Progression of the Deformation in the Southern Central Andes (37°S)

Abstract

This chapter analyzes the deformational pulses and mechanisms that affected the Southern Central Andes across 37°S. Four zones were analyzed in detail that from west to east are the Cordillera del Viento, the Tromen volcanic plateau, the Sierra de Reyes, and the Chachahuen volcanic complex. Each of these zones shows evidence of one or more deformational stages that affected the Andean margin from the Late Cretaceous onwards. Due to the contrasting structure and geology, different methodologies were applied for their study, (i) balanced structural cross sections to represent structure at depth, (ii) use of potential methods (gravimetry and magnetometry) for a subsurface constraint, (iii) geochronological analyses using U/Pb dating of detrital zircons in synorogenic sequences to determine maximum ages and source areas, (iv) morphometric analyses in the drainage network in order to analyze the activity of neotectonic structures during landscape evolution. As a result, an evolutionary model is presented for the Southern Central Andes in which four pulses of deformation were recognized in agreement with previous proposals, although with a variable distribution reflecting a complex pattern. In particular, the Cordillera del Viento area in the westernmost sector registers an uplift stage that occurred during the Late Cretaceous followed by a late Miocene reactivation. The mechanisms associated with this uplift are related to the selective reactivation of half-grabens and generation of new thrusts cutting through the extensional architecture. To the east, the Tromen volcanic plateau registers a pre-Miocene stage of uplift, later affected by a neotectonic reactivation. Localization of neotectonic activity could be related to the emplacement of asthenospheric material and consequent weakening of the upper crust, as revealed by magnetotelluric studies. In the orogenic front , the Sierra de Reyes was initially uplifted during Eocene times and subsequently suffered a strong reactivation during the Neogene. This last stage produced synorogenic successions in the Sierra de Reyes foredeep describing an unroofing sequence. Finally, the Neogene deformational stage exhumed Lower Cretaceous sequences in the foreland region before 7 Ma expanding considerably the orogenic wedge. Thus, the Chos Malal fold and thrust belt shows a foreland sequence development between the Upper Cretaceous and Eocene, that was followed by a sudden expansion of the orogenic wedge in late Miocene times and reactivation of the western sectors. Finally, Quaternary out-of-sequence thrusts define an active orogenic front at the midsection of the fold and thrust belt.