Deep heat beneath the United States traced to ancient rift with Greenland

(Press-News.org) A large region of unusually hot rock deep beneath the Appalachian Mountains in the United States could be linked to Greenland and North America splitting apart 80 million years ago, according to new research led by the University of Southampton.

The scientists argue it is not, as has long been believed, the result of plate tectonic movements causing the continent of North America to break away from Northwest Africa 180 million years ago.

The hot zone in question is the Northern Appalachian Anomaly (NAA), a 350-kilometre-wide region of anomalous hot rock that sits about 200 km beneath the Appalachian Mountains in New England.

The research, published today in the journal Geology, suggests the NAA in fact developed about 1,800 km from where it is now, when the Earth’s crust began to break apart near the Labrador Sea between Canada and Greenland.

Over time, this area of hot, unstable rock deep under the Earth’s surface has slowly moved to where it is today – at a rate of approximately 20 km per million years.

The research was undertaken by the University of Southampton in the UK, the Helmholtz Centre for Geosciences in Potsdam (GFZ), Germany, and the University of Florence in Italy.

Tom Gernon, lead author of the study and Professor of Earth Science at the University of Southampton, said: “This thermal upwelling has long been a puzzling feature of North American geology. It lies beneath part of the continent that’s been tectonically quiet for 180 million years, so the idea it was just a leftover from when the landmass broke apart never quite stacked up.

“Our research suggests it’s part of a much larger, slow-moving process deep underground that could potentially help explain why mountain ranges like the Appalachians are still standing. Heat at the base of a continent can weaken and remove part of its dense root, making the continent lighter and more buoyant, like a hot air balloon rising after dropping its ballast. This would have caused the ancient mountains to be further uplifted over the past few million years.”

‘Mantle wave’ theory

The scientists turned to a new idea they recently proposed called ‘mantle wave’ theory, which was recognised as a finalist for Science magazine’s 2024 Breakthrough of the Year.

The theory describes how hot, dense rock slowly peels away from the base of tectonic plates – much like blobs in a lava lamp – after continents break apart. These ‘waves’ ripple along the lower surfaces of the continents over tens of millions of years and can help explain rare volcanic eruptions that bring diamonds to the surface, and why some inland regions are unusually high.

Using advanced geodynamic simulations, seismic tomography data (like a medical ultrasound, but using seismic waves to image Earth's interior) and tectonic plate reconstructions, the research team traced the likely origin of the NAA to the breakup of the Labrador Sea, which occurred between 90 and 80 million years ago when Greenland separated from Canada.

Professor Sascha Brune, co-author of the study who leads the Geodynamic Modelling Section at GFZ in Potsdam, Germany, said: “These convective instabilities cause chunks of rock, several tens of kilometres thick, to slowly sink from the base of the Earth’s outer layer known as the lithosphere. As the lithosphere thins, hotter mantle material rises to take its place, creating a warm region known as a thermal anomaly.

"Our earlier research shows that these ‘drips’ of rock can form in series, like domino stones when they fall one after the other, and sequentially migrate over time. The feature we see beneath New England is very likely one of these drips, which originated far from where it now sits.”

If correct, the NAA has likely been slowly migrating south-westward across the North American lithosphere at a rate of approximately 20 kilometres per million years, which is broadly consistent with independent geodynamic predictions. The NAA’s current size (roughly 350 km across) and depth align closely with what models predict for such migrating instabilities.

Based on the team’s research, the centre of the anomaly is estimated to pass beneath the New York region within the next 15 million years.

A mirror of the NAA

The study also proposes that a similar anomalous hot zone beneath north-central Greenland may share the same origin, making it effectively a mirror image twin of the NAA – having emerged from the opposite flank of the Labrador Sea as it drifted apart.

Beneath Greenland, this thermal anomaly contributes to elevated heat flow at the base of the kilometres-thick ice sheet, influencing how the ice moves and melts today.

In this way “ancient heat anomalies continue to play a key role in shaping the dynamics of continental ice sheets from below,” said Professor Gernon.

Dr Derek Keir, study co-author and tectonics expert at the University of Southampton and the University of Florence, said: “The idea that rifting of continents can cause drips and cells of circulating hot rock at depth that spread thousands of kilometres inland makes us rethink what we know about the edges of continents both today and in Earth’s deep past.”

The team’s findings build on research indicating that deep Earth processes can continue to unfold for many tens of millions of years after surface plate boundaries have ceased to be active.

These long-lived instabilities can shape everything from regional uplift to patterns of volcanism and erosion, even across parts of the continental interiors previously thought to be geologically stable.

Professor Gernon added: “Even though the surface shows little sign of ongoing tectonics, deep below, the consequences of ancient rifting are still playing out. The legacy of continental breakup on other parts of the Earth system may well be far more pervasive and long-lived than we previously realised.”

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