2
Epeirogenic and Intraplate Movements

LARRY D.BROWN

Cornell University

ROBERT E.REILINGER*

Air Force Geophysics Laboratory

ABSTRACT

Major deformations of the Earth’s surface are largely consistent with the tenets of plate tectonics, which predict that such activity should be focused at the various boundaries along which massive lithospheric plates collide, pull apart, or slide past one another. Yet crustal deformations also occur well into the interior of these plates. Some may represent the “distributed” effects of distant plate boundaries, as, for example, the earthquakes of the intermontane western United States. Some, such as the geodetically observed uplift over a deep magma chamber in the Rio Grande rift of New Mexico, may correspond to incipient formation of a new plate boundary. Others, like the subtle, broad uplifts and subsidences in the nominally “stable” cratonic interiors, are much more puzzling. Such motions often appear estranged, if not divorced, from accepted plate-tectonic processes. Postglacial rebound, a well-known phenomenon in portions of North America and Europe, also appears to be an inadequate explanation for many observations.

Understanding contemporary motions of plate interiors is often hindered by the paucity and uncertain accuracy of relevant geophysical and geodetic observations. Yet intraplate tectonics constitutes more than a scientific enigma. Even seemingly slow vertical motions may threaten river courses or seafront properties on socially relevant time scales, and the subtle strains accumulating elsewhere may portend future earthquakes or volcanoes in the least predictable places.

INTRODUCTION

Even a cursory glance at global earthquake activity makes clear that much of the world’s tectonic activity is concentrated in relatively narrow belts (Figure 2.1). According to the theory of plate tectonics, these earthquake belts, and the similar patterns of volcanic activity, mark the currently active boundaries between large, lithospheric plates that are moving relative to one another (e.g., Dewey, 1975). The Pacific “Ring-of-Fire,” for example, is associated primarily with subduction of the lithospheric plates carrying the Pacific Ocean beneath other plates carrying Asia, North America, and South America. In the United States, the San Andreas Fault is considered a classic example of active tectonics associated with two plates (Pacific and North America) sliding past one another.

However, plate boundaries are far from being the

*  

Robert E.Reilinger is now at the Massachusetts Institute of Technology.



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Active Tectonics: Studies in Geophysics 2 Epeirogenic and Intraplate Movements LARRY D.BROWN Cornell University ROBERT E.REILINGER* Air Force Geophysics Laboratory ABSTRACT Major deformations of the Earth’s surface are largely consistent with the tenets of plate tectonics, which predict that such activity should be focused at the various boundaries along which massive lithospheric plates collide, pull apart, or slide past one another. Yet crustal deformations also occur well into the interior of these plates. Some may represent the “distributed” effects of distant plate boundaries, as, for example, the earthquakes of the intermontane western United States. Some, such as the geodetically observed uplift over a deep magma chamber in the Rio Grande rift of New Mexico, may correspond to incipient formation of a new plate boundary. Others, like the subtle, broad uplifts and subsidences in the nominally “stable” cratonic interiors, are much more puzzling. Such motions often appear estranged, if not divorced, from accepted plate-tectonic processes. Postglacial rebound, a well-known phenomenon in portions of North America and Europe, also appears to be an inadequate explanation for many observations. Understanding contemporary motions of plate interiors is often hindered by the paucity and uncertain accuracy of relevant geophysical and geodetic observations. Yet intraplate tectonics constitutes more than a scientific enigma. Even seemingly slow vertical motions may threaten river courses or seafront properties on socially relevant time scales, and the subtle strains accumulating elsewhere may portend future earthquakes or volcanoes in the least predictable places. INTRODUCTION Even a cursory glance at global earthquake activity makes clear that much of the world’s tectonic activity is concentrated in relatively narrow belts (Figure 2.1). According to the theory of plate tectonics, these earthquake belts, and the similar patterns of volcanic activity, mark the currently active boundaries between large, lithospheric plates that are moving relative to one another (e.g., Dewey, 1975). The Pacific “Ring-of-Fire,” for example, is associated primarily with subduction of the lithospheric plates carrying the Pacific Ocean beneath other plates carrying Asia, North America, and South America. In the United States, the San Andreas Fault is considered a classic example of active tectonics associated with two plates (Pacific and North America) sliding past one another. However, plate boundaries are far from being the *   Robert E.Reilinger is now at the Massachusetts Institute of Technology.

