Oligocene Extensional Tectonics at the Main Andes. Valle del Cura Basin, San Juan Province, Argentina Diego A. Winocur* and Victor A. Ramos Instituto de Estudios Andinos (CONICET-UBA), Pabellón II, Ciudad Universitaria, CABA, Argentina. *E-mail: Winocur@gl.fcen.uba.ar Resumen. La presente contribución muestra nuevos datos geocronológicos, estratigráficos y estructurales en el sector central de la zona de subducción subhorizontal pampeana (pampean flat slab) de los Andes Centrales. Dicho sector se encuentra comprendido entre los 29° y 30°LS, en el denominado Valle del Cura ubicado al NO de la provincia de San Juan, Argentina. En esta región se evidencian características extensionales para la Formación Tilito la cual es la más antigua dentro del Grupo Doña Ana. Esta secuencia volcánica es de composición mayoritariamente ácida y posee edades que van entre los 27 y los 23 Ma. Hacia este período se desarrolló un arco magmático de características calcoalcalinas asociado a subducción el cual se encontraba ubicado en la Cordillera de Doña Ana en sector chileno y su extensión hacia el este ocupó el Valle del Cura en su porción occidental. La correlación de los productos volcánicos tanto en sector chileno, como argentino muestran características extensionales para el Oligoceno Superior. Los datos radiométricos realizados en las volcanitas de la Formación Tilito, permiten correlacionar la actividad extensional de las estructuras proponiendo entonces que el arco magmático del Oligoceno superior – Mioceno inferior se desarrolló bajo condiciones extensionales. Keywords: Valle del Cura, Oligocene, Extensional, Frontal Cordillera 1 Introduction The main objective of this work is to present evidences of an extensional period at Oligocene times in the Pampean Flat Slab segment of the Central Andes. The Argentine Frontal Cordillera between 29º and 30º south latitude has a particular geological interest (Figure 1). From the tectonic point of view is located in the middle of the Pampean Flat Slab subduction zone of the Central Andes. The structure presents a great complexity due to changes in the stress patterns that have occurred since the Paleozoic to the present. The dominant structural style is thick-skinned due to the tectonic inversion of the previous extensional faults. The region was studied by many authors focused at the Tertiary volcanic sequences. The first of these studies was realized by Maksaev et al., (1984); Nasi et al., (1990); Kay et al. (1987, 1988, 1991) and Ramos et al., (1987) who recognized this sequences in the Argentinean side. New chronological data where published by Martin et al. 1997; Limarino et al. 1999; Bissig et al., 2001; Litvak and Page (2002); Mpodozis and Kay (2003); Litvak et al. 2005; Charchaflié et al. 2007; Winocur and Ramos (2008, 2011).

1.1 Stratigraphy The Oligocene stratigraphy in this sector of the Chilean Andes was defined by Thiele (1964) where outcrops Doña Ana Formation, an Oligocene sequence of andesites and basalts. Later Maksaev et al. (1984) subdivided this unit in two members providing radiometric dates for them and improving their distributions along the High Cordillera of Chile. The lower Tilito Member, consists on rhyolitic ignimbrite tuffs and lavas interbedded with dacitic flows. The K/Ar dates provided range from 27 to 22.1 Ma. The upper Escabroso Member is composed of andesitic lavas, volcanic agglomerates and breccias with ages between 26 and 18.9 Ma. Later, on Martin et al. (1997), based on more detailed field studies have observed an angular unconformity with a hiatus between 1 and 3 Ma separating these two units leading to consider as independent formation within the Doña Ana Group. In fact, according to this authors the Tilito Formation have ages between 27 and 22.1 Ma and the Escabroso Formation have ages between 21 and 17 Ma. In the Argentinean side the first to recognize these units were Ramos et al. (1987, 1989) who also recognized outcrops of basaltic necks, grouped in the Las Máquinas Basalts with K-Ar ages of 22.8 ± 1.1 Ma. These basalts were related to a back-arc activity coeval with the main volcanic arc of the Doña Ana Group. More recently Bissig et al. (2001) dated the volcanic rocks of the Doña Ana Group more precisely. A range of volcanic activity between 25.1 ± 0.4 and 23 Ma for the Tilito Formation and 21.9 ± 0.9 and 17.6 ± 0.5 Ma, for the Escabroso Formation were obtained by Ar/Ar and U/Pb methods. Charchaflié et al. (2007) also conducted Ar-Ar radiometric dating, which confirmed previous existing ages and more recently Litvak (2009) provided two new K/Ar Oligocene dates in the Cordillera de Zancarrón in Valle del Cura, assigning that outcrops at the Tilito Formation. 2 Method, Samples, Results The surveyed area show many different sectors in different units with robust evidence of synextensional sedimentation affected at the Tilito Formation (Figure 3a). This figure shows several outcrops with extensional sets of faults in a road section (Figure 1a) sited at the right margin of Guanaco Zonzo creek. The outcrops correspond to the dacitic tuffs of the Tilito Formation affected by two normal

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of N-S strike faults with about one meter observed displacement (see the hunter for scale). These volcanic sequences are dated about two kilometers to the north yielding ages of 24.5 ± 0.2 Ma by U/Pb in zircon (Charchaflíé et al., 2007). Evidence of extensional faults was founded 40 km SE of the first one, sited at the south part of Valle del Cura river. This outcrop shows a similar extensional structural setting affecting a series of volcaniclastic deposits of the Tilito Formation. The strikes are similar to the first one, but displacements are smaller than one meter (Figure 3b). This sequence corresponds to the Tilito Formation, and was dated by Bissig et al. (2001) in 23.9 ± 0.3 Ma by Ar/Ar method in biotite. 3 Discussion and Comments

Several authors have proposed an extensional regime for the Abanico and Coya Machalí basins in regions further south during the Oligocene as Godoy et al. (1999) and Charrier et al. (2002). However at these latitudes Winocur and Ramos (2008; 2011) were the first who proposed the extensional regime for the Oligocene times at these latitudes. This period was characterized by a generalized extension coeval with an active calc-alkaline volcanic arc in an extensional Andean type subduction (Winocur and Ramos, 2008; 2011). This study presents some evidences of the Oligocene extensional tectonics in the Main Andes at these latitudes. Figure 2. Schematic simplified stratigraphy of Valle del Cura.

Based on the present observations, complemented with existing geochronological and geochemical data we made conclude that for the Oligocene times a magmatic arc occupied the axial part of the Main Andes between 29º and 30º (Winocur 2010; Winocur et al. in prep.) under an extensional tectonic regime during the time of deposition of Tilito Formation. References Bissig, T., Clark, A. H., Lee, J. K. W. and Heather, K.B., 2001. The

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Figure 1: Location map of the study zone at the Valle del Cura basin between the Cordillera de Colangüil and the Chile international limit.

Figure 3: a. View a road section of the Tilito Formation affected by extensional faults at the Guanaco Zonzo Creek. b. Extensional setting affecting a volcaniclastic sequence of Tillito Formation at the Valle del Cura river.

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