Advance Mechanisms of Allochthonous Salt Sheets: Implications for Predicting Subsalt Pore Pressure

Michael R. Hudec

Bureau of Economic Geology, The University of Texas, Austin

Two mechanisms have been proposed for salt-sheet advance: tank tread and basal shear. In tank-tread advance, the base of the sheet is attached to its substrate, and the sheet advances by rolling forward as the top rolls downward to become the new base. In basal shear, the sheet advances along a shear zone at or near its base. The two models have very different implications for the deformation and overpressure of sediments just below the sheet.

Physical and mathematical models suggest that extrusive salt sheets advance as tank treads, although this may be a product of materials and boundary conditions used in the models. However, salt sheets buried beneath sedimentary roofs show evidence of significant basal shear, at least near their leading edge. Most parts of the Sigsbee Escarpment are today advancing along a thrust fault that soles landward into the base of allochthonous salt. This supports the widespread existence of base-salt shear zones along the leading edge of the Sigsbee salt canopy. Parts of the Sigsbee Escarpment also display internal shortening of the sheet roof, which would be expected if the sheet had some component of tank-tread advance but the roof was being prevented from rolling downward. However, the shortening in most sheet roofs is small compared with the displacement of the leading-edge thrust, suggesting that the tank-tread component is subordinate to basal shear.

Prevalence of basal shear implies very weak, overpressured subsalt sediments. It may therefore be possible to use the structural style observed at the front of a salt sheet to infer conditions beneath the sheet.