Differential cooling across the accreted terrain-North American cratonic boundary in Idaho-Oregon

Abstract:

The Cretaceous-age margin of western North America, as exposed along the Idaho-Oregon border, juxtaposes accreted terrains to the west and cratonic North America to the east. The Idaho batholith intrudes exclusively east of this boundary. New (U-Th)/He zircon ages from samples collected across the boundary between 44 and 45 °N latitude illustrate the variability in cooling below 200 °C and further document the abrupt nature of this margin.

Three samples collected from the accreted terrains near the Idaho border yield the oldest ages: 102.3 ± 1.6 Ma (± 1σ; Sparta Butte, OR); 88.6 ± 1.3 Ma (Snake River at Pittsburg Landing, ID); and 83.5 ± 1.4 Ma (above Pittsburg Landing, ID). These ages bracket the inferred timing of deformation within the western Idaho shear zone (WISZ), which currently demarcates this boundary. Immediately to the east, within the WISZ, samples yield ages of 76.2 ± 1.4 to 63.2 ± 1.0 Ma and record steady cooling following deformation. East of the WISZ, within the Atlanta lobe of the Idaho batholith, cooling ages vary from 65.0 ± 1.0 to 20.2 ± 0.3 Ma. The overall pattern of cooling ages is older on the margins of the Atlanta lobe and younger in the center closer to the Sawtooth Valley. Higher density sampling in the Sawtooth and White Cloud Mountains near Stanley, ID reveal internal young Miocene deformation associated with extension. Ages within the Sawtooth Mountains range from 34.2 ± 0.5 Ma (Atlanta lobe) to 20.2 ± 0.3 Ma (Sawtooth granite), whereas two samples of the Atlanta lobe from the White Cloud Mountains to the east yield ages of 68.5 ± 1.0 Ma and 64.2 ± 1.0 Ma. The cooling age pattern likely reflects exhumation of the Sawtooth Mountains along an E-dipping, low-angle normal fault during the Oligocene-Miocene. Overall, the pattern of cooling ages clearly demonstrates distinct domains of thermal stability – notably accreted terrains, WISZ, and the Atlanta lobe - with respect to cooling below 200 °C.