Age of the Appalachians Part 2: Ground Truth
One of the most common canards I hear about North Carolina geologic is, “The Appalachians are the oldest mountains in the world.” I hear the same about the Uwharries, which are not even in the contest. The age of the Appalachians is tied to the question of “When were they built?” The answer is that there was no single time in which they were built or finished. The story of the Appalachians was built through observation of crosscutting relationships, then augmented with isotopic dates: See Part 1 of this blog series. The map at the top of this article is from Jim Hibbard of NCSU and his co-authors (2006, Lithotectonic map of Appalachian Orogen from the Geological Survey of Canada) which can be downloaded here. That map gives you a quick understanding of the complexity of the Appalachians.
Mountain ranges are built in events called orogenies, which go in fits and starts. The first event in the Appalachians was the Grenville Orogeny, about 1 billion years ago. Grenville age rocks are found from Canada to Alabama, so you know that whatever produced them is a continent-scale event. This orogeny was the result of collision between proto-North America and what is now the west coast of South America. A map from Dr. Chris Scotese’s PaleoMap Project is available here. Grenville-age rocks are found beneath the Amazon. In North Carolina, Grenville-age rocks are exposed in the Blowing Rock gneiss, the Cranberry gneiss, the older rocks beneath Pilot Mountain and Hanging rock, and the Toxaway gneiss.
So which is older, North American Grenville or South American Grenville? It’s a tie. This is a recurring theme in plate tectonics: a tectonic event, like an orogeny or rifting, usually has a mirror on the other continent involved. Even though these areas are now widely separated, they used to be much closer.
Other events in the Appalachians include the Taconic Orogeny and the Acadian Orogeny, with other orogenies recognized locally. By locally, I mean in northern, central or southern Appalachians. We’ll skip over them, because they get very complicated, and go straight from the start of collisions to the end of the collisions. There’s one view of it here, with nice pictures, although geologists in the Southern Appalachians might take issue with the details. The point is, we have several episodes of mountain building, usually re-cycling parts of earlier episodes.
The final event in the Appalachians was the Alleghanian Orogeny, widely accepted to be coincident with the collision of North America with Gondwana, a conjoined Africa and South America. Another good map is here, or here. When did it start and end? This brings up a common problem in tectonics: diachronous events. Isotopic dating shows that the Alleghanian in the Northern Appalachians is a bit older than in the Southern Appalachians. Evidently the Iapetus ocean closed like a zipper instead of like an elevator door. So you can’t put a single date on the Alleghanian Orogeny for the whole mountain chain.
Collisions like thus produce scars in the form of faults, and produce granites. Alleghanian thrust faulting further built the Appalachians, and the Alleghanian Orogeny recycled parts of the earlier Grenville and Taconic and Acadian Orogenies. The collision also produced granites around 300 million years in age, now found as Mount Airy, Rolesville, Churchland, Castalia, Sims, and Lilesville granites. We can say that active collision, in North Carolina, was completely done by the end of the Permian Period, about 245 million years ago. Take a look at the granites, just chock full o’ zircon and dated with relative ease. Granites that have been deformed by squeezing, and granites that have not, straddle the age of 245 million years ago.
Then collision stopped. About 200 million years ago, North America and Africa rifted apart, creating Triassic basins and the Atlantic Ocean. Now, the East Coast is a passive margin, quietly piling up sediments, and the Appalachians are no longer undergoing active thrust faulting.
You may be asking, “Are we done yet?” Well, the Appalachians aren’t done yet. Recent sedimentary studies suggest that there has been Cenozoic uplift in the Southern Appalachians over the past 65 million years. If you look at the topography in southwestern North Carolina, you can see that it is much more rugged than what surrounds it. (If you drive Highway 64 up the Nantahala Gorge, you’ll see what I mean.) This all suggests that the mountains are bobbing up a bit, trying to reach some equilibrium between crust and mantle. This was underscored by a recent article from NCSU scientists, Sean Gallen, Karl Wegmann, and Del Bohnenstiehl, published by the Geological Society of America in GSA Today. The article is here.
As an added treat, you can hear Dr. Wegmann talk about his research, on Thursday, April 10, 2014, starting at 7 p.m. at the Daily Planet cafe. He also works on landslides, so this is a good chance to ask questions about the recent events in Oso, Washington.