Emerging Consensus on the Structure of the Lithosphere in Eastern North America

The Appalachian Orogen formed in the course of a complex continental collision culminating in the assembly of the last supercontinent, Pangea. Multiple lines of geophysical evidence agree that the lithosphere beneath it is thinner relative to the Proterozoic or older regions of North America. They diverge on most other aspects of the Appalachian lithosphere, such as its thickness, the location of thickness change, and the provenance of the lithosphere beneath the Appalachian terranes. Two new studies with different scope and data yield consistent estimates of both the thickness of the lithosphere and the way it changes from the Grenville Province of New York to the Appalachians of New England. A global thermodynamic model of Fullea et al. (2021) combines surface wave seismic tomography, gravity, heat flow and petrological constraints to evaluate temperature and density distributions in the upper mantle and sets the base of conductive lithosphere at the temperature of 1300C, with a buffer of transitional thermal properties extending beneath it. A regional survey of closely (50-70 km) spaced long-operating seismographs by Li et al. (2021) documents abrupt vertical changes in impedance (velocity times density) and rock texture (velocity anisotropy) in the upper mantle, using them to define the vertical extent of the rigid lithosphere. Both studies show that Grenville Province lithosphere extends to depths 150 170 km near its eastern edge while the Appalachian lithosphere is 110-130 km thick in Maine and central New England. The lateral change in thickness is abrupt and coincides with the eastern edge of the Grenville Province on the surface. A hypothesized extension of Grenville lithosphere beneath the Appalachians can involve only thinned rifted margins of proto-North America (Laurentia). No obvious difference is noted between Appalachian terranes with Laurentian and proto-Africa (Gondwana) affinities, implying that the Pangea-forming collision, or later events have homogenized the mantle lithosphere. Changes in impedance and anisotropy reside below the estimated thermal depth limit of the lithosphere in many, but not all, locations. This points to a complex nature of the thermal buffer region that may involve dynamically sustained rapid lateral changes in physical properties of the upper mantle.