Salt extrusion-rates in the Zagros

Back-stripping of sediments deposited in salt — withdrawal basins indicate that natural salt structures develop very slowly. Deep conformable pillows of salt can be inferred to have risen at 0.01 mm a⁻¹ for a hundred millions of years (Edgell, 1996) and intrusive diapirs typically rise at rates of 0.02 to 2 mm a⁻¹ for 20 to 2 Ma (Jackson & Talbot, 1985). However, these rise rates are for subsurface (blind) diapirs buried beneath other rocks. Rate of salt flow can increase dramatically when the salt breaches the surface and is constrained only by air or water. Diapiric salt is then subject to maximum buoyancy forces which may be supplemented by lateral tectonic forces. Huge (160 X 90 X 9 km) nappes of salt have been shown to have advanced at average rates up to 275 mm a⁻¹ from Middle Miocene to Plio-Pleistocene over the floor of the Gulf of Mexico where the Mississippi delta is sliding on salt into the open basin (Wu et al., 1990).