Constraining the timing of ductile shearing in the western Idaho shear zone along the Salmon River canyon, Idaho, USA

Abstract In the North American Cordillera, a system of margin-parallel strike-slip faults and ductile shear zones accommodated northward translation of accreted terranes between the Cretaceous and Eocene. Constraining the timing of displacement on these structures 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 shear zone that deformed the boundary between the North American margin and accreted terranes to the west. At the latitude of the Salmon River canyon (45°25′N), available timing constraints demonstrate that shearing in the WISZ most likely initiated at ca. 108–104 Ma and continued after ca. 91 Ma. To more precisely constrain the timing of ductile shearing, we present new U-Pb zircon crystallization ages from granitoid dikes that intruded the WISZ along the Salmon River canyon. Dikes that crystallized between ca. 112 Ma and 101 Ma were deformed by the steeply east-dipping ductile fabrics that were generated during WISZ shearing. Undeformed dikes that crosscut these steeply east-dipping ductile fabrics yielded crystallization ages of ca. 85 Ma, which brackets the timing of cessation of WISZ shearing. This is consistent with published ca. 83–81 Ma 40Ar/39Ar biotite ages from the WISZ that approximate the timing of cooling through the quartz crystal-plastic transition. Our new timing constraints are consistent with published estimates for the timing of shearing further to the south in the WISZ and support correlation of the WISZ with dextral-transpressional shear zones along-strike in northwestern Nevada and the Sierra Nevada batholith that share similar kinematics and timing (ca. 105–83 Ma).