COGENETIC STRUCTURES TO THE WESTERN IDAHO SHEAR ZONE: IMPLICATIONS FOR OROGEN-PARALLEL TRANSLATION OF ACCRETED TERRANES

Transpressional shear zones in the western U.S. Cordillera are key to understanding the extent and impact of orogen-parallel translation of accreted terranes. In the northern Rocky Mountains, the western Idaho shear zone (WISZ) separates numerous terranes accreted to Laurentia in the Mesozoic (e.g., Blue Mountains) from Precambrian cratonic North America. The WISZ strikes north in central Idaho, parallel to two other major shear zones (Deadwood and Sawtooth) exposed further east within the Idaho batholith region. All three shear zones appear to merge with three northwest-striking shear zones to define the Syringa structural embayment in northern Idaho: the Ahsahka, Woodrat Mountain, and Clearwater shear zones. On the basis of compatible kinematics and overlapping times of deformation, we propose that the Sawtooth shear zone, located ~90 km east of the WISZ in the Sawtooth Range (Idaho), continues north into the Clearwater shear zone; the previously identified Deadwood shear zone joins the Woodrat Mountain shear zone; and the WISZ merges with the Ahsahka shear zone. The subparallel, northwest-striking Ahsahka, Woodrat Mountain, and Clearwater shear zones share the kinematics of reverse, top-to-the-southwest transpressive shearing; the subparallel, north-striking WISZ, Deadwood, and Sawtooth shear zones share the kinematics of dextral strike-slip transpressive shearing. All of these shear zones were active in the Late Cretaceous with overlapping intervals of deformation. We, therefore, suggest that the three NW-striking shear zones in northern Idaho and the three north-striking shear zones in central Idaho represent two segments of a single orogen-parallel transpression system that was active in the Late Cretaceous. The variation of kinematics from top-to-the-southwest transpressive shearing in northern Idaho to dextral strike-slip transpressive shearing in central Idaho is consistent with northeast-directed Mesozoic plate convergence along the curved Laurentian margin in Idaho. A transpressive system comprised of linked contemporaneous shear zones suggests that the northward translation of the accreted, Mesozoic terranes was accommodated by multiple orogen-parallel structures, and the total displacement along the system exceeded the estimated ~50 km in the WISZ.