The shaping of salt diapirs

Six parameters shape the geometry of passive diapirs associated with stiff overburden: rates of salt supply (S′); dissolution (D′); sediment accumulation (A′); erosion (Er′); extension (E′); and shortening (Sh′). These parameters change in space and time, and hence influence the geometry of the structure as it forms. A complex six-parameter plot, representing the evolution history of a salt diapir, can be simplified into three separate graphs. This study recommends adding to plots of S′ against A′, a second of rate of salt supply (S′), this time with extension rate (E′), and a third of rate of sediment accumulation (A′) plotted against extension rate (E′). Integrating these three plots on a single diagram results in a complete description of the evolution history of a diapir when it is applied to three-dimensional data. However, if applied to a profile, the plot shows only the two-dimensional evolution history of the diapir. Lateral forces (extension or compression) have a significant role in moulding the geometry of a salt diapir by influencing the space which it occupies. By incorporating extension in the moulding plots of salt diapirs, this study introduces the rate of sediment accumulation multiplied by extension ((E·A)′) as a significant factor in moulding salt diapirs. By using this rate against the rate of salt supply, this study redefines the conventionally accepted interpretation of upward-narrowing, upward-widening and columnar diapirs. Upward-narrowing diapirs form when the rate of salt supply is less than the rate of sediment accumulation multiplied by extension. Upward-widening diapirs form when the salt supply is greater than the rate of sediment accumulation multiplied by extension. Columnar diapirs form when the rate of salt supply is equal to the rate of sediment accumulation multiplied by extension. This new relationship explains the absence of columnar and upward-widening diapirs in passive margins where thin-skinned extension dominates, and emphasizes the significance of lateral movement (extension and shortening) in moulding the geometry of salt diapirs.