3.4: California's Plates and Plate Boundaries

Where Plates Collide

The result of multiple moving lithospheric plates, California is home to not one, not two, but all three plate boundary types (Figure \(\PageIndex{1}\)). The North American Plate which makes up the North American continent is moving to the southeast relative to its neighboring plates. The Pacific Plate which makes up the coastal regions of California and the Pacific Ocean is moving relatively northwestward. The Juan de Fuca and Gorda plates are small lithospheric plates west of the Cascadia subduction zone that are both moving eastward toward the North American Plate (see Klamath Mountains, Cascade Range, and Coast Ranges).

Figure \(\PageIndex{1}\): The Pacific Plate-North American Plate-Juan de Fuca plate boundaries. "Relative Plate Motion" by the USGS is in the public domain.

California's Transform Boundaries: San Andreas and the Mendocino Fracture Zone

From the Mendocino Triple Junction to the Salton Trough exists one of the most famous plate boundaries on Earth: the San Andreas fault system. The San Andreas fault system is a transform plate boundary where the Pacific Plate is moving northwest relative to the North American plate (Figure \(\PageIndex{1}\)). The motion within the system is right-lateral strike-slip faulting. Average slip rates along the entire fault system range from 20 to 35 mm per year (~8 to 14 inches/year).

The San Andreas fault system begins offshore of Cape Mendocino, at the Mendocino Triple junction, where the Juan de Fuca, North American, and Pacific plates meet. The northern segment of the system parallels the California coast from the Mendocino Triple Junction off Cape Mendocino southwest to Daly City, California. It then moves onshore through the Santa Cruz Mountains until it reaches Hollister, California.

The central segment of the San Andreas runs from Hollister to Parkfield, California. Unlike the northern and southern segments of the system, the central segment seldom has earthquakes and the motion on the system is mostly aseismic creep, or continuous slip.

The southern segment starts at Parkfield, runs through the Carrizo Plain on the west side of the Great Valley, and starts to curve near the San Gabriel Mountains from a northwest-southeast orientation to a west-northwest to east-southeast orientation. This area where the fault system changes its orientation is known as the “Big Bend” and is related to divergent plate motion occurring to the south as the East Pacific Rise moves onshore in the Salton Trough. This “Big Bend Region" is one of transpression, where both strike-slip motion and reverse faulting occur (Figure \(\PageIndex{2}\)).

South of the “Big Bend,” the fault runs along the northern boundary of the San Gabriel Mountains, through Cajon Pass, along the southern boundary of the San Bernardino Mountains and the fault then ends near Bombay Beach, California. This is a transtensional area where the plate boundary is both transform and divergent (Figure \(\PageIndex{2}\)). A system of faults continues south, moving offshore at the northern end of the Gulf of California, where it eventually meets the Rivera Triple junction, at the northern end of the East Pacific Rise.

Figure \(\PageIndex{2}\): Transtension and transpression are both the result of bends in a fault. Transtension occurs where a bend creates an area of extension as well as transform. Transpression occurs where a bend creates an area of compression and transform. "Pull apart basin" by Wtbarron via Wikimedia Commons is licensed under CC BY-SA 3.0.

Geologically, the San Andreas fault system is relatively young and has only existed for approximately 30 Ma. For most of California today, the plate boundary is the San Andreas transform fault system between the Pacific plate and the North American plate. Prior to 30 Ma, the plate boundary was a subduction zone, where the Farallon plate was being subducted beneath the North American plate. On the other side of the Farallon plate, was a spreading ridge between the Farallon Plate and the Pacific plate. The eastern side of the Farallon plate was subducting under North America faster than new oceanic crust was being created at the spreading ridge with the Pacific plate. Eventually, the spreading ridge reached the subduction zone at an oblique angle (Figure \(\PageIndex{3}\) below). The relative motion between the Farallon plate and the North American plate was different from the relative motion between the Pacific plate and the North American plate. Thus, when the spreading ridge intersected western North America, a new relative motion and style of deformation formed and the San Andreas Fault system began.

