EXHUMATION AND KINEMATICS OF EOCENE METAMORPHIC CORE COMPLEXES ALONG THE LEWIS AND CLARK STRIKE-SLIP SYSTEM

Metamorphic core complexes of the northern US Rocky Mountains, Montana and Idaho, were exhumed in Eocene time by large-scale extension, and are linked to regional transtension along the Lewis and Clark transform fault system. Metamorphic core complexes north and south of the Lewis and Clark fault zone include the Priest River, Boehls Butte, Bitterroot, and Anaconda complexes. Numerous Eocene basins also opened up along the transform system at this time. Detailed U-Pb zircon and Ar-Ar geochronology from the Bitterroot complex indicates that extension started at 53+/-1 Ma. Geochronological data from the other complexes indicates as similar timing for the onset of transtension. Thermochronology (Ar-Ar and fission-track) from the Bitterroot complex footwall and other footwalls indicates that rapid extension ended at ~40 Ma, but that extension continued to ~35 Ma. Rocks exposed in the four core complexes were exhumed from different depths within the Late Cretaceous-Paleocene orogenic wedge. Metamorphic P-T data indicate a general decrease in maximum exhumation from west (20-30 km) to east (20-10 km), in the respective footwalls. Stretching lineations are similar in the mylonites of the core complexes (~104?-110?) and coincide with the general trend of the transform fault system. Extension and exhumation of middle crustal rocks along the Lewis and Clark transform occurred at the southern edge of the region in the Pacific NW that was effected by major Eocene extension. Regional transtension was most likely caused by oblique convergence between North America and the Kula and/or Resurrection plates. Deep exhumation and major extension was concentrated along the strike-slip fault system in areas that contained voluminous mid-crustal magmatism before and during the transtensional event.