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Oligocene Extensional Tectonics at the Main Andes. · PDF fileOligocene Extensional Tectonics at the Main Andes. Valle del Cura Basin, San Juan Province, Argentina ... El volcanismo

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  • 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), Pabelln II, Ciudad Universitaria, CABA, Argentina. *E-mail: [email protected] Resumen. La presente contribucin muestra nuevos datos geocronolgicos, estratigrficos y estructurales en el sector central de la zona de subduccin subhorizontal pampeana (pampean flat slab) de los Andes Centrales. Dicho sector se encuentra comprendido entre los 29 y 30LS, en el denominado Valle del Cura ubicado al NO de la provincia de San Juan, Argentina. En esta regin se evidencian caractersticas extensionales para la Formacin Tilito la cual es la ms antigua dentro del Grupo Doa Ana. Esta secuencia volcnica es de composicin mayoritariamente cida y posee edades que van entre los 27 y los 23 Ma. Hacia este perodo se desarroll un arco magmtico de caractersticas calcoalcalinas asociado a subduccin el cual se encontraba ubicado en la Cordillera de Doa Ana en sector chileno y su extensin hacia el este ocup el Valle del Cura en su porcin occidental. La correlacin de los productos volcnicos tanto en sector chileno, como argentino muestran caractersticas extensionales para el Oligoceno Superior. Los datos radiomtricos realizados en las volcanitas de la Formacin Tilito, permiten correlacionar la actividad extensional de las estructuras proponiendo entonces que el arco magmtico del Oligoceno superior Mioceno inferior se desarroll 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 29 and 30 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; Charchafli 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 Doa 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 Doa 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 Mquinas 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 Doa Ana Group. More recently Bissig et al. (2001) dated the volcanic rocks of the Doa 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. Charchafli 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 Zancarrn 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 (Charchafl 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 Machal 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 29 and 30 (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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