Africa-America split: back to the suture.

Earth scientists mapping the United States at considerable depths have located the "suture" between North America and an African fragment -- now called Florida -- that was left behind when the two continents parted 190 million years ago, according to two papers to be published in GEOLOGY. This connecting seam, which runs roughly east-southeast beneath southern Georgia, probably first formed 300 million years ago when the drifting African and North American continents collided to form the supercontinent Pangea, says Douglas Nelson, an associate researcher for the Consortium for Continental Reflection Profiling (COCORP) at Cornell University in Ithaca, N.Y.

Because Florida's oldest rocks and fossils more closely resemble those of Africa than those of the rest of the United States, earth scientists have long suspected that Florida might have once been part of Africa. But no one knew until now exactly where the fragment's geologic boundary was. Using a sonarlike technique called seismic reflection profiling (SN: 12/8/84, p. 364), Nelson and his colleagues have detected the suture and mapped its course beneath the sedimentary layers of Georgia's coastal plain. The 68-kilometer-wide, wedge-shaped suture ranges from 5 to 35 kilometers deep and is inclined 15 to 25 degrees toward the south. Because the suture runs beneath the Brunswick Magnetic Anomaly -- an area where the earth's magnetic field is unusually weak -- researchers hope that other magnetic anomalies will also turn out to indicate crustal plate boundaries and other important geologic structures.

Because the suture is deeper than most wells are drilled, its exact composition is unknown. However, Nelson says it is probably made up of debris accumulated when the two crustal plates collided head-on, forcing one plate beneath the other. Sediments on top of the diving plate would be scraped up onto the leading edge of the overriding plate, he says.

The research, which is part of a larger mission to map in three dimensions the continental plates, will help explain plate tectonics. "If we can reconstruct the history of plate motion through time," says Nelson, "we may be able to understand the process."