CONTINENTAL OVERVIEWS AND SUMMARIES
We use a short-wavelength (<250 km) gravity map and a magnetic map to define anomaly-trend zones in the conterminous United States. The zones appear to define major regions of coherent structure within the crust. The main orientations of the zone boundaries are northeasterly and northwesterly, except along the western Cordillera, where they trend parallel and normal to the western coastline and to the strike of the underlying subduction zone. Correlations between geophysical and geologic features along the eastern and western margins of the United States suggest that a major cause of the zone and boundary trends is plate interaction at the continental margins. Linear gravity highs, commonly linked into extensive chains, are interpreted as the expressions of horsts that occupy the central parts of rift systems. Major crustal deformation appears to have occurred along the boundaries of the horsts over long periods of geologic time. The oldest basement of the central cratonic region may be overprinted in many places by younger tectonic events so that primary features of the oldest basement are obscured. We also observe that the structural pattern of the continent thus defined exhibits a crude symmetry about the midcontinent rift system. The pattern appears to have evolved from 1.5 to 1.0 Ga. We point out several specific geologic feature-geophysical anomaly correlations, although a comprehensive treatment of these is not attempted. We identify two types of regional magnetic anomalies, which are associated with regions underlain by thick sedimentary sections, and which may be linked to a shallow mantle. A comparison of a gravity map of long wavelengths (>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.