Thermal runaway is a ductile localization mechanism that has been linked to deep-focus earthquakes and pseudotachylyte formation. In this study, we investigate the dynamics of this process using one-dimensional, numerical models of simple shear deformation. The models employ a visco-elastic rheology where viscous creep is accommodated with a composite rheology encompassing diffusion and dislocation creep as well as low-temperature plasticity. To solve the nonlinear system of differential equations governing this rheology, we utilize the pseudo-transient iterative method in combination with a viscosity regularization to avoid resolution dependencies. To determine the impact of different model parameters on the occurrence of thermal runaway, we perform a parameter sensitivity study consisting of 6000 numerical experiments. We observe two distinct behaviors, namely a stable regime, characterized by transient shear zone formation accompanied by a moderate (100 - 300 Kelvin) temperature increase, and a thermal runaway regime, characterized by strong localization, rapid slip and a temperature surge of thousands of Kelvin. Nondimensional scaling analysis allows us to determine two dimensionless groups that predict model behavior. The ratio tr/td represents the competition between heat generation from stress relaxation and heat loss due to thermal diffusion while the ratio Uel/Uth compares the stored elastic energy to thermal energy in the system. Thermal runaway occurs if tr/td is small and Uel/Uth is large. Our results demonstrate that thermal runaway is a viable mechanism driving fast slip events that are in line with deep-focus earthquakes and pseudotachylyte formation at conditions resembling cores of subducting slabs.

In this study, we develop two dimensional (2-D) box models to identify the most viable reasons for the destruction of the North China Craton (NCC). We examine the role of flat slab-induced hydration, high-density lower crust, and weak mid-lithospheric discontinuity in our models. Results indicate that flat slab-induced hydration weakening of the eastern part of the NCC can lead to rapid craton destruction if hydration weakening rates are sufficiently fast. This accelerated hydration rate may be attributed to the extensive carbonatite magmatism within the eastern part of the NCC, facilitating a faster pathway for water diffusion throughout the craton. Craton destruction is contingent upon the craton’s density exceeding the surrounding mantle density, and its viscosity decreasing below 1022 Pa s. We observe that the presence of a dense lower crust or a weak mid-lithospheric discontinuity fail to destroy the NCC unless it is weakened.

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.

Crustal-stored magma reservoirs contain exsolved volatiles which accumulate in the reservoir roof, exerting a buoyancy force on the crust. This produces surface uplift and sudden loss of volatiles through eruption results in syn-eruptive subsidence. Here, we present three-dimensional, visco-elasto-plastic, numerical modeling results which quantify the ground deformation arising from the growth and release of a volatile reservoir. Deformation is independent of crustal thermal distribution and volatile reservoir shape, but is a function of volatile volume, density and depth and crustal rigidity. We present a scaling law for the volatiles’ contribution to syn-eruptive subsidence and show this contributes ~20% of the observed subsidence associated with the 2015 Calbuco eruption. Our results highlight the key role that volatile-driven buoyancy can have in volcano deformation, show a new link between syn-eruptive degassing and deflation, and highlight that shallow gas accumulation and release may have a major impact on ground deformation of volcanoes.