Subsurface samples and temperature data from the Fenton Hill drilling site, adjacent to the Valles caldera, New Mexico, allow an investigation of the thermal history of this site through analyses of the behavior of isotopic dating systems in changing thermal regimes and through thermal modeling. Estimates of the age of the heating event at Fenton Hill responsible for the current elevated temperatures vary from about 1 to 4 Ma. However, new 40Ar/39Ar analyses and thermal modeling indicate that the age of heating may be much younger than previously believed. The 40Ar/39Ar analyses of microcline separates from five sample, ranging in drill hole depth from 1.13 to 4.56 km and in situ temperatures from 110¿C to 313¿C, indicate thermal events at the site at around 1030 and 870 Ma and a very recent thermal event related to the thermal development of the caldera. The maximum estimates of peak heating duration during this recent thermal event are between 3 and 60 ka. Two distinct thermal perturbations are recognized in the subsurface temperature data, a shallow disturbance, probably related to heating from a lateral flow of hot water just above the Precambrian surface at a depth of about 730 m and a deep thermal disturbance, the source of which is unknown. Model studies indicate the age of the shallow thermal disturbance to be about 10 ka. With reasonable constraints on the source of the deep thermal disturbance, it must lie within a few kilometers of the site, and its maximum age is estimated to be less than about 40 ka. Thus heating at the Fenton Hill site appeas to be much younger than the main caldera event at about 1 Ma and is probably related to a magmatic and/or hydrothermal event very close to Fenton Hill during the last few tens of thousands of years.