July 26. The African Suture

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)

Across southern Georgia, and extending east into the Atlantic Ocean and west across Alabama, the Brunswick magnetic anomaly is a long, curving magnetic low, representing a strong contrast in magnetic material deep in the earth’s crust. There is plenty of discussion about its exact nature – is it a fault zone, a string of intrusive igneous rocks, simpler changes in rock type, or something else. But fundamentally, it represents the zone along which the leading edge of Gondwana collided with the southeastern margin of North America. We call this zone of amalgamation a suture zone, where continents or other blocks have been attached to each other, like a huge medical suture using a lot of different ways to do the attaching.

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.

It wasn’t until the Jurassic Period, which we will get to next October, that Pangaea began to break apart again. As you might expect, it broke apart along its weakest zones, and some of the weak zones were the highly faulted and deformed areas where the continents had come together in Pennsylvanian-Permian time. Africa rifted away from North America approximately along the old suture between them – but not exactly. The rift split along different lines in what is now the southeastern United States, and Florida and southern Georgia and southern Alabama and the adjacent continental shelves got left behind. A bit of Gondwana had become part of North America.

—Richard I. Gibson

Links:
http://www.livescience.com/44633-brunswick-magnetic-anomaly-explained.html
http://www.geosociety.org/news/pr/2014/14-22.htm

1988 COCORP study
Brunswick anomaly

July 16. Stone Mountain, Georgia

The southeastern part of the United States bore the brunt of the collision with Gondwana. What is now the westernmost part of Africa, around Senegal and Mauritania, collided with the Carolinas and Georgia. This was a true continent-continent collision, similar to that between India and Eurasia, and the mountain uplift squeezed between the continents was a huge one. 

When continents collide, everything doesn’t just go up. Plenty of material is forced down, too. Far enough down for temperatures and pressures to change the rock, to metamorphose the rock into radically different forms. A sandstone with quartz and feldspar and iron oxide cement and other grains can be changed into a banded rock called gneiss, with the chemicals in the original rock reorganized into entirely different minerals, often arranged in thin layers whose geometry is related to the orientation of the pressure regime.

Under such conditions, melting can also happen, and molten rock, magma, is typically associated with collision zones. Because of the chemistry of the rocks involved, it’s actually pretty easy to see the differences between oceanic subduction, with a slab of oceanic crust diving down beneath a continent and giving rise to a volcanic mountain range like the Andes in western South America, and the melting of complex continental crust which tends to be more silica rich, more granitic in composition.

Photo by Kyleandmelissa22 via Wikipedia, public domain.

In Georgia, Stone Mountain outside Atlanta is a solidified granitic body that formed as a result of the collision of Gondwana and North America about 300 million years ago, near the end of the Pennsylvanian Period. Technically the rock is called quartz monzonite, but it amounts to granite with a lower percentage of quartz than most granite. Petrologists have dozens of terms for rocks to give them a way to talk about specific compositions more clearly.

The granitic rock at Stone Mountain solidified well within the earth’s crust, probably 5 to 10 miles down. The difference between chemically similar deep-seated or plutonic rocks (from the realm of Pluto) and those that solidify on the surface, such as lava flows, is their grain size. At the surface, molten rock cools and solidifies quickly, so individual crystals don’t have time to grow very large. Insulated within the earth, granites and other plutonic rocks solidify over many thousands or even millions of years, so their crystal grains are relatively large, sometimes several centimeters across.

Over time, with more uplift and more erosion, the solid granitic rock was brought to the earth’s surface. It is more resistant than the surrounding rock, so it eventually eroded into the prominent dome-like feature that stands outside Atlanta today. There is a huge carving of Confederate Generals on one side of the mountain.

—Richard I. Gibson

Reference:
Age and origin of the Stone Mountain Granite, Lithonia district, Georgia (Whitney, Jones, and Walker, 1976)


Photo by Kyleandmelissa22 via Wikipedia, public domain.