14.2: An In-Depth Look at the San Andreas Fault and the Basin and Range

Introduction

Two essential principles of physical geology are that ocean basins grow through seafloor spreading at mid-ocean ridges and close through subduction at some convergent boundaries. However, when these two processes combine, the situation becomes complex, as illustrated by the Cenozoic tectonic history of western North America. This is a rare scenario, and only a few examples of it can be clearly described in the geological record.

Figure \(\PageIndex{1}\): Plates and plate boundaries with a general sense of motion identified.

In the Mesozoic, the west coast of North America was dominated by subduction. This led to the Nevadan, Sevier, and Laramide orogenies. The last of these, the Laramide orogeny, was typified by flat-slab subduction. This happened because the rate of subduction between the North American and Farallon plates was faster than the spreading happening at the mid-ocean ridge between the Pacific and Farallon plates. In other words, seafloor spreading couldn't keep up with subduction, and the mid-ocean ridge was drawn closer and closer to the trench. This meant that the Farallon Plate was still young and hot when it reached the trench, making it buoyant and resistant to sinking. Think of it being more like a balloon filled with air than one filled with sand.

Eventually, the mid-ocean ridge itself was subducted, beginning around 30 million years ago (Ma). This event produced two major tectonic features: the San Andreas Fault System and the Basin and Range Geologic Province.

Figure \(\PageIndex{2}\): Normal (a) and flat-slab (b) subduction. (Picture by R. J. Lillie and Parks and Plates by the National Park Service.)

Basin and Range

The Basin and Range is a geomorphic province that extends across much of the southwestern United States and western Mexico. From the eastern margin of the Sierra Nevada Mountains in California, the Basin and Range extends across Nevada to the Wasatch Mountains in western Utah, characterized by a topography featuring a repeating sequence of roughly north-south-oriented, broad valleys (basins) separated by elongated mountain ranges (ranges). So persistent are the orientation and geometry of the basins and ranges that this region was described by Charles Dutton, an important 19th-century geologist, as an “army of caterpillars marching toward Mexico”.

Figure \(\PageIndex{3}\): Sketch of the Basin and Range Geologic Province (by Thatmaceguy via Wikimedia.)

It is probably the best modern example of a wide continental rift zone, a type of divergent boundary within a continent. Unlike typical (narrow) rifts, such as the East African Rift, deformation is distributed over a wide area, and normal faulting distributes tensional stresses over a broad region, creating a series of horsts and grabens.

Figure \(\PageIndex{4}\): Generalized rift diagram. A wide rift has many valleys, a narrow rift focuses the extension in one main area. (Sketch by Trista L. Thornberry-Ehrlich, Colorado State University and the National Park Service.)

Regional continental rifting is the cause of the famous Basin and Range physiography: multiple north-south trending mountain ranges separated by north-south trending valleys. Nevada, which is entirely within the Basin and Range, is actually the most mountainous state in the U.S. This is not because it contains the tallest mountains, but because it contains the most mountains, each of which is bound by normal faults.

Figure \(\PageIndex{5}\). Photo of the central Basin in Range in Nevada. The dark, linear patches are mountain ranges (horsts) and the lighter areas in between are basins (grabens). (By NASA.)

This faulting has doubled the width of the state of Nevada over the last 50 million years. Most faults in the Basin and Range are either inactive today or exhibit low activity. Only the normal faults toward the eastern (e.g., Utah’s Wasatch Fault near Salt Lake City) and western (e.g., eastern Sierra Nevada faults in eastern California) margins of the Basin and Range are highly active.

Figure \(\PageIndex{6}\): Animation of tilted-block faulting, one of the ways extension occurs in the Basin and Range (by Aturn4000 via Wikipedia.)

The faulting that started the Basin and Range is complex and not very uniform. In some places, extension dates back as far as 55 million years ago, whereas in others it remains active. The majority of the extension occurred around 20 to 10 million years ago. In general, extensional provinces in the northern Basin and Range began earlier, whereas rifting in the southern portion began later, around 30 million years ago. No single fault system has been active for more than 10 million years (Axen et al., 1993).

Figure \(\PageIndex{7}\): Geothermal resource map of the US (by the US Department of Energy.)

A significant side effect of Basin and Range extension is heat. Stretching thins the plate, relieving overburden pressure and triggering decompression melting. The stretched and thinned crust makes it easier for magma to reach the surface. This is evident in the extremely high heat flows in the Basin and Range area. Even today, 98% of the geothermal electricity in the United States is produced in Basin and Range states, with California and Nevada being the most productive.

Figure \(\PageIndex{8}\): Wheeler geologic area, Colorado, showing the eroded remnants of the Fish Canyon Tuff, a 5000 cubic kilometer eruption, one of the largest ever measured. (Photo by G. Thomas via Wikimedia.)

