TESTING HYPOTHESES ON THE ORIGIN OF THE SYRINGA EMBAYMENT IN THE SALMON RIVER SUTURE ZONE, WESTERN IDAHO, USA

A sharp embayment occurs in the Mesozoic accretionary margin of western North America, where the N-S oriented Salmon River suture (SRS) bends abruptly to the W near the 46^th parallel to form the E-W oriented segment of the Syringa embayment. Hypotheses proposed to explain the origin of this embayment range from tectonic inheritance in development of the collisional margin to Cretaceous or younger E-W oriented strike-slip offset of the older N-S boundary. New geologic mapping and LA-ICPMS U-Pb zircon age data permit testing of these hypotheses. From outboard (SW) to inboard (NE) in the area of the bend at Orofino, our new data include the following. A largely undeformed quartz diorite pluton that intrudes Permo-Jurassic Wallowa terrane sequences yielded a 125.2 ± 2.5 Ma age that matches a 160-125 Ma quartz diorite-diorite plutonic suite elsewhere in the Wallowa terrane. Farther NE, plutons that intrude Mesozoic transitional assemblage hornblende gneiss and marble sequences bracket the age of deformation along the outboard-most shear zone (the SW-vergent Ahsahka thrust) to between 115.5 ± 2.3 Ma from mylonite quartz diorite and cross-cutting 93.4 ± 1.9 Ma pegmatite. These ages are similar to those determined by previous workers along the N-S portion of the SRS near McCall, Idaho. A 68.8 ± 1.4 Ma age from weakly deformed quartz diorite to the northeast near Weippe that intrudes Proterozoic continental assemblages is consistent with ~70 Ma ages for early Bitterroot lobe plutons of the Idaho batholith and previous dating of ~73 Ma deformed plutons in the Weippe area. To the NW near Moscow, a 2-mica granite (~78 Ma) is similar in age to the northern Atlanta lobe of the batholith SE of the embayment. These new ages show that deformation occurred progressively NE and inboard from <117 to ~70 Ma across a series of shear zones developed around the bend and along the E-W segment of the embayment. Tectonic belts and metasedimentary rock packages along the SRS are continuous around the Syringa embayment and both the N-S and E-W segments of the embayment share similar magmatic, structural, and metamorphic histories. We thus argue that the embayment was formed by a combination of tectonic inheritance and late-stage oroclinal bending as the accreted terranes rotated clockwise and impinged into the continental margin during dextral translation.