SCHULTZ-ELA, DANIEL, and M. P. A. Jackson, Bureau of Economic Geology, Univ. Texas at Austin, Austin, TX

ABSTRACT: Evolution of Extensional Fault Systems Linked with Salt Diapirism Modeled with Finite Elements

On a divergent continental slope, gravity spreading causes thin-skinned extension of a salt layer and its brittle overburden. Faulting of the overburden initiates diapiric walls. We have numerically modeled the evolution of this fault-salt system with the finite element software GEOSIM-2D.

The initial fault pattern depends strongly on the boundary conditions and rheology. In particular, whether an initiating fault dips landward or seaward depends on friction at the base of the modeled salt and whether one or both fault blocks move laterally. Symmetric faulting is possible even though only one block moves laterally. For the modeled conditions, any initial asymmetry evolves into a symmetric graben with continued extension. During this stage of reactive diapirism, the graben block develops inward-dipping fracture zones. Although significant offset is prevented on these proto-faults, the fracture zones would substantially enhance permeability. The viscous layer rises beneath the graben to form a diapiric wail. In contrast to other piercement mechanisms, the geometry of this diapir and overlying faulting is completely insensitive to the relative densities of salt and its overburden. Faulting and loading of the overburden drive the salt flow rather than vice versa.

Continued extension thins the overburden as the proto-faults in the graben sequentially activate. Older, deeper faults deactivate. The diapir grows isostatically. If the overburden thins sufficiently by faulting, the crest of the diapir may actively pierce the graben and extrude. Pressure in the graben block changes from a relative low to a high as the diapir changes from reactive to active.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.