The upwelling asthenosphere accounts for the history of extensive volcanism across the Basin and Range. The origin of this remains debated, particularly given the absence of volcanism associated with the Laramide orogeny. However, once subduction stopped and a transform boundary was established, it seems that a ‘hole’ developed in the nearly flat-subducting slab, allowing material to rise from below.

This event, known as the Mid-Tertiary Ignimbrite Flare-up, happened in two waves: one in Washington and Idaho at about 54 Ma, and another in Arizona/New Mexico near the US border with Mexico at 43 Ma. Over time, the northern branch of volcanism shifted south, and the southern branch moved north, converging near Las Vegas at approximately 21 Million Years Ago (Humphries, 1995). This period is characterized by gigantic explosive eruptions, including some of the largest ever known on Earth, and is documented by extensive tuff deposits. Some of these eruptions are more than 5000 times larger than the May 1980 Mt. St. Helens Eruption.

Significance of the Basin and Range

The importance of the Basin and Range to geology is significant. Not only did the extension expose many sedimentary formations, fossils, and resources at or near Earth's surface, but it also contributed to the understanding of normal faulting. Two important and related features were first recognized through studies of Basin and Range extension: the detachment fault and the metamorphic core complex. Detachment faults, also known as low-angle normal faults, were highly controversial when they were first described. Most normal faults are steeply tilted, typically at an angle of around 60 degrees. However, the detachment faults of the Basin and Range are closer to horizontal than vertical. It's thought that these faults either started at high angles and rotated to lower angles with extension or formed at low angles.

Figure \(\PageIndex{9}\): Geology students examine the Wasatch Fault outside Salt Lake City, Utah. The Wasatch may have been active as a thrust fault before the Basin and Range and as a normal fault during the Basin and Range. (Photo by Matt Affolter.)

Movement and detachment faults also cause the formation and exposure of metamorphic core complexes. These are places where large amounts of extension (dozens of miles in some areas) have allowed deep crustal rock to bulge ductilely toward the surface. The detachment in these cases marks a boundary between highly metamorphosed basement rocks in the footwall, separated by more deformed tilted rocks of the hanging wall.

Figure \(\PageIndex{10}\): Sketch of metamorphic core complex that facilitates the upwelling of the mantle (by ManuRoquette via Wikipedia.).

Through progressive extension, the deep rocks bulge upward, and metamorphic rocks that form at depth, such as mylonite, exhibit signs of deformation in the shallow crust, including brecciation. One of the best-studied and understood detachment faults and metamorphic core complexes is in the Whipple Mountains, a mountain range of the Basin and Range, near the border of California and Nevada. Another example includes the Black Mountains near Death Valley.

Figure \(\PageIndex{11}\): Mylonite with garnet. Image from Creative Commons.

Cause of the Basin and Range Formation

The extension in the Basin and Range has two commonly cited causes that could be related or could have occurred simultaneously. One cause is extensional collapse. Beginning in the middle Mesozoic, the western edge of North America experienced prolonged compressional forces due to subduction and terrane accretion, driving the Nevadan, Sevier, and Laramide orogenies. Approximately 60 million years ago, the compression began to decrease, with a complete reduction in this force by approximately 30 million years ago. With the reduction of the forces that had squeezed and overthickened the crust, it began to "relax" and undergo gravitational collapse. As a result, this region of the crust spread out and thinned, contributing to at least some of the Basin and Range extension. One crucial piece of evidence supporting this model is the reactivation of dormant faults. Found all over the west as former thrust faults that turned into normal faults upon the transition to extensional tectonics.

The other cause of extension in the Basin and Range is due to the San Andreas Fault System, a complex yet vital component in understanding the modern plate boundary between the North American and Pacific plates.

San Andreas Fault System

Figure \(\PageIndex{12}\): Map of the San Andreas Fault in California, showing plate motion. (Image by Kate Barton, David Howell, and Joe Vigil of the USGS.)

In 1970, still in the early stages of the development of the theory of Plate Tectonics, Tanya Atwater published a paper that explained the newly discovered magnetic striping in the ocean crust off the northern California coast. She used the offshore striping patterns in ocean crust, which form from the freezing of magnetic minerals in basalt, to reconstruct an onshore tectonic history. Analysis of the striping implied that a significant event occurred around 30 million years ago: the mid-ocean ridge between the Pacific and (now mostly subducted) Farallon Plates started to subduct. Once the mid-ocean ridge started subducting, the relative motion of the North American and Pacific plates changed from convergent to transform. This change in transform motion initiated the formation of a strike-slip fault at the plate boundary, which would eventually develop into the San Andreas fault system.

The current motion, and most likely the past motion, is not exactly parallel to the fault system itself. There is some extensional component to the motion, and that divergent motion might be at least somewhat responsible for Basin and Range extension, particularly after 30 million years ago. It is also responsible for the opening of the Gulf of California, which rifted the Baja California peninsula from the mainland of Mexico, and the more than 90 degrees of rotation of the Transverse Ranges in Southern California (Ingersoll and Rumelhart, 1999). The video above, illustrated by Tanya Atwater, depicts both events and the widespread extension of the Basin and Range, particularly in Nevada.

