FOLD-AND-THRUST BELTS DETACHED on SALT

Fold-and-thrust belts detached on salt, whether in passive-margin deepwater provinces or along convergent-margin orogens, have fundamentally different structural styles than those detached on normal to moderately overpressured shale. They can be broadly divided into two subsets: those where no salt mobility preceded shortening, and those where diapirs and minibasins were established prior to the onset of shortening. In both cases, the critical taper angle is small and deformation occurs practically simultaneously over broad areas or propagates landward or toward the hinterland from the salt pinchout due to the viscous nature of the décollement.

When there is no early salt movement, shortening results in a regular wavetrain of elongate, generally symmetrical salt-cored detachment folds. The spatial and thickness distribution of the salt influences fold orientation, thrust-fault development, and strike-parallel geometries. Stacked salt layers lead to disharmonic and polyharmonic folding.

In fold-and-thrust belts where salt withdrawal and diapirism predated shortening, the variable strength of the overburden controls the deformation and yields much more complex geometries. Folds nucleate at the weak diapirs, which get squeezed and sometimes thrusted, with high lateral strain gradients common directly adjacent to the diapirs. In the case of circular diapirs connected by a polygonal array of deep salt ridges, variably oriented folds, thrusted folds, strike-slip faults, and even normal faults form as strong minibasins translate somewhat independently above the décollement and rotate about vertical axes. Elongate salt walls tend to be squeezed more at their centers than at their ends, resulting in steep salt welds linking diapirs at fold terminations. In all instances, the preexisting structural architecture strongly controls the deformation, with significant strike-parallel variations in structural style.