Two postglacial uplift curves of simple exponential form are defined for separate parts of the Søndre Strømfjord area by the altitudes and C14 ages of 21 in situ marine shell samples, each of which is ecologically or stratigraphically related to a strandline. (Mean deviations between data and statistically best-fit curves are less than 165 yr or 5.5 m) One curve represents the zone of maximum uplift, ∼125 km west of the present ice sheet margin, which has risen 145 ± 5 m since ∼8,800 yr B.P. The initial uplift rate of about 105 m per 1,000 yr is the highest reported for Greenland. The other curve represents a zone ∼50 km west of the present ice sheet, where 75 ± 5 m of postglacial uplift began ∼7,300 yr B.P. at an initial rate of about 60 m per 1,000 yr.

The Søndre Strømfjord uplift data and two sets of data from East Greenland define four exponential uplift curves with remarkably consistent exponential constants (0.7 to 0.8). The mathematical and physical consistency of these four curves has the following major implications: (1) postglacial uplift has been a relatively simple isostatic process, and has occurred at an exponentially decreasing rate that is a direct function of glacio-isostatic disequilibrium; (2) the half-life of isostatic recovery is 870 to 990 yr, and the relaxation time is 1,250 to 1,430 yr, which yields an effective viscosity between 4 × 1020 and 4 × 1021 poises for the earth materials involved in isostatic compensation; (3) these constants, derived from exponential uplift curves, do not appear to vary as a function of ice-margin retreat rates. This result indicates that the crust is very flexible laterally, and together with other data, suggests that the flexural parameter of the lithosphere may be as low as 20 km.

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