Georg Reuber

and 3 more

The Yellowstone magmatic system is one of the largest magmatic systems on Earth, and thus an ideal location to study magmatic processes. Whereas previous seismic tomography results could only image a shallow magma reservoir, a recent study using more seismometers showed that a second and massive partially molten mush reservoir exists above the Moho \citep{huang2015yellowstone}. To understand the measurable surface response of this system to visco-elasto-plastic deformation, it is thus important to take the whole system from the mantle plume up to the shallow magma reservoirs into account. Here, we employ lithospheric-scale 3D visco-elasto-plastic geodynamic models to test the influence of parameters such as the connectivity of the reservoirs and rheology of the lithosphere on the dynamics of the system. A gravity inversion is used to constrain the effective density of the magma reservoirs, and an adjoint modelling approach reveals the key model parameters affecting the surface velocity. Model results show that a combination of connected reservoirs with plastic rheology can explain the recorded slow vertical surface uplift rates of around 1.2 cm/yr, as representing a long term background signal. A geodynamic inversion to fit the model to observed GPS surface velocities reveals that the magnitude of surface uplift varies strongly with the viscosity difference between the reservoirs and the crust. Even though stress directions have not been used as inversion parameters, modelled stress orientations are consistent with observations. However, phases of larger uplift velocities can also result from magma reservoir inflation which is a short term effect. We consider two approaches: 1) overpressure in the magma reservoir in the asthenosphere and 2) inflation of the uppermost reservoir prescribed by an internal kinematic boundary condition. We demonstrate that the asthenosphere inflation has a smaller effect on the surface velocities in comparison with the uppermost reservoir inflation. We show that the pure buoyant uplift of magma bodies in combination with magma reservoir inflation can explain (varying) observed uplift rates at the example of the Yellowstone volcanic system.

Arne Spang

and 2 more

Geodynamic codes have become fast and efficient enough to facilitate sensitivity analysis of rheological parameters. With sufficient data, they can even be inverted for. Yet, the geodynamic inverse problem is often regularized by assuming a constant geometry of the geological setting (e.g. shape, location and size of salt diapirs or magma bodies) or approximating irregular bodies with simple shapes like boxes, spheres or ellipsoids to reduce the parameter space. Here, we present a simple and intuitive method to parameterize complex 3D bodies and incorporate them into geodynamic inverse problems. The approach can automatically create an entire ensemble of initial geometries, enabling us to account for uncertainties in imaging data. Furthermore, it allows us to investigate the sensitivity of the model results to geometrical properties and facilitates inverting for them. We demonstrate the method with two examples. A salt diapir in an extending regime and free subduction of an oceanic plate under a continent. In both cases, small differences in the model’s initial geometry lead to vastly different results. Be it the formation of faults or the velocity of plates. Using the salt diapir example, we demonstrate that, given an additional geophysical observation, we are able to invert for uncertain geometric properties. This highlights that geodynamic studies should investigate the sensitivity of their models to the initial geometry and include it in their inversion framework. We make our method available as part of the open-source software geomIO.