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_Journal of Structural Geology 32 (2010) 264–277 _Composite natural samples, consisting of a single layer of anhydrite embedded in matrix of rock salt, were experimentally deformed at temperature T ? 345 x14C and strain rate e_ ? 10?7 s?1. The geometry and kinematics of folds and boudins affecting the stiff anhydrite layers were described in Part 1 of this study (Zulauf et al., 2009). The microfabrics, deformation mechanisms, and the role of fluids are treated herein. Anhydrite was deformed under brittle–viscous conditions as indicated by fracturing, twinning, and local recrystallization based on strain-induced grain-boundary migration. Viscous deformation of halite was accommodated by slip on {110}C110D leading to formation and rotation of subgrains with a striking 001-maximum parallel to the long axis X of the strain ellipsoid. Differential stress obtained from subgrain size is largely consistent with stresses recorded by load cells of the machine (<5 MPa). Despite high deformation temperature and low water content in starting samples, NaCl brine and hydrocarbons expelled from fluid inclusions of both halite and anhydrite led to hydraulic fracturing and redistribution of matter. Fluids migrated along newly formed microfractures in anhydrite and along halite–anhydrite boundaries. Released hydrocarbons and NaCl brine were redeposited in open space of neck domains in the form of black organic coatings (condensate) and fine-grained halite, respectively. These fluid-controlled phenomena are common in salt domes of northern Germany, and released fluids may lead to contamination of bedrock and biosphere. The results of this study should be important for workers dealing with radioactive waste deposits in rock salt. © 2009 Elsevier Ltd. All rights reserved. 1. Introduction Rock salt in undisturbed state is characterized by low porosity (<<0.5_), low permeability (<<10?20 m2) and low water content (<0.1 vol_) (Popp et al., 2007). For these reasons, rock salt is commonly used as host rock for storage of gas and crude oil, and is also considered for the disposal of radioactive waste. However, deforming rock salt may change its properties considerably. Transient and steady-state flow properties of rock salt are thus important for analyses of salt tectonics and geotechnical engineering problems. The deformation mechanisms of naturally deformed rock salt have recently been reviewed by Urai and Spiers (2007). Low temperature dislocation glide and dislocation creep are common deformation processes. Fluids support solution–precipitation creep and induce dynamic recrystallization by fluid-assisted grain boundary migration (e.g., Urai et al., 1987; Spiers et al., 1990; Spiers and Schutjens, 1990; Hickman and Evans, 1991; Spiers and Carter, 1998; Watanabe and Peach, 2002; Schenk and Urai, 2004; Ter Heege et al., 2005b; Schleider and Urai, 2007). Rock salt is seldom free from mechanical anisotropy. Layers of anhydrite or layers of dry shale are significantly stronger than halite resulting in inhomogeneous strain. Laminated salt rocks can exhibit considerable different mechanical properties compared to pure rock salt (Li et al., 2007). Interfaces between intercalated competent layers and rock salt are particularly important because they are sites where stress, strain, and deformation mechanisms change. Moreover, contacts between rock salt and stiff layers such as anhydrite could act as pathways for migrating fluids which are important for the deposition of radioactive waste in salt stocks (e.g., Bornemann et al., 2008). The present paper presents microfabrics of experimentally deformed samples consisting of a single stiff layer of Gorleben anhydrite embedded in weaker matrix of Asse halite. The macroscopic geometry of boudins and folds has already been published (Zulauf et al., 2009). Anhydrite, oriented parallel to the major stretching axis X, was macroscopically deformed by fracturing whereas halite behaved viscously. At an 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 (Fig. 1). 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 and the normalized width of boudins (Wd ? Wa/Hi) is almost constant at 1.5 ± 1.0 (Zulauf et al., 2009). The microfabrics observed in deformed samples will be used to deduce the deformation mechanisms that operated in halite and anhydrite during experimental runs. It will be shown that fluids have played a significant role during deformation. The microfabrics further explain why contacts between competent anhydrite and weaker halite play a key role for fluid transport in salt domes. As the deformation mechanisms in rock salt are similar to those of silicate rocks deformed at high temperature and pressure, rock salt can be used as analogue material for silicates (e.g., Guillopé? and Poirier, 1979; Hiraga and Shimamoto, 1987). 2. Methods 2.1. Experimental setup We deformed composite samples consisting of natural halite as matrix and natural anhydrite as an embedded layer. All samples were delivered by BGR Hannover. For details on sample preparation, see Zulauf et al. (2009). Halite samples from the Asse borehole in northern Germany (800 m level, Speisesalz Na2SP) have been used for experiments. The internal structure of the Asse salt dome has been described by Essaid and Klarr (1982). Anhydrite samples used for our experiments were collected from the Gorleben deep borehole 1004 (Bornemann et al., 2008). Pure constrictional deformation of composite halite–anhydrite samples was performed using a new thermomechanical deformation rig described in detail by Zulauf et al. (2009). The anhydrite layer was oriented parallel to the X-axis of the finite strain ellipsoid. Deformation conditions during each run were as follows: T ? 