K–Ar analyses of rocks in the central Kootenay Arc indicate a Paleocene (or slightly earlier) thermal event, locally reaching 350 °C, that has reset this isotope clock. The area of resetting is nearly coincident with a north–south belt, 20 km wide, of metamorphic rocks resulting from an earlier (mid-Jurassic?) thermal event with temperatures reaching an estimated 625 °C. The strata involved in both high-grade metamorphism and resetting seem not to have been uplifted more than older weakly metamorphosed strata to the east. The mid-Jurassic? and Paleocene thermal events appear to be distinct in time from one another, but may both be related to some common control in the deeper crust or upper mantle.