Age of Tertiary extension in the Bitterroot metamorphic core complex, Montana and Idaho

One of the largest and most important extensional structures in the Bitterroot complex is the Bitterroot mylonite zone. An undeformed rhyolitic dike that transects mylonitic fabrics in the southern part of the zone yields a ⁴⁰ Ar/ ³⁹ Ar K-feldspar age of 46.4±0.8 Ma, establishing a minimum age for the structure. Previously published U-Pb data for deformed granitoids from other parts of the shear zone suggest that the maximum age of mylonitization is 48±1 Ma. Documentation of 46-48 extension in the Bitterroot complex suggests simultaneous deformation in the Rocky Mountains Basin and Range and Omineca extensional provinces and reinforces the idea that the Lewis and Clark fault system may have acted as an extensional transfer zone in middle Eocene time. -from Authors