Evolution of Large Venusian Volcanoes: Insights from Coupled Models of Lithospheric Flexure and Magma Reservoir Pressurization

Many large volcanic edifices on Venus exhibit surficial evidence of subsurface magma transport and storage: summit caldera faults indicating collapse into a magma chamber, and radial grabens indicating radiating dikes (although uplift may also produce the latter). These tectonic features reflect interactions between local magma-induced stresses and broader-scale stresses resulting from flexure of the lithosphere beneath the edifice load. Here, we explore the relationship between magma movement in the lithosphere and the flexural stress state via axisymmetric finite element models of the Venusian lithosphere. The lithosphere, modeled as an elastic material of thickness Te, is overlain with a conical edifice and embedded with an inflating, hence overpressured, spherical magma reservoir that perturbs the surrounding region. The volcanic edifice acts as a continuous gravitational load, flexing the lithosphere. The resulting flexural stress state beneath the edifice is characterized by high differential stresses, extensional in the upper part of the lithosphere and compressional in the lower part. These two distinct regions are separated by a neutral plane, a region characterized by relatively low differential stresses and least compressive principal stress (σ3) oriented out of the model plane. We examine models with different reservoir depths and analyze the orientation patterns of maximum stresses along the magma chamber wall. For a given chamber model, we increase the overpressure until one of the normal stresses at some point on the wall satisfies the failure criterion (here taken to be 0 MPa, the onset of the tensile regime). We find that reservoirs situated in the lower (extensional) lithosphere fail at the bottom; such a chamber is unstable, because it would not collect magma but rather expel it downward. Thus, we conclude magma chamber formation in the lower lithosphere is unlikely. In contrast, failure promoting lateral sill formation occurs near the reservoir mid-section for magma chambers located in the upper (compressional) lithosphere; continued failure in this mode would tend to produce oblate magma chambers with zones of intrusions at their margins. This may help explain the growth of the larger, more oblate chambers necessary to explain Venusian calderas with diameters comparable to or exceeding likely lithospheric thicknesses. Alternatively, at the margins of such chambers, sills propagating near the neutral plane region may eventually transform into radial dikes due to the out-of-plane orientation of σ3 in that region. Our results also explain how the evolving stress state of the lithosphere tends to redirect magma passage over time: magma supply is diverted to sill-like intrusions at depth, inhibiting summit eruptions and possibly shifting eruption locations to the lower flanks, above the distal margins of the sill-like chamber.