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_Journal of Structural Geology 32 (2010) 580e594_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Deformation and recrystallization mechanisms in actively extruding salt fountain: Microstructural evidence for a switch in deformation mechanisms with increased availability of meteoric water and decreased grain size (Qum Kuh, central Iran) Guillaume Desbois a,*, Prokop Z?vada b, Zsolt Schl?der c, Janos L. Uraia Structural geology, Tectonics and Geomechanics, RWTH Aachen University, Lochnerstr. 4-20, D-52056 Aachen, Germany b Institute of Geophysics ASCR, v.v.i.; Box14cn? II 1401, 14131, Prague, Czech Republic c Midland Valley Exploration Ltd., 144 West George Street, Glasgow, UK Article info Article history: Received 6 July 2009; Received in revised form 10 March 2010; Accepted 11 March 2010; Available online 17 March 2010 Keywords: Rocksalt Salt extrusion Gamma-irradiation Deformation mechanisms Microstructure Abstract Microstructural study of rocksalt samples from an active salt fountain (Qum Kuh, central Iran) enabled to identify the relative contribution of different deformation mechanisms on extrusive salt flow. The microstructural study combined reflected and transmitted light microscopy of gamma-irradiated thin sections, textural analysis of digitized microstructures and Electron Back Scattered Diffraction (EBSD). Deformation microstructures record the strongly variable deformation conditions of salt flow in the diapiric system from the diapiric stem towards the distal part of the mature viscous fountain. High-stress deformation conditions typical for diapiric stems are recorded in the small subgrains within the porphyroclasts of all documented samples. Recovery and recrystallization due to divergent and decelerating flow associated with differential stress drop in the salt extrusion above the diapiric orifice is reflected by abundant growth band microstructures. This study reveals also evidence for penetration of rainwater into the salt mass and documents the switch from the dominant dislocation creep into dominant solution-precipitation creep from the upper part to the distal part of the fountain. This deformation mechanism switch is provided by influx of meteoric water and grain size decrease likely controlled by subgrain rotation and grain-boundary migration recrystallization. ? 2010 Elsevier Ltd. All rights reserved. 1. Introduction The properties of rocksalt are extensively discussed in the literature. Its multiple role in sedimentary basin evolution (Littke et al., 2008; McClay et al., 2003; Rowan et al., 1999), fluid sealing capacity for gas storage (Li et al., 2005), drilling problems in petroleum industry (Muecke, 1994; Wilson et al., 2002), the possibility of waste disposal in salt (Salters and Verhoef, 1980) and salt mining give the topic enormous economic importance and emphasize the need for the proper knowledge of the mechanical and transport properties of halite rocks deforming in nature (Drury and Urai, 1990; Urai and Spiers, 2007; Schoenherr et al., 2007a,b, 2009; Urai et al., 2008). Zagros mountains (SE part of Iran) are the world-class site for well exposed salt extrusions and glaciers with up to 60 salt extrusions ranging in age from Cambrian to Pliocene displaying at least 6 different morphologies (Talbot, 1998; Jahani et al., 2007). Rocksalt * Corresponding author. Tel.: ?49 241 80 95780; fax: ?49 241 80 92358. E-mail address: g.desbois@ged.rwth-aachen.de (G. Desbois). 0191-8141 $ e see front matter ? 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.03.005 extruding on flat topography through release-bends of major strike-slip faults in Zagros (Talbot and Alavi, 1996; Talbot and Aftabi, 2004) typically forms viscous fountains. However, most rocksalt in Zagros Mountains forms salt glaciers (namakiers) flowing down from the crests of the Zagros anticlines or through one or more fault gullies or uptilted river valleys (Talbot, 1998). Salt glaciers were also extruded from diapirs in the Central European Basin during Triassic (Mohr et al., 2007). Vertical extrusion rates of salt extrusions (<10 mm a) were recently measured using InSAR technique (Interferometric Synthetic Aperture Radar) (Weinberger et al., 2006; Hudec and Jackson, 