Cenozoic Evolution of the Sierras Pampeanas Tectonomorphic Zone Between 27.5°S and 30.5°S, Argentina

Abstract

The Andean Cordilleran orogenic system stretches over 7,000 km along the western margin of South America and serves as a useful laboratory to evaluate the causes of spatial and temporal variations in orogenic processes. Although the geology of the Andean margin is fundamentally controlled by the subduction of the Nazca plate beneath the South American plate, the style of deformation, basin morphology, exhumation history, and volcanic activity along this margin are remarkably heterogeneous in both time and space. My Ph.D. work presents new data from the Miocene – Pliocene along-strike depocenters bounding the basement block uplifts of the Sierras Pampeanas and the fold and thrust belt of the Andean Precordilleran in the south Central Andes between ca. 27.5°S and 30.5°S. I use new observations from sedimentology, detrital zircon U-Pb data, and low-temperature thermochronology to evaluate the mechanisms driving basin organization, sedimentation, and exhumation. Geohistory analysis supports flexurally controlled basins between ca. 18 and 6 Ma with detritus derived exclusively from the active Precodillera to the west. Accelerated deformation in the Precordillera produced accelerated sedimentation from ca. 10 – 8.5 Ma. A deceleration of sedimentation from ca. 6 – 5 Ma was most likely controlled by heightened aridity. Around the same time, low-temperature thermochronometers record the widespread exhumation of the foreland basin system for over 300 km along strike, this may be driven by dynamically controlled uplift related to Miocene flat-slab subduction. Low-temperature thermochronometers suggest that the geothermal gradient throughout the late Miocene was ca. 35°C/km – 25°C/km and had not been significantly depressed as previously proposed. Granite-cored ranges in the Sierras Pampeanas were sampled for low-temperature thermochronology to constrain the exhumation history of the region. Modeling of both apatite fission track and apatite (U-Th-Sm)/He thermochronometers demonstrates that these rocks have been close to the surface since the late Paleozoic. Reheating during the Cretaceous is attributed to elevated geothermal gradients due to back-arc rifting. Final exhumation (1- 2 km) occurred in the mid to late Miocene and may have been controlled by the onset of flat-slab subduction. These results suggest that the Sierras Pampeanas may have had inherited positive topography that has controlled basin organization and sediment distribution patterns since the Paleozoic.