. 3 and 4). The pyroclastic deposits of basin and western San Rafael block is registered that lasted until ~450,000 years, associated with the Diamante Caldera, are 10 m 0.1 Ma (Figs. 8 and 9). Structural control in the 0.7–0.1 Ma volcanic beneath the present surface. stage describes a NW band parallel to the 1.8–0.7 eruptions and The structures in the eastern San Rafael Block exert a strong displaced some 40–50 km to the southwest (Fig. 9). Finally, a new control in the eruption of Pleistocene retroarc monogenetic retreat in volcanic activity is registered in the last 0.1 Ma to the volcanoes, through the activity of mainly the Cerro Negro and El easternmost Malargüe fold and thrust belt (Fig. 9). Westernmost Jilguero fault systems (Figs. 7 and 8). On the other hand, the eastern volcanic field, active in the previous 0.7–0.1 Ma interval, next to the Malargüe fold and thrust belt has controlled the eruption of Malargüe orogenic front, remains active at the time of the youngest younger than 100 ka material through the El Infiernillo fault system eruptions (Fig. 9). (Figs. 4 and 8). These two fault groups associated with retroarc Then a retraction in retroarc volcanic activity can be determined for volcanoes remain spatially individualized with the exception of the the last 1.7 Ma between 34° and 35°30′S at the site where the arc northernmost sector of the study area where they interact with migrated/expanded over the eastern slope of the Andes until 4 Ma. This each other (Fig. 4). There, at the piedmont sector described by retraction in retroarc volcanic activity and associated extensional Cortés (2000), a series of very oblique WNW-trending normal faults mountain collapse are therefore proposed to be linked to the of the Carrizalito fault system are joining the Malargüe fold and steepening of the subducted slab after a cycle of shallow subduction thrust belt orogenic front (Fig. 4), where the Diamante volcanic in the area. field is associated with the only polygenetic vent corresponding to the Diamante stratovolcano. Acknowledgements The extensive Mendoza Basaltic Volcanic Field here described constitutes part of an extended but low density large Pleistocene– Field work and logistics of the present study (Andrés Folguera, Holocene basaltic volcanic field developed after the main Andean Victor A. Ramos, José A. Naranjo) were financially supported by orogenic phase of contraction in the retroarc foothills of the Southern grant PICT 14144/03 of the Agencia de Investigación Científica and Andes achieved in Late Miocene times. Their vents are linked to Tecnológica of Argentina; Fondecyt Project 1960186, Conicyt, Chile Quaternary extensional relaxation of the orogen at these latitudes. (José A. Naranjo) and Science Research Project from the Ministry The unspiked K–Ar technique that can precisely date younger rocks of Education, Culture, Sport, Science and Technology, Japan (no. than 0.1 Ma ages applied in this study produced the first 13373004) (Yuji Orihashi). The authors kindly acknowledge the comprehensive geochronological data set of these volcanic fields. members of Laboratorio de Tectónica Andina for critical comments Then a precise sequence of volcanic events can be determined as well and support. Author's personal copy

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Appendix A

Major element compositions of the whole rocks of Table 1.

a a b Sample no. SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2OK2OP2O5 Total FeO /MgO Mg# K2O+Na2O Alkalinity (wt.%) a) Río Salado Group 1 Las Hoyadas 090499-1C 54.96 1.00 17.15 8.29 0.14 4.87 7.55 3.77 1.58 0.29 99.62 1.53 53.78 5.37 −0.61 2 Las Hoyadas 090499-2 54.89 0.99 17.01 8.16 0.14 5.22 7.94 3.84 1.51 0.32 100.04 1.41 55.89 5.35 −0.52 3 Las Hoyadas 090499-3A 55.61 0.97 17.04 8.08 0.14 4.92 7.23 3.87 1.54 0.33 99.73 1.48 54.68 5.43 −0.78 4 Lagunilla1 111199-2 46.60 1.28 14.51 10.79 0.18 11.70 10.50 2.72 1.26 0.44 99.99 0.83 68.22 3.99 1.17 5 Hoyada 111199-5 53.06 1.06 17.07 8.59 0.14 6.33 8.66 3.59 1.36 0.31 100.16 1.22 59.32 4.94 −0.23 6 Lagunilla 2 111199-6 53.63 0.99 17.50 8.28 0.13 5.51 8.31 3.59 1.48 0.34 99.76 1.35 56.86 5.08 −0.38

b) Papagayos Group 7 Los Leones W 081199-1 46.76 1.29 16.30 10.08 0.18 8.39 12.00 2.82 1.40 0.45 99.68 1.08 62.23 4.24 1.31 8 Pozo 081199-8A 45.68 1.44 14.67 10.71 0.18 11.05 10.81 2.84 0.93 0.53 98.84 0.87 67.13 3.81 1.14 9 Pozo 081199-9 45.34 1.53 14.20 10.88 0.18 11.47 11.46 3.02 1.18 0.54 99.80 0.85 67.61 4.21 1.83 10 Pozo 081199-10 45.80 1.60 14.88 10.85 0.18 10.98 10.93 3.05 1.30 0.52 100.08 0.89 66.72 4.34 1.84

