Modelling postglacial sea levels with power-law rheology and a realistic ice model in the absence of ambient tectonic stress

Abstract

Previous studies of postglacial rebound with power-law rheology and simple ice models over Laurentia have found that sea-level data in the transition zone (e.g. Boston) cannot be easily explained unless non-linear rheology is restricted to lie within a thin layer between the lithosphere and the upper mantle. The reason is the existence of a viscously stationary zone, which is characteristic of non-linear mantles. In this study, the simple ice model is replaced by a realistic ice history adapted from the ICE-3G model, which takes into account important effects such as retreating ice margins and eustatic ocean loading. A 3-D finite element model is used to calculate the deformation and the relative sea levels in eastern Canada and along the US east coast. The purpose is to investigate whether the realistic load model allows sea-level data near the centre of rebound and along the Atlantic east coast of Canada and the USA to be explained by non-linear rheology simultaneously. The assumptions made are that: (1) postglacial rebound sees the steady-state creep and not the transient creep; (2) either ambient tectonic stresses do not interact with rebound stress or the ambient stress level is low; (3) lateral heterogeneity of rock properties and anisotropic flow can be neglected. Results indicate that the retreating ice margin does make it easier for non-linear rheology in the mantle to explain the sea-level data in the transition zone around Boston. However, reconciling all the sea-level data near the centre of rebound and along the Atlantic east coast simultaneously is still difficult for a non-linear rheology. So far, two types of models with non-linear rheology have been found to be able to explain the sea-level data inside and outside Laurentia simultaneously. They are: (1) earth models with a thin (≤270 km) non-linear zone overlying a 1021 Pa s lower mantle; (2) earth models with a linear upper mantle with an average viscosity of 1021 Pa s overlying a non-linear lower mantle with A* = 3 × 10−35 Pa−3 s−1 and n = 3.