Earlier this year, 2014, there was a flurry of news reports in the popular science press about the Brunswick Magnetic Anomaly in southern Georgia as the expression of the old boundary between Africa and North America when they came together in Pennsylvanian and Permian time.
That’s fascinating, but it isn’t news. We’ve known this pretty much since the early 1980s or longer. There have been some new studies addressing the details of what’s going on there, but the fundamental nature of this linear zone has been known for years.
A magnetic anomaly is a departure from the broad general magnetic field of the earth. The magnetic anomalies that geologists like me care about represent geology, because they represent differences in magnetite content that can allow us to infer things about the subsurface. Most of my career, since 1975, has been focused on studying magnetic anomalies as well as anomalies in the earth’s gravity field, and trying to figure out what they mean. I made an interpretation of the magnetic map of the U.S. including the Brunswick anomaly as the suture zone between Gondwana and North America back in 1988.
|Part of Magnetic Map of North America (USGS) showing Brunswick Anomaly (curving blue zone)|
You usually see this part of the U.S. referred to as part of Africa, but the zone that includes all of Florida, some of the Florida Shelf, and the southern parts of Georgia and Alabama as well as possibly the Bahamas and points to the southeast were really part of Gondwana, occupying a triangular zone between what is now Senegal to Liberia on the African coast and Venezuela and the Guianas down to northeastern Brazil on the South American coast. This little bit of Gondwana had been part of the supercontinent for at least a few hundred million years, and it was at the forefront of part of the Allegheny-Appalachian collision in Pennsylvanian time that welded Gondwana to North America. They remained attached for close to 100 million years, forming part of the supercontinent of Pangaea.
|White line is a highly generalized boundary of the terrane that was once part of Gondwana, but is now part of North America.|
—Richard I. Gibson
1988 COCORP study