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 abstract article info Article history: Received 16 October 2008 Accepted 30 June 2009 Available online 7 July 2009 Keywords: retroarc basalts back arc extension Mendoza Payenia K–Ar dating volcanoes One of the major retroarc volcanic provinces in the southern Central Andes (34° and 37°S) is developed in the Andean foothills of the San Rafael region between the orogenic front and foreland basement uplifts of Late Miocene age. Here we present the first comprehensive geochronological study of the Quaternary volcanism, 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 composition exposed between 34° and 35°30′S. The data constrains the basaltic volcanism to between ~1.8 Ma and the Holocene. The spatiotemporal distribution of the ages indicates that eruption in the retroarc was episodic with some distinct patterns. The orogenic front of the San Rafael Block is associated with 1.8– 0.7 Ma volcanic eruptions, while the Malargüe fold and thrust belt front in the Andean foothills is related to younger eruptions produced at 0.1–0.01 Ma. Both areas are associated with Late Cenozoic normal faults that 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 Jurassic to Neogene magmatism along the western South American margin is the direct consequence of subduction of oceanic lithosphere. While arc magmatism has been associated with a single phenomenon related to the dehydration of the subducted oceanic crust at depth, volcanism at retroarc positions (Fig. 1) has been explained by different processes that encompass from development of asthenospheric win- dows, back-arc extension, eastward arc migration due to shallowing of the subducted lithosphere and lower lithosphere overheating due to slow plate displacements (see discussion in Kay et al., 1999, 2005, 2006, 2007; Risse et al., 2008). The largest—less than 5 Ma retroarc volcanic plateau in the entire Southern Andes—corresponds to the Payenia volcanic field (Fig. 1; 34°30′–38°S) (Muñoz and Stern, 1988; Stern, 1989) that covers the Andean Late Miocene orogenic front. This has been explained as related to strong asthenospheric influx due to the steepening of the subducted Nazca plate after a cycle of shallow subduction in the area (Kay et al., 2006). Recently, seismic tomographies showed abnormal “heated” sublithosphere beneath this volcanic province that supports the previous hypothesis (Gilbert et al., 2006). Poor radiometric covering has not allowed to reconstruct accurately eruptive evolution of the area, as well as associated Quaternary tectonism. Compositional variations and changes in volcanic and structural style through time along the Present south Andean arc (Fig. 1), as well as their related causes, have been discussed in numerous works (see Jordan et al., 1983; Kay et al., 2005, among others). Regional studies have shown the segmented nature of the volcanic arc from 2° N to 55° S, where around 200 stratovolcanoes and 10 potentially active calderas are present (Stern, 2004; Stern et al., 2007). This segmenta- 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 determine distinctive geochemical patterns and consequent eruptive mechanisms and type of volcanic rocks. These segments also show remarkable variations regarding general ages of main volcanic provinces and life-span of associated individual centers. Journal of Volcanology and Geothermal Research 186 (2009) 169–185 ⁎ Corresponding author. E-mail addresses: andresfolguera2@yahoo.com.ar, andes@gl.fcen.uba.ar (A. Folguera), jnaranjo@sernageomin.cl (J.A. Naranjo), oripachi@eri.u-tokyo.ac.jp (Y. Orihashi), sumino@eqchem.s.u-tokyo.ac.jp (H. Sumino), nagao@eqchem.s.u-tokyo.ac.jp (K. Nagao). 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.06.012 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores