De nombreuses études ont montré l’impact de la déformation sur le système K-Ar, et donc les âges ⁴⁰Ar/³⁹ Ar. Ces études se limitent souvent à une comparaison des âges obtenus dans des roches déformées et un protolithe indemne de déformation. La première partie de cette étude a inversement consisté à étudier la distribution de la déformation à différentes échelles et à décrire finement les gradients d’intensité de la déformation. L’étude a porté sur deux protolithes de nature granitique, associés à une différence d’âge entre leur formation et les évènements tectonométamorphiques faible (< 1 Ma ; massif d’Ikaria) ou élevée (>240 Ma ; massif du Tende). Pour le premier cas, la déformation entraine une perte de 40Ar dans les clastes des phases potassiques, interprétée comme résultant de la réduction des tailles des domaines de diffusion qui n’est pas accentuée par une intensité de déformation croissante. Pour le second cas, l’héritageen 40Ar du protolithe se traduit par la circulation de fluides et de 40Ar externe au système via les structures se déformant activement, produisant parfois un vieillissement des âges grandissant dans la phengite des structures les plus localisantes, alors que sur d’autres coupes il est observé un rajeunissement plus logique. Pour les deux cas,l’interprétation des âges obtenus dans les phases néoformées pendant la déformation est ambiguë entre refroidissement,cristallisation et mélange, et nécessite un examen détaillé des données confrontées aux températures de fermeture possibles. Les interprétations indiquent pour le cas d’Ikaria une localisation de la déformation ductile en moins de 1-3Ma le long d’un gradient de second ordre d’une dizaine de mètre d’épaisseur. La localisation de la déformation à l’échelle d’une zone de cisaillement se réalise plus rapidement dans le cas de l’exhumation post-orogénique d’un MCC(~7 Ma) que dans le cas de l’exhumation de matériel continental impliqué dans un prisme de subduction (~14-10 Ma). / Numerous studies have shown the impact of deformation on the K-Ar system, and therefore ⁴⁰Ar/³⁹ Ar ages. These studies often do not provide data characterizing deformation and are limited to a comparison of the ages obtained indeformed rocks and an undeformed protolith. The first part of this study thus consisted in studying the strain distribution at different scales and finely describing strain intensity gradients. The study focused on two granitic protoliths, associated respectively with a difference in age between the formation of the protolith and the age of the tectonometamorphic events that is low (<1 Ma ; Ikaria Island) or inversely high (> 240 Ma ; Tenda massif). In the firstcase study, deformation results in a 40Ar loss in K-bearing phases, interpreted as resulting from the reduction of diffusion domains sizes which is not accentuated by an increasing strain intensity. In the second case study, the 40Arinheritance of the protolith results in fluids and extraneous 40Ar circulation through the actively deforming structures,ages in phengite being increasingly older approaching the most localizing structures in some sections, while others behave in an opposite way, more in line with the progressive strain localization in time. For both cases, interpretation of ages obtained in the newly formed phases during deformation is ambiguous between cooling, crystallization and mixing, and requires a detailed examination of the data confronted with the possible closing temperatures.Interpretations indicate for the Ikaria case study a strain localization in less than 1-3 Ma along a second order gradient of about ten meters in thickness. Strain localization at the scale of a shear zone occurs more rapidly in the case of a post-orogenic exhumation of a MCC (~ 7 Ma) than in the case of the exhumation of continental material involved in a subduction prism (~ 14-10 Ma).

This study contributes to the understanding of major river evolution in Southeast Asia during the Cenozoic. In order to trace the evolution of a hypothesized palaeo-Yarlung Tsangpo-Irrawaddy River, this work undertakes the first systematic provenance study of detrital minerals from Cenozoic synorogenic fluvial and deltaic sedimentary rocks of the Central Burma Basin, employing a combination of high precision geochronology, thermochronology, and geochemistry analytical techniques on single grain detrital zircon and white mica. The dataset is compared to published isotopic data from potential source terranes in order to determine source provenance and exhumation history from source to sink. A Yarlung Tsangpo-Irrawaddy connection existed as far back as ca. 42 Ma and disconnection occurred at 18–20 Ma, based on provenance changes detected using a combination of U-Pb ages and εHf(t) values on detrital zircons, and ⁴ºAr/³⁹Ar dating on detrital micas. During the Eocene and Oligocene, units are dominated by U-Pb age and high positive εHf(t) values, characteristic of a southern Lhasa Gangdese magmatic arc source. An antecedent Yarlung Tsangpo-Irrawaddy River system formed the major river draining the eastern Himalaya at this time. A significant change in provenance is seen in the early Miocene, where detritus is predominantly derived from bedrock of the eastern Himalayan syntaxis, western Yunnan and Burma, a region drained by the modern Irrawaddy-Chindwin river system characterized by Cenozoic U-Pb ages and negative εHf(t) values. This is attributed to the disconnection of the Yarlung-Irrawaddy River and capture by the proto-Brahmaputra River, re-routing Tibetan Transhimalayan detritus to the eastern Himalayan foreland basin. Re-set zircon fission track ages of 14-8 Ma present in all units is used to infer post-depositional basin evolution related to changes in the stress regime accommodating the continued northward migration of India. The early Miocene initiation of the Jiali-Parlung-Gaoligong-Sagaing dextral shear zone and the continued northward movement of the coupled India-Burma plate aided in focusing deformation inside the syntaxis contributing to the disconnection of the Yarlung Tsangpo-Irrawaddy system, linking surface deformation and denudation with processes occurring at deeper crustal levels.