Uplift in southeast Alaska
The fastest measured rates of uplift in the world today are in southeast Alaska: in Glacier Bay National Park and east of Yakutat (Fig. 1). The first measurements of this rapid uplift were done the mid-20th century through tide gauge studies, which suggested that land in the Glacier Bay region was emerging at 30 mm/yr (Hicks and Shofnos, 1965). However, the cause of the uplift was still being debated when we initiated our investigations in 1998. To determine whether this uplift was driven by tectonics or glacial rebound we embarked on a field program that included 1) repeating and reviewing the original tide gauge measurements; 2) measuring the current rates of uplift with modern GPS geodetic techniques; 3) defining the regional pattern of uplift; 4) determining when this episode of uplift began; and 5) assessing the total amount of uplift that has occurred.
The uplift pattern documented here spans an area of over 100,000 square km, centered on the coastal mountains along the Gulf of Alaska. The data set depicts a regional pattern of uplift, with peaks of 30-32 mm/yr centered over upper Glacier Bay and Yakutat Icefield. The peak uplift rates are found in regions that have experienced the highest rates of ice loss. Raised shorelines that date back to 1770 ± 20 AD indicate total sea level fall in the range of 1.0 to 5.7 m. The onset of uplift measured at the raised shoreline sites correlates with when the Glacier Bay Icefield began its dramatic collapse. GIA modeling results provide robust constraints on lithospheric elastic thickness, asthenosphere thickness and asthenosphere viscosity (Larsen et al., 2005). The simultaneous onset of unloading and sea level change is a direct observation of the causal relationship between glacial unloading and the region’s uplift. Climate changes rather than tectonic forces have primarily forced these regional sea-level changes.
These adjustments to LIA glacier loading and unloading are producing significant stresses on the earth’s crust in Glacier Bay, which can affect seismicity and regional tectonics. The rising land is also continually changing the shorelines and geomorphic texture of shoreline throughout the Park and causing changes in hydrologic patterns, erosion and sedimentation. All these changes have a direct impact on the ecosystems of the Park.
An overview article on our uplift studies was published in the Alaska Park Science Journal (Vol. 6, Issue 1). You can download a pdf of just our article here.
This project is supported by the National Science Foundation Tectonics Program (EAR-0408801). GPS survey equipment and data archiving is provided by UNAVCO.
