Quantification of fault-related illite neomineralization in clay gouge allows periods of fault activity to be directly dated, complementing indirect fault dating techniques such as dating synorogenic sedimentation. Detrital “contamination” of gouge is accounted for through the use of illite age analysis, where gouge samples are separated into at least three size fractions, and the proportions of detrital and authigenic illite are determined using illite polytypism (1M d = neoformed, 2M 1 = detrital). Size fractions are dated using the 40 Ar/ 39 Ar method, representing a significant improvement over earlier methods that relied on K-Ar dating. The percentages of detrital illite are then plotted against the age of individual size fractions, and the age of fault-related neoformed material (i.e., 0% detrital/100% neoformed illite) is extrapolated. The sampled faults and their ages are the Absaroka thrust (47 ± 9 Ma), the Darby thrust (46 ± 10 Ma), and the Bear thrust (50 ± 12 Ma). Altered host rock along the frontal Prospect thrust gives an age of 85 ± 12 Ma, indicating that the 46–50 Ma ages are not related to a regional fluid-flow event. These ages indicate that the faults in the Snake River–Hoback River Canyon section of the Wyoming thrust belt were active at the same time, indicating that a significant segment of the thrust belt (100 km 2 +) was active and therefore critically stressed in Eocene time.