11.6: Cenozoic Events

Formation of the San Andreas Fault

Whew. We emplaced the Coast Range Ophiolite. Then the Great Valley Sequence and the Franciscan Complex came along, both added at about the same time. Can we take a breather? Not yet. There’s still a lot that happens in the story of how the Coast Ranges came to be.

When we last left the situation, the breakup of Pangaea had created subduction trenches on the edge of what would become North America, and major parts of California at last formed: the Sierra Nevada, the Coast Ranges, the Great Valley Sequence.

Subduction continued at the proto-North American edge as the continent continued its migration to the northwest, rotating as it did so. Magma formation ceased, the Sierran volcanoes died out and eroded into the Great Valley, and for a time, California experienced a rare time of calm.

But things were about to get interesting. About 30 Ma, aimed straight at the giant subduction trench that formed western edge of the continent, was something that would not submit and compliantly dive into oblivion. A spreading center–a location where new ocean crust is formed–headed toward California (Figure \(\PageIndex{1}\)). And unlike the old, cold, dense ophiolitic crust that normally would descend into the trench, never to be heard from again, the spreading center was youthful and hot, meaning that its rocks were less dense and relatively buoyant.

Figure \(\PageIndex{1}\): The tectonic setting of the west coast of North America in the Mesozoic was quite different than today, as shown in this interpretive map view. The Farallon oceanic plate sat between the continental North American plate to the east, and the oceanic Pacific plate to the west. The boundary between the Pacific and Farallon was divergent (a spreading center). The boundary between the Farallon and the North American was convergent (a subduction zone). This work by Callan Bentley is licensed under CC BY-NC 4.0.

When the subduction trench encountered this warm, buoyant spreading center, things got messy. Try holding a basketball underwater in a swimming pool and you’ll get the idea. The spreading center and the trench collided, wrestled, argued irreconcilably–but it was an impossible situation. The trench couldn’t stop what it was doing–the spreading center couldn’t be subducted. It’s not unreasonable to imagine giant earthquakes as the two plates fought. In terms of geologic violence and drama, thirty million years ago off the coast approximately where Los Angeles is today would have been a very interesting thing to witness.

In time, these two contending parts of the world agreed to disagree and go their separate ways, and in that moment–maybe by 28 Ma–the San Andreas fault was born. The San Andreas fault is the solution to the inability of the spreading center to subduct. The San Andreas split the old Farallon plate, creating two “triple junctions,” places where three plates come together. Today we call these the Mendocino Triple Junction and the Rivera Triple Junction. The Mendocino Triple Junction is, unsurprisingly, near Cape Mendocino, while the Rivera Triple Junction is off the tip of Baja California. These are the two tectonic ends of the San Andreas system.

But they used to be much closer together. Over time, the Mendocino Triple Junction and Rivera Triple Junction migrated away from each other in a northwest-southeast trend, growing and lengthening the San Andreas fault between them. One place that yields good information on this process, and how fast it all happened, is one of the newest National Parks in the U.S.

Pinnacles National Park

If you head to Pinnacles National Park from the east, you drive down a long, linear valley to get to the park entrance. The valley is unremarkable to non-geologists, but to those who know what to look for, this straight-line indentation in the surface of the Earth is the San Andreas fault.

Pinnacles is home to a steep and remarkable assemblage of cliffs and boulders and talus caves, all of which make for exciting, if somewhat grueling, hiking and climbing. Visitors often seek out the famous California condors, whose recovery from near extinction has been helped greatly by efforts at Pinnacles.

But what’s really interesting here is the rocks. This rhyolitic volcanic area erupted about 23.5 Ma, and it happened to do so right on the newborn San Andreas. Since that time, the ground has shifted–both sides of the fault moving to the northwest, but the east side moving slower to the northwest. This fault action has ripped apart the volcanic complex so that today we find the original edifice in two halves–one half, the Pinnacles. Another half, called the Neenach Volcano, exists 314 km (195 miles) to the south (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Pinnacles, California Offset by San Andreas Fault. The present-day location of The Pinnacles is 314 km (195 mi) from the volcano that the San Andreas sliced it from. "Pinnacles, California Offset by San Andreas Fault" by the USGS is in the public domain.

