BASINAL RESPONSE TO NASCENT ARC-ARC COLLISION: LATE TRIASSIC-EARLY JURASSIC OF THE BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON

Previous and ongoing studies of sediment composition and stratigraphic architecture in the Blue Mountains province of eastern Oregon suggest a new model for Late Triassic-Early Jurassic basinal response to nascent arc-arc collision. In the Izee-Suplee area, Upper Triassic-Lower Jurassic forearc basin strata were deformed and recycled in arcward-advancing thrust slices which also produced chert-rich olistostromal and conglomeratic detritus derived from the Baker terrane (Dickinson and Thayer, 1978; Dickinson, 1979). In the Wallowa terrane, Upper Triassic-Lower Jurassic strata were deposited, mildly deformed and locally eroded prior to Middle Jurassic time (Goldstrand, 1994; White, 1994). Representative units include the Doyle Creek Fm. (submarine fan deposits with volcanic and plutonic lithic fragments), the Martin Bridge Fm. (platform carbonate), and the Hurwal Fm. (marine shale and conglomerate with lithic fragments of radiolarian chert, marble, volcanic, sedimentary and metamorphic grains). Carbonate- and chert-clast conglomerate in the Hurwal Fm. represents detrital input from the uplifting and eroding Baker terrane (Follo, 1992).

Existing tectonic models attribute Upper Triassic-Lower Jurassic synorogenic deposits in the Wallowa and Izee-Suplee areas to isolated local uplift of the Baker terrane. We infer that these deposits reflect a larger-scale episode of regional mountain-building not previously recognized. Evidence for Late Triassic deformation and metamorphism throughout the Baker terrane (Ashley, 1995; Evans, 1995; Walker, 1995) and coincident transport of detritus from the Baker terrane into the Wallowa and Izee-Suplee areas supports a new model for two-arc, Molluca sea-type double subduction and growth of a composite accretionary prism (the doubly-vergent Baker fold-and-thrust belt; BFTB) during nascent arc-arc collision. We propose that as the BFTB grew, it migrated into and modified the Wallowa and Izee forearc basins. Strata in the Wallowa terrane may record the transition from a forearc basin (Doyle Creek Fm.) to a flexural foreland basin (Hurwal Fm.) that resulted from crustal loading in the BFTB. In this model, regional shallowing recorded in the Martin Bridge Fm. resulted from development of a flexural forebulge that was later submerged in the foredeep basin (Hurwal Fm.).