Kinematics and dynamics of salt tectonics in the Paradox Basin, Utah and Colorado : field observations and scaled modeling
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Scaled physical models were used to investigate initiation of salt tectonics driven by progradation and by thick-skinned extension and to investigate modification of pre-existing diapirs by subsequent regional extension and shortening, salt withdrawal and dissolution. Combined field mapping and physical modeling were used to investigate salt tectonics in the Paradox Basin, Utah and Colorado. In the scaled physical models, prograding sediment wedges expelled the underlying salt basinward by differential loading. Wedges were folded into expulsion rollovers. Where the base of salt was flat, the salt uniformly inflated the salt layer distally, whereas salt accumulated above the basement steps to form salt structures in basin with basement steps that faced proximally. Deformation advanced basinward and created a series of basinward-younging structures. During thick-skinned extension, a thick source layer, slow extension, low salt viscosity, and small synextensional basement grabens strongly decoupled the deformation in the overburden from the extending basement. Conversely, where salt was thin and extension was rapid, basement faulting deformed the overburden into a forced fold cored by a salt pillow which could mature into a diapir. Because salt is weak, buried diapirs and their roofs localized subsequent regional deformation. During regional extension, diapirs rose reactively below the crestal faults where the source layer was thick and extension was slow; conversely, diapirs subsided where the source layer was depleted and the extension was rapid. During regional shortening, diapirs rose actively. Reverse faults and buckle folds formed in the roof above the diapirs. Salt withdrawal and dissolution formed a fault-bounded trough in the overburden above a vanishing diapir. The trough profile was a mirror image of that of the initial diapir and had inner contractional and outer extensional zones of faults. The above experimentally based observations were combined with field and seismic data from the Paradox Basin, leading to the following conclusions: (1) The Paradox diapirs were initiated by progradation of the late Paleozoic Cutler wedges into the basin. (2) Late Eocene to Oligocene regional extension deformed the diapirs and formed crestal faults, and fault-displacement and fault-bend folds. (3) Late Cretaceous to Eocene Laramide shortening had negligible effects on the gentle folds in the basin. (4) Salt dissolution had minor effects on crestal faults above the Paradox diapirs.