Rheological Controls on Magma Reservoir Failure in a Thermo-Viscoelastic Crust

Abstract

As volcanoes undergo unrest, understanding the conditions and timescales required for magma reservoir failure, and the links to geodetic observations, are critical when evaluating the potential for magma migration to the surface and eruption. Inferring the dynamics of a pressurized magmatic system from episodes of surface deformation is heavily reliant on the assumed crustal rheology, typically represented by an elastic medium. Here, we use Finite Element models to identify the rheological response to reservoir pressurization within a temperature-dependent Standard Linear Solid viscoelastic (“thermo-viscoelastic”) domain. We assess the mechanical stability of a deforming reservoir by evaluating the overpressures required to initiate brittle failure along the reservoir wall, and the sensitivity to key parameters. Reservoir inflation facilitates compression of the ductile wall rock, due to the non-uniform crustal viscosity, impacting the temporal evolution of the induced tensile stress. Thermo-viscoelasticity enables a deforming reservoir to sustain greater overpressures prior to failure, compared to elastic analyses. High-temperature (e.g., mafic) reservoirs fail at lower overpressures compared to low-temperature (e.g., felsic) reservoirs, producing smaller coincident displacements at the ground surface. The impact of thermo-viscoelasticity on reservoir failure is significant across a wide range of overpressure loading rates. By resisting mechanical failure on the reservoir wall, thermo-viscoelasticity impacts dyke nucleation and formation of shear fractures. Numerical models may need to incorporate additional processes that act to promote failure, such as regional stresses (e.g., topographic and tectonic), external triggers (e.g., earthquake stress drops), or pre-existing weaknesses along the reservoir wall.