MAGMATIC PLUMBING SYSTEM EVOLUTION DURING SYNEMPLACEMENT SHEAR, SOUTHERN WALLOWA MOUNTAINS, OREGON

The Southern Wallowa Mountains expose numerous, large dikes associated with the Colombia River Flood Basalts (CRFB). The CRFB is Earth’s youngest and best-preserved flood basalt province, allowing for detailed study of the development of flood basalt plumbing systems. Moreover, this region of the Wallowa Mountains has experienced the greatest regional uplift exposing deeper levels of the dikes compared to those elsewhere. North of Cornucopia, the Pine River Valley runs parallel to the boundary between the Cornucopia Stock and older metasedimentary and metavolcanics. Many shear indicators are observed in the wall-rock near this boundary, including shear bands, sigmoidal hydrothermal veins, and grain-size reduction. Using unmanned aerial vehicles (UAV) we have mapped the morphology of these dikes in three dimensions on a ridge to the East of Cornucopia. We observed densely populated and anastomosing dikes generally aligned sequentially along Pine Valley. In the margins of these dikes, indicators of dextral shear in wall-rock penetrate into the dike margins, indicating that dike emplacement was contemporaneous with shear. This is an important finding, given that CRFB dikes are often considered pure tensile fractures, and provides corroborating evidence of the influence of the Pine Valley transtensional Graben, located just to the south of the field area, during the emplacement of the dikes. These observations, taken in sum, indicate that mechanical anisotropy focused the multiple magma pulses along this narrow region of intense deformation. Dike morphology in this region is often amoeboid, with rather large aspect ratios for dikes, indicating that inelastic deformation of wall-rock was a key feature for magma accommodation. Notably, with inelastic deformation, the dike is less prone to closure during depressurization phases, leaving the conduit open as long as the system remains molten. We hypothesize that this region is an exposed, fossilized fissure system that may have repeatedly fed significant volumes of magma to surficial lava flows of the CRFB. Our model may provide an effective mechanism for protracted magma mobilization from deep to shallow crust by reutilizing pre-existing dike structures.