THE MECHANICS OF RING FAULT FORMATION: NEW INSIGHTS INTO A CLASSIC PROBLEM IN VOLCANOLOGY

Elliptical calderas formed by subsidence along ring faults are a common element of many volcanic systems, and study of the geological record on Earth and other planets/moons clearly illustrates the considerable extent and magnitude of the devastation that can accompany the formation of these kilometer scale depressions and associated explosive eruption events. In recent years, numerical models have become an important and widely used tool for interpreting the field and remote sensing data that inform our understanding of ring fault formation and caldera evolution; however, a key problem with standard elastic formulations may compromise the ability of such models to provide direct first-order insight or serve as points of calibration for more sophisticated modeling approaches. Specifically, most researchers explore the initiation of ring fault formation by treating the magma reservoir as a cavity, isolated within a gravity-free host, to which a magma over/underpressure is applied. Use of a gravity-free host derives from canceling opposing magma and host rock stresses acting normal to the reservoir wall, which correctly zeroes out this component of the lithostatic (gravitational) load within the host. But, by then eliminating gravitational effects entirely within the elastic medium on the basis of a stress balance performed in only the wall-normal direction, the formulation inappropriately zeroes out the remaining two components of the three-dimensional lithostatic stress tensor in the host rock. This has critical implications that have already been explored for several common volcanic situations (cf. Grosfils, J. Volc. Geotherm Res., 2007; Long and Grosfils, JVGR, 2009; Hurwitz et al., JVGR, 2009). The primary goal of the current research effort is to expand upon previous explorations by demonstrating why a standard gravity-removed elastic formulation should not be used to assess the mechanics of ring fault formation. The results provide new insight into the volcanic conditions that allow caldera formation to occur, and can help resolve why analogue and numerical models striving to simulate comparable ring faulting conditions yield sharply different solutions (i.e., with respect to fault dip orientation, propagation direction, etc., all of which have implications for eruption conditions).