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Abstract

Salt deposited over rift sequences along passive margins flows basinward in response to thermal subsidence and non-uniform sediment loading. The flow results in extensional tectonics in the salt and the overlying sediments where the space is not constrained and in compressional tectonics where it is constrained. Salt waves form as compression propagates inward from the borders of the laterally contracting body. Upslope flow of the salt is particularly effective in creating folds having regular wavelength because of the increasing resistance. Improved seismic resolution permits recognizing the complex internal structure of the folds, whereas isolated 2D sections may be misleading. The resulting salt waves easily can be mistaken for diapirs in the conventional sense; their flanks may appear discordant when, in fact, they are not. Alternating materials of different rheological properties greatly enhance the formation of uniformly spaced waves or folds. Apart from the dominant halite, the alternating material may consist of mostly mono- or polymineralic evaporite rocks such as anhydrite, carnallite, bischoffite, or tachyhydrite, as well as alternating shale and sandstone/siltstone layers interlayered with, or overlying, the evaporites. Syndeformational sedimentation over the evaporites results in the formation of minibasins. The layers overlying the evaporites participate in the folding, with upward decreasing amplitude, so that definition of the upper boundary of the evaporite sequence on seismic sections is not always straightforward. Seismic lines from Brazil’s Atlantic margin are used to illustrate these points.

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