2007; Aftabi et al., 2010) and also estimated by palinspastic reconstruction (Davidson et al., 1996; Bruthans et al., 2006). Active salt extrusions offer a unique opportunity to reconstruct deformation conditions for rocksalt based on identification of deformation mechanisms and constitutive equations from laboratory calibrations, experiments and theory. Because the measured flow laws are based on short-term laboratory tests and performed at high differential stresses, their precision is limited when extrapolated to low stress, long-term natural conditions. One key test for correct extrapolation of mechanical behavior of rocksalt is the direct comparison of the G. Desbois et al. Journal of Structural Geology 32 (2010) 580e594 581 deformation mechanisms operative in nature and experiment (Passchier and Trouw, 2005). Gamma-irradiation decoration is the most powerful technique for microstructural investigation of halite (Urai et al., 1987; Garcia Celma and Donker, 1996; Schl?der and Urai, 2007; Schoenherr et al., 2009). The physical basis of the blue decoration of halite microstructures is the production of F-center defects by gamma-irradiation, which aggregate into sodium colloids (Van Opbroek and den Hartog, 1985). Because the colloids nucleate preferentially at solid-solution impurities and crystal-defect sites (Jaine-Lidiard model), the resulting coloration reflects the halite microstructures. The intensity of the coloration is a function of the solid-solution impurities and crystal-defect density. Decoration of grain and subgrain boundaries is achieved by chemical etching of the thin sections (Urai et al., 1987). Combination of both kinds of decoration provides a rich detail of microstructure (subgrain-rich grain, strain-free new grains, growth bands, syn-sedimentary chevrons, grain-boundary indentation truncation, edgewise propagation of subgrains, dissolution-precipitation features, core-mantle structure, pressure fringes) and used to infer qualitatively the deformation, recrystallization and fluid-transport mechanisms in studied samples (Urai et al., 1986a,b, 1987, 2008; Schl?der and Urai, 2007; Schoenherr et al., 2009). Deformation mechanisms are also conveniently identified using EBSD for CPO measurements (Schl?der and Urai, 2007) together with grain size and shape statistics, while subgrain size statistics is used for piezometric estimates of differential stress (Schl?der and Urai, 2005). While solution-precipitation accommodated grain-boundary sliding (GBS) is typically associated with low degree of CPO and abundant substructure-free equant grains (Schl?der and Urai, 2007), dynamic recrystallization by dislocation creep is typical with large variety of substructure richness in the grains and well-developed CPO (Passchier and Trouw, 2005). The grain size of samples is controlled by two competing deformation mechanisms during dynamic recrystallization: subgrain rotation (SGR) and grain-boundary migration (GBM) recrystallization. SGR tends to decrease the average grain size of the aggregates and is typical with low-angle misorientation of marginal subgrains from cores of porphyroclasts and its dominant activity is manifested by similar size of subgrains and adjacent new grains in the matrix (Halfpenny et al., 2004). In contrast, GBM generally increases grain size of the dynamically recrystallizing aggregate. GBM recrystallization in combination with SGR can also result in grain size decrease, but regardless to the size of subgrains developed in porphyroclasts (Passchier and Trouw, 2005). Previous studies (Talbot and Rogers, 1980; Urai et al., 1986b; Schl?der and Urai, 2007; Urai and Spiers, 2007; Schoenherr et al., 2009) suggest that dislocation-creep-accommodated dynamic recrystallization and pressure-solution creep both govern rheology of active salt extrusions. Various creep laws are available (e.g. Carter et al., 1993 and Spiers et al., 1990) to describe the specific deformation mechanisms. However, the salt flow is best described by a multi-mechanism creep equation and deformation map for rocksalt, which combine the flow laws for different deformation mechanisms (Spiers and Carter, 1998) and suggest that under natural conditions flow will occur either by climb-controlled dislocation creep or pressure-solution