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Active Tectonics: Studies in Geophysics FIGURE 2.1 Global seismicity. Earthquakes tend to be concentrated along relatively narrow belts that define the boundaries of active lithospheric plates. However, some earthquakes also occur well within the plates thus defined. Examples include southeast Asia and the intermontane western United States (after Barazangi and Dorman, 1968). whole story as far as active tectonics is concerned. In many parts of the world, within both ocean basins as well as continental interiors, there are concentrations of earthquake and volcanic activity that seem divorced from plate-tectonic precepts. Hawaii is a dramatic example. The volcanoes of the Hawaiian Islands-Emperor Seamount chain are generally believed to result from upwelling of magmas from a source beneath the Pacific lithospheric plate. Although the geometry of the volcanic chain appears to be due to the motion of this plate over the underlying mantle “hot spot,” the hot spot itself is arguably a phenomenon independent from the overlying global plate framework. Regions such as southeast Asia and the western United States, where plate boundaries cut into continents, seem especially prone to intraplate tectonics. The volcanic and seismic activity in such areas is conspicuous (Figure 2.1) and often dramatic (Figure 2.2). The semantics of whether tectonic activity in such areas should be be considered truly intraplate or treated as some kind of distributed effect of a distant plate boundary largely begs the issue. Regardless of nomenclature, the fundamental causes of many such phenomena remain unclear, and their place in the plate-tectonic framework unresolved. The ancient Precambrian cores of the world’s continents contain special problems for understanding active tectonics. Although the most obvious manifestations of contemporary tectonics, such as seismicity, are decidedly less pronounced than along the plate boundaries, or even in the geologically younger intraplate regions, earthquakes do occur in the craton and near its periphery. In the United States, two of the most destructive earthquakes in history occurred not along the San Andreas Fault but in the nominally “stable” eastern United States near New Madrid, Missouri, in 1811–1812, and near Charleston, South Carolina, in 1886. Seismicity continues in many parts of the eastern United States, and faults of relatively recent geologic vintage have been identified (York and Oliver, 1976). That parts of the cratonic interior are tectonically active in the present time should perhaps not be surprising in view of the geologic record. Features such as the Michigan Basin and the Adirondack Dome (Figure 2.3) are incontrovertible evidence that the cratons were subject to major vertical motions in the past that lack a clear connection to the plate-tectonic scenarios of those times. Geologic strata attest to many gentle inundations and uplifts of the interior platforms that reflect relative mo-

