EFFECTS OF MANTLE FLOW LAW STRESS EXPONENT ON POSTGLACIAL INDUCED SURFACE MOTION AND GRAVITY IN LAURENTIA

Abstract

Recent studies show that power-law rheology with stress exponent n = 3 in the lower mantle or in a thin zone just beneath the lithosphere are consistent with the sea level observations in and around Laurentia. In this study, the stress exponent n is varied from 2 to 4, and for each value of n, a range of creep parameter A * is searched to find the earth model that gives the best fit to the sea level observations in and around Laurentia. The effects of stress exponent on other geophysical observables-namely present day uplift rate, horizontal velocity, free-air gravity and the rate of change in gravity are also studied. The model used to calculate the postglacial readjustment of the Earth is a 3-D finite element model that includes realistic ice histories and eustatic water loads on stratified incompressible viscoelastic Maxwell flat-earths. Three types of earth rheology models are considered-the first one has nonlinear rheology throughout the mantle and the second has a nonlinear zone at the top of the mantle below the elastic lithosphere and a linear mantle. The third one has nonlinear rheology restricted to lie in the lower mantle. It is found that the models with nonlinear rheology in the lower mantle give better fits to the sea level data in and around Laurentia than the first two types. However, sea level data, when taken as a whole, cannot clearly discriminate between the stress exponents 2, 3 or 4. Present day uplift rate, horizontal velocity and rate of gravity change however may be useful in further discriminating mantle rheology.