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

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) (ie µ s3.4) and a fluid assisted diffusion creep law (FAL) (ie µ s/Td3), for which ie 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 Pais 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 Pais. 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.