Wednesday, November 24, 2021

Geology of the Salt Lake Valley

One of the things I wanted to do before I moved from Utah was to get a nice panoramic picture of the Salt Lake Valley from above Salt Lake City. Here is a shot of the Salt Lake Valley looking south (so east is left and west is right). In the picture the Wasatch Mountains are on the left side of the valley and the Oquirrh Mountains are on the right side of the photo.  

The reason for this, besides just being a beautiful valley, was the geological setting of the valley. The Salt Lake Valley holds an important place, geologically speaking, in the landscape of the United States. The valley is the eastern most extension of the region known as the Basin and Range. 

Coverage of the Great Basin. Image courtesy of the NPS.

The Basin and Range extends from the Wasatch Mountains in the east to the Sierra Nevada Mountains in the west. It encompasses the western half of Utah and pretty much all of Nevada, as well as parts of Idaho, Oregon, California, and Arizona.

Illustration of the plate tectonics of the west coast of North America with the Farallon Plate subducting beneath North America. Image courtesy of the NPS

The formation of the Basin and Range Province began long ago when a plate known as the Farallon Plate was being subducted under the west coast of North America. This occurred along the Californian coast and south into Mexico. 

Evolution of the western coast of North America going from a subduction zone to a transform plate boundary causing the formation of the extensional Basin & Range Province. Images courtesy of the NPS.

The subduction, where one plate goes beneath another, produced volcanoes in California and other places in the American west, and it also squeezed North America. Around 40 million years ago, most of the Farallon Plate was completely subducted beneath North America. What that did was a few things:

  1. It released the pressure that was squeezing North America, like someone releasing a squeezed sponge. 
  2. The subduction zone was no longer. In it's place a new plate boundary formed, a transform plate boundary known as the San Andreas Fault.  

As the pressure was released, the plate started to expand. This expansion produced a series of north-south running blocks, which were rotated from the pressure release. These rotated block produced the north-south running mountains we see across the region today, as well as the north-south trending faults that border all of the mountain ranges.  

As the mountain ranges were rotated upwards, they were eroded. The eroded material washed down into the gaps between the mountains, producing the sediment laden valleys we know so well today, including the Salt Lake Valley. This is the reason that earthquakes can also be particularly dangerous in this region, because all of the sediment has a tendency to shake like Jell-O in an earthquake. 

The eastern valley produced from these rotational blocks was the Salt Lake Valley, with the Wasatch Fault, the fault at the base of the Wasatch Mountains, representing the eastern most extension of the region. It is also one of the most active faults of the whole region since it is a boundary fault. 

This can also be seen in the fa├žade I designed for the University of Utah parking garage seen here with some of the various Basin and Range mountain ranges as well as their fault lines. 

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