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 tectonics, 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 adjacent to the Hope fault, northwest Montana. The Hope fault has been interpreted in the past as a mostly right-slip fault. New kinematic data,$^{40}Ar/^{39}Ar$thermochronometry, and geobarometry indicate that cooling of footwall rocks at ~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 component of 4 km developed during the Late Eocene. These results indicate that the Hope fault experienced substantial normal slip in the Late Eocene, making it coeval with other normal and detachment-style faults in the northern U.S. Cordillera. The western Lewis and Clark line, which 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.