STABILITY OF A FRICTIONAL, COHESIVE LAYER ON A VISCOUS SUBSTRATUM - VALIDITY OF ASYMPTOTIC SOLUTION AND INFLUENCE OF MATERIAL PROPERTIES

This study deals with the stability of a stratified structure composed of a cohesive and frictional overburden, a viscous substratum, and a rigid basement. That structure should be seen as a prototype for vario us salt tectonics and lithospheric plates stability analyses. The dest abilizing factors are the density contrast, the tectonic compressive s tress, and the possible erosion and deposition at the top surface. The overburden stiffness, a nonlinear function of in situ stress, has a s tabilizing role. Two solutions are extracted from the variational form ulation of the stability problem previously proposed [Leroy and Triant afyllidis, 1996]: the first is analytical and is obtained by disregard ing gravity, and the second is numerical and is based on the finite el ement method. The latter is used to assess the validity of the previou sly presented asymptotic solution. It is shown that the asymptotic sol ution is accurate even for values of the small parameter, defined as t he perturbation wavenumber times the overburden thickness, as large as 0.4. Furthermore, the possibility for the cohesive material in the ov erburden to accommodate part of the deformation by slip along a popula tion of small pervasive faults is accounted for by the introduction of a deformation theory of plasticity. Stability predictions based on th is theory indicate that structural modes, such as folding, and localiz ed faulting modes are triggered for similar stress magnitudes. The par ametric study presented includes the previously undetected influences of the stress gradient with depth and of the work hardening properties of the competent overburden. The role of erosion and deposition in de stabilizing shallow overburdens, regardless of the magnitude of the te ctonic stress, is also established. The stability predictions are then applied to a folded section through the Campos basin, offshore Brazil , revealing that the deformation theory of plasticity is necessary to explain the buckling that occurred during the Albian.