May 23. Acadian orogeny

The long-lasting complex collisions that created the Appalachian Mountains and their equivalents in Greenland, Britain, and Scandinavia continued into the Devonian. You may recall that we called that the Caledonian Orogeny, or mountain-building episode near the end of the Silurian last month, when Greenland and Scandinavia collided, as well as the bits of North America and Europe that would become most of the British Isles.

The long, narrow microcontinent called Avalonia, for rocks in the Avalon Peninsula of Newfoundland, did not encounter North America at the same time in all places. South of Maritime Canada, a lot of the accretion of Avalonia to North America didn’t happen until the Middle Devonian and later. It’s usually called the Acadian Orogeny, for Acadia, the French name of Nova Scotia where some of the colliding happened. It was likely underway in the northern terranes by late Silurian, but it culminated in a major crunch in the Devonian.

Bits of Avalonia or related terranes were accreted to North America as far south as Georgia and Alabama, and some of those rocks are in the subsurface today, while some are exposed in the Appalachian Mountains. This collision was another one that wasn’t really head-on, but more oblique in nature, involving some strike-slip faulting like that along the Pacific Coast of Canada today. But there was also enough subduction to generate magmas that are scattered through the Acadian mountain belt.

Besides the well-known orogeny in eastern North America, there was some ongoing tectonic activity in parts of Europe at about the same time. In Europe, it’s called the Variscan or Hercynian Orogeny, just to confuse matters, and there, it continued well into the Carboniferous Period that followed the Devonian. This was the result of Africa, or really Gondwana, or even more accurately, some pieces of Gondwana moving to collide with Europe.

The diagram shows the 200-million-year sequence of events that add up to the Appalachian Mountains, spread over at least four distinct tectonic collisions spanning parts of 5 periods of geologic time. It’s not over yet.

—Richard I. Gibson

Diagram from USGS