Three-Dimensional Failure Patterns Around an Inflating Magmatic Chamber

International audienceKey Points: • The overpressure required for failure at the walls of a magma chamber depends on its geometry and on the effective bedrock strength. • Plastic shear bands (faults) initiate either from near the apex or the tip depending on the symmetry and elongation of an oblate chamber. • Elastic dilation at oblate chambers tip open concurrent magma pathways whereas contraction around prolate chambers favors apex failure. Abstract Bedrock failure around an inflating magma chamber is an important factor that controls the occurrence of volcanic eruptions. Here, we employ 3D numerical models of elasto-plastic shear failure around an inflating crustal reservoir, to study how the induced failure patterns depend on the geometry of the chamber, on the host rock strength and on the gravitational field. Our simulations show that either localized of diffuse plastic failure domains develop in 3 stages. Failure initiates (stage 1) after a critical overpressure is reached, the value of which depends on effective host rock strength. Next, and with increasing applied overpressure, either distributed (for zero friction angle) or localized plastic failure zones (for 30 • friction angle) form (stage 2), until they finally connect to the surface (stage 3). Cylindrical chambers develop prismatic shear zones that simultaneously merge from the surface and chamber walls. For spherical and prolate chambers, diffuse conical zones of failure develop from the chamber's crest, whereas for oblate symmetrical chambers, shear bands initiate at the horizontal tips but bend back above the center of the chamber to reach the surface. In contrast for asymmetrical oblate chambers, shear bands initiate in their cylindrical section and vanish along the elongated direction. Here, magmatic fluids may migrate both through diffuse elastic dilation zones at the tips, and through localized shear zones from the crest. Our results thus suggest that natural obse