The rheological structure of the earth's mantle was determined based on data from the postglacial isostatic adjustment in Laurentide. According to a linear analysis with a Newtonian rheology, the apparent viscosity derived from the observed relative sea level data is about ten times larger in the central part of the glaciated region than that in the surrounding region. Namely, the apparent viscosity has spatial dependence. The observed relation [formula omitted] in Laurentide and Fennoscandia, where [formula omitted] and ζ respectively represent uplift rate and estimated remaining uplift, does not reflect the nonlinearity of the Theological property of the earth's mantle. Rather, the observed relation [formula omitted] means that the lower mantle viscosity is greater than 1024 poise, regardless of Newtonian or non-Newtonian rheology. According to the analysis for a thin channel viscosity model with a power-law creep rheology [formula omitted], where [formula omitted] and σ; respectively represent strain rate and deviatoric stress, the observed relative sea level variations and free-air gravity anomalies in the glaciated region in Laurentide can be explained almost satisfactorily. The same model is also consistent with the observed relation [formula omitted] Our calculation therefore indicates that the viscosity of the lower mantle is so high that a thin channel viscosity model is a good approximation of the mantle flow, and that the upper mantle rheology is governed by the power-law creep law with n=3 [formula omitted]. The average temperature, strain rate, deviatoric stress, and apparent viscosity estimated in the present work are 1,500 K to 1,700 K, 2.8/H2 sec-1, 0.22H bar, and 0.04H8 poise, respectively, where H represents the thickness of the low viscosity channel in cm. The relationship between strain rate and deviatoric stress is consistent with an extrapolation of high strain rate laboratory data.
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)