In this article we shall discuss what a terrane is and how the existence of terranes can be explained by the mechanisms of plate tectonics already discussed.
What is a terrane?
A terrane is a part of a landmass, bounded by tectonic faults, which is different in many ways from the main landmass to which it is attached.
Geologists ascribe this to the terranes having originally been separate from the main landmass, and being borne up against the edge of the landmass by the motion of plates. As we have discussed, oceanic crust is usually simply subducted beneath continental crust. But if some material (for example an island) can not or does not subduct (because of its greater thickness or lower density than oceanic crust, or for some other reason) then instead it will dock against the edge of the landmass.
One example of terrane formation is shown in highly schematic form in the diagram to the right. In picture 1 we see a microcontinent traveling from left to right along with the oceanic crust in which it's embedded; at the right of the picture we see the oceanic crust subducting beneath a continent.
In picture 2 the microcontinent has collided with the continent, incidentally squeezing up the sediment in the accretionary wedge between them. In this particular example, this has produced not one but two terranes: the former island and the former sedimentary wedge. The former microcontinent is now the left end of the continent, and the oceanic crust is now subducting beneath that.
The result is that after the docking has taken place we have three regions sitting side by side which are bounded by faults and which have no common geological history. Of course, after the point at which they dock, they can and will share a history; a lava flow, or an fall of volcanic ash, or deposition of sediment, can then cover them all in common; or a river could cut its way through all three. But until they join, there is no reason why they should have anything in common.
The map to the right (from the United States Geological Service) shows some of the terranes on the western coast of North America, where they have been particularly well studied. The terannes are color-coded by type.
Note that not all terranes are formed in exactly the manner of our example above; however, they all do appear to have been formed in the same general manner, i.e. by material being transported by the motion of plates to the western edge of the continent and sticking there.
Terranes: how do we know?
We have said that a terrane will have a different geological history from the surrounding areas on the same landmass. For example:
- The terrane will have different types of sedimentary rocks from the main landmass.
- The sedimentary rocks will typically contain different fossils from the main landmass.
- Also, the fossils and sediments often indicate a different environment from the main landmass; they may, for example, be indicative of a marine environment.
- Not only this, but the fossils in a terrane may indicate a different climatic zone from the landmass to which it is attached: for example, they may be tropical fossils, such as coral, found in an environment which is far from being tropical.
- The terrane will often look like it was once something in particular other than just another part of the continent; it will look like it has formerly been oceanic sediment, or a volcanic island arc, or a piece of oceanic crust.
- The basement rock of the terrane is often different from that of the main landmass in obvious ways: we may, for example, see basalt in the terrane adjoining granite in the main landmass.
- The basement rock will frequently yield different dates on different sides of the fault when absolute dating methods are applied.
- Extensive geological features such as lava flows or layers of volcanic ash may terminate abruptly at the edges of the terrane.
- The terrane will have a different paleomagnetic history from the landmass to which it is adjoined.
What is more, these features will go together. That is, the fossil evidence, the paleomagnetic evidence, the evidence from dating, the different sediments, the different basement rocks, and so forth, will all identify exactly the same fault-bounded area as being a terrane. If you really wanted to, then perhaps you could try to explain each of these features away one by one (or at least try to imagine that there's some explanation which you can't think of yet) but how would you go about explaining why these features of a terrane are coterminous?
Terranes look, in short, as though they came from somewhere else and were somehow tacked on to the main body of the landmasses to which they are now attached. And so it is reasonable to conclude that this is in fact what happened.
Furthermore, we can understand why they should have been transported and attached in this way. What would otherwise be a puzzle becomes an expectation in the light of plate tectonics. What would be more natural than to find seamounts, volcanic islands, and marine sediments scraped off the edge of a subducting plate?
This view of terranes is strengthened by the observation that the evidence shows that terranes became joined to landmasses at times and in places where on other grounds we think that subduction was taking place. For example, since the Atlantic ocean is being formed by rifting, we should not expect to find any terranes on the Atlantic side of America which date to younger than the Atlantic; and we don't. Meanwhile we find plenty of more recent terranes on the west side of America, where the Pacific oceanic crust is subducting beneath the continental crust.
For these reasons, it makes sense to explain the terranes as being caused by the motion of plates. Indeed, this successful explanation of the existence of terranes must be taken as evidence for the theory of plate tectonics, and for the subduction of oceanic crust in particular, insofar as the existence of terranes makes no sense at all without that theory and make perfect sense in light of it.
Note on vocabulary
The word "terrane" originally just meant "an area having a preponderance of a particular rock or rock groups". The terranes which we've talked about in this article were called "exotic terranes" or "allochthonous terranes", "tectonostratigraphic terranes", "accreted terranes" and so forth. However, the word "terrane" is now pretty much synonymous with an area distinct from the surrounding landmass because it has been transported by the motion of plates and has then docked against another landmass.
The reader should bear this in mind when looking at older texts: the words "exotic terrane", for example, do not imply anything more exotic than what in this text we shall simply call a "terrane".