The unloading of the earth's surface associated with the melting of the Laurentide ice sheet has resulted in present-day vertical velocities of the order of 10 mm/yr in the Hudson Bay - James Bay region as well as horizontal velocities of 2 mm/yr and tilting of 1-2 mm/100 km/yr in central and eastern Canada and the north eastern U.S.. The development of an accurate global model for Laurentide postglacial rebound has important implications in a number of scientific areas, including earthquake and climate studies. An accurate global model that predicts the on-going changes in the height of the land surface, including the ocean bottom, is needed in order to estimate contemporary changes in ocean volume. The rate of tilting of the land surface in central North America is important for explaining long-term water level changes and flow rates in the Great Lakes basin and the Hudson Bay watershed, the site of major hydro-electric projects.
Present models predicting Laurentide postglacial rebound are global in extent but are based largely on relative sea level data restricted to locations near the ocean coast. Geological evidence on the past configuration of the Laurentide ice sheet is constantly being re-evaluated and the surface loading and unloading models are becoming more realistic. Consequently, present-day crustal velocities predicted by recent models of Laurentide postglacial rebound differ by as much as 40% in places. In cooperation with the Canadian Geologic and Geodetic Surveys, we have developed a plan for direct measurement of contemporary rates in central North America using the newly-developed, high precision, geodetic techniques of GPS and absolute gravimetry. The establishment of two new continuously operating GPS receivers to work in conjunction, with existing GPS stations in Canada and the U.S. is proposed (See Map for site locations) . The new stations would be established in the area of maximum gradient of vertical crustal movement rate in central North America. Semi-annual absolute gravity measurements are proposed at four continuously-operating GPS stations and three additional, intermediate stations in the Manitoba/Central U.S. area. This will provide an independent verification of the GPS results. The geodetic observations will be used as constraints in an updated Laurentide postglacial rebound model. (Text and Figures Provided by Dr. T. Lambert and Dr. T. James, Canadian Geodetic Survey.)
