Tectonic evolution of the Syringa embayment in the central North American Cordilleran accretionary boundary

Abstract

The Syringa embayment (Idaho, USA) is 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 46th parallel. New geologic mapping, structural analysis, and laser ablation–inductively coupled plasma–mass spectrometry U-Pb zircon age data constrain the origin and prolonged evolution of this embayment. We agree with previous workers that the Syringa embayment may have initiated as an inherited Proterozoic rift boundary, producing a kink in the north-south–oriented continent margin. Structural analysis on the northwest-oriented Ahsahka shear zone that is adjacent to the accretionary boundary indicates dominantly reverse, southwest-vergent motion, as confirmed by crystallographic vorticity axis analysis on quartzites. New U-Pb zircon age dating brackets the age of deformation along the Ahsahka shear zone to between ca. 116 and 92 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 maintained kinematic compatibility during oblique dextral convergence along the margin. The Syringa embayment has an additional younger structural history, specifically 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 shear zone. Following truncation, continued orthogonal contraction was partitioned to the northeast of the Ahsahka shear zone in the northwest-trending Clearwater zone, which is bracketed by existing age data to ca. 73–54 Ma.