Natural Strain in Glacial and Diapric Rock Salt with Emphasis on Oakwood Dome, East Texas

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Jackson, M. P. A.

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In Part I, structural styles in the gravity-driven, ductile-flow processes of glaciers and diapirs are analyzed. Salt glaciers flow under minute differential stress when dampened by rainfall. Thus, the concentration of water in diapiric rock salt is of paramount importance to predicting creep rates in a repository medium. Natural strain rates for rock salt vary enormously from 10^8/s to 10^16/s; the slowest rates are those for average diapiric uplift. Contrary to widespread generalization, structural attitudes in dome-salt mines are predominantly moderate to steep, rather than vertical.

In the proposed model, diapiric folds are 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 many folds appear to have been initiated by buckling. 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.

Part II describes the results of geometric analysis and strain analysis of salt core from Oakwood Dome. The core has 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 severe truncation of the diapir crest, probably by groundwater dissolution during cap-rock formation. The uppermost 2 meters of rock salt recrystallized in the presence of water. Consideration of homologous temperatures and present maximum erosion rates suggests that the salt recrystallized at least 3 million years ago at depths 400 meters greater than present.

All the strains recorded in Oakwood halite are of the flattening type. The ratio of flattening to constriction increases upward, whereas the strain intensity decreases upward, perhaps in transition to an originally "neutral" zone in the diapir, since removed by dissolution. The orientations of maximum-extension directions in rock salt vary widely.


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