PALEOZOIC ASSEMBLY, MESOZOIC REACTIVATION AND LATE CENOZOIC DEFORMATION OF THE BASEMENT OF THE SIBERIAN ALTAI MOUNTAINS: A MULTI-METHOD THERMOCHRONOMETRIC STUDY

Situated in the NW of the active Central Asian orogenic belt, the Altai Mountains form a domal shaped intracontinental, mainly transpressive, orogen in the Siberia-Kazakh-Mongolia-China border region. Using multi-method thermochronometry we reconstructed the thermo-tectonic history of the Siberian Altai. Our study in particular focuses on the Paleozoic granitoids underlying the Teletskoye graben area. This young (< 2 Ma) extensional feature, formed along long-lived structures in the Precambrian-Paleozoic basement. Reactivation of these structures occurred since the Plio-Pleistocene and is thought to be a far-field effect of indentation of India into Eurasia. This neotectonic deformation created the Teletskoye graben and is responsible for the current Altai morphology. The results obtained in our ongoing study provide new insights in the thermo-tectonic history of the Siberian Altai. Zircon SHRIMP U/Pb ages of 460-420 Ma indicate a significantly earlier (Late Ordovician-Early Silurian) crystallization age for the granitoids than previously thought (Early to Late Devonian). Emplacement of the plutons probably represents a terminal phase in the collision of the Tuva-Mongolian and Altai-Mongolian microplates with Siberia. 40Ar/39Ar age spectra of amphiboles, muscovites and biotites indicate a continuous post-collisional cooling of the basement throughout the Devonian (~380-390 Ma) and Early Carboniferous (~350-360 Ma). Apatite fission-track (AFT) ages were found to be Mid Mesozoic and refer to the low-T cooling history of the rocks. Modeling of AFT age and confined track length data records a rapid, Late Jurassic-Cretaceous (~150-80 Ma) cooling event, related to denudation and tectonic reactivation caused by the distant Mongol-Okhotsk orogeny and important extension in adjoining West Siberian and Mongolian basins. From the Late Cretaceous until the Pliocene, the thermal history models indicate a period of stability, that was disturbed ~5 Ma ago by renewed cooling. This cooling is interpreted as denudation of the active and growing Altai mountain ranges. In the frame of this study, ongoing apatite U-Th/He analyses will be linked to these AFT dates and should further increase our understanding of the latest phases of deformation in the Altai and their relation with the India-Eurasia convergence.