One of Earth's Great Crustal Plates Cracking in Two
Scientists at Columbia University's Lamont-Doherty Earth Observatory have reported direct evidence that one of the Earth's crustal plates is cracking in two. Their research was funded by the National Science Foundation (NSF).
In a report published this week in Earth and Planetary Science Letters (vol. 133), the scientists say they have confirmed that the Indo-Australian Plate -- long identified as a single plate on which both India and Australia lie -- appears to have broken apart just south of the Equator beneath the Indian Ocean. The break has been underway for the past several million years, and now the two continents are moving independently of one another in slightly different directions.
A fundamental tenet of plate tectonics theory is that the Earth's surface is divided into rigid plates that move together and apart like pieces of a jigsaw puzzle. Scientists have long recognized 12 major plates. Now there are 13.
Scientists have known that for some 50 million years, the Indian subcontinent has been pushing northward into Eurasia, forcefully raising the Tibetan Plateau and the Himalayan Mountains. This new research suggests that starting about eight million years ago, the accumulated mass became so great that the Indo-Australian Plate buckled and broke under the stress.
"The result of this critical stage in the collision between India and Asia is the breakup of the Indo-Australia Plate into separate Indian and Australian plates," says Jeffrey Weissel, a scientist at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, New York.
"This is a newly observed way of creating a new boundary between plates," says Lamont-Doherty scientist James Cochran, who co-authored the report with Weissel, and James Van Orman, now a graduate student at the Massachusetts Institute of Technology. Van Orman, the report's lead author, was an undergraduate at Florida State University in 1993 when he began the research with Weissel and Cochran at Lamont-Doherty, as part of a summer internship program sponsored by the National Science Foundation.
"In the Central Indian Ocean, nature is conducting a large scale laboratory experiment for us, showing us what happens to the oceanic lithosphere [Earth's outer layer] when force is applied," says Weissel. Essentially pushed into an immovable object, "it can buckle like a piece of tin."
In the 1970s, scientists first discovered a broad zone, stretching more than 600 miles from east to west where the equatorial Indian Ocean floor was compressed and deformed. Drilled samples had shown that the zone had begun to buckle and crack about eight million years ago at the same time the Tibetan Plateau had reached its greatest height. Cochran was chief scientist on the ocean drilling cruise that collected this data.
More recently, researchers at Northwestern University used data on how newly created seafloor had spread outward from mid ocean ridges to the west and south of the deformed region in the Indian Ocean. They theorized that the movements of the newly created seafloor could be accommodated only if a distinct plate boundary existed between separate Indian and Australian plates across the equatorial Indian Ocean.
In relation to the Indian plate, the Australian Plate is moving counterclockwise, the Northwestern University scientists calculated. In the western part of the new plate boundary, the plates are moving away from each other. To the east, the Australian Plate is converging on the Indian Plate, they believed.
If the theory were correct, the ocean floor in the eastern part of the new plate boundary should be compressed, buckled, cracked, and eventually thrust upward along the cracks. More critically, if a separate Australian Plate were rotating counterclockwise in relation to a separate Indian Plate, the amount of compression should increase rapidly and systematically from west to east across the central Indian Ocean.
To test the theory, the Lamont-Doherty team took actual measurements of how compressed the Indian Ocean floor has become in the region believed to be the new plate boundary. Using sound waves to probe sub-seafloor rock layers, they created images of sub-seafloor structures.
The images were collected during two separate research voyages that each spanned the entire deformed zone from north to south. Weissel was aboard a 1991 cruise of the French research vessel Marion Dufresne. In 1986, aboard Lamont-Doherty's former research vessel, the Robert D. Conrad, he obtained images along a north-to-south line 185 miles to the west.
The images showed scores of systematically aligned cracks, or faults, in the oceanic lithosphere -- created as the once whole plate buckled and cracked. As the now- distinct plates continued to converge, slabs of ocean floor slid upward along the faults to alleviate the strain. The more the two plates converged, the farther the slabs slid upward. "Van Orman's summer job," says Weissel, "was to very carefully measure how far vertically the blocks of crust were thrust upward along more than 200 faults."
The measurements clearly showed that two separate plates were converging. More importantly, the thrusting observed on the French research cruise was about twice that found on the U.S. cruise. That proved that compression was more intense to the east -- confirming the Northwestern group's prediction on spreading rate and direction at the mid-ocean ridges .
"Our result therefore provides direct evidence from the deformation itself that the compression of oceanic lithosphere in the central Indian Ocean, originally regarded as 'intraplate,' is better described as constituting part of a broad boundary zone between distinct Indian and Australian plates," the Lamont-Doherty scientists wrote in Earth and Planetary Science Letters. Cochran says the research "gives insight into how strong and rigid plates are, how they respond to stress, and what their limits are before they break." Weissel adds that, "This is an important piece of work that came out of the NSF's Research Experiences for Undergraduates program. It was basically an undergraduate's summer intern project."
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