Mar. 9, 1999 HOUSTON, March 3, 1999 -- For three decades, geologists have been mystified by one of the world's largest disappearing acts: How could the boundary between two immense continental plates be geologically detectable for a long stretch, then vanish from scientific view?
The two plates in question border one another all the way from Arabia to the Antarctic plate. For years, scientists had been able to locate the northern portion of their boundary along the mountainous and earthquake-prone East African Rift Valley, but they could find no evidence of the plate boundary to the south of the valley.
This 1,300-mile-long mystery has finally been solved. For the first time, geologists have been able to locate and detect motion along the southern portion of the boundary between the west African (Nubian) plate and the east African (Somalian) plate, where they meet up with the Southwest Indian Ridge, the midocean system which marks the edge of the Antarctic plate.
"We have been able to determine where the plate boundary must be and how the plates are moving," says Rice University geologist Richard Gordon, who has been studying the region for 15 years. "Using geophysical data from the Indian Ocean south and southeast of Africa, we estimated the motion of both African plates relative to Antarctica. By subtracting these two estimates, we were able to indirectly estimate the motion between the two African plates."
Gordon, the W.M. Keck Professor of Geology and Geophysics at Rice, made the discovery with Dezhi Chu, a former postdoctoral researcher in geology at Rice and currently at Exxon Production Research Company in Houston.
Gordon and Chu report their findings in the March 4 issue of the journal Nature, in a letter titled "Evidence for motion between Nubia and Somalia along the Southwest Indian Ridge."
Their findings help geologists understand how the East African rift fits in with plate tectonics. It will also allow improvements in the global models that help accurately predict the motion between India and Eurasia where they collide and raise the Tibetan plateau and the Himalayas.
For years, geologists have realized that the secret to solving the mystery was to identify movement caused by the interaction between the two plates. In many instances, it's easy to pick up such movement. It can be on a dramatically seismic scale: continental plates are subject to some of nature's most powerful forces, capable of creating mountains, earthquakes and volcanoes. South of the Rift Valley, however, geologists had no such helpful clues.
Gordon and Chu were able to pick up slow movement of both African plates relative to Antarctica by using two types of observations made at sea. They looked for subtle variations in the strength of the magnetic field observed near the sea surface above the Southwest Indian Ridge by ships and airplanes. By comparisons to historical data, they were able to calculate how fast the African plates are moving away from Antarctica at different locations along the ridge. They also used sonar data collected by many different ships to estimate the direction of motion between Africa and Antarctica at numerous locations along the ridge.
Gordon and Chu tested their data against two possible models of the region, how a single rigid plate is predicted to behave, and how two separate plates are predicted to behave.
"The Nubian plate near the Southwest Indian Ridge moves faster than what we would expect and clockwise of what we would expect if it was a part of the same rigid plate as the Somalian plate," Gordon says. "Statistically, the motion we observed fits the model of two distinct plates much better than the model of a single, rigid plate."
By analyzing and piecing together the data, they were able to construct a careful picture of where the boundary between the plates is located, where the plates are headed and how fast they are moving.
The extremely slow motion Gordon and Chu found radiates from a pivot point located in the Indian Ocean off the coast of eastern South Africa. Just as with a pair of scissors, right around the pivot the motion is very small, and motion is greater at points farther away.
North and northwest of the pivot point, the two plates are moving apart, where the East African rift is located. South and southeast of the point, the plates are moving toward each other. The place where the motion is the fastest, only about 6 millimeters a year, is in the northern end of the separating East African rift. To the southeast of the pivot point, the speed of the plates coming together is about 2 millimeters per year. In contrast, the separation rate across the world's slowest spreading mid-ocean ridge is about 12 millimeters per year, and the median spreading rate across all the world's ridges is 50 millimeters per year.
Gordon and Chu believe the boundary between the Nubian and Somalian plates is very wide and diffused, rather than narrow and localized, as is the case across midocean ridges. This means any activity associated with the newly discovered boundary is spread over a wide area, and rumblings, however small or large, can occur over hundreds of miles away.
The results may bear on the safety of a recently constructed dam in Lesotho, where controversy is ongoing about potential earthquake hazards if a plate boundary goes through it or near it.
"This research indicates that there is a plate boundary that must pass through or near Lesotho and adjacent South Africa," Gordon says. "But on the other hand, because it is near to the point of rotation, it indicates that the deformation rates are relatively low."
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