LITHOLOGICAL AND STRAIN ANISOTROPY DURING DIKE PROPAGATION AND GROWTH, CHIEF JOSEPH DIKE SWARM, CORNUCOPIA, OREGON

The Columbia River Flood Basalt Group (CRFB) is the youngest large igneous province in the world, with an extensive and exposed plumbing system clustered in the Chief Joseph Dike Swarm (CJDS). Our study focuses on the area located in the Wallowa Mountains in northeastern Oregon, specifically in the western margin of the Pine Valley near Cornucopia. These dikes penetrate into Cretaceous-aged Nevadan Intrusives, and a variety of lithologies grouped under the Wallowa Terrain (middle Permian to Cretaceous). The unusual dike widths (~ 8 meters) and incomparable exposures allowed for detailed outcrop and unmanned aerial vehicle (UAV) mapping. Using orthomosaics and 3D models coupled with in situ structural data we have documented the complexity of the CJDS system. These methodologies provide greater insight into the magma output mechanism and magma transport in complex three-dimensional morphologies. Dikes are typically assumed to be the product of elastic-brittle deformation with lensoidal morphologies. However, dikes in the CJDS in the vicinity of Cornucopia are amoeboid and bulbous with variable aspect ratios. We interpret that the amoeboid dikes appear to have responded to regional shear stresses and wall-rock deformation in part controlled by rock mass plasticity. Many dikes in our field area also exhibit anastomosing patterns that further support that the dikes were emplaced in a dynamic environment with a heterogeneous stress field. Thick feeder dikes in continental flood basalts often intrude materials with contrasting mechanical properties. This is no different for the CJDS, where a myriad of dike morphologies are observed near lithological diversity. Fracture systems near these oddly shaped dikes often exhibit dikelets and slickenlines that give insight into the near-dike deformation regime that accommodates magma emplacement. Moreover, the mapped morphologies often signal the occurrence of multidike systems. In these, older dikes capture and reorient new ones along their margins or near the older dike center. Our work has critical implications for models of space accommodation and the evolution of the CRFB plumbing system. This work moves us one step closer to detailing the mechanisms of emplacement of dike swarms and the rates of magmatic activity in flood basalt systems.