Determining the stress regime within a volcanic edifice from igneous intrusions and deformation measurements (California)

Understanding the controls on the stress regime within a volcanic edifice is important when considering the likely locations and characteristics of future eruptive activity. In most volcanoes, magmatic dikes, which align with the most compressive stress in the crust, are oriented radially to the center of the edifice due to outward compression from a pressurized magma body. However, eruptive fissures at many basaltic shields are circumferential on the upper flanks and radial on the lower flanks. An explanation for the contrasting orientations is downsagging caused by loading from dense subvolcanic intrusions. As a test of the proposed model, deformation measurements were collected from a shield volcano with radial and circumferential eruptive vents, and the process of dike emplacement was studied using exposures at an eroded stratovolcano.; Leveling measurements at Medicine Lake volcano, in northern California, indicate approximately ∼8 mm/yr subsidence maximized at the summit, decaying symmetrically on the flanks, and remaining relatively constant on a decadal time scale. Regional Global Positioning System surveys reveal a spatially complex field of volcano-tectonic deformation, including tectonic shear east of Medicine Lake and subsidence of the volcano. The deformation is fit equally well by a deflating volume source at depth and a dense subvolcanic load; however, the loading model is preferred due to the lack of a physical mechanism for volume loss at the modeled depth.; Analysis of ∼20 silicic radial dikes at the eroded Summer Coon volcano, in southern Colorado, suggests that radially oriented rhyolite, rhyodacite and quartz latite intrusions have segmented geometries, require high driving pressures, and follow paths that are inclined near the center of the volcano and become sub-horizontal with increasing radial distance. Based on the geometry of the intrusions, eruptions from radial dikes are most likely to occur near the base of a volcanic cone and eruption volume is probably greatest from lower elevation vents. No circumferential dikes were observed at Summer Coon, possibly because the density contrast between solidified silicic intrusions and the host volcano is greater for felsic systems than for mafic.