Pangaea’s breakup may have been an outside job.
A reexamination of tectonic movements 200 million years ago suggests that the supercontinent was pulled apart by shrinking of the forerunner to the modern Indian Ocean. The new work, presented online February 27 in Geology, signals that scientists may have to rethink Pangaea’s demise, says geologist Stephen Johnston of the University of Victoria in Canada, who was not involved with the research.
“Everything we think we know about Pangaea is up in the air now,”Johnston says.
Roughly 300 million years ago, all of Earth’s major landmasses squashed together to form Pangaea, the planet’s most recent consortium of all the continents (SN Online: 6/18/12). Around 100 million years later, the supercontinent began breaking apart with the birth of the Atlantic Ocean between what would become North America and Africa.
Because the planet’s size doesn’t change, the creation of new crust at the bottom of the Atlantic had to be compensated by the destruction of crust elsewhere at a subduction zone, where surface material plunges into Earth’s interior.
Geoscientists have proposed two sites for where this subduction might have taken place: the ancestors of the modern Indian and Pacific oceans. The forerunner of the Indian Ocean, called the Tethys Ocean, shrank around this time as the early African and Eurasian continents drifted together in a scissoring motion. To the east, the western edge of North America may have steamrolled over the Paleo-Pacific Ocean.
Determining which ocean accommodated the Atlantic crust formation poses a challenge because of the planet’s shape, says earth scientist Fraser Keppie of Nova Scotia’s Department of Energy in Halifax. Flat maps distort what areas are parallel to one another and therefore conducive to forming a conveyer belt between emerging and sinking crust. Instead of a traditional map anchored at the geographic poles, Keppie created a circular map centered on a fixed point near Southern Europe. Nearby continents rotated around that point like the swinging hands on a clock.
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CIRCULAR REASONING Visualizing the movements of the continents as swinging around a fixed point reveals that the opening of the Atlantic Ocean (bottom left) was parallel to the closing of the Tethys Ocean (top right). As the Atlantic grew, the Tethys shrank to accommodate the new crust, new research proposes. Source: D.F. Keppie/Geology 2015
Looking at the circularly shifting continents from this perspective, Keppie saw clearly that both the boundary where the Tethys crust sank and the rift where Atlantic crust formed extended outward from the circle’s center. The Paleo-Pacific’s edge, on the other hand, sat along the circle’s rim, perpendicular to the other two regions. This arrangement establishes that as the continents rotated around the fixed central point, the Atlantic’s growth connected to the dwindling Tethys, not the Paleo-Pacific, Keppie says.
“When I first saw this, I was really shocked,”he says. “It was absolutely apparent that the Atlantic and the Tethys are the compensation system, not the Atlantic and the Pacific.”
Keppie proposes that the Tethys didn’t just accommodate Pangaea’s breakup — it was the driving force behind the supercontinent’s fragmentation. As gravity pulled the Tethys crust down into the subduction zone, the crust yanked on Pangaea’s Eurasian edge. If strong enough, this tug could have ripped the continent apart between Africa and North America, a weak point left over from where two earlier landmasses had stitched together to form Pangaea.
The current prevailing explanation contends that material from the Earth’s interior sprung up along the boundary between North America and Africa, forcing the two continents apart. This push, rather than pull, hypothesis makes less sense, Keppie says, because it posits a big coincidence that the new material just happened to bubble up along one of Pangaea’s seams.
Keppie’s work isn’t the final say on Pangaea’s division, Johnston notes. But it does make predictions that geologists can now test, such as an ancient fault in the Pacific where two plates scraped together. “The great part about this work is that it’s clear, simple and testable — we can go out into the field and look at the rocks in light of his model and test it,”Johnston says.