GEOLOGIC MAP OF THE PURGATORY SADDLE 7.5’ QUADRANGLE, ADAMS COUNTY, IDAHO, WESTERN IDAHO: PRE-ACCRETIONARY DEFORMATION IN THE EASTERN BLUE MOUNTAINS

Deformation and metamorphism associated with shear zones and suture zones record mountain building processes in the mid-to-lower crust during tectonic accretion. The U.S. Pacific Northwest Cordillera is underlain by crust that was constructed in the Mesozoic and Cenozoic by the accretion of complex island arc-terranes to the Laurentian continent. In western Idaho, a series of plutons, inferred to be Permian to Cretaceous, intrude a poorly constrained shear zone, known as the Oxbow-Cuprum shear zone (OCSZ). Deformation in this shear zone may represent the oldest deformation in the eastern Blue Mountains. Geologic mapping of the Purgatory Saddle 7.5-minute quadrangle was conducted to investigate the extent, structural relationships, and age of this shear zone.

The Purgatory Saddle quadrangle contains [east dipping] metasedimentary and metavolcanic rocks of the Seven Devils Group. Volcaniclastic and basalt suites of the Permian Hunsaker Creek Formation in the western portion of the Purgatory Saddle quadrangle are conformably overlain by greenstones of the Wild Sheep Creek Formation that record volcanism in the Wallowa terrane of the Blue Mountains. In the central portion of the map, quartz diorites and diorites intrude the OCSZ that is contained within the Hunsaker Creek Formation. These plutons have been inferred to be Permian in age, but there are no published ages from these magmatic suites. The OCSZ to the southwest has been inferred to predate accretion of the Blue Mountains to Laurentia. Recently, the age of continental assembly has been questioned and proposed to have occurred earlier than previously thought. This opens the possibility that the shear zone may provide information into the deformation history during continental assembly.

The northeast-southwest elongate plutons intrude the shear zone, however contain no deformation fabrics. This suggests that the plutons post-date the OCSZ. The elongate morphology of the plutons may reflect intrusion of the diorite suites along pre-existing weaknesses in the older OCSZ. While geochronology will be required to bracket deformation in OCSZ within the Seven Devils Mountains, field-based relationships support a Permian-to-Triassic age for the OCSZ, which may require a reevaluation of the deformation history of the Blue Mountains Province.