ROLE OF NE-STRIKING OBLIQUE DEXTRAL THRUST FAULTS IN THE DEVELOPMENT OF THE WALLOWA TERRANE IN THE NORTHERN CORDILLERA: IMPLICATIONS FOR TERRANE ACCRETION AND CLOCKWISE ROTATION

New geologic mapping has better defined the character and history of two major NE-striking fault systems in the northern part of the accreted Wallowa terrane of western Idaho: the Limekiln-Waha Escarpment (LW) and the Klopton Cr.-Hammer Cr.-Mount Idaho (KHI) fault zones. These faults have extensive histories that include an earlier episode of dextral-reverse oblique-slip faulting ranging from at least post-mid-Cretaceous to pre-Miocene that occurs in basement rocks of the Wallowa terrane and a second episode of normal(?) and reverse faulting that deforms cover rocks of the Miocene Columbia River Basalt Group. We focus here on the earlier history. The ENE striking, steeply dipping LW fault zone juxtaposes Triassic Wild Sheep Cr. Fm. rocks to the NW against Late Triassic Hurwal Fm. rocks to the SE. The shallow obliquity between the fault and bedding and similarity in metamorphism across the fault suggests that significant (>10 km) right lateral strike-slip displacement has occurred on this zone with limited throw. Along strike to the NE, cover rocks obscure relationships in the basement, but a significant change in strike from NW to W striking occurs on the terrane boundary between Wallowa and North American assemblages. The NE striking, moderately SE dipping KHI fault zone juxtaposes sub-to lower greenschist grade Jurassic Coon Hollow Fm. and poorly age-constrained volcanosedimentary rocks on the NW side against upper greenschist grade Late Paleozoic-Mesozoic plutonic rocks on the SE side. Kinematic indicators consistently suggest oblique right lateral-thrust displacement on the fault. Along strike to the NE, this fault zone offsets the Wallowa-North American terrane boundary as well as most post-suturing deformation (Western Idaho shear zone) in a dextral sense with 15-35 km of strike separation. The strike of the boundary also changes from N-striking to the south of the KHI to NW-striking north of it, suggesting that this fault system as well as the LW system were involved in modifying the original N-S orientation of the older terrane boundary to form the Syringa embayment. We suggest these fault systems may have acted as synthetic dextral reverse faults during clockwise rotation of the Wallowa terrane as it impinged eastwards into North America following docking and northward translation along the continental margin.