LAUSR

Abstract Although mantle rheology is one of the most important properties of the Earth, how a radial mantle viscosity structure affects lithosphere dynamics is still poorly known, particularly the role of the lithosphere, asthenosphere, transition zone, and D" layer viscosities. Using constraints from the geoid, plate motions, and strain rates within plate boundary zones, we provide important new refinements to the radial viscosity profile within the key layers of the lithosphere, asthenosphere, transition zone, and D" layer. We follow the approach of the joint modeling of lithosphere and mantle dynamics (Ghosh and Holt, 2012; Ghosh et al., 2013b, 2019; Wang et al., 2015) to show how the viscosities within these key layers influence lithosphere dynamics. We use the viscosity structure SH08 (Steinberger and Holme, 2008) as a starting model. The density variations within the mantle are derived from the tomography models which, based on prior modeling, had provided a best fit to the surface observables (Wang et al., 2015). Our results show that narrow viscosity ranges of moderately strong lithosphere (2.6–5.6 × 1022 Pa-s) and moderately weak transition zone (5–9.3 × 1020 Pa-s), as well as slightly large ranges of moderately weak asthenosphere (5–34 × 1019 Pa-s) and D" layer (4.8–18 × 1020 Pa-s), are necessary to match all the surface observables. We also find that a very strong lithosphere (>8.6 × 1022 Pa-s) along with a weak asthenosphere (