THE SHAPING OF SALT DIAPIRS

Six parameters shape the geometry of passive diapirs associated with s tiff over burden: rates of salt supply (S'); dissolution (D'); sedimen t accumulation (A'); erosion (Er'): extension (E)I 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, r epresenting 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 ext ension rate (E), and a third of rate of sediment accumulation (A') plo tted against extension rate (E). Integrating these three plots on a si n ie diagram results in a complete description of the evolution histor y of a diapir when it is applied to three-dimensional data. However, i f applied to a profile, the plot shows only the two-dimensional evolut ion history of the diapir. Lateral forces (extension or compression) h ave a significant role in moulding the geometry of a salt diapir by in fluencing the space which it occupies. By incorporating extension in t he moulding plots of salt diapirs. this study introduces the rate of s ediment 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 inter pretation of upward-narrowing. upward widening and columnar diapirs. U pward-narrowing diapirs form when the rare of salt supply is less than the rate of sediment accumulation multiplied by extension. Upward-wid ening diapirs form when the salt supply is greater than the rate of se diment accumulation multiplied by extension. Columnar diapirs form whe n the rate of salt supply is equal to the rate of sediment accumulatio n 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 latera l movement (extension and shortening) in moulding the geometry of salt diapirs. (C) 1998 Elsevier Science Ltd. All rights reserved.