Using new information gathered from three research cruises to Antarctica, a scientific team led by Steven Cande of Scripps Institution of Oceanography at the University of California, San Diego, has located the final missing geological piece in a puzzle of plate tectonics in the Southwest Pacific Ocean.
The team's report, published in the March 9 issue of the journal Nature, reveals that East and West Antarctica started to spread about 43 million years ago and then abruptly stopped 17 million years later, after the rift between them had opened about 180 kilometers (about 112 miles).
For more than 25 years, scientists who study plate tectonics have been piecing together the story of how the earth's plates have moved and developed over tens of millions of years. In telling this tale, the scientists have systematically described how the Pacific plate, the North American plate, and others have moved at different points in time, providing a historical picture critical for geologists tracing the background of earthly material. The plate tectonics researchers arrive at their conclusions by calculating the motion of the plates. As one plate moves, the edge of an adjoining plate is affected, similar to giant pieces in a plate tectonics jigsaw puzzle, called the "global plate circuit."
The region around the Antarctic Ross Sea and the West Antarctic rift system had remained a global plate circuit mystery for more than a quarter century.
"Over the years, there has been a lot of interest in knowing about the correct plate motion around Antarctica because that is the key to understanding the motions between the Pacific plate and North American plate, and for understanding the geology of East and West Antarctica," said Cande, a professor of geophysics at Scripps and lead author of the paper. "The (global plate) circuit is linked together by these huge rigid plates over thousands of miles. So, by determining the plate motions in Antarctica, we are defining the plate motions up here in California (for example)."
The West Antarctic rift system, one of the largest active continental rift systems on Earth, is the result of movement along the boundary between East and West Antarctica. Because of a lack of information concerning seafloor spreading in this area, the timing and magnitude of the plate motions leading to the West Antarctic rift system was largely unknown.
In the Nature article, Cande, along with Joann Stock of the California Institute of Technology, Dietmar Muller of the University of Sydney, and Takemi Ishihara of the Geological Survey of Japan, reports new information about the magnetics, gravity, and bathymetry in key areas of the south Tasman Sea and the northern Ross Sea.
The data, collected in three cruises in 1996 and 1997, revealed an ancient rift valley off Cape Adare. This "Adare Trough" region, Cande and the other researchers calculated,
was the site of approximately 180 kilometers of separation between 43 million and
26 million years ago.
The inclusion of this East-West Antarctic motion in the global plate circuit explains a gap between the Pacific and Australian plates. The Adare Trough region subsequently becomes the final critical missing plate boundary in the Southwest Pacific.
The researchers note that inclusion of this motion will modify plate motion history associated with the Alpine Fault in New Zealand and affect global issues such as motion between hot spots in the Pacific and Indo-Atlantic Oceans.
"The new data helps it all fit together," said Cande. "It explains the formation of the Transantarctic Mountains. It removes a puzzling gap between the Australian and Pacific plates. Essentially it helps resolve a longstanding controversy regarding the deformation in this area to the global plate circuit linking the Pacific plate to the rest of the world."
The project was funded by grants from the National Science Foundation.
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Scripps Institution of Oceanography, at the University of California, San Diego, is one of the oldest, largest, and most important centers for global science research and graduate training in the world. The National Research Council has ranked Scripps first in faculty quality among oceanography programs nationwide. The scientific scope of the institution has grown since its founding in 1903 to include biological, physical, chemical, geological, geophysical, and atmospheric studies of the earth as a system. More than 300 research programs are under way today in a wide range of scientific areas. The institution has a staff of about 1,300, and annual expenditures of approximately $100 million, from federal, state, and private sources. Scripps operates the largest academic fleet with four oceanographic research ships for worldwide exploration and one research platform.