Analogue modeling of the internal kinematics of salt diapirs
Final Report Abstract
Plane-strain experiments using plasticine as rock analogue Plane-strain coaxial deformation of a competent plasticine layer embedded in an incompetent plasticine matrix was carried out at room temperature to improve our understanding about the evolution of folds and boudins if the layer is oriented perpendicular to the Y-axis of the finite strain ellipsoid. The rock analogues used were Beck’s green plasticine (matrix) and Beck’s black plasticine (competent layer), both of which are strain-rate softening modelling materials with stress exponent n = ca. 8. The effective viscosity ? of the matrix plasticine was changed by adding different amounts of oil to the original plasticine. At a strain rate ? of 10-3 s-1 and a finite strain e of 10%, the effective viscosity of the matrix ranges from 1.2 x 106 to 7.2 x 106 Pa s. The effective viscosity of the competent layer has been determined as 4.2 x 107 Pa s. If the viscosity ratio, m, between layer and matrix is large (> ca. 20) and the initial thickness of the competent layer, Hi, is small, both folds and boudins develop simultaneously. Although the growth rate of folds seems to be higher than the growth rate of boudins, the wavelength of both structures is approximately the same as is suggested by analytical solutions. A further unexpected, but characteristic, aspect of the deformed competent layer is a significant increase in thickness, which can be used to distinguish plane-strain folds and boudins from constrictional folds and boudins. To demonstrate the impact of varying strain rates on growing folds and boudins under plane strain, we investigated a stiff layer consisting of non-linear viscous Kolb grey plasticine that is embedded in a weak matrix, consisting of non-linear viscous Beck’s green plasticine. The strain rates used range from 8 x 10-6 s-1 to 2 x 10-2 s-1. At room temperature the viscosity ratio between layer and matrix, m, increases with strain rate from 3 to 10. Different runs have been carried out at room temperature in which the layer, S, was oriented perpendicular to the principal strain axes (X>Y>Z). Our results suggest a considerable influence of the strain rate on the geometry of the deformed stiff layer including its thickness. A striking break in both the degree of layer thickening and the change in arc length of folds and boudins is obvious at ? = 1 * 10-3 s-1, which corresponds to a viscosity ratio, m = ca. 7. These results might be interesting for workers dealing with rock salt or melt-lubricated shear zones where strain rates may vary significantly. The new results further confirm that analytical solutions do not hold for very low viscosity ratios between layer and matrix. Thermomechanical experiments using halite and anhydrite A new thermomechanical apparatus has been designed and built in the frame of the present project that can be used for the full range of three-dimensional coaxial deformation at elevated temperatures but without imposing a confining pressure. The new apparatus has been used to carry out experiments under both bulk plane strain and bulk constriction. Bulk constriction was required to model the internal kinematics of axisymmetric salt diapirs. We present the first thermomechanical experiments on 3D-folding and boudinage of a natural competent rock embedded in a natural incompetent matrix, the latter undergoing dislocation creep. Composite natural samples consisting of a single layer of anhydrite, embedded in halite matrix, were constrictionally deformed at temperature, T = 345°C, strain rate, ? = 10-7 s-1, maximum viscosity, ? = 2 x 1013 Pa s, and maximum finite strain, eX = 122%. The anhydrite layer, oriented parallel to the major stretching axis, X, was deformed under brittle-viscous conditions as is indicated by fracturing, twinning, and local recrystallization, the latter based on strain-induced grain-boundary migration. Viscous deformation of halite was accommodated by slip on {110}<110> which led to formation and rotation of subgrains. A considerable 001-maximum parallel to the long axis, X, of the strain ellipsoid, is indicated by EBSD analyses of halite. Differential stresses, obtained from subgrain size of halite, are largely consistent with stresses recorded by the load cells of the machine (< 5 MPa). At advanced state of constriction (eX > 90%) a strong increase in strain hardening of halite led to a transient tension fracture that healed up and was shortened by folding during the final phase of viscous deformation. Tiny prismatic anhydrite inclusions, disseminated inside the halite matrix, were reoriented during constriction resulting in a linear grain-shape fabric. 