LOADED DOWN: UNDERSTANDING HOW VOLCANO CONE GROWTH IMPACTS DIKE PROPAGATION FROM INTEGRATION OF FRACTURE MODELS AND GEOLOGIC DATA

At large stratovolcanoes, eruptive vents may open anywhere from the summit to the lowest elevations of the flanks. To forecast future vent locations, it is critical to understand what influences the propagation pathway of magmatic dikes beneath and within large volcanoes. It has been hypothesized that the increased load on the crust from a large volcanic edifice inhibits magma ascent and favors flank eruption (e.g. Pinel & Jaupart 2000), but the relationship between cone growth and magma transport remains poorly understood because of the complex processes associated with dike propagation and the lack of geologic constraints on magma pathways within stratovolcanoes.

Here we use fracture mechanics models to analyze how the load of a stratocone impacts the in-plane propagation path and subsurface geometry of radial dikes. We focus on the conditions that favor vertical versus lateral propagation within the edifice and use the model to predict how dike geometry and vent location vary with cone size, magma density, and reservoir overpressure. We apply the model to a case study of silicic radial dikes exposed within the base of an eroded Oligocene-aged stratovolcano (Summer Coon volcano, Colorado) that was ~2.2 km tall and ~7 km in radius. The radial dikes in our study first crop out ~2 km away from the volcano center and extend within the volcanic edifice for several kilometers where their distal outcrops widen substantially. Flow indicators along dike margins suggest dominantly lateral propagation pathways within the edifice. Modeling supports that the size of the Summer Coon edifice was large enough to create a barrier to vertical propagation for all but the most buoyant, highest pressure dikes, which we suggest explains the limited dike exposure near the center of the volcano. Modeling also supports that arrested dikes can propagate laterally away from the edifice until reaching a distance where stresses again permit vertical propagation. Therefore, despite magma buoyancy and low host rock strength, stress gradients within the edifice are effective at trapping ascending dikes and promoting lateral propagation.