Résumé
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. (1) 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. (2) 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.