3D-images of the anhydrite layer, based on computer tomography, revealed rare kink folds with axes subparallel to X, and boudins which result from tension fracture. With increasing layer thickness, Hi, the width of boudins, Wa, increases linearly (Wa = -0.3 + 1.3 * Hi). The normalized width of boudins (Wd = Wa/Hi) is almost constant at 1.5 ±1.0. Fracture boudinage of the anhydrite layer cannot be described by the Fletcher- Smith equation to correlate the geometrical and rheological data. The oblique orientation of most of the boudins, with respect to the principal strain axes, results from folding of the boudins by a second generation of folds. This second generation of folds, with axes subperpendicular to the layer, is characteristic for the constrictional runs, but does not occur under bulk plane strain. Plane-strain experiments were carried out under similar conditions as described above for the constrictional runs with the layer, S, oriented perpendicular to the X-axis (S?X) and perpendicular to the Z-axis (S?Z). Mohr-Coulomb fractures (thrusts) developed in cases where S?X. The angle between the thrust plains and the YZ-plain ranges from 10° - 25°, rarely up to 60°. Salt was flowing viscously into the space between hanging and foot wall. Few noncylindrical folds developed only in cases where Hi < 1 mm. In cases where S?Z, fracture boudins developed, the number of which increases almost linearly with Hi. The normalized width (wavelength) of the plane-strain boudins (Wd =Wa/Hi) is almost constant at 1.3 ±0.7. This value is quite similar to the Wd value described above for constrictional boudins. Despite of the high deformation temperature and the low water content of the starting samples, NaCl brine and hydrocarbons were expelled from fluid inclusions of halite and anhydrite and led to hydraulic fracturing and redistribution of matter. The fluids migrated along halite-anhydrite boundaries and along newly formed microfractures which are more common in anhydrite than in halite. Combined analysis using fluorescence microscopy and Raman spectroscopy suggest that hydrocarbons were released from fluid inclusions, migrated along microfractures and finally were redeposited in open space of neck domains in form of black organic coating (condensate). Released NaCl brine led to precipitation of fine-grained halite in the central part of open neck domains. As all of these phenomena are common in salt domes of northern Germany, the results of the present study should be important for workers who are dealing with radioactive waste deposits in rock salt. Radioactive waste deposits are sites of elevated temperature which was considered in the present experiments, and released fluids may led to contamination of bed rock and biosphere.
Publications
- 2005. Coeval folding and boudinage under bulk plain strain, with the axis of no change perpendicular to the layering. Int. J. Earth Sci., 95: 178-188
Enama-Mengong, M. and Zulauf, G.
- 2007. Deformation of a halite-anhydrite sequence under bulk constriction: Preliminary results from thermomechanical experiments. In: Wallner, M., Lux, K.-H., Minkley, W. and Hardy, H.R. Jr. (Eds.), The Mechanical Behavior of Salt – Understanding of THMC Processes in Salt, pp. 63-68. Taylor & Francis Group, London
Zulauf G., Zulauf J. and Bornemann, O.
- 2007. The impact of strain rate on folding and boudinage under plane strain: Results from analogue modelling. Geotectonic Research, 95, 101-114
Enama Mengong, M. and Zulauf, G.
- 2008. Structures in an anhydrite layer embedded in a halite matrix: First results from thermomechanical experiments under plain strain. Geotectonic Research, 95/01, 111-113, Special Issue TSK 12
Mertineit, M., Zulauf, G., Kihm, N. and Zanella, F.
- 2009. Experimental deformation of a single-layer anhydrite in halite matrix under bulk constriction. Part 1: Geometric and kinematic aspects. J. Struct. Geol.
Zulauf, G., Zulauf, J., Bornemann, O., Kihm, N., Peinl, M., Zanella, F.
(See online at https://doi.org/10.1016/j.jsg.2009.01.013)
