Paper No. 80-11
Presentation Time: 4:25 PM
THE GEODYNAMIC IMPLICATIONS OF THE CENTRAL APPALACHIAN ANOMAL
KING, Scott D.1, LONG, Maureen D.2, WAGNER, Lara3, EVANS, Rob L.4, MAZZA, Sarah E.5, BYRNES, Joseph6, KIRBY, Eric7, GAZEL, Esteban8, JOHNSON, Elizabeth A.9, BEZADA, Maximiliano10, ARAGON, John C.2, LIU, Shangxin11 and MILLER, Scott R.12, (1)Department of Geosciences, Virginia Tech, Blacksburg, VA 24060, (2)Department of Earth and Planetary Sciences, Yale University, New Haven, CT 06520, (3)Dtm, Carnegie Institution of Washington, 5241 Brad Branch Road, NW, Washington, DC 20015-1305, (4)Geology and Geophysics, Woods Hole Oceanographic Institution, Clark South 172, MS 24, Woods Hole, MA 02543, (5)Department of Geosciences, Smith College, Northampton, MA 01063, (6)University of Minnesota, Minneapolis, MN 55455, (7)College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, OR 97331, (8)Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853, (9)Department of Geology and Environmental Science, James Madison University, Harrisonburg, VA 22807, (10)Earth Sciences, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, (11)Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24060, (12)Department of Geology and Geophysics, University of Utah, Salt Lake City, UT 84112
The present-day lithospheric structure beneath the central WV/VA border is anomalously thin and the area has experienced pulses of Jurassic and Eocene magmatism. Using the geophysical and geochemical observations, we evaluate geodynamic models of lithospheric loss. Seismic tomography models image a low-velocity region, the Central Atlantic Anomaly (CAA), with lateral extent of ~100 km in the uppermost mantle.Receiver function analysis, magnetotelluric data, and the observed increase in seismic attenuation provides further evidence for thin (~80-90 km) lithosphere above the CAA. Eocene volcanism can be explained by fractional crystallization, with felsic magmas forming at shallow crustal depths and mafic magmas characterized by typical ocean island basalt trace element signatures, suggesting melting of an asthenospheric source.
Inherited lithosphere structure coupled with shearing-flow can organize small-scale convection. The present-day lithosphere configuration could result from shear-flow and thermal ablation of the lithosphere with melt migrating beneath the thinnest part of the lithosphere. Edge-driven convection with a downwelling at the cratonic lithosphere to the west predicts a linear anomaly that follows the strike of the cratonic boundary, displaced 600-1000-km to the east. The present-day transition zone topography and absence of a linear volcanic trend requires any deep-seated anomaly (thermal or chemical) be an isolated blob. These models rely on thermal ablation of the lithosphere and are limited by that time scale. The relatively sharp edges of the thin lithosphere suggest the process may be ongoing. The ~100-km wide thinned lithosphere and absence of crustal modification are consistent with a lithospheric Rayleigh-Taylor (RT) instability. After detaching, the lithosphere sinks and upwelling return flow produces decompression melting. The missing lithosphere cannot be traced to features in the tomography models or transition zone topography. Regardless of mechanism, the Jurassic and Eocene volcanism require two events. This is not a natural outcome of any of these mechanisms.
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