Scaled experiments, whose design was based on recently published low values for the viscosity of natural rocksalt, suggest that under typically low geological extension rates, a salt layer 500 m thick or thicker decouples the brittle overburden from the faulted basement. Because of its low viscosity, salt cannot transmit the large differential stresses necessary for basement faults to propagate upward as faults or abrupt forced folds into the brittle overburden, unless the salt layer is thin or has been depleted. Thick salt diffuses localized deformation in the basement by flowing away from rising basement blocks, and toward sinking basement blocks. Basement extension is not transmitted to the overburden vertically but is laterally transmitted from the margins as the salt basin widens. Rather than being controlled by basement faults, the distribution of faults and diapirs in the overburden is controlled by lateral changes in overburden stratigraphy or thickness, preexisting faults, and faults that propagate laterally into the salt basin.

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