Conceptual and Numerical Models of Ring-Fault Formation in Composite Volcanoes

Some ring faults are dip-slip; others are partly faults (shear fractures) and partly ring dikes (extension fractures). Even if stresses tend to concentrate at discontinuities such as ring faults, most unrest periods in existing calderas do not result in caldera slip. Also, caldera formation is a rare event in the lifetime of any composite volcano. Here we present new conceptual and numerical models of caldera formation, including nested calderas, in volcanoes with shallow spherical or sill-like (oblate ellipsoidal) magma chambers. In all the models, the host rock (including the volcano) above the chamber is composed of 30 comparatively thin layers with stiffnesses (Young's moduli) alternating between 1 GPa to 100 GPa. The chamber itself is located in a single, thick layer. The crustal segment hosting the chamber is either 20 km or 40 km wide but has a constant thickness of 20 km. The loading conditions considered are: (1) a crustal segment subject to 5 MPa tension; (2) crustal segment subject to excess magmatic pressure of 10 MPa at the bottom (doming of the volcanic field containing the chamber); and (3) a combination of tension and doming. In all models, the magma-chamber top is at 3 km depth; the diameter of a sill-like chamber is 8 km (its thickness is 2 km), that of a spherical chamber is 4 km. We conclude as follows: (1) Excess pressure in a chamber normally results in dike injection rather than caldera formation. (2) For doming or tension, a spherical magma chamber favors dike injection except when the chamber is in a very soft (10 GPa) layer, or one with recent dike injections: then the stress field favors ring-fault formation. (3) For a sill-like chamber in a 20-km wide volcanic field, a ring-fault can be generated from either tension or tension and doming; for a 40-km wide field, doming alone is sufficient to generate a ring fault. We conclude that stress fields in composite volcanoes with sill-like chambers subject to tension, doming, or both are likely to generate ring-faults.