The Mid-continent Tilt Project observes present-day crustal tilting in Manitoba and contiguous regions to the south in the U.S. This will help Manitoba Hydro understand long term water level changes, which is important to their assessment of outflow capacity and environmental issues, including shoreline erosion. It will also provide information to assist with the development of a computer model of the most recent North American (Laurentide) ice sheet.

The main tools used to observe crustal deformation are the Global Positioning System (GPS) and absolute gravity. The dots in the figure show four GPS stations where continuous observations are being made. Absolute gravity observations are being made, twice yearly, at the four GPS stations and also at three other intermediate sites (diamonds).

The contours in the figure show the vertical crustal motion predicted by the ICE-3G postglacial rebound model. Although this model is constrained by relative sea level (RSL) observations near Hudson Bay, it is less well constrained in the interior. Crustal motion observations from this project will help the ongoing refinement of models such as ICE-3G.

Location of GPS and absolute gravity stations for Mid-continent Tilt Project All GPS stations are also absolute gravity stations Uplift rates in mm /yr predicted by ICE-3G postglacial rebound model are contoured in red larger image [GIF, 20.6 kb, 452 X 464, notice]

Terrain Sciences Division of the Geological Survey of Canada (GSC) is contributing to this project through mapping of strandlines, or relict beaches, that were formed around the large lakes that were present when the glaciers were retreating. Improved mapping provides important information on the history of tilting, which leads to better understanding the conditions that led to the present situation.

Two U.S. agencies, National Aeronautics and Space Administration (NASA) and National Oceanic and Atmospheric Administration (NOAA), are partners in this project. NASA has loaned two GPS receivers and NOAA carries out the spring absolute gravity observations. With financial assistance from Manitoba Hydro, GSC runs the fall absolute gravity campaigns, maintains the Flin Flon and Pinawa GPS stations, and collects, processes, and interprets the GPS data. The Churchill GPS station is operated by the Geodetic Survey of Canada, and the North Liberty GPS station is operated by the Jet Propulsion Laboratory, NASA.

Plot of absolute gravity values from 1987 to 1999 at Churchill, Manitoba and from 1988 to 1999 at International FAlls, Minnesota. (1 microGal = 10 nanometers/s2. The vertical bars denote the estimated 67% error level. Note the improvement in the precision of absolute gravity measurements in recent years. larger image [GIF, 363.3 kb, 882 X 1295, notice]

Absolute gravity measurements have been made for over ten years at Churchill, Manitoba and International Falls, Minnesota in a combined effort by the Geological Survey of Canada (assisted by the Geodetic Survey) and by the U.S. National Oceanic and Atmospheric Administration. The data up to 1999 show a decrease in gravity at a rate of 2.1 microGal/year at Churchill and 0.9 microGal/year at International Falls, equivalent to a land uplift rate of 14 millimeters/year and 6 millimeters/year, respectively. Falling sea level at Churchill, due to the rise of the land surface in delayed response to the removal of the massive Laurentide ice sheet, has been known and understood for many years. Now, using techniques such as absolute gravimetry, the rate of land rebound can be measured in the continental interior far from the ocean.