Geological setting of the Argentine Frontal Cordillera in the ¯at-slab segment 30800 0 ±31830 0 S latitude) N. Heredia a, * , L.R. Rodrõ Âguez Ferna Ândez b , G. Gallastegui a , P. Busquets c , F. Colombo c a Instituto Geolo Âgico y Minero de Espan Äa, Avda. Repu Âblica Argentina 30, 1B, 24004 Leo Ân, Spain b Instituto Geolo Âgico y Minero de Espan Äa, Rõ Âos Rosas 23, 28003 Madrid, Spain c Facultat de Geologõ Âa, Universitat de Barcelona, Martõ  i Franque Âs s/n Barcelona, Spain Received 1 November 2001; accepted 1 November 2001 Abstract The Argentine Frontal Cordillera, between latitudes 30820 0 and 31820 0 S, presents an eastward-verging polycyclic structure. Two large groups of rocks with markedly different structural nature can be distinguished: a Paleozoic basement with characteristic thin-skinned structure and the Andean cover showing remarkable extensional Mesozoic structures inverted by Tertiary compressional tectonic events. These groups are divided by a large tectonic unconformity separating not only rocks with different style and degree of deformation but also different paleogeographic settings. The Cenozoic tectonic inversion is linked with the subhorizontal subduction of the Nazca Plate in the Pampean ¯at-slab segment and subsequent uplift of the Andean Cordillera. The Paleozoic basement is composed of Devonian and Permo-Carboniferous marine sedimentary rocks, separated by an angular uncon- formity. These rocks were deposited in siliciclastic and mixed sedimentary platforms, respectively, and intruded by Permo-Triassic granitoid rocks. The Devonian rocks were deformed by the Famatinian orogeny in Late Devonian times. The most important Paleozoic structures are related to the Gondwanic orogeny, of Permian age. These include east-vergent thrusts and related folds that record a large shortening component. The Andean cover, lying unconformably over the Paleozoic basement, is primarily volcanic and volcanoclastic, with some interbedded continental sedimentary rocks. Two main stratigraphic groups can also be de®ned here: an older sequence linked to an extensional tectonic event and a synorogenic sequence connected to a compressional tectonic event that produced an inversion of previous extensional features. The lower unit Choiyoi Group: Permian and Triassic) is affected by normal faults with downthrown western blocks and intruded by Jurassic granodioritic rocks. The faults in this unit involve the Gondwanic basement in a typical thick-skinned tectonic style and are grouped in bands with N±S strike. The uppermost units Vizcachas, Melchor, and Olivares groups: Tertiary and Plio-Quaternary) also lie unconformably over the latter. The normal faults were inverted in the upper Miocene by the uplift of the western block, deforming the lower and upper units in a compressional context during the Andean orogenic cycle. Displacement on reverse faults, measured in the Tertiary synorogenic rocks, is usually less than 1 km. The Tertiary crustal shortening calculated from cross-sections is about 10%, in contrast with the estimated 50% shortening of the Precordillera unit. All this shows that most of the crustal shortening of the Andean Cordillera during the compressional stage has been transferred to the Precordillera unit through the lower detachment fault. These facts also show that the Cordillera Frontal unit is an uplifted block in which the extensional structures have been preserved, and that the Rodeo±Upsallata basin is a piggy-back type. q 2002 Published by Elsevier Science Ltd. Keywords: Frontal Cordillera; Andean orogeny; Gondwanic orogeny; Inversion tectonic; Andean igneous rocks Resumen El a Ârea investigada en este trabajo abarca un sector de la unidad morfoestructural conocida como Cordillera Frontal, si bien el extremo oriental de la misma comprende parte de la Depresio  n de Rodeo±Upsallata. En la Cordillera Frontal se diferencian dos grandes conjuntos de rocas con naturaleza y estructuracio Ân diferentes: un substrato Paleozoico y una cobertera Mesozoico terciaria. El substrato Paleozoico esta  constituido por rocas sedimentarias depositadas en ambientes marinos o de transicio Ân, deformadas en el Devo Ânico superior y el Pe Ârmico inferior durante los Ciclos Oroge Ânicos Famatiniano y Gondwa Ânico, e intruidas por rocas granõ Âticas permotria Âsicas. Las rocas relacionadas con el Ciclo Oroge Ânico Famatiniano tienen una edad devo Ânica y aparecen cubiertas en discordancia Journal of South American Earth Sciences 15 2002) 79±99 0895-9811/02/$ - see front matter q 2002 Published by Elsevier Science Ltd. PII: S0895-981102)00007-X www.elsevier.com/locate/jsames * Corresponding author. Tel.: 134-98-726-2171; fax: 134-98-726-2183. E-mail address: nheredia@iet.es N. Heredia).