Structural interpretation and validation of three-dimensional piercement structures

Three dimensional forward modeling is used to interpret the geometry and evolution of faulted salt domes. Field evidence indicates that active salt dome are circular in map view with relief above surrounding area. Domes usually begin with 2 or 3 major normal faults that cross the crest of the structure. Minor radial normal faults terminate against the major normal faults. A polygonal central graben may develop at the crest of the dome. The overburden strata are relatively unstrained internally. The top of the salt may be smooth or faulted. Results from the modeling show that simultaneous movement on the master faults causes them to be mutually offsetting. The structural style is controlled by the faulted or unfaulted nature of the top of salt and by the direction of the cross section. Quite different structural patterns appear in cross sections cut parallel and perpendicular to the master faults. In models with three master faults, the domes appear very asymmetrical on cross sections perpendicular to a major normal fault, but are symmetrical on cross sections parallel to a major normal fault. Due to out-of-plane volume transport, most cross sections show unrestorable structural thinning in the central. Because the section line is oblique to the dip, most sections across the flank grabens show apparent thickening easily misinterpreted as stratigraphic growth. The apparent growth problem can be fixed by restoring the cross section with true thickness calculated from vertical thickness and true dip of the bed. 3D restoration of model domes indicates that vertical simple shear is not a good restoration method for active piercement structures, instead, parallel flexural slip unfolding is a good and practical 3D restoration method for 3D piercement structures. Results obtained from model dome restoration have been successfully applied to interpret the restoration of natural domes in Texas and Germany.