Anisotropic seismic velocity in the crust and upper mantle of northeastern America from surface wave tomography

Northeastern America consists of the Grenville province, the Appalachian orogen, and the passive Atlantic margin. The area moved over the Great Meteor hotspot at 110-130 Ma and is dotted with many mafic intrusions. The rich tectonic history here makes it an ideal place to study how the continental lithosphere evolves during orogenesis, rifting, and the interaction with hotspot. This dissertation develops a high-resolution 3-D anisotropic shear-wave velocity model in northeastern America using surface wave tomography. The crust is thin near the Atlantic coast and relatively thick in the Grenville Province and the Appalachians but relatively thin from Lake Huron to the St. Lawrence River. In the upper mantle, a significant low-velocity anomaly is imaged in New England at 60-180 km depths (NAA). A low-velocity anomaly is also imaged in northwestern Virginia at 80-160 km (CAA). Both anomalies correspond to a positive radial anisotropy and coincide with previous hotspot tracks. They are probably caused by residual hot plume material combined with a warm asthenosphere at the base of the indented lithosphere due to hotspot interactions. The NAA is much broader and stronger, probably because the area interacted with two hotspots at different times. Azimuthal anisotropy at the NAA is small with fast directions in a circular pattern at 80-140 km, suggesting complex small-scale horizontal flow. A newly observed feature is the positive radial anisotropy in the mantle lithosphere beneath southernmost Ontario among Lake Huron, Lake Erie, and Lake Ontario. Interestingly, no notable velocity anomaly is observed in the area above 100 km, corresponding to the strongest anisotropy. This anomaly is at the Great Meteor hotspot track ~130 Ma and probably reflects the fossil plume head. Even though the velocity anomaly from the hot plume material largely disappears due to thermal diffusion, the associated strong radial anisotropy is frozen in the lithosphere, providing evidence for the past plume. Azimuthal anisotropy in the area is also small and complex, fitting the plume model. The hotspot, located right among the three lakes, possibly contributed to the formation of the eastern Great Lakes by thinning the lithosphere.