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, 2M₁ = detrital). Size fractions are dated using the ⁴⁰Ar/³⁹Ar method, representing a significant improvement over earlier methods that relied on K-Ar dating. The percentages of detrital...