FINITE ELEMENT MODELLING OF THE SEDIMENTARY BASIN WITH THRUST STRUCTURES

Abstract

Using computer modelling on the basis of the 2-D finite-element method, the dynamic conditions of tectonic compression of sedimentary layers are investigated. These models represent a geotectonic situation for the Ukrainian Carpathians in Early Cretaceous time taking into account a simplified cross-section. Geometrically, this cross-section include three parts: moving thrust wedge, layered sedimentary complex and fixed declined basement of the continental platform. Series of genetically inherited models, in which the number of thrust faults increases, are constructed. The detailed numerical models within the framework of continuum mechanics are used: the finite-element resolution is 500x100 m with biquadrate approximation of displacements. The stress-strain state in the non-homogeneous medium with essentially non-uniform mechanical properties according to flysch composition of load-bearing and weak layers with different thicknesses are studied. Linear range of the models: total thickness – up to5 km(included1 kmas water hydrostatic pressure) and 50–80 km in lateral direction. The non-ideal contact with slipping between layers and stress relaxation are taken into account.

The analysis of computer modelling results and investigation of the stress-strain state is carried out on the several 2D-models, presented by cross-sections, for which hypotheses of plane deformation in the gravitational field are accepted. Results of modelling displayed some deformation features and stress distribution during compression up to2.5 km. "Snapshot" maps of stress and strain fields according to thrust wedge moving by step 0.5 km are constructed. Particular attention is given to the area of tee-like contact between front and back rock massifs regarding the faults of thrust kids and base layers at the detachment horizon.

The occurrence of laterally periodic zones of high stress level control (on the basis of the brittle-fracture criterion) the formation and development of slightly sloping faults through sedimentary layers. Plastic strains for lower layers on the detachment horizon by long-time creep are modelled. Near to the faults they are accumulated and resulted  in folded structures, but ones are practically absent in the apical parts of thrust block.

Self-consistent restructuring of models that track the origin and evolution of thrusts or take into account 3D-inhomogenity can be a task for future research.

Key words: computer modelling, the finite element method, thrust structures, the stress-strain state, layered medium.