APPLICATIONS OF NON-LINEAR FINITE ELEMENT MODELING TO PROBLEMS IN STRUCTURAL GEOLOGY AND TECTONICS

Analyses in structural geology have progressed from descriptive (i.e., geometric) to kinematic (time/space evolution), and most recently towards a more mechanical understanding of the development of structures. While significant advances have occurred via physical and numerical modeling studies, many aspects of the mechanics of rock deformation are still incompletely understood. Here, we discuss the suitability of non-linear finite element (FE) models for tackling a variety of problems in structural geology and tectonics. The FE method can tackle complicated geometries, material properties and applied loads for which analytical (i.e., exact) solutions cannot normally be obtained. While common in engineering disciplines, the adoption has been slower since geologic problems are often more complex, and the generation and interpretation of FE models requires skills beyond those normally taught to geologists. Despite these difficulties, the FE method offers a significant advantage in structural geology since the complete deformation history (displacements, stresses, strains) can be obtained while employing realistic material properties and loading conditions.

Two different applications of FE modeling are described. The first is focused on the incorporation of realistic anisotropic effects into models of thrust-related folding. A flat-ramp-flat footwall is overlain by a deformable hanging wall sequence separated by a frictional interface. The influence of mechanical anisotropy is assessed via two approaches: (1) an anisotropic plasticity constitutive relationship; and (2) hanging wall layering assigned different material behaviors. These simulations can be compared to existing geometric/kinematic models or natural geologic structures (i.e., geometry and kinematics), and also provide complete mechanical histories for analysis. The second application focuses on secondary faulting and the stress/strain history during extension and subsequent inversion of a listric fault, including effects from syn-tectonic deposition and subsidence. Based on a North Sea oil/gas field example, this simulation demonstrates the advantages that FE models offer in that a cumulative stress/strain history is produced for a large scale problem involving complex geometries and multiple tectonic episodes.