Illite age analysis (IAA), the method of comparing radiometric ages of successive size fractions with varying percentages of detrital illite, has been successfully applied to several rock types, including fault gouge, shales and argillaceous limestones. IAA results are presented for five fault rocks, including four clay gouges and one cataclasite, from the exhumed Southern Appalachian foreland fold-thrust belt (eastern United States). Determining detrital versus authigenic illite is now an established procedure, utilizing X-ray analysis to quantify illite polytypes. Less established, however, is how to interpret 40Ar/39Ar ages as a function of diagenetic grade. Both a total gas age, incorporating the recoiled argon fraction after irradiation, and a retention age (omitting the recoiled fraction) are obtained for a sample. We relate their respective use to diagenetic grade and, specifically, the crystallite thickness of illite. When measured crystallite sizes are equal to or smaller than that calculated from Ar recoil we use total gas ages (here ∼5 nm), while for crystallites greater than ∼10 nm we use retention ages.

The four clay gouge ages are all the same within error and much younger than that of the cataclasite. We conclude that a major period of frontal Appalachian fault activity occurred during the early Permian (Cisuralian, 276–280 Ma) and that these foreland faults were active simultaneously as part of an internally deforming, regional thrust wedge.

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