The lack of pervasive flooding of the South Pole-Aitken (SPA) basin raises questions about the mare basalt source region models, whether deep or shallow. This paper investigates the dichotomy between the nearside Imbrium-sized flooded basins and the unflooded SPA basin by modeling the effects of giant impacts on the dynamics of the lunar mantle. We allow an initially spherically symmetric Moon model to evolve through mantle convection for a few hundred million years and then let an Imbrium-or SPA-sized impact occur. The subsequent thermal convection in the lunar mantle resulting from the thermal energy introduced by an impact is determined for the next 100 m.y. The mantle is assumed to be incompressible, with an infinite Prandtl number and a temperature-dependent viscosity. It is shown that the mantle circulation induced by an Imbrium-sized impact can only slightly push aside the radioactive-rich potassium, rare earth element, and phosphorous (KREEP) layer from beneath the surrounding crust. This concentrates the radioactive elements beneath the low-thermal-conductive ejecta blanket and provides a suitable condition for enhancement of radioactive heating and subsequent partial melting and basaltic flow to the basin. On the other hand, a SPA-sized impact is capable of generating mantle circulation vigorous enough to strip away the KREEP layer from beneath the surrounding crust within a time period of 40 m.y. This removal prevents radioactive heating and partial melting beneath the surrounding crust, thus explaining the lack of flooding of SPA basin. These results are insensitive to the time of impacts but are strongly dependent on their sizes. ¿ 2001 American Geophysical Union