Recent seismic experiments across the southern Sierra Nevada, California, show that the range lacks a thick crustal root. Xenolith studies indicate that delamination and loss of much of the lower crust may have occurred between 10 and 3 Ma. We estimate that delamination occurred ca. 3.5 Ma on the basis of a sudden pulse of mafic potassic magmatism within and just east of the Sierra Nevada from 4 to 3 Ma. This pulse interrupted the low level of volcanism that had prevailed regionally since 13 Ma. Delamination could have promoted magmatism via decompression melting, heating of the shallow lithosphere by upwelling asthenosphere, and foundering of crustal rocks into the mantle, and could have provided dynamic support to the range. Foundering of the lower crust may have occurred spontaneously, owing to negative buoyancy, or may have been triggered by extension between the Sierra Nevada and Death Valley.