FIGURE 2.10 Apparent vertical motion (millimeters per year) of the eastern United States from precise releveling (Eastern United States results from unpublished map by G.Jurkowski; Great Lakes and Canadian results from Vanicek and Nagy, 1980). Some of these patterns have been questioned because of uncertainties about the accuracy of the geodetic measurements on which they are based.

temporary uparching of the Adirondack Dome in New York State (Isachsen, 1975) likewise suspect, or are real and important neotectonic motions being revealed?

Until thorough error analyses are completed, proper reservations about the reality of significant neotectonic motion in at least parts of the stable interiors seem warranted. However, evidence for neotectonic motion in such areas does not hinge solely on geodetic data. Holocene motions along the East Coast have been inferred from tilted beach terraces (Winker and Howard, 1977) and from submarine geomorphology (Officer and Drake, 1981). Adams (1980) has even attempted to correlate the relatively detailed changes in river drainage in the deep interior of the United States to contemporary tilts indicated by leveling. Anderson et al. (1984) cited a correlation between patterns of modern subsidence indicated by leveling and post-Pleistocene deformation of wave-cut terraces near Eastport, Maine. Albeit many, if not all, of these inferences of motion could be challenged to some degree, the issue of neotectonics of the interior remains unresolved. In fact it is the continuing uncertainty about what is really going up and down in many parts of the interior that constitutes perhaps the dominant problem of active intraplate tectonics.

DISCUSSION

Although intraplate motions are remote almost by definition from plate boundaries, it does not necessarily follow that they are unrelated to plate-boundary forces. The concept that lithospheric plates are rigid is a first-order treatment at best, and the geologic record is replete with evidence that interiors respond and deform to the actions at their edges. The tectonic collage that is now southeast Asia, formed by the collision of the Indian subcontinent into the Asian underbelly, is elegant proof that plate-boundary forces exert an influence hundreds of kilometers into the interior (e.g., Molnar and Tapponnier, 1978).

Some of the intraplate stress patterns mapped by Zoback and Zoback (1980) for the United States (Figure 2.11) suggest affinities with activity at boundaries of the North American plate. The extensional stresses of the Basin and Range, for example, have been interpreted as distributed shear from the San Andreas Fault system (Atwater, 1970). East-west compression of the eastern United States has been argued to reflect plate-driving forces (Sbar and Sykes, 1973), although other explanations have also been discussed (Zoback and Zoback, 1980, 1981). Yet neither intraplate stress patterns nor seismicity are by any means simple, and the link, if any, to distant plate boundaries is more often obscure than not.

One possible reason for complexity in intraplate tectonics is reactivation, i.e., the concept that present tectonics is guided by crustal heterogeneities formed during much earlier times. Woollard’s (1958) appeal to reactivation of older geologic structures as an explanation of eastern United States seismicity has been echoed in various guises ever since. Sykes (1978) surveyed an array of evidence suggesting that intraplate neotectonics is influenced by structures inherited from earlier times, when plate boundaries may have been more directly involved (Figure 2.12). For example, Zoback et al. (1980) reported seismic reflection data that documents continued Cenozoic motion on Cretaceous faults in the Mississippi Valley region. Ancient crustal flaws may well serve to guide and focus plate-boundary forces in locally diverse, perhaps destructive ways, although the mechanics of this process are not completely understood (e.g., Zoback et al., 1980).

However, reactivation is at best only part of the picture for active tectonics in intraplate regions. Postglacial rebound is another, and hot spots are undoubtedly a



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement