Vertical crustal movements from leveling data and their relation to geologic structure in the eastern United States

Abstract

Through analysis of leveling data, rates of relative vertical motion have been determined for points of a grid of profiles over much of the eastern United States. The rates are commonly too large to be attributed to leveling errors and therefore must reflect true earth movements. With some exceptions, modern movements appear to be related to earlier Phanerozoic trends, but the rates of modern movements are much larger than average rates over the past 130 m.y. Thus movements are either episodic or oscillatory about a long-term trend. if oscillatory, the periods must be less than about llY yr and may be much smaller. The spatial patterns of the rates of relative motion, or tilts, can be correlated with certain geological features. The Atlantic and Gulf coastal plains are tilting downward away from the continental interior. Along a profile from Savannah, Georgia, to Philadelphia, Pennsylvania, the Atlantic Coastal Plain is also tilting downward to the north, with a perturbation at the Cape Fear arch. The Appalachian Highlands are rising relative to the Atlantic Coast at rates of up to 6 mm/yr. The pattern of maxima and minima in relative velocities along the profiles crossing the Appalachian Highlands province suggests elongated zones of relative uplift and subsidence paralleling either the Appalachian drainage divide or the trend of Appalachian structure. The maxima correlate strongly with topographic highs. There is a suggestion of correlation of modern seismicity with extremes of relative velocities in the Appalachian Highlands province. The interior Plains are tilting downward to the east at a rate of 2 X l0 -8 rad/yr, the implication being that glacial rebound is not an important factor south of the Great Lakes at the present time. The wavelength between successive zones of relative uplift in the Appalachian Highlands is about 300 km, suggesting that at least the entire thickness of the lithosphere is involved, and probably the asthenosphere as well. Possible explanations for these phenomena include the effects of hydrologic variations, phase transitions in the lower crust and/or the upper mantle, asthenospheric currents driven by ocean and ice loads or plate motions, and stresses derived from nonequilibrium crustal loads or crustal unloading by erosion. advantage of reducing, hopefully, the effect of cumulative errors in leveling, but may introduce new sources of error in the form of ambiguous sea level measurements, nonsynchroneity of observations, excessive smoothing, and other effects, so that warping may obscure information of value in the study of geological phenomena. For these reasons only information from relative velocity and tilt rate data is used here. Furthermore, only larger wavelength movements were examined in order to minimize the influence of local (<50 km), nontectonic processes (e.g., groundwater withdrawal, compaction due to man-made loads).Six continuous profiles of relative velocity have been prepared from NGS data. These profiles cross virtually all of the major structural provinces in t...

Recent Relative Sea-Level Change in Eastern North America

“…Absolute rates of crustal movement can be determined only if the leveling net is tied into a fixed frame of reference, usually tide-gauge stations (e.g., Holdahl and Morrison, 1974;Vanicek, 1975Vanicek, , 1976. Leveled surfaces are warped to fit the tide-gauge data, which can introduce errors due to nonsynchronous leveling and tide-gauge data, as well as excessive smoothing (Brown and Oliver, 1976 Leveling data are not without error. Constant relative movement over the time interval between relevelings is assumed, although various studies have shown that this assumption is not always correct (e.g., Brown and Oliver, 1976;Brown, 1978).…”

Applications of Geodesy to Geodynamics an International Symposium

“…The 1973 Point Mugu quake (6.0) was preceded by uplifting and then downwarping of about 4 cm during the previous 13 years [Castle et al, 1977]. However, these changes are not remarkably bigger than those inferred from leveling data in relatively non-seismic regions [Brown, 1976], and more recent experience with the Palmdale bulge has indicated that leveling must be treated with care and used in combination of other data [Kumar & Strange, 1978]. This data should include both gravity measurements and horizontal geodetic survey.…”

“…Postseismic geodetic measurements [e.g., Brown et al, 1977] are important because they indicate some aspects of the stress redistribution precursory to subsequent creep and seismicity in the zone, and because they are affected by the same tectonic context as the earthquake itself. In fact, so far earthquake models have been more successful in explaining post-seismic adjustment than the precursory buildup and the earthquake itself [e.g., Nur & Mavko, 1974;Anderson, 1975;Rundle & Jackson, 1977;Walcott, 1978].…”

Geodetic Monitoring of Tectonic Deformation: Toward a Strategy

“…These observations need to be validated because they do not accord with geological evidence, although they do accord with the expected effect of atmospheric refraction when repeat leveling surveys are made over shorter sight lengths than was common in earlier times. Present rates of uplift relative to the Atlantic coastal plain, according to sparse geodetic data, are up to 6 mm/yr (Brown and Oliver, 1976), whereas erosion rates from Late Cenozoic time to present yield a geological uplift rate of only about x.04 mm/yr (Hack, 1978). According to Isachsen (1976a;197Gb) and Isaensen et al ( 1978), the Adirondack Mountains are doming upward at the late of 3-4 mm/yr during Holocene time, whereas tide-gauge levels on Lake Champlain just east of the dome register no more than 1 mm/yr in historic time, contradicting the leveling estimates (Barnett and Isachsen, 1980).…”

“…Compilations of vertical data on a national scale (Brown and Oliver, 1976;Gable and Hatton, 1980) indicate that major crustal movements are taking place far from the recognized boundaries of the North American plate-movements that are seemingly unrelated to plate-tectonic processes, at least insofar L-we understand them at present. Other movements can be attributed to plate tectonics, but the details are not understood.…”