SHEAR AND DIALATION: EVIDENCE OF MIXED MODE FRACTURING DURING DIKE EMPLACEMENT, CHIEF JOSEPH DIKE SWARM

Magmatic dikes serve as the primary conduits for magma transport through the crust. While dikes are commonly interpreted to be the result of pure tension cracks (Mode I), large-scale pure mode fractures are unrealistic in natural systems. This study examines the emplacement mechanics of a dike within the Chief Joseph Dike Swarm (CJDS), which is thought to be the magmatic plumbing system of the main-phase flows of the Columbia River Flood Basalts (CRFB). The Waterfall Dike (WTFD), an ~8 meter wide dike emplaced within the Cornucopia Stock in the southern Wallowa Mountains, provides compelling evidence for mixed-mode fracturing involving shear failure alongside crack dilation. We observe: 1) strong quartz fabric within a hydrothermal quartz vein along the dike margins. This fabric is aligned obliquely with the dike margin and is consistent with sinistral shear; 2) microfaulting within granitic wallrock, displaying small, yet measurable sinistral displacements; and 3) overall dike morphology containing many offsets, which would require sinistral shear for crack dilation to occur. These features indicate that WTFD formed through a combination of shear and tensile stresses, rather than pure tension. Additionally, microfaulting in the wallrock suggests that space accommodation was created, in part, via rock mass plasticity (i.e., nonrecoverable deformation distributed along fractures), rather than purely elastic deformation. These displacement markers offer valuable insights into the paleostress field during the CRFB magmatism. Expanding this type of analysis to other CJDS dikes could deepen our understanding of the magmatotectonic processes in flood basalt provinces.