GIA‐Induced Mantle Circulation and Surface Horizontal Motions in North America: Effects of 3D Laterally Varying Elastic Thickness and Viscosity

Abstract The horizontal surface motions of the Earth's crust induced by glacial isostatic adjustment (GIA) are highly sensitive to lateral variations in Earth properties. We construct 3D GIA models by incorporating both varying elastic lithospheric thickness and lateral viscosity variations in the asthenosphere in a global, compressible, self‐gravitating, viscoelastic Earth model. This allows us to investigate how geological structures such as the North American continental craton with thick lithosphere and stiff upper mantle and oceanic plate regions with thinner lithosphere and weak asthenosphere influence mantle flow during and after the last deglaciation and how this relaxation is expressed in surface motions. While vertical crustal motions are always in phase with the ice loading change, horizontal motions can be out of phase with the glaciation history and vertical motions. The delay in response to deglaciation and the timing of the direction reversal after deglaciation is complete are controlled by the interplay between elastic lithospheric flexure and the mantle viscous drag on the base of the lithosphere. The weak asthenosphere channels the mantle return flow more horizontally and the stiff cratonic root directs the flow more vertically, forming a “box‐like” circulation beneath the former ice cover. Such complex interactions may be better constrained with joint geodetic and seismic anisotropy analysis and provide new perspectives for GIA‐ice sheet interactions as well as the interpretation of GIA associated seismotectonics.