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Monday, April 14, 2014

April 14. The Caledonian Orogeny





Today I’m going to tackle the Caledonian Orogeny. It’s gonna be a long one – it’ll make up for the short episodes I’ve had recently. You recall, I hope, that orogeny means “mountain building,” and mountains often result from collisions. The Caledonian Orogeny represents some really complex collisions during the Silurian, especially in Europe. The Caledonian gets its name from the Latin name for Scotland, given to it by the Romans. 

Before I get into it I want to define three terms. First, a terrane. That’s a relatively coherent block of geological material. It could be a continental fragment or an island arc or a slice of oceanic crust, or some combination of them all, but generally, it remains one structural piece over some period of geologic time, and has more or less a fairly uniform history over that time.

Next, a suture. That’s the place where two terranes come together, just as a surgical suture is where two pieces of the body are joined. It might be a fault line, or it might be a narrow belt of rocks, or it might be more complicated. Sometimes when two strong blocks like continents come together, the suture between them is a narrow belt of oceanic crust trapped between them as they came together.

And last, accretion. That’s just the process of things coming together, amalgamating. It’s a continent growing by accreting little terranes around its margins.

Simplified map of the Caledonian Orogeny


So – The Caledonian. Back in the Ordovician, on March 16, we talked about the Avalonian Terrane, a long narrow strip of continental material that lay between Laurentia – that’s the core of North America – and Baltica, the heart of Europe. Avalonia included most of what is now England, as well as southern Ireland, together with southeastern Newfoundland where the Avalon Peninsula lies and gives its name to this terrane, plus northern Nova Scotia and some bits of northern coastal New England.

Avalonia probably began as a long narrow microcontinent that rifted away from the supercontinent of Gondwana, probably from what is now more or less northeastern Africa. A modern analogy might be East Africa today, where the East African Rift System is tearing eastern Africa away from the main continental mass of Africa. Eventually, it may become a long, narrow microcontinent chugging across the Indian Ocean.

Most of what is now northern Scotland and northwestern Ireland were parts of North America, probably pretty close to what is now southeastern Greenland. North America, or Laurentia, was tilted from its present orientation and much further south, so that the Silurian equator ran pretty much along what is now the eastern United States. Baltica and Avalonia were to the south, and the sea between North America and the other two is called the Iapetus Ocean. Some call it the proto-Atlantic Ocean, but it really has nothing directly to do with the modern Atlantic.

As we discussed in the Late Ordovician on March 24 there was probably a volcanic island arc, something like modern Indonesia, off the coast of North America. Its collision with the continent near the end of the Ordovician created the Taconic Orogeny.

Avalonia was working its way across the Iapetus Ocean in complex ways. Britain, pretty much at one end of Avalonia, began to collide with the main Baltica continent probably in the Early to Middle Silurian, about 430 million years ago. Once that end of Avalonia was attached to Baltica, Europe had a long trailing peninsula, perhaps something like the modern Malay Peninsula, but bigger and longer. The Iapetus Ocean was getting narrower and Baltica, with Avalonia attached, continued to move toward North America.

Toward the end of the Silurian, the true continent-continent collision that we call the Caledonian Orogeny was in progress. The mountains that formed were prominent especially in what is now northeastern Greenland and in Norway – and the Iapetus Ocean that was once between Greenland and Norway was gone. Two continents had collided, squeezing a mountain range up between them much like the Himalayas scrunched between colliding India and Asia. There is some evidence in the rocks to indicate that the Greenland-Norway phase of the Caledonian Orogeny actually got started in the Ordovician or even earlier, but I think it’s fair to say that the culmination of the mountain building was during the Silurian. These things take many millions of years.

Further along the colliding belt, the far north of Scotland and northwestern Ireland, which were sitting there on North America, were sutured to Avalonia and touched the rest of Britain for the first time.

Southeastern Newfoundland and northern Nova Scotia, which were part of Avalonia, joined with New Brunswick and the Northern Peninsula of Newfoundland, which were already parts of North America. In North America, this is sometimes called the Acadian Orogeny, for the French name of Nova Scotia. And to be accurate, it was really only getting started during the Silurian. We’ll talk a bit more about it next month, during the Devonian, but it was part of this great complex collision often lumped together as the Caledonian Orogeny.

Don’t visualize all this as a nice straightforward head-on impact. It looks like the assemblage was at least in part a result of complex glancing blows and sliding, oblique collisions. At times, it might have been something like western North America today, where the Queen Charlotte Islands off British Columbia are sliding along a major strike-slip fault, a break where two distinct terranes slide past each other. But don’t even visualize that as a nice smooth slide, either. Think of fits and starts, with collision here, pulling apart there, and generally a lot of diverse action making a geological mess of things.

That’s what we have now in Newfoundland and Scotland – Newfoundland is partly the edge of the core of North America and partly Avalonia, and Scotland is part of North America attached to Avalonia – which had already been attached to the Precambrian core of Europe, which we’ve been calling Baltica.

All these pieces that fused together during the Caledonian Orogeny are far apart today, because of the opening of the Atlantic Ocean. That’s a rifting apart that was near, but not exactly along the old suture zone, the join line, between the continents. That’s why bits of North America got left in northern Scotland and bits of the Avalonian part of Europe got left in Newfoundland. But that rifting, that leaving behind of bits of continents, didn’t happen until the Jurassic. We won’t be talking about that until October. In fact, there was still ocean outboard of most of North America south of New England. It won’t be closed until the fat lady sings, and in this case the fat lady is Africa – or more properly, Gondwana. Gondwana is coming, but it won’t be here for almost 200 million years. We’ll get to that in June.

While the main mountain-building action was between Greenland and Norway, and down through what is now Scotland and Maritime Canada, there was another branch of the Caledonian Orogeny in northern Germany and Poland, along the linear margin of Baltica. This one might have been another that was mostly sliding rather than head-on collision. As for the rest of Europe, most of France, Spain, Italy, the Balkan region – it wasn’t there yet.

One last thing – don’t visualize all this mess as coming to a sharp end at some precise time. Additional small continental fragments seem to have been rifting off Gondwana, starting a trek across the remaining ocean basin. One such block is called the Meguma Terrane, which is most of southern and central Nova Scotia and the adjacent offshore. It collided, well after the end of the Silurian Period, with the southern edge of the Avalonian Terrane, which itself had already been accreted to North America in places. So the process was ongoing, over many millions of years.

I won’t be surprised if I got something in all that wrong – so Caledonian experts, please speak up!

—Richard I. Gibson

Primary references for this compilation:
1. Petroleum Geology of the North Sea, K.W. Glennie, ed., Blackwell Science, 1998
2. Evolution of the Arctic-North Atlantic and Western Tethys, Peter A. Siegler, AAPG Memoir 43, 1988.
3. Silurian Paleogeographic map by Ron Blakey 
You can find plenty more about the Caledonian on the web.

Map above by Woudloper via Wikipedia under Creative Commons license 

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