The impact of a volcanic edifice on intrusive and eruptive activity

In a volcanic area, the orientation and composition of dikes record the development of the magmatic system that feeds intrusive and eruptive activity. At Spanish Peaks, Colorado, curved dike trajectories issuing from a single focal area have been attributed to horizontal propagation from a pressurized central reservoir in a deviatoric tectonic stress field. These dikes, however, are nowhere in contact with the central intrusion, are younger than it by about 1 My and are not filled with the same magma. They were emplaced at shallow depths ( ≈ 1 km ), where the local stress field is very sensitive to surface loads. Here, we show that their trajectories can be set by the load of a volcanic edifice in a tectonic stress field. The orientation and distribution of the Spanish Peaks dikes have changed in the course of two million years as magmas were evolving chemically. Early dikes that were parallel to each another and filled with primitive melts document ascent in the regional tectonic stress field. They were replaced by curved dikes carrying evolved melts, which record the influence of a sizable volcanic edifice. Beneath this edifice, the induced compression prevented dense primitive magmas from erupting in the focal area and diverted intermediate magmas sideways. The growth of this large volcanic cone was probably responsible for the formation of a magma reservoir. The mechanisms that have shaped the Spanish Peaks dike swarm may control the spatial distribution and migration of eruptive centers in many active volcanic areas.