KINEMATICS OF EOCENE METAMORPHIC CORE COMPLEXES IN THE NORTHERN ROCKY MOUNTAINS

Metamorphic core complexes of the northern Rocky Mountains (MT and ID) were exhumed in Eocene time by large-scale crustal extension. Extension and exhumation in the Priest River, Clearwater, Bitterroot, and Anaconda complexes was kinematically linked via dextral transtension on the Lewis and Clark transform fault system. Detailed geochronology and thermochronology (U-Pb, Ar-Ar, fission track) from the Bitterroot complex indicates that extension started at 53+/-1 Ma and continued until after 40 Ma. New SHRIMP U-Pb zircon data from the Anaconda complex indicate a similar timing for ductile deformation. Geochronological data from the Priest River complex indicates a similar timing for the onset of major extension, and published K-Ar ages from the Clearwater complex are consistent with exhumation during the same time span. Mylonitic granodiorite and two-mica granite in the Anaconda shear zone give crystallization ages of 53 +/-1 and 48 +/1 Ma, respectively from 206Pb/238U analyses of zircon. Stretching lineations in mylonites of the core complexes south of the Lewis and Clark fault zone are oriented ~104?-110? and coincide with the general trend of the transform fault system, which is consistent with proposed kinematic links. Extension and exhumation of middle crustal rocks along the Lewis and Clark fault zone occurred near the southern boundary of areas within the Cordillera that underwent major Eocene extension driven by regional dextral transtension. Exhumation of deep crustal rocks in this tectonic setting was concentrated along the strike-slip fault system in areas that contained voluminous Eocene mid-crustal magmatism, and were underlain by primarily Proterozoic crust.