THE RUSH PEAK SHEAR ZONE, CUDDY MOUNTAINS, WESTERN, IDAHO: EVIDENCE FOR MULTIPLE EPISODES OF REACTIVATION

The Rush Peak shear zone (RPSZ), located along the southern Cuddy Mountains, western Idaho, appears to have experienced multiple episodes of reactivation. Mylonitic shear zones within the Mesozoic plutonic basement of the Olds Ferry Terrane have been observed at the intersection of Rush Creek and the Rush Peak fault (RPF) trace. Anastomosing cm-dm thick mylonitic shear bands collectively comprise the RPSZ. The total thickness of RPSZ is unclear because it is faulted by the RPF. The exposed portion is ~50m thick. Individual mylonitic bands generally strike NE (050-080) and incline steeply to moderately to the SE. Outcrop observations of shear sense indicators (S-C fabric and C’ shears) and petrographic evidence suggest sinistral strike-slip movement. Slickensides and slickenfibers on subvertical ENE striking fractures are common adjacent to the RPF. Kinematic analyses of these features indicate similar sinistral strike-slip movement in association with fracture development. A rhyolitic dike approximately 30-50m wide parallels the RPSZ for the length of the RPF. However, there is no evidence of deformation in the dike, therefore, it is interpreted to be Tertiary(?) in age. The Rush Peak fault, a coincident structure with the RPSZ, is an ENE striking, ~12km long fault that juxtaposes Mesozoic basement rock on the north against Miocene volcanic rocks of the Columbia River Basalt Group on the south. Late Cenozoic vertical displacement across the RPF exceeds 1800m in the Rush Peak vicinity creating the steep range-front of the Cuddy Mountains. The RPF is unique in that it trends ENE in a dominant NNW tectonic regime. A fault scarp near Rush Creek cuts late Quaternary alluvium and left-laterally displaces a boulder levee ~7m. We suggest that the current tectonic regime is taking advantage of the relatively weak nature of the RPSZ and has reactivated it in a brittle mode.