Figure \(\PageIndex{13}\): Stress map of California and nearby regions, showing the Eastern California Shear Zone. (Map by the USGS.)

In the southern section, the San Andreas has occupied at least two other locations (Powell and Weldon, 1992); a new position for the plate boundary appears to be forming in eastern California and western Nevada, within the Eastern California Shear Zone and Walker Lane Fault Zone. This prediction is based on earthquake data indicating that this linear trough accommodates 15-25% of the plate motion between the Pacific and North American plates (Guest, B., 2007).* This zone is responsible for famous and large earthquakes like the 1872 Mw ~7.6 Owens Valley Earthquake, 1992 Mw 7.5 Landers Earthquake, and the 2019 Mw 6.4 and 7.1 Ridgecrest Earthquakes. If the plate boundary shifts eastward, California may peel away from the mainland United States, much like Baja California did from Mexico.

* Guest, Bernard; Niemi, Nathan; Wernicke, Brian (1 November 2007). "Stateline fault system: A new component of the Miocene-Quaternary Eastern California shear zone". Geological Society of America Bulletin. 119 (11–12): 1337–1347. Bibcode:2007GSAB..119.1337G. doi:10.1130/0016-7606(2007)119[1337:SFSANC]2.0.CO;2. ISSN 0016-7606. Wikidata Q70050019.

Final Thoughts

A seemingly simple question, “What plate boundary is on the western edge of the continent?” is a difficult one to answer. In California, the San Andreas Fault is the plate boundary, running east of Los Angeles and west of San Francisco. However, it can be argued that the plate extends east to Salt Lake City, Utah, as part of the Basin and Range. To the south, the San Andreas merges into a complex series of faults, such as the San Jacinto Fault, and eventually transitions into a mid-ocean ridge as Baja California has rifted away from Mexico and become a peninsula. North of San Francisco, near the town of Eureka and Cape Mendocino (the westernmost part of the state), the plate boundary makes a westward turn into the ocean, ending the San Andreas Fault and transitioning into the Mendocino Fracture zone and the southern boundary of the Juan de Fuca Plate, a residual piece of the Farallon Plate. North of Cape Mendocino, the plate boundary between the Juan de Fuca and North America is convergent, forming a subduction zone. This stretches toward British Columbia, forming famous volcanoes like California’s Mount Shasta, Oregon’s Mount Hood, and Washington’s Mount St. Helens and Mount Rainier. In Canada, north of the Juan de Fuca Plate, the boundary between the North American Plate and the Pacific Plate is again a strike-slip fault, the Queen Charlotte Fault, before curving around to form a subduction zone again near Juneau, Alaska. The difficulty in answering the plate boundary question becomes even more complex when travelling back in time: a mid-ocean ridge that was subducted around 30 million years ago initiated the formation of a transform that would eventually produce the San Andreas Fault. It's an interesting problem that will keep geologists occupied for years to come.

Key Terms

• Basin and Range Geologic Province - a province in the western US containing multiple north-south trending mountain ranges separated by north-south trending valleys

Further reading

https://www.nps.gov/articles/basinrange.htm NPS site on the Basin and Range

Atwater, Tanya. “Implications of plate tectonics for the Cenozoic tectonic evolution of western North America.” Geological Society of America Bulletin 81, no. 12 (1970): 3513-3536.

Axen, Gary J., Wanda J. Taylor, and John M. Bartley. “Space-time patterns and tectonic controls of Tertiary extension and magmatism in the Great Basin of the western United States.” Geological Society of America Bulletin 105, no. 1 (1993): 56-76.

Davis, Gregory A. “Rapid upward transport of mid-crustal mylonitic gneisses in the footwall of a Miocene detachment fault, Whipple Mountains, southeastern California.” Geologische Rundschau 77, no. 1 (1988): 191-209.

Humphreys, E. D., Post-Laramide removal of the Farallon slab, western United States, Geology, 23, 987-990, 1995.

Ingersoll, Raymond V., and Peter E. Rumelhart. “Three-stage evolution of the Los Angeles basin, southern California.” Geology 27, no. 7 (1999): 593-596.

McPhee, John, “Basin and Range” HarperCollins (1981)

Miller, Marli B., and Terry L. Pavlis. “The Black Mountains turtlebacks: rosetta stones of Death Valley tectonics.” Earth-Science Reviews 73, no. 1-4 (2005): 115-138.

Powell, Robert E., R. J. Weldon, and Jonathan C. Matti, eds. The San Andreas fault system: Displacement, palinspastic reconstruction, and geologic evolution. Vol. 178. Geological Society of America, 1993.