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

Abstract

S U M M A R YWe 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 10'' Pas. 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 X 10" Pa s. The value of 3 X 10" 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 X 10" 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.

A Reanalysis and Reinterpretation of Geodetic and Geological Evidence of Glacial-Isostatic Adjustment in the Churchill Region, Hudson Bay

“…proposed by Peltier (1998;1999) for region SE are markedly smaller than previous estimates for region RG (Peltier, 1994;Mitrovica and Peltier, 1995;Peltier, 1996). Similar to Mitrovica et al (2000), they and Hager used the RSL data without correction, but limited the time window to 6.8 ka, resulting in a relaxation time of (6.3 ± 1.0) ka for region RG.…”

Radial profile of mantle viscosity: Results from the joint inversion of convection and postglacial rebound observables

“…There are a number of problems associated with this procedure, particularly in regard to our present effort at establishing decay times. First, large temporal gaps exist in the geological data, and this requires extensive (and subjective) interpolation [see Mitrovica and Peltier, 1995]. Second, the decay times associated with the RSL trends may not be an accurate representation of the true observational constraints.…”

The dynamics of Cenozoic and Mesozoic plate motions

“…For example, the primary constraint on mantle viscosity remains observations of Fennoscandian and Laurentide postglacial rebound [Haskell, 1935;Nakada and Lambeck, 1989;Mitrovica and Peltier, 1995], and although some controversy remains concerning the gross radial viscosity structure of the mantle [e.g., Nakada and Lambeck, 1989; Tushingham and Peltier, 1992], all lines of evidence seem to point toward a highly viscous lower mantle [Mitrovica and Forte, 1997]. In fact, Mitrovica [1996] has shown that the disagreements within the community may be traced to a misinterpretation of the Haskell [1935] constraint and that rebound data are indeed consistent with a highly viscous lower mantle.…”