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Extensional Origin of Breached Paradox Diapirs, Utah and Colorado: Field Observations and Scaled Physical Models
Physical modeling and field observations were used to investigate the origin of the breached salt structures in the northeastern Paradox Basin. Previous studies considered the salt anticlines to be preexisting diapirs enhanced by Laramide compression; crestal faults were ascribed mainly to salt dissolution and salt withdrawal. In contrast, we argue that both folding and normal faulting resulted mainly from regional extension.
Our experiments simulated the effects of salt dissolution, regional contraction, and regional extension on preexisting salt diapirs of various shapes. (1) Salt dissolution and withdrawal formed complex structures in subsiding roofs above vanishing diapirs; the diagnostic features are an inner contractional zone balanced by outer extensional zones. (2) Regional contraction rejuvenated the diapirs; reverse faults flanked the actively rising diapirs and extended beyond their ends. (3) During regional extension, normal faults were localized along salt walls, which were variably rejuvenated. Initially high parts of walls rose reactively, whereas initially lower parts subsided below crestal grabens.
Structural patterns above the Paradox diapirs are incompatible with those predicted by models of regional contraction and salt dissolution and withdrawal. Instead, fault patterns above the Paradox diapirs are consistent with the extensional models. En echelon fault arrays and fault deflection off the northwest ends of Moab Valley and Salt Valley salt structures suggest that regional extension was directed north northeast oblique to the northwest-trending salt walls and anticlines. During extension, Lisbon Valley anticline rose where the source layer was thick; whereas mature diapirs (for example, Gypsum Valley) subsided where the source layer was depleted. Extension formed fault-displacement folds and fault-bend folds. Tectonic and erosional unroofing and flow of salt from withdrawal synclines to the diapirs tilted strata away from the diapirs.
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