Abstract
Stress-dependent nonlinear upper mantle rheology has a firm base in rock mechanical tests, where this nonlinearity results from dislocation creep of minerals. In the last few decades there has been some attention to nonlinear, power-law, materials for application in scaled analogue experiments for tectonic processes. However, studies describing the rheology of analogue materials with the same nonlinear dependency on stress as observed for lithospheric mantle materials at relevant stress levels, are still lacking.In this study we have developed and rheologically tested materials based on combinations of silicone polymers and plasticine, with the aim of obtaining a material that can serve as a laboratory analogue to the power-law rheology of olivine aggregates at lithospheric mantle conditions. From our steady-state creep tests we find that it is possible to obtain such a power-law material, with effective viscosities over relevant model stress ranges [5–4000 Pa] that allow for nonlinear deformation at laboratory time scales.We apply the developed material to a process where localized deformation of the lithosphere can be expected: slab break-off. We study this process using analogue models, where we apply the new nonlinear material to the lithospheric mantle domains, while we use Newtonian glucose to represent the low viscous asthenosphere. Now that we properly manage power-law behavior in our analogue lithosphere materials, we are able to model localized lithospheric tearing.
Highlights
Permanent deformation of the Earth’s lithosphere can occur in brittle or ductile modes, where the latter prevails for increasing temperatures and depths (Goetze and Evans, 1979; Kohlstedt et al, 1995)
For ductile steady-state deformation we can discriminate between Newtonian rheology, where strain rate is proportional to stress and viscosity is constant, and nonlinear rheologies, where viscosity has a nonlinear dependency on stress (Ranalli, 1995)
In our paper we develop materials based on silicones and plasticine mixtures - informed by previous works of Weijermars (1986); Sokoutis (1987); Zulauf and Zulauf (2004) and Boutelier et al (2008) - and study their behavior to determine how these can serve as mechanical analogs for nonlinear ductile creep in the lithospheric mantle
Summary
Permanent deformation of the Earth’s lithosphere can occur in brittle or ductile (viscous) modes, where the latter prevails for increasing temperatures and depths (Goetze and Evans, 1979; Kohlstedt et al, 1995). Ductile deformation involves a permanent change of shape without fracturing, and can be caused by various microphysical creep mechanisms, each of which results in deformation rates with different dependencies on external conditions such as stress or temperature, and microstructure of the material (Ranalli, 1995). In settings with highly varying stresses, power-law creep may result in high rates of deformation, as the strain rate will localize stronger than the stress itself. Such a system weakening will be especially efficient when it results in geometrical changes of tectonic units such that stress becomes increasingly localized by a positive feedback, as is observed in slab necking or tearing processes in numerical and analytical models (Andrews and Billen, 2009; Schmalholz, 2011). In combination with strain softening a strongly stress-dependent rheology is a precondition for stable shear zones (Rutter, 1999)
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