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Show Bruce G. Bills NASA Goddard Space Flight Center Greenbelt, MD 20706 Geodynamic Paleolimnology of the Great Basin Climatic change since the late Pleistocene has produced very significant changes in the distribution of surface water in the Great Basin. At 15,000 years before present, the Great Basin contained numerous large lakes, with Bonneville and Lahontan both over 20,000 km2, and 15 other lakes with surface areas over 500 km2. The gravitational loads due to the water impounded in the largest of these lakes were sufficient to cause significant deformation of the Earth's crust. The shorelines which formed as level surfaces, while the large lake loads were present, are arched upwards in the center of the basin during more arid times, when. the loads are much reduced. Lakes thus have the potential for both causing and recording crustal deformation. The spatio- temporal pattern of this deformation, which is recorded in the basin- wide variations in height along shorelines of known age, can be used as a diagnostic tool to reconstruct the long term strength of the solid Earth beneath the load. Glacial loads are often used in a similar way, but lake loads present two significant advantages: they tend to better preserve geomorphic and sedimentary records of their past fluctuations, and the upper surface of the lake water load closely follows an equipotential surface. There is a dynamic coupling between the flow of water on the surface of the Earth and the viscous flow of rock within the crust and upper mantle. The hydro- isostatic crustal deformation, which is a direct result of water loads, can substantially influence the magnitude and location of the loads themselves, by causing diversions or avulsions of rivers and by influencing heights of inter- basin thresholds. Proper utilization of the crustal deformation pattern to obtain constraints on rheology of the crust and upper mantle requires accurate knowledge of the loading history. Since the largest lakes in the Great Basin were actually complex hydrologic networks, composed of many multiply connected basins, successful reconstruction of their loading histories requires both significant amounts of well dated shoreline materials and a hydrologic model which accounts for precipitation, evaporation and inter- basin flow. Any such reconstruction will implicitly contain a great deal of paleoclimatic data, including at a minimum, regional patterns of evaporation and precipitation as a function of time. Lake Bonneville was the largest of the late Pleistocene lakes in the Great Basin. At its greatest extent it had a surface area of 45,500 km2 and had a drainage basin area of 128,400 km2. The isostatic warping of the Bonneville basin shorelines has been quite intensively studied, starting with the pioneering work of G. K. Gilbert in 1890, and remains a topic of current research. The maximum amplitude of vertical deformation is roughly 70 m, and the deformation produced by the Bonneville load extends beyond the limits of the lake itself. Several smaller neighboring lakes have paleo- shorelines which are tilted up towards the center of the Bonneville load. These include Bear Lake to the northeast, and Lakes Waring, Clover, Spring and Maxey to the west. The pattern of rebound has been measured on the Bonneville (- 1550 m), Provo (- 1450 m) |