CRUSTAL ARCHITECTURE OF THE EASTERN BLUE MOUNTAINS PROVINCE AND SALMON RIVER SUTURE ZONE RESOLVED THROUGH INTEGRATED GEOLOGIC MAPPING, GEOCHRONOLOGY, AND GEOPHYSICAL SURVEYS

The boundary between the Precambrian Laurentian rocks and accreted volcanic island arc terranes of Blue Mountains Province is located in western Idaho. This boundary is represented by high-grade metamorphic rocks of the Salmon River suture zone (SRSZ), and deformed intrusive rocks of the dextrally transpressive western Idaho shear zone (WISZ). To investigate the structural relationship of the pre-, syn-, and post-accretionary tectonic features, we integrate geologic mapping, U-Pb zircon geochronology, and geophysical (gravity and magnetic) surveys to investigate: 1) the link between the pre-accretionary deformation associated with the Oxbow- Cuprum shear zone within the Blue Mountains Province and plutonism, and 2) the structural relationship between the WISZ and the SRSZ.

Geologic mapping of the Purgatory Saddle 7.5-minute quadrangle in western Idaho delineates a series of elongate, undeformed plutons previously suggested to be Permian-Jurassic in age that intrude a mylonitic shear zone (Oxbow-Cuprum shear zone). U-Pb zircon geochronology (Ruth Lake pluton) yielded a 222.1 ± 5.8 Ma age on plutonism, which places a minimum age for shear zone development in the Wallowa terrane of the Blue Mountains Province.

Complete Bouguer and isostatic residual gravity anomaly maps resolve the extent of the western Idaho shear zone and the boundary of the accreted terrane and Precambrian North America, previously resolved by geochemical and structural analysis. The high angle of terrane boundaries suggests that surficial boundaries represent steep, crustal scale features, as resolved by gravity measurements. 2.5-D crustal scale gravity and magnetic models suggest 1) a southern extent of the SRSZ unexposed at the surface due to thick Columbia River Basalt cover, 2) east-west shortening of the SRSZ increases from the west to east, due to overprinting from mid-crustal deformation associated with the near vertical WISZ, and 3) ~7 km offset in the Moho below the WISZ, also proposed by the Earthscope IDOR project.