Mid-Tertiary K-Ar ages have been measured from metamorphic rocks that, on the basis of stratigraphic and other evidence, originated at a younger time. The young ages indicate that the rocks were heated prior to uplift during Basin and Range faulting. These data, plus the large volumes of volcanics, indicate widespread crustal heating here during the Tertiary. In the Catalina Mountains of Arizona, 10 of 14 K-Ar ages on mica and feldspar are between 25 and 30 m.y. The remaining, very coarse grained, muscovites are older, and presumably had greater argon retention. Ten other mica samples from the province show dates of 20-57 m.y. but these rocks are low in K-minerals. These high ages can be reconciled by assuming that the mica minerals have incorporated a common level of extra Ar-40 independent of K content.