Abstract
The deformation pattern of the paleoshorelines of extinct Lake Bonneville were among the first features to indicate that Earth's interior responds viscoelastically to changes in surface loads (Gilbert, 1885). Here we revisit and extend this classic study of isostatic rebound with updated lake chronologies for Lake Bonneville and Lake Lahontan as well as revised elevation datasets of shoreline features. The first order domal pattern in the shoreline elevations can be explained by rebound associated with the removal of the lake load. We employ an iterative scheme to calculate the viscoelastic lake rebound, which accounts for the deformation of the solid Earth and gravity field, to calculate a lake load that is consistent with the load-deformed paleotopography. We find that the domal deformation requires a regional Earth structure that exhibits a thin elastic thickness of the lithosphere (15–25 km) and low sublithospheric Maxwell viscosity (∼1019 Pas). After correcting for rebound due to the lake load, shoreline feature elevations reveal a statistically significant northward dipping trend. We attribute this trend to continent-scale deformation caused by the ice peripheral bulge of the Laurentide ice sheet, and take advantage of the position of these lakes on the distal flank of the peripheral bulge to provide new insights on mantle viscosity and Laurentide ice sheet reconstructions. We perform ice loading calculations to quantify the deformation of the solid Earth, gravity field, and rotation axis that is caused by the growth and demise of the Laurentide ice sheet. We test three different ice reconstructions paired with a suite of viscosity profiles and confirm that the revealed trend can be explained by deformation associated with the Laurentide ice sheet when low viscosities below the asthenosphere are adopted. We obtain best fits to shoreline data using ice models that do not have the majority of ice in the eastern sectors of the Laurentide ice sheet, with the caveat that this result can be affected by lateral variations in viscosity. We show that pluvial lakes in the western United States can place valuable constraints on the Laurentide ice sheet, the shape of its peripheral bulge, and the underlying mantle viscosity.
Highlights
The western U.S experienced a mean increase in precipitation during the last glacial cycle, which led to the formation of a series of pluvial lakes that filled the Basin and Range Province (e.g., Benson et al, 1990; Mifflin and Wheat, 1971)
We revisit the deformed elevational pattern of Lake Bonneville and Lake Lahontan shoreline features to investigate the different contributions to their deformation
The first order signal is the unloading of these extinct lakes, which leads to a domal deformation pattern in the lake shorelines
Summary
The western U.S experienced a mean increase in precipitation during the last glacial cycle, which led to the formation of a series of pluvial lakes that filled the Basin and Range Province (e.g., Benson et al, 1990; Mifflin and Wheat, 1971). Bonneville (30–10 ka), an extinct pluvial lake that occupied the eastern Great Basin (Fig. 1B). At its maximum extent (~18 ka) (Oviatt, 2015), the lake had a volume of around 10,300 km (Chen and Maloof, 2017), comparable to present-day Lake Superior. During Lake Bonneville’s existence, the smaller Lake Lahontan (Fig. 1C) occupied the western part of the Great Basin and experienced a similar increase and decrease in lake volume, reaching its maximum extent at ca. During Lake Bonneville’s existence, the smaller Lake Lahontan (Fig. 1C) occupied the western part of the Great Basin and experienced a similar increase and decrease in lake volume, reaching its maximum extent at ca. 16–15 ka (Benson et al, 2013; Reheis et al, 2014) (Fig. 1A)
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