Thirty years ago Mariner 10 revealed extensional troughs that form giant polygons in the floor material of the Caloris impact basin, Mercury. The polygonal troughs occur in the interior of the basin and overprint wrinkle ridges formed in an earlier stage of compression. In contrast, lunar and martian basins exhibit extensional troughs that are circumferential and confined to basin margins. Loading by basin-filling material can explain the extensional and compressional features seen in deformed lunar and martian basins, but not the existence of the Caloris polygonal troughs. Here we suggest that the Caloris troughs formed from late-stage basin uplift and extension due to lateral flow of a relatively thick crust toward the basin center. If such lateral flow occurs, the resulting timing, location, and magnitude of the extensional stresses predicted by our model are consistent with those inferred from the polygonal troughs. These results are not strongly dependent on the degree of lateral flow or the assumed crustal rigidity. For a dry plagioclase rheology and likely radiogenic heat fluxes, the crustal thickness around Caloris is 90–140 km. Similar late-stage uplift and extension probably do not occur in basins on the Moon and Mars because their crusts are too thin to allow analogous lateral flow.