fined to predict a complex pattern of relative motion of land and sea the world over (e.g., Clark, 1980).

A persistent suspicion is that the stresses associated with postglacial rebound may be sufficient to trigger seismic activity in deglaciated regions (e.g., Stephansson and Carlsson, 1980). However, the correlation between rebound and earthquakes seems tenuous at best.

Land uplift following glacial unloading has also been reported on a more local scale. Hicks and Shofnos (1965) correlated an anomalous drop in sea level in southeastern Alaska with recent retreat of a small ice sheet. The rate of this local land uplift is on the order of 4 mm/yr.

Ice is not the only crustal load capable of driving surface deformation. Crittenden (1963), for example, reported both geologic (deformed shorelines) and leveling evidence of continued local uplift in the area of former Lake Bonneville. This rebound is inferred to follow the removal of the water load associated with climatic changes in postpluvial periods. Depression of the land surface by filling of new reservoirs is well known (e.g., Longwell, 1960). Holzer (1979) even reported evidence that drawdown of aquifers in southern Arizona has been followed by a rebound effect. In addition, Opdyke et al. (1984) attributed post-Pleistocene uplift in northern Florida to crustal unloading associated with limestone dissolution in Karst areas.

Redistribution of crustal loads by erosion, sedimentation, and faulting is fundamental in geology. Sedimentary infilling clearly augments thermal subsidence to form some of our major basins (e.g., Sleep, 1971), and the importance of crustal loading by thrust sheets is being recognized as a major factor in the formation and evolution of foreland basins and basement arches (Quinlan and Beaumont, 1984). Perhaps a contemporary example is found in the south central United States, where leveling results (Figure 2.8) have been interpreted to show uplift of a forebulge associated with sedimentary loading of the Mississippi delta (Jurkowski et al., 1984; Nunn, 1985).

EPEIROGENY

Sedimentary strata that overlie large areas of the stable interiors like the central United States and eastern Europe record a history of broad upwarping and downwarping relative to sea level (e.g., King, 1977). In some cases, large basins or domes have formed, apparently unrelated—except by age—with distant plate boundaries. Formation of these relatively gentle tectonic features is called epeirogeny, a term that still carries an aura of mystery. Indeed, there is no widespread agreement on what causes these interior motions.

One of the most surprising results to arise from the analyses of precise leveling data is that many of these platform areas seem still to be going up or down at geologically rapid rates. For example, a map of vertical crustal motion in eastern Europe (Figure 2.9) published not long ago shows parts of the Russian platform going up and down at differential rates of several millimeters per year. Leveling in the eastern United States (Figure 2.10) likewise seems to suggest that vertical neotectonic motion is the norm, not the exception, in these areas, in spite of a long-term geologic record of relative tranquillity (e.g., Brown and Oliver, 1976). Although some of these apparent motions may be remnants of postglacial rebound, as in the Baltic Shield or the Great Lakes area of the United States, most lack a clear-cut neotectonic explanation.

However, before ascribing these motions to some new, unheralded form of intraplate tectonics, it is important to recognize that there are major outstanding questions about the accuracy of the geodetic measurements on which most such studies are based. Recent work has shown, for example, that systematic errors are more serious than previously thought and that apparent changes in elevation once believed to be neotectonic in origin are now perceived by some to be artifacts of observational errors (Strange, 1981).

Unfortunately, assessing the influence of geodetic errors on estimates of vertical crustal motion in areas like eastern North America is still in an early stage, and initial results are too few and inconclusive (e.g., Fadaie and Brown, 1984). Yet even a cursory glance at Figure 2.10 provides grounds for skepticism. In this figure are two independent estimates of crustal motion, one based on water level and leveling in southeastern Canada and one based on leveling and sea-level data for the eastern United States. Although the rates are similar, these estimates show a disturbing degree of inconsistency where they join along the United States-Canada border. Until these data sets are reduced jointly and uniformly it is perhaps unfair to expect complete agreement; yet, the question remains as to whether some of these patterns are more the result of statistical smearing of unrecognized and inadequately treated systematic errors than real ground motion.

A prominent trend of the map in Figure 2.10 is the apparent uplift of the Appalachians relative to the coastal regions and interior plains. Yet this relict mountain belt is generally thought to have last been active over 200 m.y. ago (Williams and Hatcher, 1983). Since certain types of leveling error are known to correlate with height, is the Appalachian “uplift” really the accumulation of such errors? Is the similarly inferred con-



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