Articles techniques et présentations

The lithospheric mantle often undergoes enrichment in incompatible chemical elements during ocean closure and collisional orogeny. This enrichment is most pronounced during the subduction of sedimentary rocks and crystalline crust derived from continents. Uranium and thorium are radioactive isotopes that belong to the group of high-field-strength elements (HFSE), whereas the radioactive isotope of Potassium is part of Large-Ion Lithophile Elements (LILE). All three radioactive isotopes are of incompatible elements and, therefore, they can exhibit increased concentrations in the post-orogenic lithospheric mantle compared to the ordinary lithospheric mantle. The unusually high content of uranium, thorium and potassium in the post-orogenic mantle lithosphere is well reflected in the composition of potassic and ultrapotassic magmas, which can sometimes be extremely rich in these radioactive elements. This enrichment is relatively well-documented by numerous geochemical studies and, therefore, must be considered during the numerical modelling of rifting processes. To understand the primary mechanism of intracontinental rifting, various lithosphere-scale 2D models were used during the 2D conductive thermal modelling and 2D modelling of coupled heat transfer and elastic deformations of the lithosphere (comprising the crystalline crust and uppermost mantle). This 2D thermo-mechanical modelling was performed using COMSOL Multiphysics. In the purely conductive 2D thermal modelling, the “Heat Transfer in Solids” module of COMSOL was only used to simulate stationary and time-dependent heat transfer by conduction, which is presumed to be the dominant heat transfer mechanism at the regional scale within the solid lithosphere. Additionally, two COMSOL modules, “Heat Transfer in Solids” and “Solid Mechanics (Linear Elastic)”, were used to conduct a fully coupled thermal modelling that takes into account the thermal expansion of the lithosphere due to the decay of radioactive elements. According to the purely conductive 2D thermal modelling, the anomalously high content of radioactive elements in the post-orogenic lithospheric mantle leads to a time-dependent temperature increase, creating favourable conditions for intracontinental rifting more than 20-100 million years after the orogenic processes. The time gap between the orogeny and the significant temperature increase within the lithosphere is governed by two main factors: (1) the quantity of the radioactive isotopes of thorium, uranium and potassium and (2) the size of the hosting them lithospheric mantle (Maystrenko and Slagstad, 2020). According to the 2D numerical thermo-mechanic modelling, the post-orogenic temperature rise not only weakens the lithosphere but also induces thermal expansion of the lithosphere, which can be sufficient to trigger the initial stage of intracontinental rifting without the need for external, regional-scale, extensional, tectonic forces. Hence, this is a novel concept of intracontinental rift initiation as a consequence of time-dependent temperature rise and thermal expansion of the post-orogenic lithospheric mantle due to the decay of radioactive, incompatible, chemical elements. Such a relatively straightforward mechanism of rift formation significantly enhances our understanding of both local rift processes and global tectonic cycles on the Earth. This, still theoretical, mechanism can be effectively simulated using thermos-mechanical modelling with the assistance of the finite-element analysis software package, COMSOL Multiphysics.