Progression of the Deformation in the Southern Central Andes (37ºS)

Abstract

This chapter analyzes the deformational pulses and mechanisms thataffected the Southern Central Andes across 37°S. Four zones were analyzed indetail that from west to east are the Cordillera del Viento, the Tromen volcanicplateau, the Sierra de Reyes, and the Chachahuén volcanic complex. Each of thesezones shows evidence of one or more deformational stages that affected the Andeanmargin from the Late Cretaceous onwards. Due to the contrasting structure andgeology, different methodologies were applied for their study, (i) balanced structuralcross sections to represent structure at depth, (ii) use of potential methods(gravimetry and magnetometry) for a subsurface constraint, (iii) geochronologicalanalyses using U/Pb dating of detrital zircons in synorogenic sequences to determinemaximum ages and source areas, (iv) morphometric analyses in the drainagenetwork in order to analyze the activity of neotectonic structures during landscapeevolution. As a result, an evolutionary model is presented for the Southern CentralAndes in which four pulses of deformation were recognized in agreement withprevious proposals, although with a variable distribution reflecting a complexpattern. In particular, the Cordillera del Viento area in the westernmost sectorregisters an uplift stage that occurred during the Late Cretaceous followed by a lateMiocene reactivation. The mechanisms associated with this uplift are related to theselective reactivation of half-grabens and generation of new thrusts cutting throughthe extensional architecture. To the east, the Tromen volcanic plateau registers apre-Miocene stage of uplift, later affected by a neotectonic reactivation.Localization of neotectonic activity could be related to the emplacement ofasthenospheric material and consequent weakening of the upper crust, as revealedby magnetotelluric studies. In the orogenic front, the Sierra de Reyes was initiallyuplifted during Eocene times and subsequently suffered a strong reactivation duringthe Neogene. This last stage produced synorogenic successions in the Sierra de Reyes foredeep describing an unroofing sequence. Finally, the Neogene deformationalstage exhumed Lower Cretaceous sequences in the foreland region before7 Ma expanding considerably the orogenic wedge. Thus, the Chos Malal fold andthrust belt shows a foreland sequence development between the Upper Cretaceousand Eocene, that was followed by a sudden expansion of the orogenic wedge in lateMiocene times and reactivation of the western sectors. Finally, Quaternaryout-of-sequence thrusts define an active orogenic front at the midsection of the foldand thrust belt.