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Exhumation différentielle de la Cordillère Orientale (Déflexion d'Abancay, Pérou) : Une syntaxe tectonique andine ?

Abstract : I focus my thesis on the understanding of the structuration of the Altiplano northern edge: the Abancay Deflection. This region marks abruptly the latitudinal segmentation of the Peruvian Central Andes and presents striking features such as a positive topographic anomaly in comparison of its surroundings, relief obliquity, faults et rivers deviations. Numerous plutons, deeply incised by the Apurimac and Urubamba Rivers, outcrop in the core of the Abancay Deflection. This is a key that permit us the quantification of the rock kinetics at depth in that region. The Abancay Deflection is located over two morpho-tectonic areas: The Eastern Cordillera northward and the Altiplano to the south. The crustal-scale Apurimac fault system separates these two regions. Despite these significant characteristics, the Abancay Deflection is poorly documented and no recent study deals about its long-lived structuration in the Andean geodynamic context since 40 Ma. I developed a multidisciplinary approach based on low-temperature thermochronology, thermo-kinematics modeling and quantitative geomorphology to determine exhumation rates in the studied area, to unravel the mechanisms behind its structuration and to define its recent morphological evolution for different spatial and temporal scales. I demonstrate that the Abancay Deflection registered globally steady and uniforms exhumation rates of 0.2±0.1 km/m.y. between 40 and ∼5 Ma, contemporary to the Central Andes surface uplift. This exhumation pattern marks a large-scale tectonic shortening and/or a lower crustal flow. In this context, the Abancay Deflexion was an integrant part of a paleo-Altiplano extending northward (∼10°S vs. ∼14°S nowadays) partially or totally internally drained. Although the northern Eastern Cordillera and the Altiplano kept identical exhumation rates after ∼5 Ma, the southern Eastern Cordillera experienced a rapid increase of these ones (1.2±0.4 km/m.y.) creating a latitudinal differential exhumation pattern. I explain this very sharp and local increase of exhumation by the common work of the incision by capturing via regressive erosion the paleo-Altiplano through the Urubamba River together with a tectonic uplift tilting the Eastern Cordillera through the backthrusting activity of the inherited crustal-scale Apurimac fault system. According to these interpretations, and regarding the morphological and tectonics similarities, I propose a new geological definition for the Abancay Deflection and that this area is a tectonic syntaxis similar to the Himalayan or Alaskan syntaxes.
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Benjamin Gerard. Exhumation différentielle de la Cordillère Orientale (Déflexion d'Abancay, Pérou) : Une syntaxe tectonique andine ?. Sciences de la Terre. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAU030⟩. ⟨tel-02466520⟩

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