Monday, November 24, 2008

Ophiolites and ocean crust: slow tectonic shifts in textbook science

For more than fifteen years, I've been making my Plate Tectonics students read Assembling California by John McPhee. In part, I'm just using my dastardly professorial powers to inflict one of my favorite writers on helpless students. In part, I just want students to read lines like "You need a new geologist. You need a Californian." But mostly, I want students to think about ophiolites and the ways that scientists change their minds.

All of the textbooks that I've used simplify the ophiolite sequence in the same basic way. It's oceanic crust, stranded on land where we can see it and touch it and measure its structures and sample its minerals. Sediments, pillow lavas, sheeted dikes, massive gabbro, layered gabbro, layered peridotite, deformed mantle rock. The field geologist's answer to the layers recognized by seismologists on the ocean floor. The record of sea-floor spreading, one of the many pieces that fell into place as plate tectonic theory came about.

Getting that ocean floor onto continents where geologists can study it without submersibles or seismic waves is tricky. I remember arguing about California's Coast Range Ophiolite with my officemates (probably at Friday Beer after the high-pressure-metamorphism seminar). It's inland of the old subduction complex of the Franciscan Formation, so did that mean that it represented the closure of an old ocean basin? Or was it shoved up onto the continent as some kind of flake? Or, well, how do you do that mechanically, anyways? (Big picture tectonic arm-waving works better after beer. Increase pore fluid pressure and all that.)

Sometime in those nineteen years during since that seminar, the consensus understanding of ophiolites has changed. It hasn't been something that makes headlines, but in study after study, it's turned out that the trace element geochemistry has the wrong fingerprints for the ocean floor. Most ophiolites - even the famous ones like Troodos on Cyprus or the Semail ophiolite in Oman - most ophiolites formed above subduction zones.

I had heard rumors about this from igneous geochemists before, but an article in last month's GSA Today went further. If the geochemistry says that ophiolites aren't rocks that formed at mid-ocean ridges, well, we really shouldn't be using them to study what happens at mid-ocean ridges. We can't even use the sheeted dike complexes - sheets of cooled magma that intruded one another so that only half of any original dike is left - to talk about sea-floor spreading. At mid-ocean ridges, the rates of magma production and spreading are tied together, but above subduction zones, they're the result of two different processes. It's possible for the plate above a subduction zone to spread, if the downgoing plate sinks faster than the over-riding plate slide across it. (That's happening, for instance, near the Mariana Trench. And it's one of those complicating bits of tectonics that doesn't match the stories that get told in introductory classes, and that can serve as a GOTCHA! for partly-informed skeptics. But we know about trench rollback, and it's been incorporated into geologists' understanding of tectonics for a couple decades, even if we don't explain it that well to students.) The spreading of the over-riding plate might look like a mid-ocean ridge in the rock record - it's all extensional tectonics, after all - but the relationship to magmatism is different. Not the same relationship, not the same process. Still interesting... but not a way to study the way that oceans grow. Sorry.

So touching an ophiolite isn't touching a bit of old ocean floor, after all. And we don't need to argue about how the rocks got on top of the subduction complex if they usually are formed there. And a lot of tectonic arm-waving was for naught.

And the textbook science changes.


Robinson, P.T., Malpas, J., Dilek, Y., and Zhou, M., 2008, The significance of sheeted dike complexes in ophiolites: GSA Today, v. 18,. p. 4-10. (Available for free online here.)


BrianR said...

"So touching an ophiolite isn't touching a bit of old ocean floor, after all."

But isn't the backarc spreading still 'ocean floor' even if it's not the same as mid-ocean ridge?

kurt said...

Thanks for a nice blog again this week, Kim. Yours is my favorite on the internet. :-)

Kim said...

Brian -

Well, it's igneous rock formed below sea level, so maybe. But it's not the stuff that forms our big oceans. (In fact, aren't most back-arc basins named Something Sea? Bering Sea? Philippine Sea (though I don't know if it's got a well-organized mid-ocean ridge as well as the messy back-arc spreading)?)

And the authors' point is that you shouldn't study them as if back-arc basins are driven by the same forces that drive sea-floor spreading, either.

BrianR said...

Kim says: "And the authors' point is that you shouldn't study them as if back-arc basins are driven by the same forces that drive sea-floor spreading, either"

Fair enough, I see what they mean.

ScienceWoman said...

