July 5. The Alleghenian Orogeny

I’ve been saying that Gondwana was coming for months, for millions of years. It’s finally arriving. The earlier tectonic collisions in southeastern and east-central North America were between the North American continent and various stringers – island arcs, microcontinents, branches of Baltica, and further north, in Newfoundland and Labrador and Greenland, there was a true continent-continent collision between North America and Baltica.

But the really big crunch was between what was now a combined supercontinent – Laurasia or Laurussia, the assembled North America and Europe or Baltica – and the really big supercontinent of Gondwana, which comprised most of the rest of the world’s large continental blocks.

What is now northwest Africa and northern South America, plus a triangular zone between them, was approaching the southeastern margin of North America, what is now more or less New Jersey to the Carolinas and west to Arkansas and Oklahoma. Gondwana began to arrive about 325 million years ago, six or seven million years before the end of the Mississippian, and the crunching really got underway during the Pennsylvanian.

This mountain-building event is often called the Alleghenian Orogeny, named for the Allegheny Mountains in West Virginia and Pennsylvania where the effects of the orogeny are preserved. And the rocks there were more specifically deformed at the time we’re talking about now, the very late Mississippian and the Pennsylvanian. The term Appalachian Orogeny can be used to refer to the entire spectrum and time span of the events that created the diverse ranges of the Appalachian Mountains, and that would reach from late Ordovician at least into the Permian, 200 million years or more. So Alleghenian Orogeny is a better, more specific term.

How can we distinguish the related, episodic events from each other? The simple way is to look at which rocks are deformed. If Ordovician rocks are folded and faulted, but Devonian rocks are not, then the deformation must have occurred after the Ordovician rocks were formed but before the Devonian layers were laid down. It’s the old law of superposition – deposits or events that come earlier cannot lie over, or affect, rocks that come later. It can get pretty complicated of course, and unraveling the sequences of events is sometimes pretty challenging. It keeps geologists busy!

Because this collision really was the big one, it caused considerably greater deformation of the rocks than the earlier events had. The layers of rock in the Appalachian Basin, in western Pennsylvania and West Virginia, were squeezed into a series of elongate folds. Further south, in Virginia, the Carolinas, and Tennessee, the push was strong enough to break the rocks, and there are some really big faults there that were formed at this time.

There are lots of consequences to the collision between Gondwana and Laurasia, which we’ll touch on as we get further into the Pennsylvanian Period this month. The whole process will continue well into the Permian, a total of at least 60 million years just for this culminating collision. For comparison, India began to collide with Asia about 40 million years ago – and it’s definitely not done causing deformation, which we see today as earthquakes scattered through the region.

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John Ewing was born July 5, 1924, in Lockney, Texas. He had a long career with the Woods Hole Oceanographic Institution, Lamont-Doherty Geological Observatory, and Columbia University. His work focused on marine geophysics, and he’s considered to be the inventor of the air gun for marine seismology. He used geophysical data to unravel the structure of oceanic crust and the transitions between oceanic and continental crust.

—Richard I. Gibson

Illustration from USGS