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Active Tectonics: Studies in Geophysics FIGURE 2.2 Faulting associated with the 1944 Dixie Valley, Nevada, earthquake (from Stewart, 1980). tion of land and sea (Sloss, 1963), motions that are usually referred to as epeirogenic to distinguish them from the more severe forms of deformation associated with mountain building. Geodetic surveys over the cratons of North America and Eurasia appear to indicate that these regions are still going up and down at remarkable rates, although the significance of some of these observations is obscured by an ongoing debate over their accuracy. Any consideration of the active deformation of intraplate interiors must, of course, recognize the importance of vertical motions associated with the retreat of the ice sheets since the last major continental glaciation. Studies of the contemporary uplift of Fennoscandia as documented by leveling observations and tilted beach terraces are now classic (e.g., Niskanen, 1939). The effects of postglacial rebound are not restricted to areas in proximity to the former ice sheets but are global in extent and of particular importance in coastal areas. In the following discussion, we outline some of the evidence for active tectonics in intraplate areas, review some of the obstacles to our fuller understanding of their causes, and assess their potential impact on society. EARTHQUAKE DEFORMATION While the underlying causes of intraplate earthquakes remain problematical, some are clearly associated with large and, in some cases, destructive surface movements. Such earthquakes are not limited to continental areas (Wiens and Stein, 1984). Surface faulting is but one of the more obvious expressions of earthquake deformation (Figure 2.2). In some cases, geodetic measurements delineate more subtle patterns of motion. It is reasonable to assume, and the limited evidence available suggests, that intraplate earthquakes are characterized by a deformation cycle with preseismic, coseismic, postseismic, and interseismic phases similar to that inferred for interplate events (Thatcher, Chapter 10, this volume). Coseismic, and to a lesser extent, postseismic deformation have been documented by geodetic measurements for a number of intraplate earthquakes in the United States (Table 2.1). Movements of several centimeters are common, and several meters are possible. Undoubtedly similar movements have accompanied other intraplate events both in the United States and elsewhere but have gone undetected owing to the lack of appropriately timed or spaced observations. Where sufficiently detailed geodetic observations are available, coseismic movements are roughly consistent with the deformation predicted by simple dislocation theory (e.g., Savage and Hastie, 1966). Intraplate postseismic deformation has been attributed to afterslip on the earthquake fault (e.g., Savage and Church, 1974) or to subsequent viscoelastic relaxation (Koseluk and Bischke, 1981).

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Active Tectonics: Studies in Geophysics FIGURE 2.3 The Adirondack Dome (Courtesy Land Care, Inc., Boonville, New York). Plate interiors often contain isolated geologic structures that are difficult to relate to ancient plate boundaries. The Michigan Basin is another classic example. TABLE 2.1 Vertical Deformation Associated with U.S. Intraplate Earthquakes Earthquake Magnitude Coseismic Postseismic References Amplitude Extent, km Amplitude Extent, km Hebgen Lake, Montana (1959) 7.5 6 m (15 cm) 20×50 (80×140) 30 cm (7 cm)a 100 (40)a Myers and Hamilton, 1964 Savage and Hastie, 1966 Reilinger et al., 1977 Reilinger, 1985 Savage et al., 1985 Borah Peak, Idaho (1983) 7.3 1.5 m 50     Stein and Barrientos, 1985 Dixie Valley, Nevada (1954) 7.1 1.5 m (1–2 m)a 10 7.5 cm 8 Savage and Hastie, 1966 Savage and Church, 1974 Valentine, Texas (1931) 6.4 10 cm     50 Ni et al., 1981 Yellowstone Park, Wyoming (1975) 6 10 cm       Pitt et al., 1979 Oroville, California (1975) 5.7 10 cm 15     Savage et al., 1977 aHorizontal movement, 1971–1984.

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Active Tectonics: Studies in Geophysics Preseismic deformation, with its obvious implications for prediction, has proven elusive to capture in the more active interplate zones (e.g., Thatcher, 1981), so it is not surprising that it has not yet been unambiguously observed in the intraplate environment. Measurement of coseismic and postseismic deformation is guided by the known time and location of a specific earthquake. In contrast, preseismic motion is likely to be observed more by chance than design, as when repeated geodetic surveys, often carried out for other reasons, happen to cover the location of and a time interval immediately preceding some later earthquake. Interseismic deformation, the slow buildup of strain during the time periods between earthquakes, is difficult to measure for similar reasons, but also because it accumulates at rates much slower than the other deformation phases. In his review of evidence for horizontal strain accumulation from geodetic measurements, Savage (1983) found significant strain rates in a number of areas, including the Watsatch Fault near Salt Lake City, Utah, the area near Hebgen Lake, Montana, the main seismic belt of western Nevada, and the Seattle, Washington, area. Some of these deformations may well reflect the secular accumulation of interseismic strain. Earthquakes in the stable interior pose a special problem. It has proven extremely difficult to associate intraplate earthquakes, even large ones, with specific faults in areas like the eastern United States. The easternmost example of Table 2.1 is that of inferred coseismic deformation associated with the 1931 Valentine, Texas, earthquake, which is arguably still within the seismic regime of the active Rio Grande rift system (Ni et al., 1981). Although Schilt and Reilinger (1981) reported some curious vertical motions near the New Madrid seismic zone that may be earthquake related, the connection remains speculative. Aggarwal and Sykes (1978) related low-level seismicity in the New York area to the Ramapo Fault, while recent studies near Charleston (Talwani et al., 1984) may have identified subsurface faulting associated with the major nineteenth century earthquake there. Zoback et al. (1980) found FIGURE 2.4 Damage caused by the 1886 Charleston, South Carolina, earthquake [from U.S. Geol. Surv. Prof. Paper 1028 (1977)]. The historical record makes clear that earthquakes are a threat even in the nominally “stable” interior.

