Ring-faults in composite volcanoes: structures, models and stress fields associated with their formation

Abstract

Abstract Most ring-faults of collapse calderas are either circular or slightly elliptical in plan view, vertical or steeply dipping, and have vertical displacements from several hundred metres to a few kilometres. Most ring-faults are dip slip. Many, however, are partly dip-slip faults and thus shear fractures, and partly ring-dykes and thus extension fractures. Ring-faults have been modelled using analytical, analogue and numerical methods. While analytical models throw light on some aspects of the conditions for ring-fault initiation, they are too general for detailed analysis of ring-fault development. By contrast, analogue and numerical models, combined with rigorous testing on field data, have in recent years provided many new ideas and improved understanding of ring-fault initiation and development. Here we present numerical models for ring-fault formation in isotropic (non-layered) and anisotropic (layered) host rocks. Magmatic underpressure (below lithostatic) or excess pressure (above lithostatic) as the only loading normally favours dyke injections rather than ring-fault formation. A spherical (circular) chamber in a volcanic field subject to (centimetre-scale) doming, tension, or both, is unlikely to trigger ring-fault formation unless the chamber is located in a very soft (low Young’s modulus) layer or has recently injected dykes. The numerical models indicate that a ring-fault (and a ring-dyke) is most likely to form, in layered as well as non-layered host rocks, in a local stress field generated by a shallow sill-like chamber in a volcanic field subject to doming, tension, or both. The diameter of the chamber must be much smaller than the diameter of the volcanic field subject to doming; in many of the numerical models the volcanic field diameter is three to five times that of the chamber diameter. For a 20-km-wide layered volcanic field, either tension or tension combined with doming may result in ring-fault formation. For a 40-km-wide layered volcanic field, tension is not necessary; doming alone is sufficient to trigger ring-fault formation.