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As a consequence of the growth of accretionary orogenic belts in central Asia, the high elevation of the East Sayan Mountains concomitant with the plutonic activities in Jom-Bolok and Azas Plateau volcanic fields provide a rare opportunity to unravel lithospheric deformation induced by large-scale tectonic processes such as the passage of thermal plumes. Here we use receiver functions (RFs) to obtain high-resolution images of the 410 km (d410) and 660 km (d660) discontinuities and to measure mantle transition zone (MTZ) thickness. The average apparent depression of the d410 and d660 for a circular area under northern Mongolia and East Sayan are 14 km and 51 km respectively, leading to a significant thicker-than-normal MTZ with a mean value of 37 km. Our results, when incorporated with previous geochemical characteristics, suggest heterogeneous deep mantle materials highlighted by the great depression of the d660, revealing that possible foundered lithospheric remnants have dripped into the MTZ beneath the East Sayan Mountains. Negative thermal anomalies generated by the recycled lithosphere in the MTZ elucidate the prominent lateral undulation of the MTZ discontinuities, and a MTZ thinning beneath the southwest part of the study area is ascribed to the upward small-scale mantle convection initiated by the foundered lithospheric materials. We suggest that the descending lithosphere is due to the hot mantle plumes interacting with base of the mantle lithosphere which provided a viable perspective for the origin of the widespread magmatisms with distinct geochemical signatures in the region.