Thursday, January 15, 2009

My problem with Wilson Cycles

Chris over at Highly Allochthonous has just done an unusually good job at explaining what Wilson Cycles are, and the role they play in tectonic reconstruction. And he did it without using the most evil diagram in tectonics.

I don’t like the Wilson Cycle idea. I don’t deny that ocean basins open and close, but I think that it is too neat, too pat, and inadequately describes most real orogenies.

Take another look at that linked diagram. What is missing? I’ll give you a hint. If you live in Turkey, California, New Zealand, or China, it could kill you. If you still don’t know, think about which dimension that diagram considers expendable.

Crustal thickening and thinning is fairly easy to identify in the geologic record. The both initiate sedimentation, igneous activity, and/or metamorphism. Lateral tectonics, on the other hand, doesn’t leave nearly as big a mess behind. As a result, it can be (and often is) overlooked much more easily that a rift valley, or a mountain of ecolgite. And the open-and-shut presentation of Wilson cycles further de-emphasizes strike-slip motion.

I think that’s a mistake. Continents don’t just move back and forth; they also slide past each other and spin around. In most cases, continental collisions don't involve happen in the same orientation as the previous rifting (I'm ignoring the Alps tonight). Most of today’s active plate margins have some degree of lateral faulting associated with them. But the Wilson cycle doesn’t call attention to this fact.

5 comments:

CJR said...

Hah. I dislike that figure too. Or at least, I dislike the fact that it is never really presented for what it is - an extremely idealised depiction of the whole process. Even the Iapetus-Pangaea-Atlantic cycle doesn't look like this - and that's probably the best-fitting example out there (Wales in the Ordovician was probably a lot like New Zealand today - there was definitely a large element of transpression, anyway).

BrianR said...

Very good point ... makes me wonder how much of the ancient record is stike-slip, or at least a component of strike-slip, that we haven't recognized.

andrew said...

I have never cottoned to the "Wilson cycle" because the word suggests an engine, whereas all we have in the data is a couple of ragged coalescences and breakups. It's like "mantle convection," which inspired decades of diagrams showing kitchen-kettle convection cells. Heck, the word "plates" has always been misleading.

Chuck said...

The mantle has a higher Rayleigh number than a kettle, so it should convect better...

jude said...

I don't see what's so wrong with the expanding earth theory anyway. My friend's dog's waistline expands. My income tax debt fines expand. So why not the earth?


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