Two-dimensional finite-element models are used to study how
sedimentation and redistribution of sediments on the upper surface
affects the development of subsurface salt diapirs. A rising diapir
creates a bulge flanked by topographic lows in a generally accumulating
sedimentary pile. We find that the rate at which this topography is
flattened by erosion and redeposition controls the style of diapirism.
This is because the redistribution of material from topographic highs to
flanking lows is equivalent to changing the effective forces acting on
the salt. Redistributing a potential topography modulates diapiric
growth rate.
The main effects of including surficial sediment redistribution in
numerical models of diapirism are: (1) diapirs grow 10-100 times faster;
(2) diapirs may rise above their level of neutral buoyancy and extrude;
(3) diapirs assume `'finger'' or `'stock'' like shapes rather than
`'mushroom'' or balloon-on-string shapes; and (4) layers in the
surrounding sediments remain nearly horizontal and only steepen sharply
near the diapir. In effect, the rate of redistribution of surficial
overburden strongly controls the mode of diapirism. Sediment
redistribution (referred to as erosion for brevity) is modeled using a
one dimensional diffusion equation. We show the results of two different
erosion rates: infinitely slow (no erosion) and extremely fast (which
redistributes surficial sediments but does not remove them from the
system). We show that the shapes of model diapirs rising beneath
surfaces subjected to rapid erosion simulate salt diapirs in the Gulf of
Mexico. Columnar diapirs indicate rapid deposition on the shelf and
plug-like diapirs slow sedimentation on the abyssal plane. Diapirs
rising beneath surfaces with negligible erosion have the `'mushroom''
shapes interpreted for salt diapirs in central Iran.