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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Journal of Volcanology and Geothermal Research 186 (2009) 169–185 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores 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. 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.06.012 Author's personal copy 170 A. Folguera et al. / Journal of Volcanology and Geothermal Research 186 (2009) 169–185 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 composi- tion 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 A. Folguera et al. / Journal of Volcanology and Geothermal Research 186 (2009) 169–185 171 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.