Spatial variations in Te in the southern Appalachians, eastern United States

Various studies in the oceans have shown that the flexural rigidity, or equivalently effective elastic thickness (T-e), of the lithosphere is determined by the load and plate age. The results of studies in the continents have, however, been more controversial. Determinations of T-e made using spectral studies and based on the Bouguer anomaly coherence technique and surface and subsurface loading suggest that North America has a high T-e "core" (> 100 km) which is flanked by lower values. In contrast, studies based on the free air admittance suggest that T-e of North America is < 25 km, and of the order of the seismogenic layer thickness. It has been proposed that this discrepancy results from the fact that estimates based only on Bouguer coherence may be biased upward due to topographical erosion which introduces "noise", especially at short wavelengths. In order to address this question further, we have used a maximum entropy based coherence method to determine the wavelength relationship between gravity and topography in the southern Appalachians, a region where the Bouguer coherence, free-air admittance, and forward modeling techniques have already been applied. Our studies reveal a variable T-e structure with a mean T-e of 51 km and values which have a range 20 to 100 km. The mapped T-e fabric has a distinct NE-SW.trend which appears to follow the tectonic elements of the southern Appalachians. In particular, the foreland is generally associated with higher T-e values than the flanking orogenic belt. Correlations at smaller scales are difficult, however, to establish. The T-e fabric does not reflect the complex terrains that make up highly deformed regions within the orogenic belt. Our spectrally determined T-e estimates are in close agreement with ones based on forward modeling. They are a factor of 4 higher, however, than results previously based on free-air admittance. We attribute this to the fact that we have used the Bouguer coherence technique which accounts for both surface and subsurface loading. Our results suggest that while other factors such as erosion may upwardly bias T, in some regions, buried loads satisfactorily explain the spectral estimates in the southern Appalachians, irrespective of whether they are based on Bouguer coherence or free-air admittance. There is solid geological evidence in the southern Appalachians which is readily attributable to the presence of subsurface loads. This is a strong argument, we believe, to suggest that T-e in the continents is high and can have values which range from 20 to 100 km.