Abstract
This work illustrates that Lyell’s approach to inferring geological processes from field observations can be improved by using field measurements of their rates to scale dynamic models. A controversy in the 1970s about whether crystalline rock salt can flow over the surface echoed an earlier controversy resolved with Lyell’s help about whether ice could flow and carry exotic blocks. This work argues that the external shapes, internal fabrics and structures and rates of flow of current salt extrusions in the Zagros Mountains are keys not only to understanding past and future extrusion of salt, but also to the extrusion of metamorphic cores of orogens.
Salt is shown to extrude first in hemispherical domes, which later spread to the shapes of viscous fountains until they are isolated from their source. They then rapidly assume the shapes of viscous droplets, which they maintain until they degrade to heaps of residual soils. Salt emerges as a linear viscous Bingham fluid, and strain rate hardens downslope to a power-law fluid (n = 3) beneath a thickening carapace of brittle dilated salt.
The velocities of salt constrained in emergent diapirs by measurements of extruding salt sheets in Iran are remarkably rapid compared with rates estimated for buried equivalents elsewhere in or under which it is planned to store nuclear waste or to extract hydrocarbons.
- © The Geological Society 1998