Query \(\PageIndex{1}\)

At Pinnacles National Park you’ll find the northern half of a once whole volcano which was split in two and ended up 314 km (195 mi) north of its other half. This occurred at the Neenach Volcano which is long extinct, and whose other half can be found 25 miles west of the city of Lancaster. This video discusses this oddity and how it involves the San Andreas fault.

Video \(\PageIndex{2}\): The California Volcano Which Split in 2 & Moved 200 Miles Apart

At Pinnacles National Park you’ll find the northern half of a once whole volcano which was split in two and ended up 195 miles north of its other half. This occurred at the Neenach Volcano which is long extinct, and whose other half can be found 25 miles west of the city of Lancaster. This video discusses this oddity and how it involves the San Andreas fault.

Where Things Get "Creepy"

The San Andreas Fault system today experiences a variety of slip behavior. South of the town of Parkfield, large earthquakes have occurred and it appears that this segment of the San Andreas Fault is locked (Figure \(\PageIndex{3}\)). North of San Juan Bautista, the fault has produced large earthquakes like the M 7.9 San Francisco earthquake in 1906, and the M 6.9 Loma Prieta earthquake in 1989. This northern section of the fault is also currently locked. Between the two locked segments, from San Juan Bautista to the town of Parkfield, the fault experiences both seismic slip, producing small earthquakes of M5 or smaller, and aseismic slip, or “creep”. Aseismic slip does not generate seismic waves and can only be detected from geodetic observations (GPS). While creeping faults may not always generate sizable earthquakes, they do slowly deform anything constructed above them.

Figure \(\PageIndex{3}\): Map of the San Andreas fault showing locked (blue) and creeping (yellow) sections. Black lines are other active faults. This work by Coffey et. al is licensed under CC BY.

In addition to the San Andreas Fault, a number of faults within the strike-slip system also experience creep. Take the Hayward Fault, which underlies one of the most populous regions of the San Francisco Bay Area and experiences aseismic creep in certain locations at a rate of ~5 mm/year. Constructed in 1923, University of California Berkeley’s Memorial Stadium sits right atop one of such creeping locations (Figure \(\PageIndex{4}\)). From 2010-2012 the stadium underwent a $321 million renovation to increase its seismic safety after receiving a “poor” rating in 1998. Aseismic creep continues to displace the historic football stadium.

Figure \(\PageIndex{4}\): Aerial view (top) and LiDAR image (bottom) of UC Berkeley Memorial Stadium and surrounding area. The trace of the Hayward fault is indicated by red solid (known) and dashed (inferred) lines. "This work" by Horst Rademacher is licensed under CC BY-NC 4.0

Another major branch of the San Andreas Fault system, the Calaveras Fault, which extends 123 km (76 mi) from Danville to Hollister, is also creeping aseismically. It has slowly shifted sidewalks, retaining walls, and even the foundations of homes creating some interesting architectural features (Figure \(\PageIndex{5}\).

Figure \(\PageIndex{5}\): The creeping Calaveras fault has shifted this home such that the columns on the front porch are no longer vertical. This work by Allison Jones is licensed under CC BY.

The Salinian Block

If you drive past filming locations for Alfred Hitchcock’s movie The Birds, past the abandoned site of a canceled nuclear reactor sitting nearly right on top of the San Andreas fault, and go west until the road ends at the Bodega Head parking lot, you will find granitic rock.

This is a weird place to find that. Usually in California we think of plutonic rock as Sierran, associated with emplacement tens of kilometers deep, where the poor heat conduction allows large crystals to slowly assemble out of the magma. Major uplift has exposed these rocks in the Sierra Nevada (link)… but what’s with Bodega Head?

Travel a bit further south from Bodega Head and you find the high ridge that is the backbone of the Point Reyes National Seashore. Granitic rock, again. The San Andreas close and to the east, again. Point Lobos? Granitic rock again. Further south, and we find the same pattern again and again, pockets of granitic rock proximal to the San Andreas, usually to the west side of the fault. What’s going on?

