NSF PR 95-47 - July 7, 1995
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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 LamontDoherty'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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