AGE AND MAGNITUDE OF DIP-SLIP FAULTING DEDUCED FROM DIFFERENTIAL COOLING HISTORIES - AN EXAMPLE FROM THE HOPE FAULT, NORTHWEST MONTANA

Determination of the age of fault motion poses a challenge in tectonic s, yet rarely produces satisfactory results. We describe a new method in which the age and magnitude of dip-slip faulting are estimated from contrasting cooling histories of footwall and hanging wall rocks adja cent to the Hope fault, northwest Montana. The Hope fault has been int erpreted in the past as a mostly right-slip fault. New kinematic data, Ar-40/Ar-39 thermochronometry, and geobarometry indicate that cooling of footwall rocks at similar to 40 Ma resulted from dip-slip movement . This movement caused vertical separation of about 3 to 5 km between footwall and hanging wall rocks, suggesting that a minimum dip-slip co mponent of 4 km developed during the Late Eocene. These results indica te that the Hope fault experienced substantial normal slip in the Late Eocene, making it coeval with other normal and detachment-style fault s in the northern U.S. Cordillera. The western Lewis and Clark line, w hich in part may share a common tectonic history with the Hope fault, should be re-evaluated for its role in transferring Tertiary extension between the Priest River and Bitterroot core complexes.