Exhumation and Cooling History of the Salmon River Suture Zone and Western Idaho Shear Zone

Andrea E. Richardson, Washington State University
Sean P. Long, Washington State University; University of Nevada, Reno
Matthew P. McKay, Missouri State University
Matthew T. Heizler, New Mexico Bureau of Geology & Mineral Resources
Stuart N. Thomson, University of Arizona
Kevin Konrad, Oregon State University
Peter W. Reiners, University of Arizona
William K. Barba, Washington State University

Abstract

The western portion of the Jurassic-Paleogene North American Cordilleran orogen was constructed by the progressive accretion of island arc terranes and associated basins. Constraining the timing of individual terrane accretion events provides important context for understanding the spatio-temporal evolution of the Cordillera. Here, we investigate the timing of accretion-related ductile thrusting in the Salmon River suture zone (SRSZ) of western Idaho, which accommodated the suturing of the Wallowa island arc terrane to the North American margin. We present new thermochronometry (40Ar/39Ar hornblende, muscovite, and biotite; zircon and apatite fission-track; and apatite (U-Th)/He), which we integrate with published geochronology, thermobarometry, and thermochronometry to constrain the burial, displacement, and exhumation histories of thrust sheets in the SRSZ. Accretion-related prograde metamorphism in the Pollock Mountain thrust sheet spanned from 141-124 Ma, and displacement on the Pollock Mountain thrust initiated between 124-119 Ma and likely continued until 113-109 Ma. Displacement on the Rapid River thrust initiated between 124-117 Ma and likely continued until 105 Ma, and displacement on the Heavens Gate thrust was likely underway by 109-104 Ma. The Early Cretaceous suturing of the Wallowa terrane was the final event in the accretion of the Blue Mountains Province, and post-dated major Jurassic accretionary events in the Klamath Mountains and Sierra Foothills terrane provinces to the southwest and in the Intermontane terrane group to the north. The dextral-transpressional western Idaho shear zone, which overprinted the easternmost portion of the SRSZ, cooled rapidly from 500 to 300 degrees Celsius between 90-85 Ma, which is consistent with published constraints that bracket shearing between 108-85 Ma.

Session

Rates and dates