Neogene to Quaternary extensional reactivation of a fold and thrust belt: The Agrio belt in the Southern Central Andes and its relation to the Loncopué trough (38°–39°S) Emilio A. Rojas Vera a, ⁎, Andrés Folguera a , Gonzalo Zamora Valcarce b , Mario Giménez c , Francisco Ruiz c , Patricia Martínez c , Germán Bottesi d , Victor A. Ramos a a Laboratorio de Tectónica Andina, Universidad de Buenos Aires. CONICET. Buenos Aires, Argentina b Repsol exploración, Madrid, España c Instituto Volponi, Universidad de San Juan. CONICET. San Juan, Argentina d YPF – S.A exploración, Buenos Aires, Argentina abstract article info Article history: Received 31 August 2009 Received in revised form 28 June 2010 Accepted 30 June 2010 Available online 17 July 2010 Keywords: Extensional reactivation Andean fold and thrust belt Southern Central Andes Gravity The Andean orogenic front between 38 and 39°S is formed by extensional structures delimiting the Loncopué trough. These structures are superimposed to Late Cretaceous–Late Miocene compressive structures that formed the Agrio fold and thrust belt. Gravity data, and limited borehole and seismic data complimented with previous field studies were used to construct two structural cross sections across the area of Neogene extensional deformation and remnants of the previous compressive structure. One of them was restituted for Late Cretaceous times previously to extensional relaxation of the fold and thrust belt and Late Oligocene– Early Miocene and Pliocene to Quaternary stages. Different modeling techniques and filters for gravity anomalies show that i) a series of deeper and larger depocenters, where Late Oligocene to Early Miocene sequences are buried, were located particularly at the Present trough boundaries; while ii) smaller and probably shallower depocenters were concentrated at the axial-eastern trough following structures at surface with young morphological expression. The former are interpreted as representative of the initial extensional stage when Late Cretaceous compressional basement structures were relaxated, while the latter could be the product of incipient-young extensional reactivation of the Agrio fold and thrust belt. General coincidence between both areas of extensional relaxation at the western Agrio fold and thrust belt implies that a common basement discontinuity is recurrently being reactivated through time, probably in coincidence with the eastern main Loncopué fault boundary. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The Andes between 38° and 39°S are formed by the subduction of the Nazca plate beneath the South American plate that produced shortening in the last 120 Ma. The main morpho-structural features developed under this regime are from west to east: the Coastal Cordillera and the Chilean Central valley on the western side of the Andes, while on the eastern side the Main Neuquén Andes, the Loncopué trough and the Agrio fold and thrust belt were developed (Fig. 1). Particularly, the Loncopué trough is located between 36°30′ and 39°S, interposed between the Agrio fold and thrust belt and the Main Cordillera at the transitional area between the Southern Central and Northern Patagonian Andes (Fig. 1). Older contractional deforma- tions at the Agrio fold and thrust belt were mainly absorbed in Late Cretaceous times (Ramos, 1998; Zamora Valcarce et al., 2006), mildly reactivated in Late Miocene times. The Loncopué trough has a rather symmetric morphologically negative anomaly has 200 km in length in a longitudinal N–S direction, and 40 km W–E, and is bounded by two main fault systems: one located at its eastern flank formed by west- and east-facing escarpments, and the other located at its western flank formed by east-facing escarpments (Fig. 2)(Ramos, 1978; Rojas Vera et al. 2008). The afore mentioned easternmost fault system juxtaposes Mesozoic folded sequences, corresponding to the western Agrio fold and thrust belt, and Quaternary volcanic rocks accumulated at the Loncopué trough (Ramos, 1978). The westernmost fault system affects the Present arc zone. The basement is buried beneath Neogene to Quaternary volcanic products, and there are not exposed any structural relations in older strata. This fault system is formed by N- trending escarpments segmented by NW-trending ones (Fig. 2). Since the late 90s some studies have pointed out, based on poor quality seismic information, that identified linear escarpments outlining the Loncopué trough were indeed extensional structures affecting Tertiary to Mesozoic strata at depth (Zapata et al., 1999; Jordan et al., Tectonophysics 492 (2010) 279–294 ⁎ Corresponding author. Intendente Güiraldes 2160 Ciudad Universitaria | Pabellón II, 1er piso. Laboratorio de Tectónica Andina. Depto. Geología. C1428EGA – Ciudad Autónoma de Buenos Aires. Argentina. Tel.: +54 11 4576 3329x238. E-mail address: erv081@yahoo.com.ar (E.A. Rojas Vera). 0040-1951/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2010.06.019 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto