Power-law viscous materials for analogue experiments: New data on the rheology of highly-filled silicone polymers D. Boutelier * , C. Schrank, A. Cruden Department of Geology, Earth Sciences Centre, University of Toronto, 22 Russell Street, Toronto, ON M5S 3B1, Canada Received 6 April 2007; received in revised form 5 September 2007; accepted 12 October 2007 Available online 19 November 2007 Abstract The selection of appropriate analogue materials is a central consideration in the design of realistic physical models. We investigate the rhe- ology of highly-filled silicone polymers in order to find materials with a power-law strain-rate softening rheology suitable for modelling rock deformation by dislocation creep and report the rheological properties of the materials as functions of the filler content. The mixtures exhibit strain-rate softening behaviour but with increasing amounts of filler become strain-dependent. For the strain-independent viscous materials, flow laws are presented while for strain-dependent materials the relative importance of strain and strain rate softening/hardening is reported. If the stress or strain rate is above a threshold value some highly-filled silicone polymers may be considered linear visco-elastic (strain independent) and power-law strain-rate softening. The power-law exponent can be raised from 1 to w3 by using mixtures of high-viscosity silicone and plas- ticine. However, the need for high shear strain rates to obtain the power-law rheology imposes some restrictions on the usage of such materials for geodynamic modelling. Two simple shear experiments are presented that use Newtonian and power-law strain-rate softening materials. The results demonstrate how materials with power-law rheology result in better strain localization in analogue experiments. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Rheology; Analogue modelling; Power law creep; Strain localization 1. Introduction Plate tectonics implies the operation of efficient strain localization processes in order to produce and maintain litho- spheric shear zones. Localization phenomena can be generated or initiated within the brittle part of both oceanic lithosphere and continental crust as well as in the uppermost continental lithospheric mantle that is potentially brittle (Brace and Kohl- stedt, 1980). Strain localization is also observed in ductile shear zones but the mechanisms contributing to this process are not yet completely understood. Localization in the ductile regime may be promoted by strain weakening mechanisms such as dynamic recrystallization or shear heating (Poirier, 1980; White et al., 1980; Monte ´si and Zuber, 2002). However, the weakening functions associated with these mechanisms are still debated (Rutter, 1999; De Bresser et al., 2000; Monte ´si and Hirth, 2003). In the lowermost mantle lithosphere, viscous creep by dislocation glide is believed to be the dominant de- formation mechanism, while deeper in the upper mantle, dis- location creep may in turn be replaced by diffusion creep. The conditions (depth, temperature) for this transition are still not well determined (Karato and Wu, 1993). If deformation is accommodated by dislocation or diffusion creep, no strain weakening is expected. However, dislocation creep is a strain- rate softening mechanism, which promotes system weakening (Hobbs et al., 1990; Rutter, 1999). This is because diffusion creep has a Newtonian constitutive relation with strain rate pro- portional to stress, while for dislocation or power-law creep the strain rate is proportional to an exponential power of the applied stress, with the exponent being w3e4. The development of a system instability (e.g., necking, folding) around a composi- tional or geometrical heterogeneity is thus promoted by disloca- tion creep because of the higher sensitivity of the strain rate to stress (Hobbs et al., 1990; Rutter, 1999). Simply, a local stress * Corresponding author. Tel.: þ1 416 978 0833; fax: þ1 416 978 3938. E-mail address: boutelier@geology.utoronto.ca (D. Boutelier). 0191-8141/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2007.10.009 Journal of Structural Geology 30 (2008) 341e353 www.elsevier.com/locate/jsg