HORIZONTAL MOTIONS DUE TO POSTGLACIAL REBOUND

North American horizontal post‐glacial rebound velocities are computed for three five‐layered, incompressible, spherical Maxwell earth models, using a simple glacial loading cycle centered on Hudson Bay. The reference earth model has a 120‐km thick elastic lithosphere and upper‐ and lower‐mantle viscosities of 1021 Pa·s and 2 × 1022 Pa·s, respectively. The second model has a lower viscosity upper‐mantle (5 × 1020 Pa·s) and a higher viscosity lower‐mantle (1023 Pa·s), and the third has a 250‐km thick lithosphere. The reference model and the low‐viscosity upper‐mantle model calculations are in good agreement with uplift and gravity data, although they may somewhat overestimate the present‐day central uplift rate. The thick lithosphere model fares less well because it fails to exhibit the well‐pronounced peripheral bulge sinking that is required by eastern North America tide gauge data. The reference model generally has the largest horizontal rebound velocities, reaching a maximum of 4 mm/yr towards Hudson Bay at 2400 km from the load center. The effect of a thick lithosphere or of a low‐viscosity upper‐mantle is to reduce present‐day horizontal velocities, especially in the formerly glaciated region. The sensitivity of horizontal motions to lithospheric thickness and upper‐mantle viscosity may assist in discriminating between different models of the earth's rheology once sufficient high‐quality geodetic data exist. Copyright 1990 by the American Geophysical Union.