Size, depth and related structures of intrusions under stratovolcanoes and associated geothermal systems

Abstract

The subvolcanic structural configuration of faults, stress field, permeability controls and intrusion(s) shape, size and depth are primary factors controlling the location and quality of a geothermal system. Subvolcanic stocks under stratovolcanoes are typically 1 to 3 km in diameter and emplaced at depths of 1 to 4 km. Exposed stocks in the Western Cascades of Oregon are of similar size and are emplaced at depths of 1 to 2 km. Shallow magma chambers in general are typically 4 to 9 km deep, and several times larger than the subvolcanic stocks. Likely subvolcanic structures are radial compression fractures around the magma chamber, and above the chamber extension faulting, stockwork fractures and breccia pipes. Pre-existing or regional structures and stress field will influence the location and shape of magma bodies and associated thermal systems. This study combines data from areas of active stratovolcanoes and older eroded volcanic areas to formulate a detailed structural model of the plumbing system under a stratovolcano. The model is correlated with factors controlling associated geothermal systems. A geothermal system may occur at depths of 1–2 km under a stratocone's base. Deeper systems associated with shallow magma chambers are likely to be present.