Cool post! I think I'd vaguely heard about spreading around subduction zones from my hard rock friends, but the connection to ophiolites is news to me. Thanks for explaining it so clearly. Now I don't need to feel guilty about not reading the article.

Elli said...

Maybe I'm wrong, but I thought that if you look at the side-wall of the ocean floor as revealed by transform fault scarps you saw the same basic structure as what we teach for an ophiolite order. Now I'm going to sit here and try to think where I was when I heard that talk... Jeff Karson? Yay! And I even have a paper to back me up!

Stewart, Karson & Klein, 2005, Four-dimensional upper crustal construction at fast-spreading mid-ocean ridges: A perspective from an upper crustal cross-section at the Hess Deep Rift. Journal of Volcanology and Geothermal Research, 144, 287-309.

So, maybe the famous ophiolites come from back-arc rifting instead of mid-ocean ridges, but they could be formed by the same basic processes.

And I love these posts that actually make me think!

Tuff Cookie said...

Well, crap. Now we're just going to have to build an unobtanium ship and chuck it into an MOR so we can see what's really going on.

I'm wondering how this is going to change models for magma ascent at MORs, too. I suppose we're just going to have to hunt down a chunk of crust that has the right geochemical signatures for a spreading ridge and look at the structures there...but, wow. If this idea holds up, there are going to be whole chapters dropped from textbooks.

Kim said...

I don't think there's much being thrown out about what goes on at mid-ocean ridges. There are other ways of studying them than looking at ophiolites on land (as Elli alluded to). And the arguments seemed to explain the variations in ophiolites (such as ophiolites with no sheeted dikes), rather than to argue that the understanding of oceanic crust is wrong.

I think this paper (like many in GSA Today) was more of a synthesis of things that the ophiolite research community had been thinking, but which had been discussed in GSA special papers or Contributions to Mineralogy and Petrology or other journals that non-specialists are less likely to read. So some things in textbooks probably need re-writing - but really, getting pieces of normal oceanic crust onto land was a problem, even if it had become something that we just tried not to think too hard about.

BrianR said...

Kim says: "I think this paper (like many in GSA Today) was more of a synthesis of things that the ophiolite research community had been thinking, but which had been discussed in GSA special papers or Contributions to Mineralogy and Petrology or other journals that non-specialists are less likely to read."

Yeah, I just finished reading it (enjoyed it) and was reminded of talking to others more knowledgeable in this field than me about this topic over the last several years. Interesting stuff.

Chuck said...

Just because a back arc basic is being pulled apart by different forces than an oceanic basin doesn't mean that upwelling mantle that undergoes partial melt behaves differently. I mean, it's good to realize that the ones on land are all back arc ocean floor, but I think the only people likely to cry foul are those whose MOR theories are contradicted by ophiolite structures. Otherwise, a spreading ridge is a spreading ridge.

Kim said...

Don't the weird geochemical things that happen when subduction fluids get involved change the geochemistry? I thought that was the reason the igneous geochemists could tell them apart in the first place.

And are all the ophiolites from organized back-arc ridges? Some are described as arc basement and some are described as fore-arc magmas now.

And are back-arc spreading centers more prone to re-organizing than MORs are? (Shift position, shift orientation?) My gut sense is that the driving forces may vary more, so back-arc basin rifts may be messier than MORs. (This is speaking as someone who prefers more silicic rocks, though, and who has no intuition for the rheology of basaltic and ultramafic rocks.)

andrew said...

Thanks for bringing this up; I read the GSA Today piece too. I'm hoping it's a highly condensed version of the information in the GSA Specioal Paper 373, "Ophiolite Concept and the Evolution of Geological Thought." So far I've only glanced at Dilek's introductory essay, and it touches on most of what his GSA Today paper did.

So ophiolites aren't classic MORB crust, and given the influence of subduction-related fluids, they're an imperfect analog of it. So we'll just have to drill some more and do more fieldwork, darn it! I thought the paper was an exciting one that I can make good use of.

Chris Laning said...

Coolness! I will heartily endorse your Evil Professorial instincts about reading John McPhee.

I leapt upon Assembling California with glad cries, since I live in the same town as the geologist he was working with, and was thrilled to discover all that very cool stuff practically right in my back yard. I have passed some of the book's landmarks many times.

Very interesting to know how thoughts are changing, too. I'm in an awkward position -- armchair enthusiast, geology minor in college many years ago when seafloor spreading was new, and I'm always happy to find people willing to catch us nonspecialists up on what's new and interesting.