c) Los Tolditos Group 11 Chato 091199-8 44.96 1.33 15.14 11.54 0.18 10.95 11.85 2.57 1.35 0.64 100.52 0.95 65.26 3.90 1.78 12 Rodeo 091199-9 45.73 1.23 14.30 10.94 0.18 10.23 13.73 2.14 1.03 0.68 100.20 0.96 64.93 3.17 0.71 13 Rodeo 091199-10B 46.24 1.49 15.90 10.72 0.18 8.32 12.20 3.18 1.23 0.48 99.94 1.16 60.59 4.41 1.72

d) Diamante Volcano Group 14 Diamante 091199-13 47.29 1.57 15.10 11.61 0.17 10.01 10.00 3.33 1.03 0.42 100.53 1.04 63.04 4.34 1.36 15 Diamante 091199-14 49.28 1.43 16.95 9.48 0.17 6.43 11.21 3.34 1.58 0.43 100.29 1.33 57.32 4.90 1.15 16 Diamante 091199-15B 54.06 1.10 18.16 8.33 0.15 3.55 8.35 3.86 2.23 0.33 100.13 2.11 45.75 6.08 0.53 17 Diamante Basement 091199-16 60.64 0.65 17.83 5.53 0.17 1.47 6.20 4.29 2.61 0.24 99.63 3.39 34.45 6.92 −1.17 18 Diamante 091199-17 45.28 1.63 15.20 11.43 0.18 10.37 10.78 3.34 1.38 0.59 100.18 0.99 64.25 4.71 2.41 19 Diamante 101199-10A 46.85 1.16 14.72 10.65 0.15 10.91 11.67 2.71 1.01 0.37 100.20 0.88 66.97 3.71 0.84 20 Diamante 101199-10B 45.54 1.43 16.00 10.57 0.18 9.37 12.23 3.15 1.30 0.51 100.28 1.02 63.68 4.44 2.06 21 Diamante 101199-12 46.12 1.55 18.00 10.94 0.17 5.94 12.81 3.04 1.22 0.36 100.14 1.66 51.79 4.26 1.65 22 Diamante 101199-13 52.51 1.09 19.06 8.67 0.18 2.55 8.14 4.12 2.25 0.42 98.99 3.07 36.74 6.44 1.24

e) Medio Group 23 Ao Hondo NW 101199-2 53.63 1.03 17.40 8.57 0.14 5.66 8.28 3.71 1.40 0.32 100.14 1.36 56.66 5.10 −0.28 24 Ao Hondo 101199-4 46.33 1.32 15.40 10.53 0.18 10.89 10.51 3.09 1.46 0.48 100.19 0.87 67.19 4.54 1.86 25 Loma del Medio 101199-6A 46.65 1.38 14.75 10.47 0.17 10.79 11.18 3.04 1.39 0.42 100.24 0.87 67.10 4.41 1.63

f) Las Malvinas Group 26 Negro 111202-1A 47.80 1.43 16.20 9.97 0.17 7.30 10.60 3.32 1.60 0.41 98.79 1.23 59.17 4.98 1.51 27 El Puntudo 111202-2 47.07 1.51 15.48 11.19 0.17 9.10 10.98 3.19 1.16 0.17 100.02 1.11 61.69 4.36 1.37 28 Guadal 111202-3D 47.56 1.41 15.53 11.35 0.18 9.82 9.93 3.36 1.16 0.17 100.47 1.04 63.14 4.50 1.41 29 Puntano 111202-4 48.21 1.46 14.55 11.03 0.17 10.41 9.00 3.15 1.10 0.19 99.26 0.95 65.15 4.28 0.74 30 Solo 111202-5 46.34 1.33 14.94 10.70 0.18 10.21 11.67 3.01 0.25 0.05 98.70 0.94 65.39 3.31 0.37

g) Guadal Volcano 31 La Carbonilla 121202-1A 48.32 1.83 17.50 11.35 0.17 5.35 10.52 3.44 1.15 0.36 99.99 1.91 48.25 4.59 1.14 32 La Carbonilla 121202-1B 49.11 1.40 16.06 9.88 0.17 7.60 10.00 3.51 1.69 0.23 99.65 1.17 60.36 5.22 1.42

h) Aisol Volcano 33 Nihuil 121202-2 48.20 1.59 15.45 11.85 0.18 8.45 9.88 3.38 1.05 0.31 100.35 1.26 58.53 4.42 1.07

a 2+ ⁎ Mg#=100 Mg/(Mg+Fe ) calculated with FeO =0.9 Fe2O3. b Alkalinity =(K2O+Na2O)−0.37 (SiO2 −39).

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Fig. 9. Chronological zonation of retroarc volcanoes at the Mendoza Basaltic Volcanic Field and related extensional structures (shaded area indicates extent of monogenetic eruptions at the speciﬁc time span and related Quaternary structure). Note a general SW trend during the Quaternary from the San Rafael block to the Andean front. Author's personal copy

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