Limits on the tectonic significance of rapid cooling events in extensional settings: insights from the Bitterroot metamorphic core complex, Idaho-Montana

Rapid cooling events deduced from thermochronologic data are often interpreted to be coincident with the age of extensional denudation in metamorphic core complexes. ⁴⁰ Ar/ ³⁹ Ar data for mylonitic and nonmylonitic footwall rocks from the Bitterroot core complex, Idaho-Montana, US, Challenge this view. Newly obtained hornblende ⁴⁰ Ar/ ³⁹ Ar cooling ages for samples collected from the mylonite zone suggest 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 ⁴⁰ Ar/ ³⁹ Ar data from the footwalls of metamorphic core complexes to suggest the precise age of tectonic denudation. -from Authors