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.

Reference:

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.)