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 106 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 × 10−8 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.