TECTONO-STRATIGRAPHIC ARCHITECTURE OF UPPER TRIASSIC THROUGH MIDDLE JURASSIC STRATA IN EASTERN OREGON: IMPLICATIONS FOR TECTONIC EVOLUTION OF THE BLUE MOUNTAINS

The Blue Mountains Province of Eastern Oregon and Western Idaho is a complex assemblage of Early Mesozoic oceanic island arcs and accretionary prisms, ophiolitic fragments, and post-tectonic stitching plutons. While it is widely recognized that Mesozoic structure and stratigraphy of the Blue Mountains record the evolution of oceanic island-arc systems and their subsequent accretion to North America, significant controversy exists regarding tectonic models for these events. Critical unresolved questions include polarity of subduction, timing and consequences of polarity reversals, and the role and other geologic expressions of collision events (arc-arc and/or arc-continent).

We propose a speculative new tectonic model for the Triassic-Jurassic evolution of the Blue Mountains region based on a synthesis of previous studies and detailed analysis of published geologic maps (e.g., Dickinson and Vigrass, 1965; Vallier, 1977; Dickinson and Thayer, 1978; Brooks and Vallier, 1978; Brooks, 1979; Avé Lallemant et al., 1985; Follo, 1992; White et al., 1992; and others). Tectono-stratigraphic architecture in the John Day, Huntington, and Wallowa Mountains areas of Oregon suggests that syn-depositional deformation of Carnian through Sinemurian (Late Triassic to Early Jurassic) strata in the Olds Ferry, Izee, and Wallowa terranes may represent arcward (antithetic) emplacement of accretionary prism strata via a “tectonic flake” geometry during arc-arc collision. In this model, early stages of arc-arc collision generated emergent accretionary prisms during Carnian time in the Izee-Suplee area (Dickinson, 1979) and during Norian time in the Wallowa Mountains (Follo, 1992). Complex Norian through Sinemurian stratigraphy and structure in the John Day area may represent regional partitioning of collisional strain along synthetic and antithetic syn-depositional faults in the accretionary prism and forearc. A review of the literature indicates that Norian through Sinemurian (~220-195 Ma) deformation in the Huntington area is recorded by a regional angular unconformity, suggesting that collisional strain resulted in protracted uplift, folding and erosion in this area. These hypotheses are preliminary and will be tested in future work.