RHEOLOGY OF ROCK-SALT

We review progress in experimental determinations of transient and steady-state flow properties and processes of natural rocksalt aggreptes, focusing primarily on results from Avery Island, Louisiana, domal salt. The steady-state flow field-established from constant stress and constant strain-rate tests at temperatures from 50 to 200-degrees-C, strain rates from 10(-5) to 10(-9) s-1 and differential stresses, sigma, from 20.7 to 2.5 MPa-has been separated into two flow regimes, each fit by a power-law relation. At the higher stresses and strain rates this relation is epsilon = 1.6 x 10(-4) exp (-68.1/RT . 10(-3)) sigma5.3 (A) for which the pre-exponential constant is expressed in MPa-5.3 s-1 and the apparent activation energy is in J mol-1. Creep rates predicted by equation (A) do not differ appreciably from those predicted previously. The relatively low stress, low strain-rate data are very well fit by epsilon = 8.1 x 10(-5) exp (-51.6/RT . 10(-3)) sigma3.4 (B) which, at comparable conditions, predicts creep rates higher and equivalent viscosities lower than does equation (A) by two orders of magnitude. The change in behavior from (A) to (B) is ascribed to a change in rate-limiting mechanism from cross-slip of screw dislocations to the climb of edge dislocations in this dry material. Subgrain formation dominates microstructural development during steady-state flow of salt and, from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies, flow stress levels between 0.6 and 1.4 MPa have been estimated. These values do not bear directly on arguments conceming the nature of forces initiating salt pillow growth because, apparently, evidence relating to the early deformational history has becn overprinted. At that stage, fluid-assisted grain boundary diffusional processes might dominate dislocation creep, leading to a linear stress-strain rate mechanical response. Considering buoyancy forces alone, behavior described by equation (B) rather than (A) reduces the relief necessary to initiate pillow growth but a 1000-fold amplification is required to produce stress differences near 1 MPa. That forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions. Differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures.