Stress fields controlling the formation of nested and overlapping calderas: Implications for the understanding of caldera unrest

Abstract

Commonly, the formation of a collapse caldera does not necessarily imply the end of the volcanic activity in the area. In many cases, successive calderas may form close to the previous collapse depression or intersecting it leading to overlapping collapse structures. Occasionally, subsequent caldera collapses may take place at the interior of the first caldera creating nested collapse structures. During the last years several authors have investigated numerically how the stress field around magma chambers may favour the formation of collapse calderas assuming that the host rock surrounding the magmatic reservoir behaves as a linear elastic homogeneous medium. The numerical models presented in this work study how a caldera collapse may modify the stress field of a volcanic area and hence the conditions for the formation of future collapse calderas. Our models take into account the effect of the collapse structure considering it as a mechanical discontinuity. We also investigate the mechanical influence of the intra- and extra-caldera deposits on the formation of new calderas. All the numerical models are two-dimensional assuming plane strain and considering that the surrounding crust behaves as a linear homogeneous elastic material. The computational domain corresponds to a cross-section of the upper crust (50 × 25 km) and magma chambers are modelled as sill-like cavities located at a certain depth below the Earth's surface. The existing collapse caldera depression is 8 km wide and 2.75 km deep, however we consider the caldera infill (i.e. intra-caldera material) to be 1.25 or 1.75 km thick. We assume as loading conditions an underpressure of 10 MPa imposed at the chamber walls, that is, negative excess pressure in the chamber. In some numerical runs we have considered the existence of a previous ring fault by introducing a thin and elongate vertical weak zone at the caldera margins. We find that the stress field around shallow-level magma chambers favouring the formation of either nested or overlapping caldera structures depend on: 1) the dimensions of the previous caldera collapse structure, 2) the size of the new magma chamber susceptible to generate the subsequent caldera-forming event, and 3) the mechanical properties of the surrounding host rock and the syn- and post-collapse materials. Moreover, results obtained in this paper may be relevant to the interpretation of unrest episodes in active caldera systems. Our numerical results confirm that the formation of multiple calderas (nested, overlapping, or separate) imply the development of new shallow magma chambers different from the one that lead to the first collapse caldera. In consequence, interpretation of magmatic unrest episodes in active collapse calderas should not be based on the assumption that the responsible for the current unrest is the same system that originated the caldera.