The crust and upper mantle structure of the northeastern United States (NEUS) was studied by combining teleseismic and regional body-wave observations with surface-wave dispersion measurements. The velocity models suggest that structures as much as 200 km deep and even deeper can be correlated with surficial geologic and tectonic features and that the Grenville and Appalachian orogenic belts show marked differences in crustal structure. This has the important implication that major orogenic belts have effects that reach well into the lithosphere and are stable for extended periods of time, perhaps as long as 1 b.y.

Regional travel times recorded across the NEUS seismic network indicate that the northern Appalachians are characterized by a well-defined two-layer crust with a relatively high velocity lower layer. The crust of the Grenville Province in New York State is slightly thinner than that of the New England Appalachians, and it is vertically homogeneous, with nearly constant P and S velocities.

Lateral variations in structure were studied using time-term analysis and teleseismic P-wave residuals. The suture between the Grenville and Appalachian Provinces in the northeastern United States probably occurs along a north-northeast-trending belt extending from northwestern Vermont to southwestern Connecticut. High crustal velocities and/or crustal thinning, a linear gravity high, a serpentinite belt, Precambrian uplifts, and the Taconic thrusts are found along much of this belt.

Comparison of seismic velocities with resistivity measurements suggests that the lower crust of the Grenville Province may be composed of rocks with hydrous mineral phases, resulting in lowered velocities and a higher Poisson's ratio. Alternatively, the rocks of the lower crust beneath the Grenville Province may be similar to those found on the surface, while higher velocity mafic mineralogies are prevalent in the lower crust of the Appalachians. This is consistent with the hypothesis that the Grenville crust underwent substantial reactivation, thickened, and became vertically uniform during the Grenville orogeny. In contrast, the rocks of the Appalachian belt probably were associated with a cycle of oceanic opening and closure, which suggests an ensimatic origin of the lower crust in this region.

Recent COCORP results indicate that the orogenic belt of the southern Appalachians is allochthonous and is underlain by a basal decollement that extends eastward to the coastal plain. However, contrasts in structural style and tectonic evolution between the northern and southern Appalachians suggest that it may not be possible to extrapolate the COCORP results to the northern Appalachians (north of about 41° latitude) and that the suture between the Grenville Province and the Appalachian orogenic belt in New England is located in the vicinity of, or just east of, the serpentinite belt.

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