Constraints on mantle viscosity based upon the inversion of post-glacial uplift data from the Hudson Bay region

Abstract

SUMMARY We outline two parametrizations for post-glacial relative sea-level (RSL) histories associated with previously glaciated regions. The first parametrization is based on a site-dependent normalization of the RSL history, while the second involves the estimate of a site-dependent (logarithm of the) inverse decay time for the exponential-like form which characterizes these histories. Both parametrizations are shown to yield data sets which are relatively insensitive to the details of the late Pleistocene surface load history, and therefore inferences of mantle viscosity based upon them will be particularly robust. We apply the two parametrizations to consider the RSL record at a number of sites across the Hudson Bay region. In this regard our inferences (which are derived from both forward and inverse calculations) are based upon the actual RSL age-height pairs obtained by survey, rather than the highly subjective set of RSL ‘trends’adopted in previous studies. One of the main goals of the analysis is to assess the validity of a set of previously published and highly contradictory inferences of the radial profile of mantle viscosity based on the Hudson Bay RSL record. Forward analyses using models with isoviscous upper and lower mantle regions (as adopted in the vast majority of previous analyses) indicate that the parametrized versions of the RSL record in Hudson Bay, excluding data from the Cape Henrietta Maria site, are best fitted by a lower mantle viscosity near 1021 Pa s. The same conclusion holds when data from only northern Hudson Bay are considered. The RSL record in southern Hudson Bay is not self-consistent (if the error bars adopted herein are reasonable); however, the parametrized versions of the RSL curves from each site in this region can be reconciled by a model with a lower mantle viscosity somewhere in the rather moderate range 0.5–3.0 × 1021 Pa s. The value of 3 × 1021 Pa s represents a lower bound on the lower mantle viscosity required to fit the RSL records at Cape Henrietta Maria; this record is characterized by a longer decay time than those associated with other sites in the data base. This lower bound is in contrast with previous suggestions that the RSL record at Cape Henrietta Maria requires a lower mantle viscosity of 30 × 1021 Pa s. Inverse analyses described herein indicate that the RSL record from the entire Hudson Bay constrains the average viscosity in the radial region extending from the lower reaches of the upper mantle to mid-lower mantle depths; the inferences listed above are therefore more properly ascribed to this region (rather than the entire lower mantle). We have found that the data are sensitive to moderately deeper variations in the radial viscosity profile as one considers sites situated further north in Hudson Bay. This provides one means for a spherically symmetric model to yield decay times which vary across the Hudson Bay; however, the data do not provide an unambiguous requirement for such a variation.