COMBINING GEOMORPHOLOGICAL AND GEODETIC DATA TO DETERMINE POSTGLACIALTILTING IN MANITOBA

Estimates of postglacial rebound in central North America from Laurent ide deglaciation to the present time are uncertain as a result of lack of data from the continental interior. A more precise knowledge of po stglacial tilt history will assist studies of the evolution of the maj or lakes in Manitoba and will facilitate the engineering and environme ntal management of the present-day hydrological system. This paper exp lores the benefits of combining geomorphological data with high-precis ion, real-time geodetic data (GPS positioning and absolute gravity) an d lake-gauge tilt data now being collected for postglacial rebound stu dies in Manitoba and adjacent regions in the US. Presently-available d ata sets representing these data types are (I) tilting of the 9.5 kyr B.P. Campbell strand line south and west of Lake Winnipeg, (2) the rat e of decrease in absolute gravity values measured from 1987 to 1995 at Churchill, Manitoba, and (3) the present-day regional tilt rate deriv ed from water-level gauges in southern Manitoba lakes. These data are compared to theoretical predictions based on the published ICE-3G load ing history and on a model of Earth rheology characterized by a 1066B density and elastic structure, an upper-mantle viscosity of 10(21) Pa s, a lower-mantle viscosity of 2 x 10(21) Pa s, and a lithosphere thic kness of 120 km (Tushingham & Peltier, 1991). All three data types sho w disagreement in Manitoba with ICE-3G and the 'standard' Earth model. ICE-4G does better but could not be investigated in any detail. The c onstraints on model parameters provided by the different data types we re investigated by varying, one at a time, three key parameters, (1) t he thickness of the lithosphere in excess of 120 km, (2) the lower man tle viscosity, and (3) the thickness of Laurentide ice over the Prairi es, to obtain better fits to the data. The present data do not appear to constrain lithosphere thickness in excess of 120 km very well. Whil e both the Campbell strand line data and the Churchill absolute gravit y data are consistent with an increase in lower-mantle viscosity, the present-day, lake-gauge data are not. All three data types are consist ent with a thinning of the Laurentide ice-sheet over the Prairies rela tive to the ICE-3G model. Simultaneous adjustment of model parameters with the advantage of anticipated new data in Manitoba and adjacent re gions in the US will lead to better understanding of the trade-offs be tween Earth rheology and ice sheet history and hence to an improved La urentide postglacial rebound model.