Group II (143–117 Ma) and Group I kimberlites (108–74 Ma) intrude across the southwest boundary of the Kaapvaal craton, sampling the same volume of the subcontinental lithospheric mantle (SCLM) in two time slices. Major and trace element analyses of 3699 peridotitic garnet xenocrysts were used to construct paleogeotherms for 17 kimberlite localities, and to place each garnet at its depth of origin. The Ti contents of each garnet and the calculated XMg of its coexisting olivine were projected onto a southwest-northeast section across the craton boundary, and splines were used to interpolate between the virtual boreholes in each age group. The sections show that the cratonic SCLM extends at least 75 km southwest of the mapped craton boundary, suggesting a dipping contact. Marked differences between the time-slice sections show that between 117 and 108 Ma the SCLM on both sides of the craton boundary was heated and chemically refertilized by infiltrating asthenosphere-derived melts, thinning the depleted layer by ~40 km. The thermal and geochemical changes record a significant tectonothermal event that may be related to changes in the stress field associated with opening of the South Atlantic, or with mantle upwelling.