Search

[1] In paper 1 we described a methodology to model coupled fluid flow and deformation in composite salt and siliciclastic tectonic systems and investigated their compaction and overpressuring behavior prior to and during continental margin-scale gravitational spreading. Compaction-driven Darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional-plastic yield and mechanical deformation. Viscous flow of the underlying salt is independent of fluid pressure. Paper 1 presented prototype models that are limited to single uniform sediment lithologies, either sandstone-type or shale-type, that undergo mechanical and volumetric viscous compaction. In this paper we present models with layered sandstone-type and shale-type lithologies designed to better approximate the more complex stratigraphy of the Gulf of Mexico, our natural example. A first set of models demonstrates that layered lithologies can produce fluid pressure regimes similar to those observed in sedimentary basins. We then introduce an improved formulation of viscous compaction that includes a stronger dependence on porosity and depth (used as proxy for temperature), thereby more effectively self-limiting viscous compaction. A second set of models with the improved viscous compaction formulation demonstrates that the onset of gravity spreading is mainly controlled by overpressuring in the landward end of the salt basin and that resulting shortening in the distal part is partly accommodated by horizontal compaction. Models with moderately high fluid pressure best reproduce conditions considered to have been necessary for large-scale gravitational spreading in the northwestern Gulf of Mexico, which led to the formation of the Perdido Fold Belt.

[1] A methodology is presented to model coupled fluid flow and deformation in rifted continental margin composite salt and siliciclastic tectonic systems; and we investigate their compaction and overpressuring behavior associated with continental margin-scale gravitational spreading. Compaction-driven Darcy fluid flow in clastic sediments is coupled through the effective pressure to their frictional-plastic yielding and mechanical deformation. Viscous flow of underlying salt is independent of fluid pressure. Numerical models are adapted to the Oligo-Miocene phase of large-scale gravitational failure in the northwestern Gulf of Mexico, and represent the first study of this system that includes dynamically evolving fluid pressure. Here we present the methodology and prototype models with single uniform sediment lithologies and simple parameterizations of their properties. The models serve to illustrate the interactions among compaction, generation of fluid overpressure, and gravitational failure and spreading. Mechanical and viscous compaction behavior of sandstone-type and shale-type sediments are investigated. Results demonstrate that mechanical compaction can generate moderate overpressure in thick shale-type material, whereas high overpressure requires viscous compaction. In sandstone-type material, only viscous compaction can generate significant overpressure, though this requires tens of millions of years. Changes in the stress regime during gravitational-driven deformation enhance compaction and overpressure. Although illustrative of the methodology and basic processes, none of the prototype single-lithology models satisfactorily reproduces Oligo-Miocene fluid pressure and deformational regimes of the Gulf of Mexico. Numerical models of layered sediments together with an improved formulation of viscous compaction, presented in part 2 of this set of companion papers, are more successful.

[1] The Perdido Fold Belt (PFB) is a prominent saltcored deep water structure in the northwestern Gulf of Mexico. It is characterized by symmetric, kink-banded folds of a � 4.5 km thick prekinematic layer and its vicinity to the extensive Sigsbee Salt Canopy. We use 2-D finite element numerical models to study the evolution of the PFB as a gravity-driven fold belt both in a local context and in the context of the larger-scale passive margin, influenced by adjacent allochthonous salt structures. We show that parameters such as overburden strength, salt geometry, or salt viscosity determine timing, extent, and location of the modeled fold belt. Simplified models of the Gulf of Mexico show that toe-of-slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden. In this scenario, the PFB likely represents the terminal folding of a much larger, diachronously formed fold belt system. Citation: Gradmann, S., C. Beaumont, and M. Albertz (2009), Factors controlling the evolution of the Perdido Fold Belt, northwestern Gulf of Mexico, determined from numerical models, Tectonics, 28, TC2002, doi:10.1029/2008TC002326.