PROGRESSIVE KINKING OF THE NORTH AMERICAN CORDILLERAN ACCRETIONARY BOUNDARY IN THE SYRINGA EMBAYMENT, IDAHO

The Syringa embayment, Idaho, occurs in the Mesozoic accretionary margin of western North America, where the north-south oriented lithospheric boundary bends abruptly to an east-west orientation near the 46^th parallel. New geologic mapping, structural analysis, and LA-ICPMS U-Pb zircon age data constrain the origin and prolonged evolution of this embayment. The northwest-striking Woodrat Mountain shear zone is the original suture and likely reactivated an inherited Proterozoic rift boundary in the Laurentian continental margin, thus kinking the original north-south oriented Salmon River suture zone that continues to the south.  Structural analysis on the northwest-oriented Ahsahka shear zone that occurs southwest and outboard of the Woodrat Mountain shear zone indicates dominantly reverse, southwest-vergent motion, as confirmed by crystallographic vorticity axis analysis on quartzites. Our U-Pb zircon geochronology of samples from deformed and undeformed plutonic units brackets the age of deformation along the Ahsahka shear zone to between ~116 and 89 Ma. These dates indicate that deformation on the Ahsahka shear zone occurred simultaneously with deformation on the western Idaho shear zone to the south. Consequently, the Ahsahka and western Idaho shear zones are a continuous, albeit kinked, mid-Cretaceous shear zone system that partitioned oblique dextral convergence into orthogonal contraction on the Ahsahka shear zone and dextral transpression on the western Idaho shear zone. At ~90 Ma, the youngest published age constraint for the western Idaho shear zone, the linked Ahsahka-western Idaho shear zone system was terminated by further kinking in the embayment due to movement on a pair of northeast-trending dextrally transpressive structural zones, the Limekiln and Mount Idaho zones. The Mount Idaho deformation zone truncates the Ahsahka -western Idaho structural system. Following truncation, continued orthogonal contraction was partitioned to the northeast of the Ahsahka shear zone in the northwest-trending Clearwater zone, which was active after intrusion of a 73 Ma mylonitic pluton and ended by the time of 66-54 Ma intrusion of the main phase of the Bitterroot lobe of the Idaho batholith based on published U-Pb geochronology.