Mantle Rheology Across Tidal to Decadal Timescales
Abstract
Long-period deformation of the solid Earth provides a unique window into mantle rheology between seismic and glacial-isostatic-adjustment (GIA) timescales, where constitutive behavior remains poorly constrained. Laboratory experiments increasingly support broad transient anelastic relaxation, while geodynamic studies disfavor the extremely low steady-state viscosities that would otherwise be required to explain some long-period observations. The Extended Burgers Material (EBM) offers a linear viscoelastic framework that captures this transient relaxation while asymptotically approaching Maxwell behavior at long timescales.Using newly developed Love-number capabilities in the Ice-sheet and Sea-level System Model (ISSM), we compile and model complex degree-2 tidal Love-number constraints k, h, and l from satellite laser ranging, satellite gravity, Earth-rotation observations, GNSS, and DORIS over periods from 12 hours to 18.6 years. We first test whether the observations can be explained by radial steady-state viscosity structure alone using a large ensemble of flexible layered Maxwell models. These models reproduce much of the gravitational and radial response, but systematically fail to reproduce the real horizontal response, even when very low viscosities are permitted in the lowermost mantle or introduced in the transition zone and asthenosphere.We then explore EBM rheologies with distinct upper- and lower-mantle properties. These models reproduce the joint frequency dependence of Love numbers significantly better while retaining steady-state viscosities compatible with GIA constraints. The results point to an important role for transient lower-mantle relaxation, although correlations between relaxation amplitude and timescale, and the long-period cutoff of the transient response, remain incompletely resolved. Better constraints on this behavior are directly relevant to GRACE and GRACE-FO, as well as GNSS, SLR, DORIS, altimetry, and other remote-sensing observations that depend on accurate modeling of solid-Earth deformation across overlapping timescales.© 2026 California Institute of Technology. Government Sponsorship Acknowledged.