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Andrew Alden

Monday and Tuesday at GSA Fresno

By May 22, 2013

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The heart of the GSA Cordilleran Section meeting is over; the Wednesday session will be just half a day, giving me time for a side trip or two before the post-meeting field trip. It has been fun hearing from and talking with some of our local West Coast experts and the visiting experts who have come to do the same.

The sessions I attended on Monday focused mostly on California, which always interests me because that's where I live. One "meeting within the meeting" is a continuous series of talks about subduction zones—more generally, tectonic convergent zones—and the special rocks they make. California's extensive set of subduction-related rocks has made it a "type area" for this particular scientific problem (I present a tour of some of them on this site). Speakers talked about everything from the behavior of earthquake faults, to the ways that continents have their edges sharpened against other tectonic plates, to the hard-won insights from mapping the highly scrambled rock successions left behind on the margins of convergent zones.

The afternoon session I sought out was about the fossils of the Irvingtonian age (which I mentioned in yesterday's post). Now I feel a bit more familiar with that side of my local geology. And it was fun to see the people I hung out with on Sunday stepping up to give organized presentations to each other.

Today I attended talks on a tectonic problem that a small but dedicated group of researchers is working on: the story of the Gulf of California—that narrow stretch of sea between Baja California and the Mexican continent—how it formed, what series of events affected its history, what light it sheds on the geologic history of its neighboring lands and greater North America. And in the afternoon I caught up with the cutting edge of research into ophiolites: pieces of oceanic plates that are somehow pushed ashore.

These are both intricate problems, but they have different outlooks. The Gulf of California is a blurry picture that is inevitably getting clearer as new data comes in from the field. Ophiolites are a blurry picture that is getting more complicated with more detail. Researchers are still settling on the right concepts for insightful progress, and the right names to label them with. As Yilderim Dilek told the audience, "You can't form ophiolites from one template; they're like different flavors of ice cream." Nevertheless, Eldridge Moores pointed out, all of them always involve some form of subduction—a continent and a fault cutting downward through it—to be preserved. That doesn't always mean the textbook kind of subduction, he pointed out, just some version of the basic geometry. That permits ophiolites to be recognized in the oldest rocks on Earth without forcing us to say, against much evidence, that modern-style plate tectonics existed back then. There's nothing happier than a roomful of baffled scientists.


June 18, 2013 at 12:27 pm
(1) Scott Bennett says:

Andrew– Thanks for attending our session about the Gulf of California and the greater Pacific-North America plate boundary. Indeed, our research community is assembling a nice integrative story about the formation of the Gulf of California. And what a treat that Tanya Atwater gave the opening talk in our session!

Scott Bennett
USGS, UC Davis

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