Regional mantle viscosity constraints for North America reveal upper mantle strength differences across the continent

Abstract

Abstract. We present regional constraints of mantle viscosity for North America using a local Bayesian joint inversion of mantle flow and glacial isostatic adjustment (GIA) models. Our localized mantle flow model uses new local geoid kernels created via spatio-spectral localization using Slepain basis functions, convolved with seismically derived mantle density to calculate and constrain against the regional free-air gravity field. The joint inversion with GIA uses two deglaciation of ice sheet models (GLAC1D-NA and ICE-6G-NA) and surface relative sea level data. We solve for the local 1D mantle viscosity structure for the entire North America (NA) region, the eastern region including Hudson Bay, and the western region of North America extending into the Pacific plate. Our results for the entire NA region show one order of magnitude viscosity jump at the 670 km boundary using a high seismic density scaling parameter (e.g., δlnp/δlnvs = 0.3). Seismic scaling parameter demonstrates significant influence on the resulting viscosity profile. However, when the NA region is further localized into eastern and western parts, the scaling factor becomes much less important for dictating the resulting upper mantle viscosity characteristics. Rather the respective local mantle density heterogeneities provide the dominate control on the upper mantle viscosity. We infer local 1D viscosity profiles that reflect the respective tectonic settings of each region's upper mantle, including a weak and shallow asthenosphere layer in the west, and deep sharp viscosity jumps in the eastern transition zone, below the suggested/proposed depth range of the eastern continental root.