FORWARD MODELING OF FAULTED DOMES: GEOLOGIC INSIGHTS FROM 3-D KINEMATIC MODELS

Salt-related piercement structures represent one of the greatest challenges for kinematic modeling and structural restoration because of the three dimensionality of the displacement field and the presence of numerous faults. To investigate the geometry and kinematics of active piercement domes, we have developed a 3-D flexural-slip forward model of a faulted dome. The model incorporates the following features of active salt domes: the dome is circular in map view, rises above its surrounding area, and is ruptured by radial normal faults. Domes usually begin with 2 or 3 major normal faults that cross the crest of the structure. The number and the orientation of the major normal faults control the structural style of the domes. 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. In the resulting models, the major normal faults cross each other and move simultaneously. As a result, all the major faults are mutually offsetting. Minor normal faults do not develop in the horst blocks, but may develop in the graben blocks. Models with 2 major normal faults form one horst, one graben, and two half grabens. Models with 3 major normal faults develop three horsts, three grabens, and a highly strained central graben above the crest of the dome. In cross sections from the model domes, the observed structural style is controlled by the faulted or unfaulted nature of the top of salt and by the direction of the cross section. If the top of salt is faulted, the dip of the grabens increases with the dip of the horsts until the horsts reach a critical dip angle, beyond which the grabens begin to subside. If the top of salt is unfaulted, the major graben blocks of the dome have the same dip as the horst blocks, but must contain minor blocks that have a smaller dip angle. Quite different structural patterns appear in cross sections cut parallel and perpendicular to the major normal faults of the model domes. In models with 3 major 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. Model domes compare closely to natural examples from Texas and Germany.