2.3: California’s Geologic History

Through its history, California has transitioned from a passive continental margin, before the breakup of Pangae, to an active margin with the transition to subduction zone activity and the formation of the Cordilleran volcanic chain. The Cordilleran Ranges is the name for the volcanic arc that formed the Sierra Nevada Range and the Peninsular Ranges extending south into Baja California. Subduction ended when the ancient Farallon Plate was overrun as North America moved westward, overriding the northern end of the spreading center in the Eastern Pacific basin. This led to the formation to the modern transform plate boundary associated with the San Andreas and the opening of the Gulf of California.

Lecture Time: California’s Geology

Watch a lecture video that introduces the basic geology of California’s dynamic past and future. (Video length is 38:40).

The Farallon Plate

The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west coast of the North American Plate—then located in modern Utah—as Pangaea broke apart and after the formation of the Pacific Plate at the center of the triple junction during the Early Jurassic. It is named for the Farallon Islands, which are located just west of San Francisco, California (figure 2.5).

Figure 2.5: Illustrating the Shrinking of the Farallon Plate Beneath the North Americana & Caribbean Plates. Image is used under a CC-BY 4.0 license.

Over time, the central part of the Farallon Plate was completely subducted under the southwestern part of the North American Plate. The remains of the Farallon Plate are the Juan de Fuca, Explorer, and Gorda Plates, subducting under the northern part of the North American Plate; the Cocos Plate subducting under Central America; and the Nazca Plate subducting under the South American Plate.

The Farallon Plate is also responsible for transporting old island arcs and various fragments of continental crustal material rifted off from other distant plates and accreting them to the North American Plate. These fragments from elsewhere are called terranes. Much of western North America is composed of these accreted terranes.

California’s Geologic Timeline

The oldest rocks in California, a marble, date back 1.8 billion years to the Proterozoic and are found in the San Gabriel Mountains, San Bernardino Mountains, and Mojave Desert. The rocks of eastern California formed a shallow continental shelf, with massive deposition of limestone during the Paleozoic, and sediments from this time are common in the Sierra Nevada, Klamath Mountains, and eastern Transverse Range.

Active subduction began in the Triassic during the Mesozoic, producing large granite intrusions and the beginning of the Nevadan Orogeny as well as more dryland conditions and the retreat of the ocean to the west. Throughout the Jurassic the Nevadan Orogeny accelerated with large-scale granitic intrusions and erosion into deep marine basins. These basins steadily filled with sediment, with one famous example preserved as the Great Valley beds in the Coast Ranges. Simultaneously island arcs and small sections of continental crust grafted onto the edge of North America, building out the continent.

During the Cenozoic, the volcanic and deep-water sedimentary Franciscan rocks were accreted to the edge of California and vast areas of marine sedimentary rocks deposited in the Central Valley and what would become the Transverse and Coast Ranges. Examples of filled basins included the Los Angeles Basin, the Eel River Basin around Eureka, and the 50,000-foot-thick sedimentary sequences of the Ventura Basin. As stated earlier, the San Andreas Fault became most active after the Miocene, during the Oligocene, resulting in up to 350 miles of offset in some locations.

Field Trip: California’s Oldest Rocks

Let’s head on a field trip to find some of the oldest rocks in California! Either scan the QR code or visit this link to join Professor Patrich as he searches for the oldest rocks near Mecca, California. (Video length: 3 min).

California’s Geologic Faults

The Pacific Plate is a major section of the Earth's crust, gradually expanding by the eruption of magma along the East Pacific Rise to the southeast. It is also being subducted far to the northwest into the Aleutian Trench, and trenches along Japan, Kamchatka, Philippines and Hawaiian (figure 2.6). In California, the plate is sliding northwestward along a transform boundary, the San Andreas Fault, toward the subduction zone. At the same time, due to divergence along the Mid Atlantic Ridge, the North American Plate is moving southwestward relative to the Earth's core, but southeastward relative to the Pacific Plate, due to the latter's much faster northwestward motion. The westward component of the North American Plate's motion results in some compressive force along the San Andreas and its associated faults, thus helping lift the Pacific Coast Ranges and other parallel inland ranges to the west of the Central Valley, in this region most notably the Diablo Range (figure2.8). The Hayward Fault shares the same relative motions of the San Andreas. As with portions of other faults, a large extent of the Hayward Fault trace is formed from a narrow complex zone of deformation which can span hundreds of feet in width.

Figure 2.6: Relative Plate Motions of Western Portion of North America. Image is used under a CC BY-SA 3.0 license.

Central & Northern California Faults - The Hayward Fault Zone

The Hayward Fault Zone is a right-lateral strike-slip geologic fault zone capable of generating destructive earthquakes. This fault is about 119 km (74 mi) long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs through densely populated areas, including Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Castro Valley, Hayward, Union City, Fremont, and San Jose.