depending on the grain size (Schl?der, 2006; Urai et al., 1986b, 2008). Activity of these deformation mechanisms is also strongly promoted by inter-granular fluid (Urai and Spiers, 2007), that was incorporated in the salt mass during its deposition (Schl?der and Urai, 2005) or by influx of meteoric water into extrusive salt, which together with small grain size provide suitable conditions for dominant solution-precipitation creep and accelerate salt flow in the salt glaciers (Jackson, 1985; Urai, 1987). In contrast, for relatively large grain size of domal salts (2e20 mm), dislocation-creep-accommodated dynamic recrystallization is dominant. Schl?der and Urai (2007) suggested that the salt flow in the extrusion of Tertiary rocksalt in the Eyvanekey plateau was heterogeneous and was accommodated primarily by several mylonitic shear zones, where salt deformed exclusively by solution-precipitation creep and grain-boundary sliding (GBS) at grain size of 0.6 mm, while in protomylonites between the shear zones with grain size of 2e6 mm, salt deformed by combination of dislocation and solution-precipitation creep with GBS. Schoenherr et al. (2009) reported that the rocksalt microstructure of surface-piercing diapirs of Ara Salt in Oman records an overprint of dislocation creep microstructures originated in the diapiric stem by recrystallization and solution-precipitation creep producing static g' Ключевые слова: sample collected, schl?der, rocksalt microstructures, misorientation proles, urai, merle zvada, growth band, crystallographic orientation, larger grain, ree, journal structural, grain boundary, isbn, pressure solution, edgewise propagation, diapiric stem, salt, static, ebsd map, dark blue, gamma-irradiated sample, basin, elongated subgrains, paterson, band, klodawa salt, kuh, ebsd, fountain sample, society, subgrain-rich grain, alsop, eds, ebsd measurement, structural, rocksalt, spiers, grain size, science, doi, transport property, prior, meteoric water, transmitted, earth, deformation mechanism, subgrain-free rim, study, dynamic, differential, combined activity, carter, iran, subgrain-poor rim, urai spiers, tectonics, deep blue, pattern, extrusive salt, dynamic recrystallization, thmc processes, qum kuh, halite microstructures, geological, rich, eds dynamics, journal structural geology, guillop?, distal fountain, deformation, talbot, relative contribution, talbot aftabi, structural geology, extrusion, jackson, kukla, fountain, der, large variety, sample, area, middle fountain, salt mass, pressure-solution creep, migration, nature, pressure shadow, subgrain-rich, higher magnication, recrystallization, ?ow, deformation condition, aftabi, condition, rock, desbois journal, water, large, rim, transmitted light, peach, pressure, zagros, central, mechanical behavior, deformed, sub-euhedral, homogeneously blue, salt fountain, diapir, left corner, step size, subgrain boundary, surface, ebsd pattern, small grain, microstructural evidence, grain, small, geology, differential stress, uid inclusion, solution, gbm, wa, size, subgrain, banded rim, red formation, schl?der urai, orientation, internal strain, entire diapiric, subgrain size, tectonophysics, rigid clast, subgrain-free grain, source layer, geophysical, hardy, oman schoenherr, halite, entire volume, dominant, special, elsevier, salt diapirs, distal, surface-piercing diapir, geological society, journal, misorientation, grain-boundary, partially sealed, sub-euhedral grain, salt extrusion, spiers schoenherr, white, urai urai, microstructural evolution, microstructural, light, mm, london, rheology, subgrain-rich core, substructure richness, earth surface, stem, micrograph, elongated, re?ected, solution-precipitation, subsurface, creep, ara salt, marginal subgrains, microstructural analysis, stretching lineation, collected, bulletin, american, subgrain rich, porphyroclasts, mechanism, rogers urai, gamma, boundary, subgrains, blue, solid-solution impurity, desbois, urai schoenherr, core, diapiric, merle, stress, horizontal, white rim, etching technique, qum, map, gansser, sgr, schoenherr, solution-precipitation creep, sample distal, microstructures, reected light, subgrain-free, schulmann, recrystallization mechanism, growth, strain-free grain, ?uid, middle, den hartog