We use 2D finite-element models to study the evolution of the PFB by gravitational spreading during sediment progradation. The models comprise a viscous salt layer overlain by a frictional-plastic passive margin sedimentary sequence. Model experiments include sediment compaction, flexural isostasy, loading by the overlying water column, and parametric calculations of the effects of pore-fluid pressures in the frictional-plastic sediments.
Limit analysis calculations of the stability of this system reveal that the PFB can have formed by gravity spreading alone if moderately high pore-fluid pressure ratios of approx. 0.8 developed. Under these circumstances, the numerical finite-element model fold belt shows good correlation with the kink-type folds, geometry and dimensions of the PFB. Timing, rate, and extent of folding in the model fold belt are shown to be controlled by the system parameters of overburden strength, continental slope width, salt thickness, and salt viscosity. Variability in the latter two generates two end-member fold-belt types: toe-of-slope folding that propagates progressively seaward, and; synchronous toe-of-salt folding above the distal section of the salt layer. The numerical model results are interpreted to imply that landward Cretaceous toe-of-slope folding started long before the Oligocene-Miocene folding of the PFB. This allowed proximal allochthonous salt structures to develop through breached anticlines in the propagating fold belt while distal sedimentation continued. Only later did folding propagate and fold the PFB. By implication, the modern PFB is only the nose of a much older and larger fold belt.