Effects on post-glacial rebound from the hard rheology in the transition zone

Abstract

We analyse the influences of a viscosity increase in the transition zone between 420 and 670 km on the geophysical signatures induced by post-glacial rebound, ranging from the perturbations in the Earth's rotation to the short wavelength features associated with the migration of the peripheral bulge. A self-gravitating model is adopted, consisting of an elastic lithosphere, a three-layer viscoelastic mantle and an inviscid core.\ud \ud The horizontal displacements and velocities and the stress pattern are extremely sensitive to the viscosity increase and to the chemical stratification of the transition zone. The hardening of the upper and the chemical density jumps in mantle below the 420 discontinuity induces a channel effect which contaminates the horizontal deformation both in the near-field and in the far-field from the ice-sheets. These findings indicate that intraplate geodetic data can be used to put bounds on the viscosity increase in the transition zone and on the amount of chemical stratification in the mantle.\ud \ud The stress field induced in the lithosphere by the Pleistocenic ice-sheet disintegration is a very sensitive function of mantle viscosity stratification. The existence of seismic activity along passive continental margins of previously glaciated areas requires a substantial viscosity increase in the mantle, with the viscosity of the transition zone acting as a controlling parameter. A viscously stratified mantle is responsible for a delayed upward migration of stress in the lithosphere which can account for the seismicity today

Mantle Rheology, Viscomagnetic Coupling At the Core-Mantle Boundary and Differential Rotation of the Core Induced By Pleistocenic Deglaciation

“…The influence of this zone, essentially composed of garnet (Anderson & Bass 1986), on the axial rotation of the Earth has already been investigated (Hong, Yuen & Wu 1990;Spada et al 1992). Thus we focus on the influence of this layer on the radial displacement at the CMB and on the rotation of the core induced by the last deglaciation.…”

“…If we denote by V,,, the zonal degree two rotational external potential, P;" the zonal rotational pressure in the core, with the superscript 'c' denoting the core, and V;, a surface zonal loading potential, x(a, A) and u,(b, A) will be written in the frequency domain where the Love numbers k , k' and k , denote the geopotential perturbations in non-dimensional form induced by the potentials V2(,, V;, and zonal rotational pressure P;, and where g is the gravity at the Earth surface, and pN the density of the fluid core. h", hI and h" denote the Love numbers corresponding to the radial displacement at the CMB induced by V2,), P& and V&y Peltier (1974) and recently Spada et al (1992) have shown that the viscoelastic Love numbers have the following frequency form (e.g.…”

Long-term polar motion on a quasi-fluid planetary body with an elastic lithosphere: Semi-analytic solutions of the time-dependent equation

“…As mentioned above, we have to calculate the Love numbers themselves and the parameters linked to the Love numbers such as T 1 and a 0 . In this study, the Love numbers are calculated on the basis of the analytic approach by using the fundamental matrix (e.g., Spada et al, 1992b;Vermeersen et al, 1996) for an incompressible multi-layered structure. The Love numbers are determined based on internal structure variables, including density, elasticity, and viscosity profiles.…”

Lamé's problem of gravitational viscoelasticity: the isochemical, incompressible planet

“…Special solution functions for models similar to ours have been published by Wu & Peltier (1982), Spada et al (1992) and Amelung & Wolf (1994). Here, we inspect the formulae in the (r, n, s) domain given by the first authors, which apply to a model identical to ours.…”