Inferring Subsalt Structures from Suprasalt Structures: Results from Numerical Modeling of Extension Around Salt Sheets

Daniel D. Schultz-Ela

During extension a salt sheet partially decouples structures developing in overlying and underlying strata. This decoupling hampers direct inference of subsalt structures from the better imaged near-surface structures. Numerical modeling of this extensional system provides some guidelines.

Salt sheet geometry and displacement rate were systematically varied in finite-element models with a viscous salt analog and a strain weakening elasto-plastic rock analog. Encased, detached salt sheets overlay strata floored by a decollement or a continuous salt layer. Faulting was initiated at the thinnest strata or at sharp bends in the salt contact. The degree of coupling between extending structures above and below a salt sheet increased with a decrease in salt thickness, an increase in salt viscosity, an increase in extension rate, or a decrease in overburden thickness. In more strongly coupled cases, a near-surface symmetric graben indicated absence of a directly underlying subsalt structure. A near-surface half-graben, with variable degrees of faulting in the rollover, almost i variably pointed to a causative subsalt normal fault located laterally offset toward the hangingwall. The offset distance decreased as coupling increased. Subsalt normal faults commonly occurred as pairs defining subsalt grabens. Extreme extension across these salt-filled grabens thinned the strata to nothing in some models, creating the erroneous appearance of a feeder to the salt sheet. A basal decollement in models limited subsalt structural development to grabens. Flow in models with an additional basal salt layer allowed combinations of grabens, half-grabens, and horsts to develop beneath the salt sheet.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995