Modeling gravitational instabilities in the partially molten crust with a Volume-Of-Fluid method

This work aims at investigating the thermo-mechanical conditions required for the development of convective instabilities and polydiapirism in the partially molten root of orogenic belts. First, we tested the volume-of-fluid method (VOF) implemented in codes OpenFOAM (open source) and Jadim (in-house IMFT code). Comparison of theoretical and numerical solutions of Rayleigh-Taylor and Rayleigh-Benard instabilities show that Openfoam is most satisfactory in terms of speed and mass conservation (Louis-Napoleon et al., 2020).

Then, we applied the VOF method to investigate specifically the formation of metamorphic domes in Naxos, Greece. These domes are characterized by nested structures of 2 km sub-domes in a 10 km major dome, and contain zircon grains that recorded dissolution-recrystallization cycles of 1 to 2 Myrs attributed to thermal cycles (Vanderhaeghe et al., 2018). We tried to show that these imbricated domes could result from a combination of convective and diapiric episodes, considering the hot orogenic crust as a system of horizontal layers with power-law temperature-dependent viscosities with internal heating. In both 2D and 3D, small domes are systematically destroyed by convection when it appears.

Therefore in a second step we accounted for the specific lower viscosities induced by partial melting as well as compositional small heterogeneities (inclusions). These inclusions are supposed to represent sub-scale clustering of partially molten heterogeneous material with light-soft and heavy-resistant density and viscosity with respect to the "average" crustal domain. Parametric tests allow to define the conditions for the development of convection cells, diapirs and segregation-sedimentation of the inclusions. Two scenarios are then found to potentially explain the formation of the Naxos domes. A first scenario in which melting viscosity is not accounted for, but the inclusions are initially “active”, generates local convection cells around rising clusters of inclusions. Diapirs then emerge above the local convective cells and accumulate at the base of the upper crust. The second scenario takes into account melting viscosity, the inclusions’ properties are active only when temperatures exceed the melt front, and basal heating is progressively shut down. The light inclusions then rise and form domes above larger convection cells, if their rheological properties are frozen. Both these scenarios do not exclude the role of external lateral forces a posteriori to finalise the exhumation process. More generally, we found that the domes characteristics are determined by their mode of formation. We propose a dimensional analysis to distinguish suspension from sedimentation regimes.