Rheological structure of the earth's mantle derived from glacial rebound in Laurentide.

Abstract

The theological 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 ζ∝ζ3-4 in Laurentide and Fennoscandia, where ζ and ζ respectively represent uplift rate and estimated remaining uplift, does not reflect the non-linearity of the rheological property of the earth's mantle. Rather, the observed relation ζ∝ζ3-4 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 e∝σ3, where e 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 ζ∝ζ3-4. 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 (e∝σ3). The average temperature, strain rate, deviatoric stress, and apparent viscosity estimated in the present work are 1, 500K to 1, 700K, 2.8/H2sec-1, 0.22H bar, and 0.04H3 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.