Physical modeling of structures formed by salt withdrawal; implications for deformation caused by salt dissolution

University of Texas at Austin, Bureau of Economic Geology, Austin, TX, United States

By creating 15 physical models, we investigated deformation above subsiding tabular salt, salt walls, and salt stocks. Dry quartz sand simulated a brittle sedimentary roof above viscous silicone representing salt. The modeled diapiric walls had linear planforms and rectangular, semicircular, triangular, or leaning cross sectional shapes; the stock was cylindrical. In models where the source layer (or allochthonous salt sheet) was initially tabular, a gentle, flat-bottomed syncline bounded by monoclinal flexures formed above a linear zone where the silicone was locally removed. Above all subsiding diapirs, the deformed roof was bounded by an inner zone of steep, convex-upward reverse faults and an outer zone of normal faults. Above subsiding diapiric walls, extensional and contractional zones were balanced. Above the subsiding salt stock, conical, concentric fault zones comprised inner reverse faults and outer normal faults. Sediments were added both before (prekinematic) and during (synkinematic) salt withdrawal. In entirely prekinematic roofs, reverse fault zones and normal fault zones both widened with time. Reverse faults propagated upward from the corners of the withdrawing diapirs. New reverse faults formed in the footwalls of reverse faults, each nearer the center of the deepening roof trough. Conversely, new normal faults formed successively outward from the sagging trough. Synkinematic deposition retarded faulting, but the pattern of inner reverse and outer normal faults was repeated; however, reverse faults formed successively outward, whereas normal faults formed inward. New conceptual models suggest that salt dissolution forms similar structures to those physically modeled for salt withdrawal. The appropriate physical models resemble natural dissolution structures above tabular salt. Extension alone above diapirs is not caused merely by salt withdrawal or dissolution, but by regional extension or active diapirism.

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