December 22. Sierra Nevada and Wasatch Front

Today’s episode is a follow-up to the Basin and Range discussion the other day, and a follow-up to the Sierra Nevada Batholith which we talked about back in the Jurassic, in October. The Sierra Nevada Mountain Range in California coincides pretty well with the Jurassic-Cretaceous Sierra Nevada Batholith, the deep roots of the subduction and magmatic arc system that was established out there by at least 150 million years ago. 

But the modern range is vastly younger than that. The Sierra Nevada is basically one huge fault block, tilted to the west, with the fault marking its eastern front. It’s a big normal fault, with the mountains up to the west and the region to the east dropped down. That fault is essentially the western margin of the Basin and Range Province, the extended, faulted suite of uplifts and basins that continues all the way east to the Wasatch Mountains in central Utah. 

The action on the Sierra Nevada Fault started about the same time as the Basin and Range became active, in the Miocene Epoch, 15 to 20 million years ago. And most of the uplift is in the past 5 million years. There’s easily 10,000 feet of displacement on the fault and probably quite a bit more. 

Intermountain Seismic Belt
marked by dashed black line.

It may be that the size and strength of the huge granitic batholith provided a mechanical boundary to the part of the crust that was able to break into the Basin and Range Province. The other side of the Basin and Range, the Wasatch Mountains, mark the western edge of the Colorado Plateau, which is also a region that is fundamentally different in terms of the nature of the crust from the Basin and Range. The Colorado Plateau’s crust is thicker, cooler, and mostly older than the Basin and Range. So, thinking back to the episode a couple days ago, this helps us look at the unique broken nature of the Basin and Range as likely connected to its mechanical properties, such as thickness, composition, and age.

Today, there is some seismic activity within the Basin and Range, but it’s pretty spotty. In California, the main tectonic focus is along the San Andreas Fault Zone. But the Sierra Nevada Fault is almost certainly still active. The uplift of the Sierra Nevada isn’t simply on the one fault, and in 1872, a magnitude-7.8 quake hit the Lone Pine area, in the Owens Valley east of the Sierra. The movement wasn’t on the Sierra Nevada Fault, but on the nearby and related Owens Valley Fault, which is part of the system of faults continuing to uplift the Sierra Nevada. Some segments had vertical offset of as much as 20 feet.

On the other side, the Wasatch Mountain Front, which forms the backdrop of Salt Lake City and all of central Utah, is part of an earthquake belt called the Intermountain Seismic Zone. Its heritage is also about the same timing as the Basin and Range, the Miocene, but there has been some level of discontinuity there, at the western edge of the Colorado Plateau, for many tens of millions of years.

The activity along this belt is usually nothing like that on, say, the San Andreas Fault, but great earthquakes do occur, including the Hebgen Lake Quake in 1959, which had a magnitude of about 7.4. The Intermountain Seismic Belt goes from southern California, through southern Nevada and central Utah, eastern Idaho and western Wyoming, through western Montana and into Canada. If you drive Interstate 15, you’re pretty much following this zone. There hasn’t been a big earthquake on the Salt Lake City segment of the Wasatch Fault for about 1,300 years. That doesn’t mean one is due; typically, the historical record of earthquakes is far too short to infer any kind of regular periodicity, and it can be challenging to date prehistoric quakes with accuracy. Nonetheless, seismologists always try to figure this out for general hazard preparedness, and there do appear to have been sizeable earthquakes on some part of the long Wasatch Fault zone about every 350 years. A big one will happen there, sometime.

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Our anniversary today is an earthquake, on December 22, 856 A.D., at Damghan, Iran. It had an estimated magnitude of 7.9 and killed about 200,000 people, one of the most deadly earthquakes ever. It occurred in the Alborz Mountains, the range in northern Iran along the shores of the Caspian Sea. The South Caspian Basin is a small bit of oceanic crust that is trapped in the ongoing Cimmeride and Alpine-Himalayan collisions that we’ve talked about previously. Here, it seems likely that the dense bit of oceanic crust is subducting, at least somewhat, beneath the Iranian continental block. Volcanoes in the Alborz Range, including Damavand, support that idea. And so do the earthquakes.

—Richard I. Gibson

Sierra Nevada
Sierra Nevada Fault Scarp (1898)
Intermountain Seismic Zone

October 10. Sierra Nevada Batholith

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

These bodies of granite are called batholiths, a word that means “deep rock” for the fact that they crystallize at great depth. The blobs of granitic magma in eastern California rose for millions of years, well into the Cretaceous Period. They ultimately formed the Sierra Nevada Batholith, a complex of granitic bodies 400 miles long and 100 miles wide. If there were volcanoes above these deep-seated granites, most of the rock record of them has been eroded away. The granite in the Sierra Nevada is now exposed at the surface because of much later uplift and erosion – miles of rock had to be removed to get down to where the granite was. So the modern mountain range is the result of very different kinds of tectonic forces than the ancient, probably volcanic chain that was there during Jurassic and Cretaceous time.

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. Gibson

LINKS:
More about the batholith

And more 

Geologic History of California


Geologic map of California