If you visualize North America and Africa pulling apart along the new-born Atlantic Ocean, it’s obvious that in relative terms, Africa is moving east or southeast and North America is moving west or northwest. If there is anything out there beyond the continents, it could get caught in a collision with those two big masses.
That’s what happened in western North America during the Jurassic. We’d been having collisions out there earlier, as long ago as the Devonian, when the Antler Orogeny happened, and last month, on September 15, we talked about the Sonoma Orogeny that added some terranes to what is now northwestern Nevada. During the Jurassic, things got serious.
This scenario has pretty much been going on since the Jurassic. The east coast of North America is still moving west, to make way for the new oceanic crust produced at the Mid-Atlantic Ridge, and the Atlantic Ocean is still getting wider. In the west, one thing after another has been colliding.
|Subduction (from USGS)|
By mid-Jurassic time, something like 175 million years ago, active subduction was taking place in what is now eastern California. It had probably begun by late Triassic time, about the same time as extension in eastern North America began to produce the Newark Grabens, and it continued through the Jurassic and into the Cretaceous. The area must have been much like the modern Andes, with a large slab of oceanic crust plunging beneath the western leading edge of North America as it plowed westward. The standard picture of subduction is dense oceanic crust sinking beneath more buoyant continental crust, reaching depths where temperatures are hot enough for some partial melting. But the process is driven by water and other volatiles that are driven off and migrate upward, reducing the melting temperature of the rocks enough to begin melting, which then rise like blobs in a lava lamp toward the surface. If they reach the surface, they can erupt in volcanoes. If they solidify deep inside the earth, in the roots of the volcanoes, they can form huge bodies of igneous rocks. If the molten material derived from mostly uncontaminated oceanic crust, the material will be basalt, such as erupts in Hawaii and Iceland. If the uprising material goes through continental crust, it will melt more silica-rich materials and will likely end up with a granitic composition.
|Mesozoic Batholiths in red|
The white granite rocks of Yosemite National Park, including Half Dome, as well as Mount Whitney and most of the entire length of the Sierra Nevada are parts of the Jurassic Sierra Nevada Batholith.
The subducting oceanic plate that gave rise to the Sierra Nevada Batholith is called the Farallon Plate. Its early interaction with North America, during the Jurassic, was probably relatively straightforward, that standard view of subduction I discussed earlier. But it got complicated over time, and we’ll talk about some of those complications over the next couple months. Most of the Farallon Plate is gone – subducted into the earth, its materials recycled into the crust and mantle. One large remnant is offshore Washington, Oregon, and far northern California, where it is still subducting and giving rise to the Cascade Volcanoes. It’s probably fair to think of eastern California during the Jurassic as something like the Cascades today, although the extent and volume of the now-exposed granites might suggest that it was more like Cascades on steroids – a huge and long-lived subduction system.
The mountain-building event associated with this subduction and the intrusion of the Sierra Nevada Batholith is called the Nevadan Orogeny. Although there had been earlier accretions that added small terranes to western North America, this was really the first major pulse of mountain construction in the ongoing creation of the Cordillera, the western mountain ranges of North America. Their development continues to this day, so we’ll certainly talk more about them over the rest of our trip through earth history. And even during the Jurassic, the uplift of mountains in the far west had consequences much further east, which we’ll talk about later this month.
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Today’s geologic birthday is Hugh Miller, born October 10, 1802, in Cromarty, Scotland. He’s probably best known as the author of The Old Red Sandstone, first published in 1841. He is considered to be a pioneer in the paleontology of Scotland and Britain.
—Richard I. GibsonLINKS:
More about the batholith
Geologic History of California
Geologic map of California