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Active Tectonics: Studies in Geophysics FIGURE 2.5 Area affected by the 1886 Charleston earthquake compared with other major U.S. earthquakes (modified after Rankin, 1977). Earthquakes in the eastern United States are felt over a larger area than an equivalent-sized earthquake in the west owing to more efficient propagation of seismic energy. Intensities are given using the Modified Mercali scale. evidence of surprisingly large horizontal strains in southeastern New York, which may reflect ongoing ductile deformation at depth. However, an examination of the limited number of horizontal resurveys in other areas of the eastern United States associated with seismicity failed to detect any significant motion (Krueger et al., 1983). This paucity of surface manifestation, together with the comparatively low recurrence rate of earthquakes and relatively short historic record, makes seismic-hazard mapping an especially difficult endeavor in the stable interior. Yet earthquake hazard can by no means be neglected in these areas. Major earthquakes in the New Madrid, Missouri, area in 1811–1812 and near Charleston, South Carolina (Figure 2.4), are well-known testaments to the uncertain vulnerability of major population centers in such nominally quiescent regions (Sykes, 1978). Potential seismic hazard in the stable interior is exacerbated by the more efficient transmission of seismic energy. An earthquake in the eastern United States is likely to be felt over or damage a much larger area than an equivalent earthquake along the San Andreas Fault, for example (Figure 2.5). Mitigating hazards from earthquakes in the stable interior remains one of least tractable yet most important problems in contemporary tectonics. MAGMA INFLATION Another dramatic demonstration of intraplate tectonics are the oceanic volcanic chains such as Hawaii, which unequivocably confirm that not all volcanoes lie over subduction zones. Movements associated with such volcanoes have been closely monitored for some time (e.g., Wilson, 1935) and are generally thought to reflect the episodic inflation and deflation (eruption) of subsurface magma chambers (e.g., Decker, 1969). Observations of ground motion near volcanoes, together with attendant seismic activity, have proven to be effective predictors for future eruptions. Somewhat less well known, perhaps, are other results that suggest subsurface magma injection in areas of not-so-recent volcanism. One example of surface deformation attributed to magma at depth comes from the Rio Grande rift of central New Mexico. A variety of geophysical measurements near Socorro, New Mexico, indicate the existence of a thin, but extensive, mid-crustal magma layer (Sanford et al., 1977). Precise leveling over this magma body indicates vertical uplift of about 15 cm over a 40-yr period (Reilinger and Oliver, 1976). Recent measurements indicate (Figure 2.6) that the cen- FIGURE 2.6 Uplift in central New Mexico measured by repeated leveling (Larsen et al., 1985). This movement is attributed to inflation of a mid-crustal magma body. This inflation may also be responsible for seismicity of this area.

