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Evolution tectonique des Andes centrales (33°S) : implications sur la mécanique des chaînes de subduction


IPGP - Îlot Cuvier


Soutenances de thèses


Magali Riesner

Tectonique et mécanique de la lithosphère (TECTO)

Large mountain belts standing out on Earth are commonly distinguished in two types: collision-type and subduction-type mountain belts. While mountain- building mechanisms associated with collision-types are, to the first order, quite well understood, the mechanisms leading to subduction-type mountain belts remain controversial. In particular for the central Andes, which are the active archetype of subduction-type mountain belts, two different mountain-building tectonic and mechanical models are proposed. The widely accepted model is that of Andean growth sustained by east-vergent thrusts, propagating toward the continent, i.e. antithetic to the subduction. Based on the discovery of large thrusts synthetic to the subduction along the west Andean mountain front, an al- ternative bi-vergent model has been recently proposed. The present work brings new constraints to this on-going debate and discusses the two models proposed for the Andes at ?33°S latitude, where the belt appears to yield simpler and smaller deformation than further north within the Altiplano. We acquired new and carefully selected geological and thermochronological data to test the two proposed models. Our tectonic approach enables us to precisely establish and quantify the Andean structure and kinematics at 33°S latitude, both at the scale of the forearc and at crustal-scale. The crustal-scale model allows for deriving the cumulative shortening across the Andes. Our detailed investigations of the various structural units, either in terms of shortening or of timing of deformation allow assessing the overall kinematics of the deformation of the Andes at 33°S. This kinematics implies a constant shortening rate across the belt over the last ?25 Myr with, however, a sequencial transfer of the deformation through time between the various structural units. Following these results, we propose a bi-vergent model, with a primarily westward structural vergence - i.e. synthetic to the subduction zone - but with most of the present- day deformation accommodated on antithetic structures along the eastern front. Afterwards, we test the consistency of the kinematics constrained by structural observations with newly acquired thermochronological data in a thermokinematic model of the Andes at 33°S latitude. All the data and observations can be reconciled in a direct and simple thermokinematic model imposing however, a low underthrusting rate of the forearc crustal block under the growing mountain range, which we interpret to be related to its limited flexure. Finally, to explore the potential lateral propagation of the Andean deformation from its core toward the South, we use thermochronological data newly sampled or compiled from published studies at several latitudes in a precise structural framework. The implications of our results are then discussed to better understand subduction-type mountain-building.