7t h Int ernat ional Symposi um on Andean Geodynamics (ISAG 2008, Nice), Ext ended Abst ract s: 461-464 461 The Loncopué Trough: A major orogenic collapse in the western Agrio fold-and-thrust belt (Andes of Neuquén, 36º40´-38º40´S) Emilio Rojas-Vera 1 , Andrés Folguera 1 , Gonzalo Zamora-Valcarce 2 , & Victor A. Ramos 1 1 Laboratorio de tectónica andina, CONICET-UBA, 2º Pabellón, Ciudad Universitaria, Buenos Aires, Argentina (emilio@gl.fcen.uba.ar) 2 Repsol-YPF, Exploración Neuquén, Argentina KEYWORDS : neotectonics, extensional retroarc basin, strike-slip faults, orogenic collapse, basament-controlled structure Introduction The Pliocene to Quaternary Loncopué trough is located in the Andean retroarc zone between 38º 40’ and 36º 40’S approximately, east of the main Andes and the Present volcanic arc west of and the Agrio fold and thrust belt (Figure 1). Neotectonic activity has been proposed in the area by several authors (Ramos and Folguera 2005, García Morabito et al. 2005, Foguera et al. 2006) who based on morphotectonic features have interpreted as controlled by normal faults located between the Agrio fold and thrust belt and the Loncopué trough (Figure 1). However, scarce to none evidence of faulted Quaternary materials has been described. More recently, Yuan et al. (2006) determined a crustal attenuated area beneath the Loncopué trough using receiver function techniques. Zapata et al. (1999), Zamora Valcarce et al. (2006) and Jordan et al. (2001) based on limited seismic reflection and field data, proposed a basement west-dipping normal fault controlling a Late Oligocene depocenter to the west beneath the Loncopué trough, in coincidence with that area of reported young tectonic activity. However, Cobbold and Rossello (2003) interpreted this limit as produced by a major backthrust that overrides the Mesozoic sequences over Tertiary strata. In this stuy we present field evidence on the transtensional nature of the limit between the Agrio fold and thrust belt and the Loncopué trough, as well as the complex structure of the axial part of the trough that was formed by a series of ten-of- kilometers wide pull-apart depocenters that evolved during Quaternary times. Agrio fold and thrust belt This deformational belt has been divided in two sectors with contrasting geology and structural styles. The western sector near the Loncopué trough (Figure 1) is characterized by the strong inversion of Late Triassic to Early Jurassic normal faults uplifting synrift sequences of the Neuquén basin. Compressional deformation begun in the Late Cretaceous times (Zapata et al. 1999, Zamora Valcarce et al., 2006). Thin-skinned deformation becomes more important at the eastern sector where post Late Jurassic sequences are detached from Kimmeridgian evaporites which form the main decollment in the area. Late Miocene contraction has modified Late Cretaceous uplifts as revealed by the development of synorogenic depocenters (Zapata et al 2002, Zamora Valcarce et al. 2006). Loncopué trough Extensional tectonics associated with the Loncopué trough is superimposed to the western Agrio Fold and Retour au Sommaire Sommaire / Contents / Contenido