Report of Investigations No. 143 Natural Strain in Diapiric and Glacial Rock Salt, with Emphasis on Oakwood Dome, East Texas

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Jackson, M.P.A

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University of Texas at Austin. Bureau of Economic Geology


Structural styles in the gravity-driven, ductile flow processes of glaciers and diapirs are analyzed. When dampened by rainfall, salt glaciers flow rapidly under minute differential stress. Thus, the concentration of water in diapiric rock salt is crucial to predicting creep rates in a repository medium. Natural-strain rates of rock salt range greatly from 10-8/s to 10-16/s; rates of average diapiric uplift are slowest. Contrary to widespread generalization, structural attitudes in dome salt mines are predominantly moderate to steep (60 to 80) rather than vertical.Diapiric folds may be triggered by (1) shear stresses induced by upward flow, (2) shear stresses induced by boundary effects of the salt source layer, and (3) normal stresses induced by convergent flow. Folding in salt stocks follows the similar-fold model, although some folds may have begun as buckles. Closed interference structures are sheath folds formed by intense constriction of originally gentle fold culminations and depressions. Construction of plunge-isogon maps from mapped linear structures allows the flow directions of diapiric salt to be deduced. Geometric and strain analyses of salt core from Oakwood Dome show that the well bore penetrated the hinge zone and lower limb of an inclined, overturned antiform, which probably represents a salt tongue that has spread outward from the diapir center. Structural evidence indicates truncation of the diapir crest, probably by ground-water dissolution during cap-rock formation. The uppermost 2 m of rock salt recrystallized in the presence of water. Homologous temperatures and present maximum erosion rates suggest that the salt recrystallized at least 3 Ma ago at depths 400 m greater than present. All the strains recorded in Oakwood halite are of the flattening type. The ratio of flattening to constriction increases upward, whereas strain intensity decreases upward. This strain pattern may represent a transition to an originally neutral zone in the diapir, since removed by dissolution. Orientations of maximum-extension axes in rock salt vary widely.


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