August 20. Appalachian folding

Map of fold belt in central Pennsylvania from Pennsylvania Geological Survey

We talked about the Appalachian-Ouachita orogeny a lot last month, as Gondwana collided with what is now eastern North America in one of the main acts of the assembly of Pangaea. Collision was certainly underway during the Pennsylvanian Period, last month, but it also certainly continued into the Permian. Even after the land masses were attached, compression continued. Consider India and Eurasia – they’ve been colliding for at least the past 30 million years, and the consequences of the ongoing collision are seen in earthquakes throughout the Himalayan region today, and well beyond. The Appalachian orogeny was a similar event. 

It seems that especially the deformation, folding and faulting, that extended well into the continent and beyond the zone of active collision took place in the early to middle Permian Period. Some of the broad folds that dominate central and western Pennsylvania today were probably formed during the Permian. They still control the topography, 275 million years later, as erosion preferentially focuses on less resistant rock layers, leaving the more resistant beds as long high-standing ridges. You can get such mountain ridges even in a syncline, a down-folded zone in the rock, because of alternating high- and low-resistant beds.

The geologic map of Pennsylvania shows a beautiful zig-sag pattern in the rock units, because the folds, anticlines and synclines, have been tilted. Visualize a sheet of paper, squeezed so that it forms a scoop-like bend. Then point the end away from you down. Then, cut the paper off along a line parallel to the floor. The edge of the paper will make a broad U or V shape, depending on how tightly you bent the paper. That’s the zig-sag effect of folded rocks that have been tilted so that they plunge down into the earth.

I think the consensus is that most of the deformation in the Appalachian-Ouachita belt of North America was pretty much over by late Permian time. At that point, Pangaea with a major mountain range running through part of it was pretty much just sitting there. I’m not saying there were no earthquakes – I bet there were plenty. But the intense folding and faulting that resulted from two continents impinging on each other was pretty much done. And actually, as we’ll hear in a few days, it’s possible that the supercontinent was already beginning to break apart as early as the late Permian. Nothing lasts forever!

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Today’s birthday is Eduard Suess. His name came up a couple weeks ago as the Austrian geologist who used the widespread nature of glossopteris fossils to suggest the existence of the supercontinent of Gondwana. He was born August 20, 1831.

—Richard I. Gibson

Appalachian folding 

Map of fold belt in central Pennsylvania from Pennsylvania Geological Survey