Presently, the crust in the region from the Rocky Mountains to the Mississippi River and from the Canadian border to Oklahoma is all thicker than average. Sometimes termed overthickening, this anomalously high thickness has been measured through a variety of methods and compiled by various studies (Sheehan et al. 1995, Sandvol et. al. 2001). The present crustal thickness of the US can be seen in the figure below.

At its maximum, the crust in this region is 51 km thick compared to a global average continental crust thickness of 33 km. Considering the possibility that the crust of the Great Plains was 33 km thick before the Laramide, up to 18 km must be accounted for (Bird, 1984, McQuarrie and Chase 2000).

There are some very good reasons that it is often accepted that the crust was much thinner before the Laramide. Shallow water sediments deposited in the Rocky Mountains during the Cenozoic were deposited during a time of a high global sea level. Considering that sea level was about 200 meters higher at the time and the current elevation is about 2 km, we are left with about 1,800 meters of current topographic relief to account for. With simple Airy root compensation (see science page for definition of an Airy root), an additional crustal thickness of 18 km would be completely reasonably to explain support of such topography. An Airy root isn’t always a good assumption and there are regions in the world near sea level where the crustal thickness is anomalously high but none the size of the Western US (Mooney et al. 1998). Also, these regions of thick crust near sea level are typically colder and denser making them less prone to “float” like the Western US.

McQuarrie and Chase (2000) in their work and in citing other previous work show the eastern edge of the thickened crust in the western US running through Utah by the Sevier Orogeny (85-50 Ma). This same paper discusses some of the mass balance problems involved with adding up to 15 km of crust to the Great Plains. The timing of this event is further reinforced by Livaccari’s (1991) analysis of Laramide crustal thickening.

If this increased thickness in the Great Plains is so substantial, how thick was the crust to the west before this thickening? Their calculations show that a decrease in thickness of up to 30 km would be required in the Basin and Range. This seems large but when considered with the total contraction across the Basin and Range, these numbers are reasonable. We can see that the assumption that high crustal thicknesses in the Rocky Mountains and great plains were developed in the last 75 million years or so. This assumption with measures of current crustal thickness allows us to quantify the total increase in thickness as 15 to 18 km. With the amount of thickening and the timing known, we can now examine the possibilities of crustal thickening mechanisms.