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_Journal of Structural Geology 32 (2010) 342–349_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com/locate/jsg Rheology and microstructure of synthetic halite calcite porphyritic aggregates in torsion F.O. Marques*, L. Burlini, J.-P. Burg Department of Geosciences, Swiss Federal Institute of Technology (ETH-Zurich), CH-8092 Zurich, Switzerland Article info Article history: Received 14 May 2009 Received in revised form 11 December 2009 Accepted 4 January 2010 Available online 11 January 2010 Keywords: Torsion experiments Two-phase (composite) aggregate Halite (rock salt) Calcite Rheology Microstructure Abstract Polymer jacketed porphyritic samples of 70% halite ?30% coarse calcite were subjected to torsion deformation to investigate the effects of a mixture of coarse calcite on the microstructure and mechanical properties of a two-phase aggregate. The experiments were run at 100 and 200 x14C, with a confining pressure of 250 MPa and a constant shear strain rate of 3E-4 s?1. Ultimate strengths of single-phase halite synthetic aggregates at 100 and 200 x14C were ca. 32 and 8 Nm, respectively, and of the two-phase aggregate 39 and 18 Nm, respectively; this shows that the two-phase aggregate was much stronger, especially at 200 x14C. Stepping strain rate tests show that the two-phase aggregate behaved as power-law viscous, with stress exponents of ca. 19 and 13 at 100 and 200 x14C, respectively. From these high exponents, we infer that the active deformation mechanisms were not efficient enough to rapidly relax the applied stress. Halite stress exponents at 100 and 200 x14C are typically much lower, in the order of 4–6, which means that the calcite porphyroclasts were obstacles to halite plastic flow and hampered stress relaxation. The drop of the stress exponent with temperature indicates that the main deformation mechanism(s) was temperature sensitive. Matrix halite deformed plastically, while calcite rotated rigidly or deformed in a brittle fashion, with grain size reduction by fracturing (e.g., bookshelf and boudinage). We conclude that halite was softer than calcite in the investigated temperature range. Strain was homogeneous at the sample scale but not at the grain scale where the foliation delineated by plastically flattened halite contoured the rigid calcite clasts. The microstructures experimentally produced at 100 and 200 x14C are very similar and find their counterparts in natural mylonites: rolling structures, s and d porphyroclast systems, bookshelf and boudinage in brittle calcite porphyroclasts, and ductile y and c0 micro shear bands in the halite matrix. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction A major task of geoscientists has been to construct models that can explain the behavior of the complex and heterogeneous lithosphere, while capturing its effective large-scale rheological behavior. The fundamental models are the constitutive equations built from laboratory rock experimental data (e.g., Weertman’s equation, Weertman and Weertman, 1975). Although much experimental work has been done, a better knowledge of the mechanical properties of the lithospheric constituents is still lacking, in particular for the mechanical properties of composites, albeit natural rocks are essentially polymineralic aggregates. * Corresponding author. Present address: Departamento Geologia and CGUL-IDL, Faculdade Cieências, Universidade Lisboa, 1749-016 Lisboa, Portugal. Tel.: +351 217500000; fax: +351 217500064. E-mail addresses: fernando.ornelas@erdw.ethz.ch, fomarques@fc.ul.pt (F.O. Marques). 0191-8141 $ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.01.001 The mechanical and microstructural behavior of polymineralic rocks can be described by three end-member types (e.g., Handy, 1990): (1) strong minerals form a load-bearing framework; (2) two or more minerals with low relative strengths control bulk rheology; (3) one very weak mineral governs bulk rheology, while the stronger minerals form porphyroclasts. Deforming rocks exhibit, at all scales, complex rheological responses ranging from strong quasi-rigid-like to weak quasi-?uid-like-effective behavior, as a function of temperature, stress, strain rate, ?uids and or rock composition. In many cases, both behaviors are found in the same deformed rock as, for example, in porphyritic granites deformed in greenschist facies conditions, where ‘‘hard’’ feldspar porphyroclasts are enclosed (in low concentration, little interaction) in a ‘‘soft’’ plastically deformed (?ne grained) matrix of quartz and mica. Similarly, we investigate the effects of adding a (coarse, mostly equant grains) strong phase (quasi-rigid-like) to a weak (?ner grained) plastic matrix (quasi-?uid-like) on the overall behavior of a composite aggregate. This is still an open and relevant question because the solid-state rheology of rocks depends to a great extent on the F.O. Marques et al. Journal of Structural Geology 32 (2010) 342–349 343 relative proportions