CONSTRAINING THE TIMING OF DEFORMATION IN THE WESTERN IDAHO SHEAR ZONE ALONG THE SALMON RIVER CANYON, IDAHO, USA

In the North American Cordillera, a system of margin-parallel strike-slip shear zones accommodated northward translation of accreted terranes between the Late Cretaceous and Eocene. Constraining the timing of displacement on these shear zones is important for understanding the evolution of strain accommodation along the Cordilleran margin. The western Idaho shear zone (WISZ) is a north-striking, dextral-transpressional ductile shear zone that deformed the boundary between the western North American margin and accreted terranes of the Blue Mountains province to the west. Deformation in the WISZ took place during emplacement of the western portion of the Idaho batholith, and several studies have dated pre-, syn-, and post-magmatic intrusions to constrain the timing of WISZ ductile shearing. Studies in the southern part of the WISZ have constrained shearing to have started no earlier than ~105 Ma and ended no later than ~90-88 Ma. However, at the latitude of the Salmon River canyon (45°25’ N), previous studies have interpreted that WISZ deformation may have initiated as early as ~115 Ma, definitely continued after ~90 Ma, and may have lasted until ~86 Ma. To constrain the timing of termination of WISZ shearing, we present new U-Pb zircon crystallization ages from four granitic dikes that intruded the WISZ along the Salmon River canyon. These three dikes crosscut the steeply east-dipping, linear-planar ductile fabrics that were developed during WISZ shearing, and do not exhibit evidence for solid-state deformation. The dikes yielded crystallization ages of ~86-84 Ma and therefore constrain the cessation of ductile shearing on the WISZ at this latitude to be no later than ~86 Ma. This is consistent with previous timing estimates for WISZ shearing at our studied latitude as well as with published ⁴⁰Ar/³⁹Ar biotite ages of ~83-80 Ma in the WISZ that provide a first-order estimate for the timing of cooling through the ~300 °C quartz crystal-plastic transition temperature. These timing constraints demonstrate that the WISZ is likely one segment of a larger system of dextral-transpressional shear zones, as similar kinematics and timing (~100-85 Ma) are recorded in parts of the Sierra Nevada batholith and in plutons in NW Nevada.