Numerical modelling of caldera dynamical behaviour

The study of the mechanics of caldera formation can yield valuable insight into the behaviour of near‐surface magma chambers and their mechanical interaction with the crust. We have addressed the problem of physical processes responsible of long‐term ground deformation from the emplacement of magma in the crust to a final resurgent stage. A mechanical model based on a thermally coupled elastovisco‐plastic rheology and a finite deformation formulation has been used, and solved via a finite element approximation. The history of an ideal caldera is modelled in the following two stages. A doming phase corresponding to the growth of the magma chamber. This stage is constrained by values of amplitude, half‐width and time constant of regional doming and by a range of realistic over‐pressures. This uplift is linked to the plastic strain field within the crust, which strongly depends upon the pressure and temperature fields. A resurgence phase which results from a collapse stage, with an instantaneous change in shape of the topography. The magma chamber is passive and this stage is basically similar to a crater relaxation process. The rebound amplitude and half‐width are controlled by the length‐scale of the collapse. A gravitational rebound is a very likely mechanism explaining large caldera uplift without over‐pressure mechanisms. For most intracrustal loading processes, the crustal deformation is critically influenced by the relative importance of the upper crustal rheology (pressure‐dependent plasticity) versus the lower crustal rheology (temperature‐dependent viscosity). Because the upper part of the crust can deform without time‐dependent dissipation, it may be responsible for large uplift variations that can occur for moderate loading variations. By contrast the creep of the middle crust acts as a regulating medium.