STRUCTURAL CHARACTER OF THE WALLOWA-BAKER TERRANE BOUNDARY: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF THE BLUE MOUNTAINS PROVINCE, NORTHEAST OREGON

The boundary between the Baker and Wallowa terranes in the Blue Mountains province of NE Oregon is characterized by an imbricate fault zone consisting of slices of subduction-related, meta-igneous and metasedimentary rocks faulted into chert-argillite mélange. The meta-igneous slices exhibit a relatively broad range in SiO2 content (~48-67 wt. %) and vary in texture from hornblende gabbro/diorite to lithic-clast volcaniclastic breccia. Anastomosing, greenschist-facies cataclastic shear zones (<5 cm thick) occur throughout the meta-igneous rocks, but are most abundant near major fault contacts. 206Pb/238U zircon ages from a (meta)tonalite and a (meta)diorite are 226.0 ± 2.0 Ma and 231.4 ± 1.0 Ma, respectively. These rocks have low initial 87Sr/86Sr (0.7033-0.7034) and strongly positive initial εNd values (+7.7 to +8.5). Associated Norian-age metasedimentary rocks have a broader range of initial 87Sr/86Sr values ranging from 0.7046-0.7061, initial εNd values of -3.2 to +6.5, and calculated Nd model ages of 0.7 to 1.4 Ga. The matrix of the mélange consists primarily of Elkhorn Ridge Argillite and includes widespread chert-argillite broken formation with scattered metalimestone olistoliths. Both Tethyan and McCloud Permian fauna have been reported, and Coward (1985) reported radiolarians possibly as young as Early Jurassic. These rocks have initial 87Sr/86Sr values ranging from 0.7073 to 0.7094, initial εNd values between -4.7 and -7.8, and calculated Nd model ages of 1.4 to 1.6 Ga.

We interpret the various features of this broad, imbricate fault zone as a fundamental tectonic boundary that separates the far-traveled, Wallowa island-arc terrane from the Baker accretionary-complex terrane. Fault-bounded meta-igneous and associated metasedimentary rocks in this zone are likely derived from the Wallowa island-arc terrane, and were faulted into chert-argillite mélange during an inferred arc-arc collision. This zone of faulting and slicing is not significantly overprinted by younger deformation and thus may preserve many primary structural features related to subduction and collisional accretion. We propose that this terrane boundary is an exposed, “on-land” example of a zone of imbrication and tectonic mixing of arc-crust and oceanic lithosphere analogous to the ongoing Molucca Sea collisional zone.