GEOLOGY AND GEOPHYSICS OF THE GREATER BLUE MOUNTAINS PROVINCE, SALMON RIVER BELT, AND WESTERN IDAHO SHEAR ZONE: LATE PALEOZOIC THROUGH CENOZOIC EVOLUTION OF A KEY LOCATION OF THE CORDILLERAN OROGEN

The greater Blue Mountains Province (BMP), Salmon River Belt (SRB), and western Idaho shear zone (WISZ) of Oregon, Idaho, and Washington have proven to be some of the most geologically enigmatic regions in the entire U.S. Here, a Mesozoic-age accretionary boundary between Pz–Mz terranes and the Precambrian continental margin has been structurally truncated and obscured through post-accretion deformation and magmatic intrusion. Thus, defining the edge of continental North America and ascribing spatio-temporal significance to structures and metamorphism has been a vexing problem. Over the past 15 years, a renaissance of work in the BMP, SRB, and WISZ has led to application of modern techniques in mapping and structural analysis, isotope geochemistry and geochronology, and active and passive geophysical methods. Recent studies of this accretionary boundary have focused on its (1) original formation, (2) subsequent evolution, and (3) current expression. The boundary originally formed via accretion of the BMP terranes to western North America. Interpretations of accretion-age based on disparate studies range from ca. 165–118 Ma. The boundary was significantly modified in western Idaho where BMP terranes are deformed in the SRB. The western edge of the SRB is an east-dipping, intra-Wallowa terrane normal fault. This is in-turn overlain by greenschist to amphibolite facies rocks representing a tops-west, out-of-sequence thrust system in which deformation youngs, and metamorphic grade increases, upsection to the east. Tectonites of the SRSZ appear to be equivalents of BMP supracrustal rocks, although protolith ages and terrane affinities remain poorly understood. The SRB thrust system is bound on the east by the WISZ, a narrow, sub-vertical zone of tectonized Late Cretaceous plutons coincident with the Sr-isotopic boundary defining the edge of the North American continental margin. The WISZ experienced significant transpression ca. 100–90 Ma and is likely correlative with other shear zones in the western U.S. Cordillera. Miocene and younger E-W extension overlaps and post-dates formation of the Columbia River Basalt Group. Extensional block faulting has reactivated some SRB thrust faults and ductile structures in the WISZ. Normal faults are also well developed in the hanging walls of SRB thrust faults.