Can Basement Faults Propagate through Salt? An Analytical Solution
VENDEVILLE, BRUNO C., and HONGXING GE
We present an analytical model in which initially flat and horizontal salt
and overburden layers deform in response to slip along a subsalt normal fault.
The deformation history typically includes three stages. During the brief
(<15,000 yr) initial stage, fault slip lowers both salt and overburden overlying
the basement hangingwall. The resulting topographic relief triggers salt flow
from footwall to hangingwall. During the longer next stage, typically lasting 5
to 15 Ma, salt flow accommodates up to 95% of further basement-fault slip
without increasing the topography significantly (~1 -10 m, or 0.1-1.0 degrees ,
across a 10-km- wide block). Deformation is largely decoupled. Finally, during
the last, strongly coupled stage, the thinned salt layer can no longer flow fast
enough to entirely accommodate additional basement-fault slip. Both salt and
overburden deform by monoclinal flexure above the basement fault. Model results
indicate that the transition between decoupled and coupled deformation occurs
when salt thins to less than 50 to 75 m.
Sensitivity analysis indicates that the influence of initial salt thickness and rate of basement-fault slip far outweighs that of other geologic parameters, such as relative or absolute basement-block width, salt viscosity, water depth, or fault dip. Syntectonic deposition favors coupling only where sedimentation fills more than 85% of the surface relief.
Model results demonstrate that a salt layer 100 m thick or thicker effectively insulates sediments from the underlying, deforming basement.