Structural evolution of the Salmon River suture zone, Idaho, United States of America: Implications for the tectonics of obliquely-convergent plate boundaries

The Salmon River suture zone is the boundary between the accreted island arc complexes and the North American craton in west-central Idaho. This plate boundary has been the locus of several pulses of igneous activity and multiple, temporally discrete deformational events. Initial suturing occurred in the Early Cretaceous and was followed in the Late Cretaceous by the development of an intra-arc shear zone (the western Idaho shear zone). Deformation on this shear zone commenced prior to emplacement of the Payette River tonalite (92.2 ± 1.3 Ma; U-Pb zircon). 40Ar/39Ar thermochronology on biotite indicates that high temperature deformation ceased by ∼82 Ma. In the absence of other strain markers, we use the shape preferred orientation of rigid, potassium feldspar populations to constrain the kinematics of transpressional deformation. Late Cretaceous deformation was characterized by a 30–60° angle of oblique convergence. Movement on the western Idaho shear zone resulted in 20–45 km of dextral offset and 10–75 km of east-west shortening, resulting in the partial exhumation of these mid-crustal rocks. After magmatism ceased, reverse movement along steeply dipping structures east and west of the shear zone resulted in further exhumation. We hypothesize that the crustal-scale material anisotropy introduced into the lithosphere by the western Idaho shear zone controlled the development of modern, active extensional structures in western Idaho. The location, orientation, intensity, and kinematics of neotectonic extension in west-central Idaho are strongly influenced by the Cretaceous western Idaho shear zone. A comparison of crustal strain estimates and mantle fabrics at modern, obliquely convergent plate margins in California and New Zealand suggests that crustal deformation is being driven by flow in the mantle. Thus, Late Cretaceous transpressional deformation along the western Idaho shear zone was most likely driven by mantle flow.