Effects of mechanical layering on magmatic reservoir failure and magma propagation within the Venusian lithosphere

English Abstract:

Failure of magmatic reservoirs and propagation of magma is controlled in part by the state of stress within the lithosphere. Such stresses are induced by a range of loadings (e.g., gravitational, magmatic and tectonic). In addition, the response of the lithosphere to these loadings depends on its physical properties. Magmatic reservoirs and lithospheric stress states on planetary bodies have been studied using homogeneous lithospheres mainly composed of crustal material. However, planetary lithospheres may include substantial fractions of mantle material. The mechanics of a heterogeneous lithosphere may influence the failure of a magmatic reservoir and the propagation of the magma. To explore this scenario, we created two-layered axisymmetric elastic models made of mantle and crustal components using the COMSOL Multiphysics, in which a stiffer and denser mantle is underlying a softer and lighter crust. A spherical reservoir was created at the contact between the two layers. In these models, we analyzed magma reservoir stability, the amount of overpressure needed to reach failure, and the type of resulting intrusions within the two-layered lithosphere for three distinct environments: 1- lithostatic; 2- upward flexure due to a rising mantle plume; and 3- downward flexure due to a basaltic shield volcano. The results show that the difference in stiffness (>10%) between the crust and the mantle focuses the failure of the magmatic reservoirs at the mantle-crust contact. The resulting failure is driven by the in-plane stress tangential to the chamber, favoring lateral sill injections. In cases with flexure, magma chambers may become unstable (i.e., require no additional overpressure to fail) depending on the crust/mantle ratio. In some cases, we observed that failure of the magma chambers can be driven by the (out-of-plane) hoop stress favoring radial dike intrusions. The stability of magmatic reservoirs and the type and orientation of magmatic intrusions on Venus are influenced by the state of stress and heterogeneities within the lithosphere. Using our result, we can infer the potential crust/mantle ratios of the Venusian lithosphere in a particular tectonic environment in order to sustain stable and viable magmatic reservoirs and generate radial dikes as observed on the surface of Venus.