Rapid cooling events deduced from thermochronologic data are often interpreted to be coincident with the age of extensional denudation in metamorphic core complexes. 40Ar/39Ar data for mylonitic and nonmylonitic footwall rocks from the Bitterroot core complex, Idaho-Montana, United States, challenge this view. Previous work brackets the age of the principal detachment in this area, the Bitterroot mylonite zone, between 48 ± 1 and 46.4 ± 0.8 Ma. Newly obtained hornblende 40Ar/39Ar cooling ages for samples collected from the mylonite zone are within this age range, consistent with mylonitization at amphibolite facies conditions. However, muscovite, biotite, and potassium feldspar 40Ar/39Ar ages of ∼45-40 Ma for rocks from the same area define a rapid cooling event that is significantly younger than the Bitterroot mylonite zone. If the 45-40 Ma event is related to tectonic denudation by movement on the detachment, then we can infer a time lag of ∼1-3 m.y. between the age of unroofing and the time of thermal reequilibration. These data suggest caution in the use of 40Ar/39Ar data from the footwalls of metamorphic core complexes to suggest the precise age of tectonic denudation.

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