Numerical and analogue modeling of geological structures

One of the main goals of structural geology and tectonics is to unravel the direction and sense of movement on major displacement or shear zones on our dynamic planet. To attain this goal, geologists use kinematic indicators, which are structures with a symmetry related to the sense of shear. Shear bands are well known examples of such structures, but it has recently become clear that these are just one important case of a wide range of "flanking structures" developed due to local disturbance of the flow field adjacent to material discontinuities. Discontinuities in rocks occur on all scales, from microscopic size to structures transecting or limiting lithospheric plates, and this is reflected in the range of flanking structures developed. The aim of this project is to study flanking structures using physical analogue and numerical finite element models to understand their mechanical development. The full three-dimensional geometry of the analogue structures will be constructed using a geometric modelling computer program for direct comparison with three-dimensional visualization of natural structures. The results will provide a more fundamental understanding of the mechanical formation and spatial distribution of an important and widespread class of deformation structures that have often been overlooked or even misinterpreted. The study has a practical field application in assessing the sense of movement and overall kinematics of major fault zones as well as direct economic applicability in understanding roll-over or hanging wall antiforms above normal and reverse faults, tectonic structures that are important traps for hydrocarbons. This project represents part of an international cooperation between the University of Vienna, the ETH-Zürich and the University of Mainz.

The Project FlaSh investigated together with groups at the ETH Zürich (Switzerland) and the University of Mainz (Germany) a new family of geological structures, called flanking structures, which are characteristic fold structures that develop along natural faults. Although the structures have been described only recently by the leaders of the project FlaSh, flanking structures are found at all scales (from microscopic scale to the scale of satellite images) in many different geological settings (e.g. in metamorphic rocks from great depth in the crust as well as in unmetamorphosed lake sediments). The main aim of the project FlaSh was to mechanically explain the development of flanking folds along faults. Analogue experiments (i.e. models with for example plasticine or silicone that behave during several hours of deformation in the laboratory like natural rocks at great depth during millions of years of deformation) were calibrated with mathematical analytic solutions and finite element computer models, and the model results were compared with natural examples of flanking folds. The studies lead to the important conclusion that folding of host rocks near a fault is the result of a slip gradient along the fault. Natural isolated faults reveal a maximum displacement somewhere near the center of the fault, which diminishes towards the margin of the fault, where the displacement is zero. This slip gradient creates strain in the host rocks, which react by folding. If the faults are not isolated but interact with neighboring faults, the slip distribution on the individual faults can become highly complex. Because flanking structures are a direct consequence of the slip distribution along the faults, they can also become very complex near a system of interacting faults. Folded sealing horizons near the faults may provide important preconditions for the formation of oil traps and therefore the spatial arrangement of large scale faults and sealing horizons in sedimentary basins are of primary interest for the exploration of hydrocarbons. Numerical modeling of slip on such fault systems helps to predict the folding of sealing horizons into flanking structures and therefore may in future support the interpretation of three- dimensional seismic explorations. The slip distribution on natural faults is an important parameter for estimating the seismic hazard in areas of potentially active crustal deformation. Because the slip distribution is reflected also by flanking structures, their understanding might contribute to the understanding of earthquake patterns in space and time.