Abstract

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.

First Page Preview

First page PDF preview
You do not currently have access to this article.