The Hayward Fault is parallel to the San Andreas Fault, which lies offshore and through the San Francisco Peninsula. To the east of the Hayward Fault lies the Calaveras Fault. In 2007, the Hayward Fault was discovered to have merged with the Calaveras Fault east of San Jose at a depth of 6.4 kilometers (4.0 mi), with the potential of creating earthquakes much larger than previously anticipated. Some geologists have suggested that the Southern Calaveras should be renamed as the Southern Hayward

While the San Andreas Fault is the principal transform boundary between the Pacific Plate and the North American Plate, the Hayward-Rodgers Creek Fault takes up its share of the overall displacement of the two plates (Figure 2.7).

Figure 2.7: Map of Central & Northern California’s Faults. Image is in the public domain.

Southern California Faults

Most of central and northern California rests on a crustal block (terrane) that is being torn from the North American continent by the passing Pacific plate comprised of oceanic crust. Southern California lies at the southern end of this block, where the Southern California faults create a complex and even chaotic landscape of seismic activity. About two-thirds of this movement occurs on the San Andreas fault and some parallel faults- -- the San Jacinto, Elsinore, and Imperial faults (figure 2.8). Over time, these faults produce about half of the significant earthquakes of our region, as well as many minor earthquakes.

Figure 2.8: Model of Southern California Showing the Motion of the Pacific & North American Plates. Graphic by the USGS is in the public domain.

The last significant earthquake on the Southern California stretch of the San Andreas fault was in 1857, and there has not been a rupture of the fault along its southern end from San Bernardino to the Salton Sea since 1690. It is still storing energy for some future earthquake.

But we don't need to wait for a "big one" to experience earthquakes. Southern California has thousands of smaller earthquakes every year. A few may cause damage, but most are not even felt. And most of these are not on the major faults listed above. The earthquake map below, figure 2.9, shows that earthquakes can occur almost everywhere in the region, on more than 300 additional faults that can cause damaging earthquakes, and countless other small faults.

This is mostly due to the "big bend" of the San Andreas fault, from the southern end of the San Joaquin Valley to the eastern end of the San Bernardino mountains. Where the fault bends, the Pacific and North American plates push into each other, compressing the earth's crust into the mountains of Southern California and creating hundreds of additional faults (many more than shown in the fault map). These faults produce thousands of small earthquakes each year, and the other half of our significant earthquakes. Examples include the 1994 Northridge and 1987 Whittier Narrows earthquakes.

Figure 2.9: Southern California’s Fault names and locations. Image by BrucePL is used under a CC BY-SA 4.0 license.

Field Trip: The Geology of the Calico

Let’s head on a field trip to the Calico Ghost Town in Yermo, California. Either scan the QR code or visit this link to join Professor Patrich as he explores and shares the geology of this iconic place! (Video length: 3 min).

The Evolution of the San Andreas Fault

The San Andreas Fault System has gradually evolved since middle Tertiary time, around 28 million years ago. The San Andreas Fault System grew as a remnant of an oceanic crustal plate and its spreading ridge, was beginning to reach the subduction zone off the western coast of North America. The result was the development of a crustal fracture zone with right-lateral offset that propagated along the continental margin. This action also slivered off pieces of the North American Plate and added them to the Pacific Plate. In the San Francisco Bay Area, much of the offset along the San Andreas Fault System has occurred along the East Bay faults, Calaveras and Hayward fault, rather than along the San Andreas Fault itself in the Peninsula region.

Figure 2.10: The San Andreas Fault. Image by the USGS is in the public domain.

A series of block diagrams seen in figure 2.7, shows how the subduction zone along the west coast of North America transformed into the San Andreas Fault over the period from 30 million years ago to the present. Starting at 30 million years ago, the westward-moving North American Plate began to override the spreading ridge between the Farallon Plate and the Pacific Plate. This action divided the Farallon Plate into two smaller plates, the northern Juan de Fuca Plate (JdFP) and the southern Cocos Plate (CP). By 20 million years ago, two triple junctions began to migrate north and south along the western margin of the west coast. [Triple junctions are intersections between three tectonic plates; shown as red triangles in the diagrams.] The change in plate configuration as North American Plate began to encounter the Pacific Plate resulted in the formation of the San Andreas Fault. The northern Mendocino Triple Junction (M) migrated through the San Francisco Bay region roughly 12 to 5 million years ago and is presently located off the coast of northern California, roughly midway between San Francisco (SF) and Seattle (S). The Mendocino Triple Junction represents the intersection of the North American, Pacific, and Juan de Fuca plates. The southern Rivera Triple Junction (R) is presently located in the Pacific Ocean between of Baja California (BC) and Manzanillo, Mexico (MZ).

Evidence of the migration of the Mendocino Triple Junction northward through the San Francisco Bay region is preserved as a series of volcanic centers that grow progressively younger toward the north. Volcanic rocks in the Hollister region are roughly 12 million years old whereas the volcanic rocks in the Sonoma-Clear Lake region north of San Francisco Bay ranges from only a few million to as little as 10,000 years ago. Both volcanic areas, as well as older volcanic rocks in the region, are offset by the modern regional fault system.

Figure 2.11: Assembling California Over the Past 30 Million Years. Image by the USGS is in the public domain.