The Viscosity of Earth's Mantle: Newtonian or non-Newtonian

Thursday, March 21, 2002 - 9:10am - 10:10am
Keller 3-180
W. R. Peltier (University of Toronto)
Knowledge of the viscosity of Earth's iron-magnesium-silicate mantle is vital insofar as understanding tectonophysical processes is concerned. In particular the strength as well as the style of the mantle convection process is fundamentally controlled by the magnitude of the momentum diffuesivity for a given temperature difference between the core-mantle boundary and the Earth's surface. A significant issue furthermore remains as to whether the mechanism by which mantle material flows is non-Newtonian, as would be expected given the polycrystalline nature of the material involved, or whether it might be effectively Newtonian in consequence of the low differential stress regime in which mobility occurs.

One possible means of probing the Newtonian-ness of Earth's mantle is to employ a variety of solid Earth physical phenomena which depend upon the magnitude of the creep resistance to see whether observations over a significant range of phenomenological timescales are satisfied by the same model of the creep resistance. To this end it proves interesting to compare the mantle viscosity inferred on the basis of the relatively fast timescale glacial isostatic adjustment process to that inferred on the basis of analyses of the process of mantle convection, these two processes differing from one-another in characteristic timescale by five orders of magnitude ( the characteristic timescale of glacial isostatic adjustment is O(1000 yrs) whereas the characteristic timescale of the convection process is O(100,000,000 yrs)).

There are several issues that one must address in attempting to carry out a meaningfull intercomparison of this kind, not the least important of which is that little concensus exists as to the model for viscosity that best reconciles the observations of either of these fundamental geodynamic processes! I will first present the results of a new series of analyses of the radial variation of mantle viscosity based upon anlyses of the observables related to the glacial isostatic adjustment process, analyses which directly probe the relative viability of the two models in the current literature that have been suggested as candidates, one of which has a relatively modest increase of viscosity across the 660 km seismic discontinuity and the competing model that has a much larger increase across this same horizon. This analysis will make use of recent space geodetic constraints as well as absolute gravimeter measurements of g-dot on a traverse across the Canadian Shield. These analyses will be shown to entirely rule out the model with high viscosity contrast across the 660 km discontinuity.

It proves useful to enquire as to whether the model of the depth variation of Newtonian viscosity delivered by these analyses of the process of glacial isostasy is also able to deliver accord with observable properties of the mantle convection process. Neglecting all influence of the surface plates, it is found that a priori models of mantle mixing, in which the cmb temperature is fixed to the high value required by high pressure experimental constraints, inevitably predict anomalously high radial heat transfer unless the flow is assumed to be very strongly layered by the influence of the endothermic phase transformation at 660 km depth. Alternatively, one may assume that the viscosity that governs the process of mantle mixing is approximately one order of magnitude higher than that which governs glacial isostasy. Unless the heat transfer inhibiting influence of the surface plates is more significant than is most often assumed, this constitutes a strong argument that Earth's mantle creeps via a mechanism that is non-Newtonian.