This is an aeromagnetic map of the eastern margin of North America showing, among other things, Brunswick magnetic anomaly (BMA) and East Coast magnetic anomaly (ECMA).COURTESY OF THE GEOLOGICAL SOCIETY OF AMERICA
Geoscientists have identified a chunk of Africa stuck onto the southeastern United States.
A long mysterious zone of unusual magnetism that stretches from Alabama through Georgia and offshore to the North Carolina coast appears to be the suture between ancient rocks that formed when parts of Africa and North America were pressed together 250 million years ago. If so, Africa could have left a lot more behind in the American southeast when the conjoined continents rifted apart and formed the Atlantic Ocean.
“There are some large faults in the magnetic data,” said geologist Robert Hatcher of the University of Tennessee in Knoxville, regarding what is called the Brunswick Magnetic Anomaly and other magnetic features in the region. “They have not been active for a very long time. They are strike-slip faults like the San Andreas today. But there’s also younger fall with opposite direction.”
The faults appear to be the remains of the collision and then messy divorce of Africa and North America.
“There was an attempt to rip away Florida and southern Georgia,” said Hatcher. “So you have a failed rift there. We know there’s a suture there between African crust and newer crust from the Appalachians. There are pieces of crust that started in Africa.”
A rift is what happens when the crust is pulled apart. When that happened 200 million years ago, 50 million years after African and North America collided, it appears to have started near the old collision zone, but then shifted to weaker crust to the east.
That rift zone split open and caused volcanic eruptions which created new oceanic crust — what is today the crust of the Earth under the Atlantic Ocean. The rifting continues today at what’s called the Mid-Atlantic Ridge.
“The age of the suture zone is believed to be about 250 million years old, but that’s not very well constrained,” said geologist Elias Parker, Jr., of the University of Georgia in Athens. He published a paper reviewing what’s known about the Brunswick Magnetic Anomaly in the latest issue of GSA Today.
The big challenge in sorting out the history of the southeast U.S. is that there are intriguing magnetic signals, as well as gravimetric measurements, but there is not enough deep borehole studies or seismic data to confirm the faults and the proposed scenarios.
“There are deeper faults and more shallow features,” said Parker. “It makes the interpretation really challenging.”
Among the seismic projects that could help increase the resolution of the structures, said Parker, is the Southeastern Suture of the Appalachian Margin Experiment (SESAME) and the Suwanee Suture and Georgia Rift Basin Experiment.
“This is just the start to understanding the structure of the southeast U.S.,” said Parker. “What I’m trying to do is come up with a simple explanation for this.”