Extreme uplift in southeast Alaska associated with Little Ice Age glacial isostatic rebound.

Christopher F. Larsen, Roman J. Motyka, Jeffrey T. Freymueller, and Keith A. Echelmeyer.

Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Dr, Fairbanks, Ak 99775.

Abstract

Extreme uplift and sea level changes in southeast Alaska have been documented by 45 Global Positioning System (GPS) measurements, 18 tide gauge measurements, and studies of raised shorelines at 16 sites. The present magnitude and distribution of regional sea level rates in the Glacier Bay area (up to 25 mm a-1) are similar to that found in an earlier study of tide gauge measurements (Hicks and Shofnos, 1965) but our new GPS data delineated a second center of rapid uplift east of Yakutat with peak rates of 35 mm a-1. Raised shoreline studies document total sea level change since ~1750 AD, with a peak total uplift of 5.7 m found in upper Lynn Canal. The onset of the ongoing uplift episode that raised these shorelines is coincident with the start of the collapse of the Glacier Bay Icefield. The distribution of total sea level change is in general agreement with uplift rate measurements, with greater sea level change found at the sites closest to the peak uplift rates in upper Glacier Bay.

A viscoelastic two-layer Earth model subjected to an ice load model built upon observations of glacial change was used to predict uplift rates at the tide gauge and GPS sites as well as total uplift at the raised shoreline sites. An Earth model consisting of a 60 ± 30 km thick elastic lithosphere overlaying a mantle half-space with viscosity (5.5 ± 0.5) x10¹⁹ Pa s can match the uplift rates of the tide gauge and GPS observations at the 1s level. The same model creates significant misfit if used to predict the raised shoreline data set, which can be modeled only if a mantle viscosity of (1.4 ± 0.1) x 10¹⁹ Pa s is adopted. This viscosity value conversely produces significant misfit with the GPS and tide gauge measurements. However, all three data sets are consistent with a three-layer Earth model. The combined model is constrained by a total of 77 uplift measurements, which at the 95% confidence level require a low viscosity asthenosphere (h_A = (1.4±0.3) x 10¹⁹ Pa s and thickness km) beneath a km thick elastic lithosphere and overlaying an upper mantle half space with a viscosity of 4 x 10²⁰ Pa s.