Abstract

Scaled physical models illustrate the importance of progradation as a trigger for salt tectonics and formation of allochthonous sheets. Regional extension and contraction were excluded in the models. In our experiments, prograding wedges above a tabular, buoyant salt layer with a flat base expelled the salt basinward, forming the following structures proximally to distally: (1) sigmoidally distorted initially planar wedges, (2) relict salt pillows and salt welds, (3) basinward-dipping expulsion rollover and crestal graben, (4) rollover syncline, (5) landward-facing salt-cored monocline, and (6) distal inflated salt layer. This deformation zone amplified and advanced basinward during progradation; however, no diapiric salt structures formed. Over a buoyant salt layer whose basement had steps facing landward, progradation initially formed a broad anticline where salt flow was restricted across each basement step. Distal aggradation pinned the anticline and enhanced differential loading. The anticline actively pierced its crest, which had been thinned by faulting and erosion. Thereafter, the diapir grew passively, locally sourcing allochthonous salt sheets. This deformation cycle repeated over each basement step so that the age, amplitude, complexity, and maturity of salt-related structures decreased basinward. As each allochthonous salt sheet was buried and evacuated by sediment loading, arcuate peripheral normal faults formed along the sheet's trailing edge, detached wrench faults formed along its lateral edges, and active piercement at its leading edge allowed the sheet to break out and climb stratigraphic levels. This process formed a multitiered complex of salt sheets that migrated basinward with time. Restorations of examples from various salt tectonic provinces support our model results. Immense landward-dipping pseudofaults could arise entirely by salt expulsion rather than regional extension. Diapiric families and peripheral sinks are reinterpreted as the result of progradation squeezing salt basinward and laterally.

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