In late Mesozoic times, Eastern Asia was affected by a large-scale extensional event that extended from Transbaikalia to the NW to northeastern China to the SE. However, before this work, extensional deformation of the eastern Mongolian basement was not documented. Thus the aim of this study was to describe the modes of extension that marked eastern Mongolia. Through structural analyses of three representative regions at the scale of eastern Mongolia, we show that extension is marked by exhumation of a metamorphic core complex, emplacement of syn-thinning granite intrusions, and development of a horst and graben system. Numerous geochronological data (obtained from U-Pb on zircons, U-Th-Pb on monazites and xenotimes, and 40Ar/39Ar on micas and amphiboles) constrain the exhumation of the metamorphic core complex and the emplacement of the granite intrusions between ca. 138 Ma and 125 Ma. This time range appears similar to that generally attributed for the deposition of syn-rifts series within overlying basins and to those available for exhumation of metamorphic core complexes in the adjacent regions. We further propose that extension resulted from a major change in kinematic of the paleo-Pacific domains around 140-135 triggering the horizontal spreading of the abnormally hot eastern Asian continental lithosphere. Finally, on the basis of field observations and geochronological arguments, we suggest that extension might have favoured the onset of transfers of uranium-rich fluids from the basement to the overlying basins.
