ROLE OF FAULTS AND LAYER INTERFACES ON THE SPATIAL VARIATION OF STRESS REGIMES IN BASINS - INFERENCES FROM NUMERICAL MODELING

The spatial variation of the computed state of stress is studied in a series of numerically solved boundary-value problems of basin-scale de formation. These numerical experiments illustrate simple situations of decollement tectonism under compression. The study focuses on the loc al variation of the state of stress in a sedimentary overburden where major discontinuities such as large-scale faults and layer interfaces are present. The states of stress are computed using either the distin ct element method or the finite element method. Classic elasto-plastic ity laws are used for the behaviour of the continuum, and the disconti nuities are modelled as Coulomb contact surfaces. A geologic interpret ation of the numerical experiments is proposed to highlight the role p layed by major structural discontinuities in the local variations of t he stress regime. The models are two-dimensional in space and assume p lane strain, but the full three-dimensional stress-strain constitutive relation is used. Therefore, the stress regime variations can be repr esented by constructing the contour map of the Wallace-Bott stress rat io. This simple technique, together with the construction of diagrams of principal stress magnitude across the model, enables us to rapidly examine the stress-field pattern and further discuss the deformation m odes of faulting and fracturing, which may take place at a small scale . We have found that the local variations of the stress tensor are in good agreement with common geologic interpretation of brittle deformat ions in foreland and thrust systems. We demonstrate how the presence o f layer interface and the difference in mechanical properties of forma tion lithology may strongly affect the variation of stress in space. T his modelling approach has important issues when interpreting well tes ts to gain access to in-situ stresses.