Chapter 18: A crust/mantle structural framework of the conterminous United States based on gravity and magnetic trends

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Abstract
We use a short-wavelength (1,000 km) with similarly filtered maps of crustal thickness and Pn-velocity shows a significant correlation between major gravity features and the lateral distribution of mantle-surface velocity (gravity highs/high velocity, and gravity lows/low velocity). There does not appear to be a correlation with crustal thickness except at the continental boundaries and possibly in the northeastern United States. We further show that the great gravity low of the western United States corresponds laterally with an S-wave velocity low in the uppermost mantle. Using the vertical extent of the low-velocity zone as the vertical dimension of the gravity anomaly source, we estimate a decrease in the density of the source of less than 1 percent. The corresponding decrease in the shear modulus is about 17 percent. The positive correlation between gravity features and mantle velocities implies that the isostatic compensation of the first-order topographic features of the continent is in the uppermost part of the mantle. The large decrease in the shear modulus suggests that the likely cause of the isostatic compensation in the western United States is an abnormally high uppermost-mantle temperature that produces thermal buoyancy of the elevated topographic mass. We use a combination of geophysical crustal zones, mantle properties, and regional geology to propose that the conterminous United States be divided into four tectonic regions: (1) an Appalachian province that includes the eastern Gulf Coastal Plain and Florida, (2) a western Gulf Coastal Plain province, (3) a central cratonic province that extends from the Appalachians to the Rocky Mountain front, and (4) a western province extending west from the Rocky Mountain front.

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