Tectono-Thermal Evolution of the Thrust Belt in the Central Andes between 23-24 °S

Abstract

The Andean belt in South America is a first-order topographic feature on Earth. Significant differences in the topography, amount of shortening, styles of deformation, magmatism, among other features make the Andes an ideal natural laboratory to study how this type of orogeny operates through time. Fundamental questions remain regarding the mechanisms responsible for the formation of the Altiplano-Puna Plateau, a high (>3650 m) and thick crustal zone (60-70 km) in the hinterland of the central part of the Andes (17-27 °S). Two mechanisms have been proposed to be responsible for the formation of the plateau. In the Altiplano, a rapid Miocene surface uplift has been associated with delamination events after or during crustal thickening due to underthrusted crust; while in the Puna Plateau, surface uplift has been linked to a slow and steady growth due to shortening and crustal thickening. However, the kinematic history of the thrust belt in the Puna Plateau remains poorly constrained, limiting our understanding of what characteristics in the thrust belt led to the formation of the Plateau. In order to provide constraints on the growth of the Andes through time at Puna latitudes (23-24 °S), this dissertation focuses on: (1) Is shortening at northern Puna latitudes able to explain the crustal thickness? (2) What is the kinematic evolution of the thrust belt at 23-24 °S? and (3) How does the kinematic evolution of the thrust belt relate to lithospheric processes in the region? These questions were addressed using field methods, structural analysis of outcrop to regional scale structures, two low-temperature thermochronometers (apatite fission tracks and apatite (U-Th)/He), thermal modeling, U-Pb geochronology, and structural modeling. In order to provide constraints for the kinematic model, this study concentrates on determining the mechanisms, timing, and distribution of deformation in the Puna Plateau and identifying the main periods of exhumation and thickening during the growth of the orogen. In Appendix A, low-temperature thermochronologic data recorded in the Late Cretaceous to Miocene deposits and the main thrust fault in the thrust belt show that exhumation was non-uniform, started as early as 86 Ma but mainly occurred between ca. 50 and 28 Ma. These data, together with previously documented growth strata and regional unconformities, support that periods of propagation of the orogenic front at ca. 65 Ma and 50-40 Ma are separated by periods of internal deformation and exhumation. In Appendix B, structural, thermochronologic and geochronologic data show that a significant part of the shortening in the Puna Plateau is Paleozoic. Additionally, results from this appendix demonstrate that the Puna Plateau started to shorten and thicken at ca. 45 Ma, accumulated shortening mainly during the Eocene and Oligocene and experienced both forward and internal deformation during an unsteady but overall eastward propagation of the thrust belt. In Appendix C, structural and thermochronologic ages from the Eastern Cordillera east of the Puna Plateau were integrated with previous geochronologic, thermochronologic, sedimentologic, stratigraphic and structural data from the Puna Plateau and Eastern Cordillera in a kinematic model that explains the evolution of the thrust belt during the last ca. 45 Ma. This model estimates 219 km of shortening only in the Puna and Eastern Cordillera. This shortening, when combined with shortening estimates from the Cordillera de Domeyko and Salar de Atacama areas in the earlier part of the thrust belt, increases to 240 km. These results show that shortening is responsible for 90% of the crustal thickness at Puna Plateau latitudes (23-24 °S). When comparing the kinematic model to the characteristics of the lithosphere at 23-24 °S, this study suggests that periods of internal deformation and thickening in the Puna Plateau could have been facilitated by delamination events able to weaken the lithosphere and result in making space under the plateau for additional shortening. Additionally, the analysis of along-strike variation in the kinematic history of the thrust belt, structural styles, amount of shortening and upper plate stratigraphy between 19 and 25 °S suggests that a layered and laterally continuous stratigraphy able to accommodate significant horizontal shortening might have been a key factor in the formation of the Altiplano-Puna Plateau.