Radiometric dating of shallow crustal faults, which occur in the crust’s brittle regime, has remained a major challenge, because the low temperatures typical of these faults prevent complete syntectonic recrystallization achieved in deeper faults. Many shallow fault zones preserve both primary (detrital) and newly-grown (authigenic), fine-grained phyllosilicates. Radiometric ages of these fault zones, therefore, reflect a mixture of both mineral populations, whereas we are interested in the age of the newly-grown phase formed as a result of faulting. Rather than (erroneously) assuming that little or no detrital material is left in very fine grain size fractions of fault rocks, we analyze the variable ratios of authigenic and detrital mica in different clay size fractions using quantitative X-ray analysis and modeling of spectra. The detrital mica component is characterized by 2M1 polytype, whereas the authigenic form is 1M/1Md polytype (typically mixed-layer illite/smectite, I/S). Recoiled 39Ar of clay samples is trapped using vacuum-encapsulation methods, giving Ar ages that are functionally equivalent, but more precise that K-Ar analysis. Our determinations for six foreland faults in the southern Canadian Rockies cluster into two discrete populations, ~53Ma and ~73Ma, that are represented by the McConnell and Rundle thrust systems, respectively. These data suggest an episodic rather than progressive deformation history during foreland thrusting. This episodic history seems reflected in the structural geometry of the thrust belt and has new implications for the uplift and erosional history of the Rocky Mountains.