REUSE OF MAGMA ASCENT ROUTES IN AN EVOLVING MAGMATIC PLUMBING SYSTEM, CHIEF JOSEPH DIKE SWARM

The Columbia River Flood Basalts (CRFB) contains thousands of thick exposed dikes (8m mode), which provide insights into the general mechanisms of dike emplacement. The Chief Joseph Dike Swarm (CJDS), intrudes metavolcanics and metasedimentary host rock of the Wallowa Terrane in Pine Valley, located near Cornucopia. Here, incomparable exposures allows for detailed outcrop, satellite and unmanned aerial vehicle (UAV) mapping. Using orthomosaics and 3D models coupled with in situ structural data, we have documented the nature of the CJDS dike interactions. Our observations indicate that new dikes use previously formed dikes as mechanisms for propagation in an anastomosing multidike pattern. High resolution mapping of densely populated multidikes within the southern Wallowa mountains indicate that older dikes may not have completely solidified when their margins were used by the new dikes as a pathway. In other cases, new dikes propagated through the middle of preexisting dikes, perhaps in situations where the preexisting dike was well bonded. We interpret this relationship as indicating younger dikes reusing the pathway forged by older dikes due to an inherent mechanical anisotropy, caused by their thermal state and their mechanical relation with the host rock. Subsequently, the principal stress axis rotated parallel to preexisting dikes. Our model where Cornucopia multidikes are emplaced in a plastic, dynamic stress environment offers a suitable explanation to the dike morphologies observed in the CJDS. This work allows us to view a greater scope of possibilities regarding the conditions of dike emplacement in systems with high magmatic output rates.