UPPER MANTLE DYNAMICS BENEATH NEW ENGLAND AND EASTERN NEW YORK: RISING CURRENTS AND FALLING FRAGMENTS

The upper mantle beneath the Appalachian Orogen in eastern North America contains regions where seismic waves speed are significantly reduced compared to the craton to the west. They are hundreds of kilomerers across, cut across the trend of the Appalachian terranes and likely post-date the Paleozoic assembly of Pangea. The most prominent of these anomalies, the North Appalachian Anomaly (NAA) beneath southern New England, has been alternatively explained as a localized disruption of lithospheric fabric, the after-effects of the Great Meteor Hot Spot passage, and present day asthenospheric upwelling due to edge-driven convection.

Seismological observations accumulated over the last decade offer improved constraints on the geometry and the properties of the NAA. Tomographic images based on EarthScope TA data Identify a compact NAA, anout 400 km across, that is largely limited to the volume east of the Appalachian Front and place it mostly beneath the relatively thin lithosphere of the northern Appalachian Orogen.

Nearly 10% shear wave speed reduction relative to the values in the stable craton is required to explain the observed lateral variation in travel times of teleseismic S waves. Amplitudes of co-located anomalies in shear and compressional speed at depth require the NAA to have a thermal origin. Furthermore, a comparison of spectra of teleseismic shear waves shows that seismic attenuation is considerably higher beneath the NAA, also implying high temperature within it. The short lateral scale of heterogeneities in attenuation values requires a highly heterogeneous asthenosphere with large differences over distances of ~100 km and suggests interaction with, and possibly erosion of, the overlying lithosphere. Indicators of upper mantle anisotropy suggest a regionally consistent pattern of deformation, with ~N80E orientation, in good agreement with the absolute plate motion. A clear departure from this pattern is seen beneath the NAA where subvertical asthenospheric flow disrupts the pattern. Taken together, new constraints on the upper mantle structure and texture characterize northeastern North America as a highly heterogeneous volume where elevated temperature in sublithospheric mantle likely drives localized upwellings and promotes small-scale unrooting of the overlying lithosphere.