The Rocky Mountains are the largest mountain belt in western North America, stretching for over 3,000 miles and providing valuable mineral resources and water recharge. When and how they were created are questions that Professor of Geology Jade Star Lackey offers paradigm-shifting answers to in a new paper.
“Magmatic surge requires two-stage model for the Laramide orogeny,” which Lackey co-authored with Josh Schwarz of California State University, Northridge, was published recently in the journal Nature Communications.
We talked to Lackey about the research he conducted in the local San Gabriel Mountains and implications of the new data. Answers have been edited for clarity and length.
What is your paper about?
This is our paper where we dated a lot of the granites in the San Gabriel Mountains here in Southern California. We started seeing ages that didn’t make sense with current models for when magmatic activity ceased along the western mountain belts.
There’s a prominent term in the title of the paper called the Laramide orogeny. Laramie, Wyoming, is a long way from the coast here. “Why did the Laramide orogeny happen?” is this big burning question that’s sparked the imagination of geologists for decades.
The conventional thinking was that something big and disruptive—called an oceanic plateau—was present on that Pacific Plate and that it basically got stuffed into Southern California, shut down magmatism and blew apart the nice belt of granitic rocks like we see in the Sierra Nevada. We know about this plateau collision because the other half of it, the Hess Rise, is still out on the Pacific Ocean.
We found that a lot of granites in the mountains behind us are younger than expected, often 70-72 million years old, which conflicts with the notion of a sudden cessation of magmatism at about 80-85 million years ago. The Laramide was a longer-lived affair, and what we proposed was that you had to have a two-shot deal, with the plateau arriving more slowly and colliding with Southern California later. That’s when you relay the disruptive forces from the edge of the continent and inward, popping things up in slower fashion.
How did you date the rocks?
To do the work, we get these little zircon crystals out of the rocks. A magma zircon will, as it grows, sop up any uranium that’s around, and then it locks it into this good, solid crystal form. When the uranium decays it builds up into lead. It’s just tiny amounts, imperceptible to us. But because it’s all concentrated in these tiny little crystals, if we separate 20 or 30 of them out of a granite, you’ll get these crystals that we can analyze for age here at Pomona’s Oxtoby Isotope Lab.
We couldn’t have done that 10 years ago. Geochronology that we can do routinely now is so much faster. Now we can walk into the lab with a bunch of these little crystals and get a story of time using the zircon clocks to say, “What is the age of this granite?” It’s pretty awesome.
What are implications of this study?
Most people wouldn’t have estimated that these granites were as young as they are. It forces us to add about 10 million years of geologic time onto an event that we thought was pretty quick. It is going to force people to think more about how we understand the response of this sort of tectonic process.
What I like about it is we get a story that affects the whole of western North America. But to see the particulars of the timing, it’s coming through Southern California. It underscores that things that happen locally have profound implications for what’s going around in the big tectonic landscapes of how the planet operates.
It opens up possibilities for trying to unravel other paradoxes in the geology of the western United States. It gives the geologic community something to think about and maybe to help reconcile some of these other processes, understanding how the continental plates and the oceanic plates interact. If things get disrupted, what does that look like in the rock record?