Hotspot Signature beneath Eastern North America from Surface Wave Tomography

Abstract:

The New England area passed the Great Meteor hotspot during 115-140 Ma and an inferred hotspot at 50 Ma. We have developed shear wave velocity and anisotropy models in eastern North America from Rayleigh and Love wave tomography to understand the lithosphere-hotspot interactions. The data are from teleseismic earthquakes recorded at 220 broadband seismic stations of the USArray Transportable Array. The Grenville province is characterized by high velocity to 240 km depth in the upper mantle, weak radial anisotropy, and an average ENE-WSW fast direction. The most outstanding feature is a low-velocity anomaly beneath New England that correlates with strong positive radial anisotropy (Vsh>Vsv). This anomaly consists of a narrow column above 90 km depth and broadens westward at depths of 120 to 200 km. The average fast direction of the anomalous area is roughly EW at periods less than 100 s and NE-SW at long periods, indicating two layers of anisotropy. The 2-D model reveals a rotation pattern of fast direction at intermediate periods, reflecting complicated horizontal mantle flow. The complicated anisotropy structure in the low-velocity area is consistent with small shear wave splitting delay times found in this place. The shallow low-velocity anomaly and its deep westward extension align on the track of the Great Meteor hotspot, and the shallow part of the anomaly is also on the track of an inferred hotspot. This anomalous upper mantle is probably the consequence of two previous hotspot interactions. We propose possible scenarios here. First, the Great Meteor hotspot thinned the lithosphere, forming an EW channel at its base. Then, the second hotspot further eroded the disturbed lithosphere on the east side, forming a lithosphere cavity. The indented, irregular lithosphere base invoked complex asthenosphere flow in the anomalous region.