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GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions
Vol. 49 (1999), Pages 350-357

Modeling Allochthonous Salt Structures Integrating Microstructural Observations and Laboratory-Derived Salt Rheologies

R.A. Mazariegos (1), N.L. Carter (2), and J.E. Russell (3)

(1) The University of Texas-Pan American, 1201 West University Dr., Edinburg, TX 78539

(2) Salt Micromechanics, Inc., P.O. Box 1442, Crescent City, CA 95531

(3) Texas A&M University, Department of Geology and Geophysics, College Station, TX 77843

ABSTRACT

The evolution of allochthonous salt sheets of different geometries and at different depths were analyzed by means of a 2-D finite-volume numerical model. The model incorporated two laboratory-derived rheological laws for rocksalt, a low-stress, dislocation-creep power law (PL) (ïe µ s3.4) and a fluid assisted diffusion creep law (FAL) (ïe µ s/Td3), for which ïe is the strain rate, s is the equivalent stress, T the absolute temperature and d the grain size.

Salt sheets, buried at depths of 1 km and 3.3 km in sediments of 3.0x1017 to 3.0x1019 Païs viscosity, subjected to differential loading were modeled for a time evolution of 2 My to 5 My. At the end of the simulation, for the shallow salt sheet, the PL model developed a well defined "depressed salt zone"; by contrast, the FAL model developed two well defined "depression zones" or intra-salt basins and the salt sheet extended a total of 12.4 km as compared to 6.0 km for the PL. For the deeper sheet, deformation is primarily lateral at the two lower sediment viscosities and vertical at msed=3.0x1019 Païs.

Using microstructural observations made on rotary sidewall cores from a salt sheet in the Gulf of Mexico, stresses within the salt were estimated using the relation d= 214 s-1.15, where d is the salt subgrain diameter (in mm) and s- is the differential stress in megapascals. A stress function was obtained and incorporated into the model to predict the velocity field within the salt sheet. Significantly, a highly localized velocity anomaly was identified on one flank of a highly deformed allochthonous salt structure.


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