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Active Tectonics: Studies in Geophysics tral uplift over the magma chamber is flanked by a zone of subsidence, the overall pattern suggesting withdrawal of magma from one reservoir and injection into another (Larsen et al., 1985). An intriguing aspect of this uplift is that it occurs in an area where there have been no historical eruptions, although volcanic rocks in the rift attest to such activity as recently as 100,000 yr ago (Lipman and Mehnert, 1975). Whether this ground motion portends some future eruption or is part of some normal cycle of magma transfer at depth with little chance of breaking out at the surface has yet to be ascertained. Geodetic observations have been linked to possible intracrustal magmatism in other parts of the Rio Grande rift (e.g., Reilinger et al., 1979), in Yellowstone National Park (Pelton and Smith, 1982), and, most recently, near Mammoth Lakes in eastern California (Savage and Clark, 1982; Castle et al., 1984). In the latter example, changes in elevation and horizontal strain have been interpreted to indicate magmatic resurgence of the Long Valley caldera by inflation of an approximately 10-km-deep magma chamber. This inflation may be responsible for a series of four magnitude-6 earthquakes in the area in 1980. Because there have been several explosive eruptions and extrusion of rhyolite domes in this area during the past 400 yr, and because Long Valley is less than 200 miles from major cities like Sacramento and San Francisco, deformation and seismic activity are being monitored in order to predict possible future activity. The local and regional doming near Yellowstone is difficult to relate to standard plate-boundary processes. Magma injection is thought to result from a “hot spot” that can be traced into the North American continent along the Snake River Plain (e.g., Suppe et al., 1975). In this sense it is clearly an intraplate phenomena. On the other hand, it could be argued that the apparent magma uplifts near Mammoth Lakes and in the Rio Grande rift really represent plate-boundary processes. The former may be a relict of the subduction-related volcanism, which for the most part ceased when the West Coast converted from a convergent to a strike-slip margin. Likewise the New Mexico activity could well represent the beginnings of a new plate boundary, a rift that may evolve into a new ocean basin by splitting off the southwestern United States. Semantics notwithstanding, such examples must be considered in order to understand intraplate phenomena. After all, neither lies upon a currently active plate boundary per se. More to the point, the geologic processes that they represent may well pertain to other “intraplate” phenomena whose association with similarly defunct or precursor boundaries is simply not yet so apparent. CRUSTAL LOADING Contemporary deformation of the stable interior is virtually synonymous in many minds with postglacial rebound. The broad doming of recently deglaciated parts of North America and Scandinavia has long been documented by geologic studies of warped beach terraces and geodetic measurements of continued uplift (Figure 2.7). This motion is perhaps the best understood, geomechanically speaking, of any type of intraplate deformation, although controversy still revolves around distinguishing those possible deep-earth rheologies that are most consistent with the observed rebound effects (e.g., Kaula, 1980). An important aspect of recent studies of postglacial phenomena, especially sea-level changes, is that this is a global phenomenon, not restricted to the immediate area of glacial retreat. Concepts such as the collapse of a peripheral bulge (e.g., Walcott, 1972) have been re- FIGURE 2.7 Postglacial rebound of Fennoscandia (Balling, 1980). Contours represent uplift in millimeters per year measured by precise leveling.

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Active Tectonics: Studies in Geophysics FIGURE 2.8 Arching of the Gulf Coastal Plain north of New Orleans indicated by precise leveling data. This arching may be a forebulge-type response to sediment loading of the Mississippi delta (Jurkowski et al., 1984). FIGURE 2.9 Contemporary vertical crustal motion (millimeters per year) of eastern Europe inferred from precise releveling (after Mescherikov, 1973).

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Active Tectonics: Studies in Geophysics 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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Active Tectonics: Studies in Geophysics 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

