Tectonic and geodynamic controls on Oligocene–Miocene paleogeography and basin subsidence in southern Patagonia

Abstract

The Cenozoic subsidence and uplift history in southern Patagonia have been influenced by tectonic shortening, sub-lithospheric forces, and climate. The Patagonian Andes mountain-building resulted mainly from shortening and glacial erosion, whereas the extra-Andean foreland was mainly influenced by flexural and subcrustal-driving forces, which drove marine transgressions and plateau-like topography. However, there are some regional features in Patagonia that cannot be fully explained considering the proposed geodynamic scenario. The Deseado Massif in south-central Patagonia is formed mainly by Jurassic rocks overlapped by Paleogene and Neogene basalts. The lack of Oligocene and Neogene strata in this area has been associated with a paleo-relief formed previous to the Oligo-Miocene marine transgressions. This study aims to explore and investigate the likely mechanisms related to the formation of this paleo-topography. This requires analyzing and revising the Cenozoic paleogeography, subsidence and uplift history of southern Patagonia, and its connection with crustal tectonics and the influence of the asthenospheric mantle. This study utilized a combination of methodologies, including analysis of decompaction and backstripping to investigate subsidence and uplift history. These results were compared with flexural loading models constructed from the most recent structural reconstructions. Our investigation shows strong temporal and spatial differences in the subsidence distribution. The comparison of subsidence models suggest that tectonic shortening and loading during the Oligocene and Early Miocene (30–20 Ma), derived from palinspastic studies, had to be much lower than proposed by structural reconstructions. During Neogene, in contrast, the tectonic loads might explain the subsidence records relatively well. From our analyses, the maximum flexural relief could explain the exposition of the Deseado Massif region with respect to the rest of the extra Andean valleys during the Oligo-Miocene marine incursion. To the north of the massif, this uplift was recorded by the tectonic subsidence curves, which show a symmetric upwarping zone that could be associated with reduction in the accommodation space during the bulge zone development. Based on these observations, we propose that the uplift of the Deseado Massif took place during the Oligo-Miocene period, driven by flexural uplifting. Subsequent Neogene subsidence and uplift histories were superimposed on this preexisting flexural topography generating the modern paleogeography. The results have implications for the tectonic history and paleogeographic reconstructions of southern Patagonia, and shed light on the interactions between tectonic and geodynamic processes during the Oligocene-Miocene and their consequences in the foreland evolution of Southern Patagonia.