of weak and strong minerals, and their shape and distribution. The use of mixed strong and weak phases poses a practical problem for experimental simulations, because in the laboratory most common natural ductile matrices (calcite or quartz) do not deform plastically at low temperatures typical of the greenschist facies, the conditions very common to many ductile shear zones involving the plastic deformation of calcite and quartz. In trying to overcome this problem, we used as analogues (after e.g., Williams et al., 1977; Hobbs et al., 1982; Wilson, 1983; Burg and Wilson, 1987) a soft plastic (at low temperature and laboratory strain rates) matrix made of halite and hard coarse grain calcite clasts to investigate the behavior and mechanical properties of a two-phase porphyritic synthetic aggregate with contrasting rheology and behavior of the constituents. In order to assess the contribution of calcite to the rheology of the halite coarse calcite aggregate, we compare it with the experimental behavior of synthetic single-phase halite aggregates deformed under similar experimental conditions. Up to now, most low temperature work with 2-phase aggregates has been done in axial compression and low strain. However, a great deal of deformation of rocks takes place in ductile shear zones dominated by simple shear (e.g., Ramsay, 1967; Burg, 1999). It is thus very advantageous to make available microstructural and mechanical data resulting from simple shear deformation to large strains. With this aim, we carried out torsion experiments to simulate strain in shear zones and achieve high shear strains typical of natural high shear strain zones (mylonites). Previous experimental work has used 2-phase aggregates to analyze the effect of randomly distributed hard grains on the mechanics of rock salt (e.g., Price, 1982; Bloom?eld and CoveyCrump, 1993; Kawamoto and Shimamoto, 1998), or to gain a better understanding of mylonites (e.g., Ross et al., 1987; Jordan, 1987), or to study foliation development in quartz-mica rocks in pure shear ?ow (e.g., Williams et al., 1977; Hobbs et al., 1982; Wilson, 1983; Burg and Wilson, 1987). Price (1982), on the basis of triaxial compression tests, observed relatively small strength increases with increase in anhydrite content up to 50% at 200 x14C, 200 MPa (from 14.1 MPa differential stress for 100% halite, to 18.5 MPa for 50% halite), and much larger increases in strength above 50% anhydrite content (from 18.5 MPa for 50% halite, to 84.2 MPa for 0% halite). Jordan (1987) used both axially symmetric compression and simple shear, and concluded that the strength of the bulk aggregate decreases as the foliation intensifies. Ross et al. (1987) used simple shear to deform halite-anhydrite synthetic aggregates at 300 x14C and 200 MPa, and concluded that the aggregate strength strongly increased with anhydrite content. Bloom?eld and Covey-Crump (1993) and Covey-Crump et al. (2006) have studied the behavior of mixtures, at different proportions of halite and calcite in uniaxial compression tests. Bruhn and Casey (1997) deformed aggregates of calcite and anhydrite in compression and found that for the equal proportions aggregate the strength is lower than for the pure end-member aggregate. Barnhoorn et al. (2005) deformed a similar type of aggregate, to very large strain in torsion, and found that the admixture of a second phase can lead to strain localization, whilst the single-phase end-members deformed homogeneously. This is particularly interesting since anhydrite undergoes a switch from dislocation to diffusion creep from low to high strain, but this did not lead to strain localization (Heidelbach et al., 2001). Delle Piane et al. (2009) deformed aggregates of ?negrained calcite and mica in torsion, where mica behaved as hard phase and calcite as soft. At high strain calcite was segregated in high strain zones where deformation localized. Common to all previous works mentioned above is the similar grain size of the constituents and the use of metal jackets. In the present experiments the samples were porphyritic, with calcite porphyroclasts up to two times coarser than the halite groundmass. This places the used samples in end-member type 3 of Handy (1990). As discussed below, this could be in part responsible for the differences in microstructure and rheology observed between the present and previous experimental results. In addition, the use of metal jackets makes it very difficult to compare rheology with the present experiments, because polymer jackets do not interfere with sample resistance to flow. With the present experiments, we intended to find answers to the following questions: (1) what is the behavior of each phase in the aggregate? (2) How do they interfere? (3) What are the differences in mechanical properties between the two-phase aggregate and single-phase halite? What is the role of calcite? (4) Does strain localize in the halite coarse calcite aggregate? 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