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Active Tectonics: Studies in Geophysics 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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Active Tectonics: Studies in Geophysics FIGURE 2.12 Intraplate tectonics may be influenced by inherited tectonic structures. In this example, relict transform faults left behind from the opening of the Atlantic may serve as weaknesses that concentrate modern intraplate stresses, thus constituting potential foci of seismic activity (Sykes, 1978). zontal motions associated with plate tectonics (Minster and Jordan, 1978). Moreover, the geologic record is full of cyclicity (Vail et al., 1977). SOCIAL IMPACT The social ramifications of contemporary intraplate deformations are various. Some are relatively obvious: co-seismic deformations can damage nearby structures and disrupt or sever support services such as water or communication lines. Other types of deformation may have more subtle impact. The apparently slow vertical motions in the stable interior may yet be significant in planning long-term engineering projects. For example, a differential rate of only a centimeter per year will accumulate to a meter in a century. In a coastal region, a meter change in average elevation can translate into major shifts in effective shoreline positions, positions that have fundamental economic and legal significance (Bossler, 1984). Intraplate deformation can also have indirect significance. For example, movements that precede and are related to earthquakes clearly have value in prediction. Generally speaking, to the extent that intraplate motions provide clues for our understanding of the state and evolution of stress in plate interiors, they are potentially critical guides for mitigating hazards from more obvious phenomena such as earthquakes and volcanoes. NEEDS AND TRENDS Current understanding of the active tectonics in intraplate regions is mixed. On the one hand, broad concepts have been developed for areas like Hawaii (hot spots), Scandinavia (postglacial rebound), the Basin and Range of the western United States (distributed shear), and southeast Asia (India as a rigid indentor) that provide useful frameworks for monitoring, relating, and mitigating hazards from specific phenomena like earthquakes or volcanoes. Yet observations from other areas, such as the eastern United States, remain difficult to relate to any encompassing tectonic theory. Although much has been learned about intraplate phenomena, basic questions remain. How are stresses in the plate interior related to forces at plate boundaries? To what extent are contemporary tectonics influenced by pre-existing structures? How accurate are the geodetic measurements that are often our primary source of information on contemporary deformations in such areas? How do we recognize the influence of subtle levels of active tectonics in the geologic (geomorphic) record? What are the best observational strategies for monitoring motions in areas of varying degrees of tectonic activity? What is an appropriate level of effort for tectonic investigations in intraplate areas in relation to the more active interplate zones? It is easy to become complacent on the issue of intraplate tectonics. After all, damaging earthquakes are usually few and far between, and the secular motions seem too slow to warrant much concern, even if they prove to be real and not artifacts of measurement error. However, the relatively low frequency with which intraplate events seem to affect our lives does not alter the fact that they can have and will continue to have major economic and social impact. There is more than sufficient reason to justify efforts to fill our considerable gaps in understanding intraplate motions. Future progress in studying intraplate tectonics requires many things. Many areas of the United States lack any geodetic resurveys with which to estimate contemporary deformation. This is especially true of horizontal measurements outside the well-known seismic zones of the western United States. Issues of accuracy in geodetic measurements must be satisfactorily resolved. Monitor-

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Active Tectonics: Studies in Geophysics ing programs that most effectively address issues of contemporary dynamics must be designed and funded at an adequate level. Furthermore, it must be recognized that these monitoring programs must be long term. New technologies must be brought to bear and integrated with conventional techniques. And certainly more research is needed to integrate and cross-calibrate seismologic, geodetic, and geomorphic measures of active tectonics to broaden our base of observations. In many respects these demands are being addressed. For example, the National Aeronautics and Space Administration, the U.S. Geological Survey, the National Geodetic Survey (NGS), and others have been developing and/or applying new techniques such as very-long-baseline interferometry (VLBI), satellite laser ranging (SLR), and the Global Positioning System (GPS) to monitoring crustal deformations (e.g., Coates et al., 1985). The NGS has made an important effort to develop new corrections for geodetic leveling (e.g., Holdahl, 1981), and research has become much more meticulous as far as evaluating the accuracy of measurements (Jackson et al., 1980). Yet much remains to be done. Funding agencies and investigators alike must realize that proper resolution of critical issues concerning active tectonics of plate interiors will require a long-term commitment because of the very nature of the processes involved. ACKNOWLEDGMENTS Contribution No. 6 from the Institute for the Study of the Continents. REFERENCES Adams, J. (1980). Active tilting of the United States midcontinent: Geodetic and geomorphic evidence, Geology 8, 442–446. Aggarwal, Y.P., and L.R.Sykes (1978). Earthquakes, faults and nuclear powerplants in southern New York-northern New Jersey, Science 200, 425–429. Anderson, M.A., J.T.Kelley, W.B.Thompson, H.W.Borns, Jr., D. Sanger, D.C.Smith, D.A.Tyler, R.S.Anderson, A.E.Bridges, K. 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