The first-approximation model showing the occurrence of epicontinental sedimentary basins due to convective instability of the thermal lithosphere

Abstract

Modern computational technologies make it possible to simulate practically any concept developed by geologists to investigate the processes of formation of the structures under study, including diametrically opposed ones. Today’s trend is to create complex ‘realistic’ models. Such models are based on a large number of parameters with properly set values and simulate the settings that can be viewed similar to the real situations. However, the adequacy of both the models themselves and the concepts used as the basis for simulation remains the issue of debate. Apparently, it is required to specify a general approach to theoretical constructions in geodynamics, which should ensure that the scope of applicability of the models can be correctly evaluated. Such an approach can be implemented by successive approximations based on the fundamental results of the theory of simple liquids with damping memory, the most general description of irreversible deformation of materials under non-isotropic stress. It is critical to correctly formulate a model in the first approximation. It should be fairly simple and based on reliably established experimental facts, give adequate and clearly interpretable non-trivial results and allow further logical refinement of the details, i.e. the next approximations. This article presents an attempt to strictly follow the requirements and consistently construct a model that can show the occurrence of large epicontinental sedimentary basins, the origin of which has been in the focus of geological studies for many years. Our model is based on the following reliably established facts: (1) at the surface of the planet, in continental areas there is an approximately 300-km-thick thermal boundary layer (TBL), wherein the temperature drop amounts to ~1300–1500 °C; (2) the material of the lithosphere, including the crust, is irreversibly deformed during slow geological processes; (3) the continental crust is the thick layer that is less dense than the material of the mantle. The numerical experiments demonstrate free convection in the upper mantle, which induces countercurrents in the light crust and leads to the occurrence of sedimentary basins above the ascending flows and uplifts above the descending flows, which form platform shields during the transition to the quasi-stationary mode. The parameters of the typical structures formed in the lithosphere and the crust and the sedimentary basins proper are estimated. Revealed are the stages of their evolution, which correlate with the available geological and geophysical data, except for the effects caused, in our opinion, by the higher temperature of the mantle and the dynamics of the resultant melt. (Our next publications will describe modeling with account of decompression melting of the mantle material and separation, migration and freezing of the resultant melt.) The proposed first-approximation model can be used to describe a wide variety of geodynamic processes of similar scales.