Abstract

Recent discoveries of hydrocarbons beneath allochthonous salt sheets have heightened interest in the location and geometry of subsalt structures, including faults offsetting the base of salt sheets. Salt extrudes to form flat-topped sheets with discordant or stepped lower contacts that then deform in response to sedimentation and regional or local stretching. We simulate the interacting growth of subsalt and suprasalt structures during regional extension using two-dimensional finite-element models. Fault arrays above and below salt comprise combinations of three characteristic patterns: simple grabens, reactive grabens, and drape grabens. Thickness variations in the roof of a salt sheet strongly influence the initial location of suprasalt faults. Fast extension or thin salt enhances coupling between subsalt and suprasalt structures, summarized by these predictive guidelines: (1) subsalt faults are laterally offset from suprasalt drape grabens, and the offset and graben symmetry increase as coupling decreases, (2) symmetric reactive grabens tend to overlie zones of no subsalt faulting, and (3) suprasalt sags with little faulting overlie subsalt grabens. Flow in a deeper salt layer increases structural relief beneath a shallower sheet but decreases surface relief. The modeling exposes several possible interpretation pitfalls concerning true and apparent regional elevations, apparent feeder stems, subsalt ramps and flats, and effects of salt withdrawal vs. extension.

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