345 x14C, e_ ? 2–4 ? 10?7 s?1, eY? Z ? ?33_, and ex ? 122_. The initial thickness of the anhydrite layer Hi varies from 0.8 to 2.5 mm. In that run where the initial thickness of the anhydrite layer was set at 2.5 mm, deformation of the sample was limited to a finite strain eY? Z ? ?17_ and ex ? 44_, respectively, because the prescribed stress limit of the machine was reached. 2.2. Microfabric studies Fig. 1. (a) Section of deformed sample cut parallel to X-axis and perpendicular to anhydrite layer. Initial thickness Hi of anhydrite layer ? 1.5 mm. Note boudinage of anhydrite layer and strong shape-preferred orientation of small anhydrite inclusions in halite matrix. Black bar ? 1 cm. (b) and (c) Close-up views of areas marked by black frame in (a). Necks between the anhydrite boudins which are not entirely filled with viscous halite are different. Some are mineralized by fine-grained halite which should have precipitated from fluid phase (red arrows). Others are still open and coated by dark organic matter (black arrows). Thin and thick sections were prepared from undeformed halite and anhydrite samples and from sections of deformed samples oriented parallel and perpendicular to the X-axis of the finite strain ellipsoid, respectively. To reveal grain and subgrain boundaries in halite, we used the method described by Urai et al. (1987). After polishing with colloidal diamond dispersed in oil (0.25 and 1 mm), sections were lightly etched for 30 s using a 95_ saturated NaCl solution containing 0.8 wt_ FeCl3 ? 6H2O. Subsequently, the etchant was removed from the surface using a powerful jet of n-hexane, and the section was dried in a stream of warm air. Less deformed natural slabs and experimentally deformed slabs with initial layer thickness Hi of 1.5 and 2.0 mm were treated by gamma-irradiation at Forschungszentrum Jülich using a technique similar to that described by Schleider and Urai (2005). Irradiation was carried out for 155 days at 100 x14C with an average dose rate of 0.5 kGy h to a total dose rate of 2 MGy. Initially clear halite developed various shades of blue, and the color intensity in halite crystals is heterogeneous because Ключевые слова: high-strain domain, spiers, ?uid inclusion, grain size, large crystal, bornemann, schle der, behavior, den, deformed sample, sealing matrix, zulauf, skrotzki, organic matter, reected light, image, differential stress, hydraulic fracturing, raman spectrum, thesis, geol, open space, nacl brine, anhydrite inclusion, oriented parallel, wa, gorleben, ?uids, layer, grain boundary, ?lled, journal structural, thmc processes, axis, anhydrite crystal, fabric, grain-boundary migration, brittleviscous regime, minkley, subgrain boundary, reoriented, experiment, striking parallel, sample, elsevier, halite, starting sample, load cell, raman spectroscopy, twin, microfabrics, parallel, salt, mm, soc, rigid anhydrite, initial sample, size, transmitted light, mpa, mechanical anisotropy, twiss, zulauf journal, cut parallel, deformed halite, showing, pure, salt dome, primary, halite matrix, grain-shape fabric, deformation fabric, experimentally deformed, subgrain, open, experimentally, deformation, struct, plane, york, grain, undulatory extinction, asse halite, tectonophysics, neck, initial thickness, salt stock, light, ?uid, individual, urai, note, deformation mechanism, radioactive waste, bgr hannover, london, direction parallel, neck domain, microstructural evolution, slip, journal, black, ?uorescence, viscous deformation, ne-grained halite, strain rate, rate, synthetic rocksalt, irregular boundary, gorleben anhydrite, study, boundary, grain-boundary, germany, temperature, mechanism, chessboard pattern, opaque phase, dark matter, carter, matter, perpendicular, rheology, anhydrite layer, x-axis, formation, viscous halite, gure legend, twinning, space, single-layer anhydrite, result, re?ected light, strain, red curve, stress, mineral, signicant role, based, composite, uid, small inclusion, cut, long axis, average stress, microscopy, subgrain size, structural, sandwiched, ller, structural geology, rock salt, major role, constriction, eds, geophys, rock, phase, bright, deformed, uid inclusion, initial, hannover, franssen, jockwer, rectangular grid, re?ected, asse, close-up view, boudins, geology, case, strain hardening, inclusion, rocksalt, anhydrite, mechanical, competent anhydrite, parallel x-axis, long-prismatic crystal, sinistral displacement, matrix, parallel polarizers, primary constituent, bed rock, uid phase, ebsd pattern, organic, constrictional folding, migration, halite showing, und, salt tectonics, tech, dislocation creep, kali, mit, recrystallization, der, experimental, transmitted, crystal, dark, northern germany, hardy, subgrains, nal phase, actual stress, domain, primary anhydrite, black coating, radial crack, asse sample, large, mechanical behavior, local recrystallization