Northern Appalachian Anomaly

Résumé:

A new geology paper by Thomas Gernon and colleagues proposes that the Northern Appalachian Anomaly—a puzzling low-velocity zone under New England—is a migrating Rayleigh-Taylor instability triggered by Labrador Sea rifting about 80 million years ago.

This geodynamic legacy challenges conventional views of a mechanistic link to the Central Atlantic passive margin.

Étude:

The Northern Appalachian Anomaly (NAA) is a prominent low-seismic-velocity zone, ∼400 km in diameter, in the asthenosphere beneath New England (northeastern USA). Previous studies interpreted this shallow feature, occurring at a depth of ∼200 km, as a thermal anomaly tied to edge-driven convection along the North American continental margins. Those studies recognized, however, that upwelling here is highly unusual given that the passive margin has been tectonically quiescent for ∼180 m.y. We propose an alternative model, based on geologic observations, geotectonic reconstructions, and geodynamic simulations, that the anomaly instead represents a Rayleigh-Taylor instability linked to the breakup of the distant Labrador Sea continental margin. A Labrador Sea origin at breakup, ca. 85−80 Ma, would imply the migration of a chain of Rayleigh-Taylor instabilities at a rate of ∼22 km/m.y., close to expected rates from geodynamic models. A migrating-instability origin for the anomaly can reconcile its spatial characteristics, depth profile, and position near a long-inactive continental margin. A corollary is that the north-central Greenland anomaly, a mirror-image of the NAA, also potentially originated at the time of breakup. Further, The Central Appalachian Anomaly may fit this model if it represents an early-stage instability linked to rifting onset in the Labrador Sea. The NAA and other associated anomalies viably represent a legacy of continental rifting and breakup along the distant Labrador Sea margins.