third. None of these, however, seems to explain all the observations, such as the geodetic indications of contemporary intraplate ups and downs or geologic structures like the Michigan Basin. Although some of the geodetic evidence may be open to doubt, the geologic record is unequivocal. Thus the question remains as to whether there exist heretofore unrecognized mechanisms of intraplate deformation. Menard (1973), for example, postulated the existence of asthenospheric bumps to explain anomalous ocean-floor bathymetry. Jacoby (1972) looked to densification of the mantle due to magma separation as a possible means of inducing vertical motion by increasing continental buoyancy. McKenzie (1984) considered the intrusion of magma into the lower crust as another means of driving vertical motion. These and other possibilities deserve critical consideration and testing with observations.

One of the problems with geodetic indications of vertical movement within plates has to do with their apparent rates, typically on the order of a few millimeters per year (Figures 2.9 and 2.10). Although these would seem to be minute motions, they are extremely large from the geologic perspective. One millimeter per year corresponds to 1 km every million years. For comparison, rates of contemporary erosion in intraplate areas are usually estimated to be at least an order of magnitude less (Schumm, 1963). If such rates were sustained, we should expect imposing mountain ranges to be thrown up within relatively short periods, geologically speaking. That we do not see such topography leads to the argument that these motions are oscillatory or episodic, so that there is no net accumulation of relief. In some respects, such an explanation seems a bit ad hoc, and skeptics might infer that the inconsistency between some geodetically measured rates and the more subdued geologic record is further reason to question the accuracy of the former. However, it should be remembered that rates of vertical motion on the order of a few millimeters per year are much smaller that the commonly accepted rates of a few centimeters per year for the hori-

FIGURE 2.11 State of stress in the United States (Zoback and Zoback, 1980).

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