The Influence of Rheology on the behaviour of thermal plumes (Invited)
Abstract
The bulk Earth's mantle is assumed to behave as a Newtonian fluid, with a strongly temperature dependent viscosity. However at least the top cold thermal boundary layer, i.e. the lithosphere, presents a much more complex mechanical behaviour, where the viscosity may not only depend on temperature, but also on shear rate and on local stress. While the origin of the mantle complex rheology has been largely discussed, especially since plate tectonics cannot be produced out of convection in a Newtonian mantle, the physical understanding of how such a rheology influences the dynamical behaviour of thermal instabilities, such as mantle plumes and diapirs in a complex mantle, remains sparse. To study this question, we performed laboratory experiments on thermal instabilities out of a localized heat source. The fluid employed is Carbopol, which presents a yield stress, whereby irreversible deformation occurs only if the local stress exceeds a critical value, and a shear thinning behaviour with a shear thinning exponent around n=2. We show that two local non-dimensional parameters determine whether the instability may rise. The first parameter is the ';yield number' Psi, which compares the stresses due to the buoyancy of the hot pocket formed around the heater to the yield stress of the fluid. Slow convection in this hot pocket then involves the presence of a shear rate, which is therefore creating viscous stresses. The ratio of the yield stress to these viscous stresses is defined as the ';Bingham number' Bi. As soon as the buoyancy induced stress is large compared to the yield stress (Ψ > 6.85) and the viscous stresses become larger than the yield stress (Bi ≤ 1), the plume may rise. By the same token, the plume stops as soon as one of the two parameters becomes subcritical. We show, that thermal instabilities in such a fluid behave fundamentally differently from those emplaced in a Newtonian fluid. In a Newtonian fluid, a hot - and less viscous - instability rises with a big head followed by a thin stem, whereas a cold - and therefore more viscous - instability presents a finger-like shape. A hot plume in a yield-stress and shear thinning fluid presents strong localization of deformation at the edges of the thermal instability. Due to the rheology of the fluid, this leads to a less viscous region at the edges of the plume, i.e. the interior of the plume is much more solid than the fluid at its edges, which acts as a lubricating layer. The less viscous edges and the more viscous interior of the instability exhibit a finger-like shape. Our study therefore shows that as soon as the mechanical behaviour of the rocks changes, the dynamic of plumes and diapirs is expected to change as well. An upwelling that is either not buoyant enough or cannot create strong enough viscous stresses, will not be able to rise if the rheology is non-Newtonian. Therefore our study gives dynamical constraints for i) the maximum yield stress that would allow for an instability of a given size with a given buoyancy difference to rise in the Earth's mantle and ii) an order of magnitude for the maximum viscosity at a given shear rate and shear thinning exponent
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFMDI13A..01M
- Keywords:
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- 8121 TECTONOPHYSICS Dynamics: convection currents;
- and mantle plumes;
- 8120 TECTONOPHYSICS Dynamics of lithosphere and mantle: general;
- 8162 TECTONOPHYSICS Rheology: mantle;
- 8164 TECTONOPHYSICS Stresses: crust and lithosphere