Figure \(\PageIndex{3}\): These four diagrams illustrate the shrinking of the formerly very large Farallon Plate, as it was consumed beneath the North American and Caribbean Plates. Present day, only the Juan de Fua, Rivera, and Cocos plates remain (remnants of the Farallon Plate). Large solid arrows show the present-day sense of relative movement between the Pacific and North American Plates. "Farallon" by the USGS is in the public domain.

The San Andreas fault system has continued to lengthen over time. Remnants of the Farallon plate, now called the Juan de Fuca, Rivera, and Cocos plates, are continuing to subduct north and south of the San Andreas.

Another Large Transform Boundary: The Mendocino Transform

The San Andreas Fault System is a well-known transform plate boundary in California, but it is not the only one impacting the state. The Mendocino Transform (sometimes called the Mendocino Fracture zone) is another plate boundary offshore California that is capable of producing large earthquakes (such as a magnitude 7.2 earthquake in April of 1992). The Mendocino Transform (Figure \(\PageIndex{4}\)) is the fault segment which connects the offshore Juan de Fuca Ridge to the Mendocino Triple Junction. The Mendocino Triple Junction is therefore the intersection of two transform fault systems with a subduction zone. Even though this fault system is offshore of California, medium or large earthquakes occurring on it potentially could have as great an effect on northern California as earthquakes on the San Andreas system can have on central and southern California.

Figure \(\PageIndex{4}\): Location of the Mendocino Transform. (Copyright; author via source)

California's Convergent Boundary: Cascadia Subduction Zone

The Cascadia subduction zone, which is offshore of northern California, Oregon and Washington, is created by the subduction of the tiny Juan de Fuca Plate beneath North America (Figure \(\PageIndex{5}\)). This ocean-continent convergent boundary is what creates the volcanoes of the Cascade Range. Notable volcanoes in this range include Mt. Rainier and Mt. Saint Helens in Washington (the latter of which, in 1980, famously produced the largest volcanic eruption in the contiguous United States of modern times), Mt Hood in Oregon, and Medicine Lake, Mount Shasta and Lassen Peak in California. Lassen Peak is famous for its 1915 eruption, which prior to 1980 was the only volcano to erupt in the contiguous United States since the formation of the United States.

Compared to the earthquake activity of the neighboring transform boundaries, and other subduction zones in the world, the Cascadia Subduction Zone has been relatively quiet; the most recent major Cascadia Subduction Zone earthquake was in 1700 A.D. That is not to say that it is not associated with significant earthquake risk. All the largest earthquakes in history are associated with subduction zones. Examples include the 2011 earthquake and associated tsunami in Japan in 2011 and the 2004 Indonesian earthquake and tsunami in the Indian Ocean. The Cascadia Subduction Zone has the potential for similar earthquakes and tsunamis.

Figure \(\PageIndex{5}\): Cascade volcanic arc, a continental volcanic arc formed where the Juan de Fuca plate is subducting beneath the North American plate. This work by Karla Panchuk is licensed under CC BY-NC-SA.

California's Divergent Boundary: Salton Trough and Gulf of California

Between Baja California and mainland Mexico is the Gulf of California, which is the northern extension of the spreading ridge of the East Pacific Rise (Figure \(\PageIndex{6}\)). This spreading ridge extends onshore through Mexico and into the Salton Trough of southern California, where the tectonic motion then transfers from extension to transform motion, transtension, on the San Andreas fault system. This is different from the northern terminus of the San Andreas fault system which ends at the Mendocino triple junction.

Figure \(\PageIndex{6}\): An aerial view of the Gulf of California with Baja California to the west and mainland Mexico to the east. The Gulf of California as well as the Salton Trough to the north are all an extension of the East Pacific Rise, a divergent boundary. "Divergence in the Gulf of California" by Callan Bentley is licensed under CC BY 4.0.

Video \(\PageIndex{1}\): Plate Tectonics and California Geology

For a concise summary of California's modern plate boundaries and their development, check out the following video:

Query \(\PageIndex{1}\)