Journal of Geodynamics 49 (2010) 287–295 Contents lists available at ScienceDirect Journal of Geodynamics journal homepage: http://www.elsevier.com/locate/jog The Loncopué Trough: A Cenozoic basin produced by extension in the southern Central Andes Andrés Folguera a,∗ , Emilio Rojas Vera a , Germán Bottesi b , Gonzalo Zamora Valcarce b , Victor A. Ramos a a Laboratorio de Tectónica Andina, Universidad de Buenos Aires, CONICET, Argentina b YPF, Dirección de Exploración y Desarrollo de Negocios, Argentina article info Article history: Received 4 August 2009 Received in revised form 13 January 2010 Accepted 13 January 2010 Keywords: Cenozoic extension Synextensional sedimentation Southern Central Andes abstract The Loncopué Trough is located in the hinterland Andean zone between 36 ◦ 30 ′ and 39 ◦ S. It constitutes a topographic low bounded by normal faults and filled by lavas and sediments less than 5 Ma old. Repro- cessed seismic lines show wedge-like depocenters up to 1700 m deep associated with high-angle faults, correlated with the 27–17 Ma Cura Mallín basin deposits, and buried beneath Pliocene to Quaternary successions and Late Miocene foreland sequences. The southern Central Andes seem to have been under extension in the hinterland zone some 27 Ma ago and again at approximately 5 Ma ago. This last exten- sional period could have been the product of slab steepening after a shallow subduction cycle in the area, although other alternatives are discussed. Orogenic wedge topography, altered by the first exten- sional stage in the area, was recovered through Late Miocene inversion, and was associated with foreland sequences. However, since the last extension (<5 Ma) the Andes have not recovered their characteristic contractional behavior that controlled past orogenic growth. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction The Andean retroarc zone between 35 ◦ and 40 ◦ S(Fig. 1), and particularly between 36 ◦ 30 ′ and 39 ◦ S where the Loncopué Trough is developed, constitutes an outstanding site to address non-steady mountain building since the Andean phase of deformation started some 100 Ma ago. Here we present field and geophysical data from that specific site to exemplify extensional reactivation of the fold and thrust belt that have produced substantial topographic losses. The southern Central Andes (28–40 ◦ S) can be divided in two sectors based on their evolution during the last 17 Ma. Since that time, the northern region between 28 ◦ and 33 ◦ S has experienced the Pampean flat subduction associated with basement foreland uplifts. It is limited eastward by a neotectonic compressional front located some 750 km from the trench (Fig. 1)(Costa and Vita- Finzi, 1996; Ramos et al., 2002). Seismic tomographies at those latitudes show fast P wave-velocities implying a cold setting above the horizontal subducted slab (Gilbert et al., 2006). Contrastingly, a southern segment between 35 ◦ and 40 ◦ S has abandoned the Payenia shallow subduction zone since 5 Ma, as evidenced by migration of the volcanic arc more than 500 km toward the foreland (Fig. 2A) (Kay et al., 2006a). Seismic tomography at approximately ∗ Corresponding author. E-mail address: andresfolguera2@yahoo.com.ar (A. Folguera). 35.5–36 ◦ S(Wagner et al., 2005; Gilbert et al., 2006) and receiver function profiles at 39 ◦ S(Yuan et al., 2006)(Fig. 1) show (i) large subcrustal low-velocity arrivals interpreted as mantle melts and/or volatiles and (ii) an attenuated lower crust. This scenario has been interpreted as derived from asthenosphere injection in a broad- ened asthenospheric wedge more than 400 km away from the trench in the retroarc area, triggering extensional deformation at the upper crust (Fig. 2B) (Folguera et al., 2007, 2008). A 2D prelimi- nary electrical resistivity model at 36.5 ◦ S(Burd et al., 2008) shows an “uprising highly conductive plume” with geometry compati- ble with the asthenospheric anomaly modeled from gravity data (Folguera et al., 2007). The inferred extensional deformation in this segment would have expanded to the east, defining a Quaternary deformational front (Figs. 1 and 2B) where previous compressional structures would be partly collapsing diachronically since ∼5 Ma (Folguera et al., 2006a). This scenario has been discussed and even contradicted by other studies, the main discrepancies based only on surficial data (Lavenu and Cembrano, 1999; Backé et al., 2006; Melnick et al., 2006a,b; Rosenau et al., 2006; Galland et al., 2007). This work, through interpretation of reprocessed seismic lines and field work, proposes a crustal-scale extensional nature of one of the main less than 5 Ma depocenters in the southern Central Andes, as well as its polyphasic nature, indicating repeated instability of the orogenic wedge through time. Finally, its origin is discussed in light of current tectonic models discussed for this Andean segment during the Neogene. 0264-3707/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2010.01.009