From downbuilding to contractional reactivation of salt-sediment systems: insights from analog modeling.

Abstract

This work studies salt-detached fold-and-thrust belts involving minibasins by means of physical analogue modeling. The experimental set up consist of a series of minibasins and diapirs built by downbuilding into a regular polygonal framework. The minibasins-diapir framework were then submitted to contraction, and for some examples accompanied by different rates of syncontractional sedimentation. We aimed at evaluating the influence of an initial salt basin geometry (i.e. equal thickness vs. along-strike tapered) on the development of the salt-sediment system, and how this influences the geometries and kinematics of fold-and-thrust belts. We also tested how these are influenced by different syncontractional sedimentation rates. Results show that major differences on the early salt structures occur during downbuilding as a result of original salt budget (i.e. from pillows to diapirs), with a positive correlation between amount of original salt and salt structure development. Initial contractional deformation is localized on the weaker salt bodies, favoring salt extrusion. Shortening is then transferred forwards once vertical salt feeders are welded (i.e. secondary welds), while basal (primary) welds are sheared, rolled or delaminated. Changes on structural styles occur abruptly along-strike as controlled by degree of development of the precontractional salt structures. Relatively low syncontractional sedimentation rate delays forward propagation of deformation and promotes minibasins tilting. With larger sedimentation rates, a thicker cover inhibits minibasins deformation and secondary welding and, promotes a more coherent mechanical beam detached on the basal weld. Our modeling is compared to natural fold-and-thrust belts such as the Zagros and the European Alps.