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

SAGRIPANTI, LUCÍA; ROJAS VERA, EMILIO; FENNELL LUCAS; ANDRES FOLGUERA; VICTOR A. RAMOS

Growth of the Southern Andes

Springer

Año: 2016; p. 115 - 132

The following doctoral thesis analyzes the deformational pulses and mechanisms that occurred in the Southern Central Andes at 37ºS. Four zones were selected for that purpose, being from west to east the Cordillera del Viento, the Tromen volcanic plateau, the Sierra de Reyes and the Chachahuén volcanic complex. Each of these zones shows evidence of one or more deformational stages that affected the Andean margin since the Late Cretaceous onwards. Due to the contrasting structural settings and geology of each zone, different methodologies were applied, from i) balanced structural cross sections to represent structure at depth, ii) use of potential methods for a subsurface constraint, iii) geochronological analyses using U/Pb dating in detrital zircons to determine maximum ages and source areas, iv) morphometric analyses done in the drainage network in order to analyze the activity of neotectonic structures during landscape evolution, and v) sedimentary profiles with recognition of fossils which allow constraining the age and depositional environment of the area. As a result of this study 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 that reflect a more complex pattern than previously expected. In particular, the Cordillera del Viento area 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 without a direct control. To the east, the Tromen volcanic plateau registers a pre-Miocene stage of uplift that was affected by a neotectonic reactivation that modifies the pattern of the modern fluvial network. This reactivation may be locally related to the rise of asthenospheric material under the region as revealed by magnetotelluric studies. In the orogenic front, the Sierra de Reyes was initially uplifted during Eocene times to subsequently suffer a strong reactivation during the Neogene. This last stage produced an unroofing sequence registered in the Sierra de Reyes foredeep. Finally, the Neogene deformational stage has produced an anomalous development towards the foreland region where Lower Cretaceous sequences were deformed during the intrusion of a sill swarm at 7 Ma that was coeval with this deformation. Thus, the Chos Malal fold and thrust belt shows an in sequence development between the Upper Cretaceous and Eocene, that was followed by a reactivation in the internal sectors through a series of out-of-sequence thrusts since the Miocene to the Quaternary, defining an active orogenic front at the mid section of the fold and thrust belt.