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Interactions entre processus géodynamiques, tectonique régionale et climat : l’exemple des Andes australes au Néogène

Abstract : The aim of this thesis is to better understand the processes at the origin of the evolution and present-day topography of the Fitz Roy region, located in the Southern Andes, at the border between Chile and Argentina (49°S). This region is characterized by a particular geodynamic context that may play a major role on surface processes. Indeed, the Chile Ridge separating the Nazca and Antarctic oceanic plates, is being subducted beneath the South American plate since 14 My. The obliquity between the ridge and the subduction trench is responsible for a continuous northward migration of the triple junction that is located today at 46°30’S. Furthermore, the Southern Andes is covered by the 3rd largest ice sheet, that constitutes the relict of a much broader glacier formed during the late Miocene. Nowadays, Patagonia is affected by a strong climatic contrast, with a semi-arid climate on the eastern side chain, while the core and the western side of the chain are covered by glaciers and subject to strong precipitations. The tectonic/climatic setting is unique in the world and constitutes a natural laboratory to study of the interactions between ridge subduction, regional tectonics and climatic conditions on topographic evolution through time.To explore the impact of spreading ridge subduction on the overlying continental plate, analogue modeling of subduction of a ridge were conducted. The models highlight direct effects on the geometry of the slab et thus on the position of the magmatic arc. The arrival of the spreading ridge at trench, when the overriding plate moves rapidly trenchwards, induces a decrease of the superficial dip of the slab. This flattening results in an increase of the distance between the trench and the magmatic arc, prior to the ridge subduction. The eastwards migration of the magmatic arc is consistent with the observed magmatic activity migration in Patagonia during the early Miocene, a few million years before the arrival of the Chile Ridge at trench around 14 My. These models also suggest that the subduction of the ridge induces a horizontal shortening of the overriding plate, as observed in Patagonia during the early-middle Miocene. Shortening ceases when the ridge subducts.To explore the role and of tectonic and climate (glaciation) on the topography and exhumation rates, thermochronological data have been acquired in two localities: the Fitz Roy granitic massif, emplaced during the lower Miocene around 16,5 My at a depth of about 8 km and the overlying Cretaceous sandstones located to the East. Apatite fission track data (AFT) from granites provides a mean signal of 14,8 My. This suggests that the pluton cooled rapidly after its intrusion below 110 °C. The signal obtained in sandstones is younger at 12,5 My. We suggest that the fast denudation is related to the arrival of the ridge at trench and precedes its subduction, which take places at about 12 My at the latitude 49°S. Although we have not been able to constrain the exact timing of shortening in the fold and thrust belt, related to ridge subduction. We suggest that this deformation related to the ridge subduction may have ceased between the lower and the middle Miocene, based on other studies realized to the North and to the South of our study area.The (U-Th)/He data in apatite (AHe) present a signal between 7 and 6 My, that highlights the effect of the first glaciations of Patagonia that induced an acceleration of the relief denudation. We suggest that glacial erosion play a major role on the topography and relief denudation during the late Miocene. Glaciers of Patagonia at the latitude 49°S did not have a protector role of the relief, as suggested by Thomson et al. (2010).To conclude, evolution of relief in Patagonia is the result of both tectonic and climatic processes. In our study area, at a 49°S latitude, the effect of each process has been highlighted because they are not contemporaneous but separated by about 6 to 7 My.
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Submitted on : Monday, April 27, 2020 - 9:44:10 AM
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Méline Salze. Interactions entre processus géodynamiques, tectonique régionale et climat : l’exemple des Andes australes au Néogène. Tectonique. Université Bourgogne Franche-Comté, 2019. Français. ⟨NNT : 2019UBFCD015⟩. ⟨tel-02555073⟩



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