The Salinian Block encompasses much of the coastline of California, from near Santa Barbara to as far north as Bodega Head. It has many rocks that are not granitic, of course, but while we can form a sandstone just about anywhere, is it an unusual thing to come across granitic exposures so far from a place where they might have formed. The Mesozoic volcanic arc and the magma chambers that fed it were much further to the east, forming the batholiths that now make up the Sierra Nevada and Peninsular Ranges. What could bring granitic rocks so far from where granitic rocks form?

One theory involves the Garlock fault, which is a left-lateral fault slicing the southern terminus of the Sierra Nevada. Movements along the Garlock could tend to move southern Sierran rocks to the west. The Garlock intersects with the San Andreas, which is a right-lateral strike slip fault. One can imagine periodic exchanges, like passengers changing trains, where the Garlock pushes portions of granite to the west, across the San Andreas, where these blocks could now board the Pacific Plate and head northwest, in the direction of Alaska. It wouldn’t be all of the southern Sierra Nevada, just episodic blocks sent cruising northward. And we do find Salinian granite as discrete, relatively small blocks: Bodega Head, the ridge of Point Reyes, Point Lobos (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Decomposing Salinian block granite on the coast just south of Point Lobos. This work by Allison Jones is licensed under CC BY 4.0.

It’s a neat story. But many researchers have taken issue with this idea to explain the Salinian Block. They point to greater similarities in the rock to the Mojave region, and even further south to the granitic emplacements of Baja California, an idea especially supported by paleomagnetic data. At this point, we need to conclude there is not enough data on this Salinian Block issue to pronounce a conclusion. Sometimes in science we must step back to allow for clarifying research, rather than rushing to a convenient and hasty, but erroneous, conclusion.

San Andreas-Related Volcanics

A series of Cenozoic volcanic fields exist within the Coast Ranges of California. They get progressively younger to the northwest and have been diced up by right lateral faults of the San Andreas Fault system, making their history somewhat difficult to unravel. From south to north and oldest to youngest they are the: Quien Sabe/Mt Burdell, Berkeley/Tolay, Sonoma, and Clear Lake volcanic fields (Figure \(\PageIndex{5}\)). These volcanic fields also, not coincidentally, get generally younger to the north. The Quien Sabe and Burdell Mountain volcanic fields have been dated to ~11 Ma, both emplaced around the same time, but they have been separated by ~175 km of right lateral slip of the East Bay fault system. Similarly, the slightly younger Berkeley and Tolay volcanic fields formed at ~10-8 Ma and have been offset by . The Sonoma Volcanics were formed between 8-2.5 Ma and have been displaced at least 28 km along the Rodgers Creek fault.

These volcanic fields are temporally and spatially associated with the northwestward migration of the Mendocino Triple Junction suggesting that, rather than the product of subduction related flux melting (like the Cascades) they are the result of mantle upwelling into an opening in the subducting Farallon plate.

Figure \(\PageIndex{7}\) indicates the locations of these young volcanic fields as well as the locations of the Mendocino Triple Junction over the last 12 million years.

Figure \(\PageIndex{7}\): Schematic map of northern California, showing the Coast Range volcanic centers, major regional faults, and approximate position of the Mendocino triple junction over the past 12 million years. CLVF = Clear Lake Volcanic Field (green), SVF = Sonoma Volcanic Field (teal), BHVF = Berkeley Hills Volcanic Field (blue), QSVF = Quien Sabe Volcanic Field (purple). Modified from McLaughlin and Ohlin (1984); Stanley and Rodriguez (1995). This work by Allison Jones, a derivative of the original, is licensed under CC BY. Access a detailed description.

The Berkeley Hills volcanic field is yet another slightly older volcanic field. This one is Late Miocene-Pliocene in age and is located in the Berkeley Hills in Alameda and Contra Costa Counties. Some of its best exposures can be found in the Robert Sibley Volcanic Regional Preserve and near the Caldecott Tunnel. The primary volcanic unit here is the Moraga Formation which hosts basaltic lava flows, shallow intrusions, volcanic breccias and volcaniclastic sediments. It is underlain by the Orinda formation, interpreted to be 12-14 Ma river deposits, which lies unconformably over the Great Valley Group. The Moraga formation is overlain by younger sedimentary units. These units are all folded into a syncline - the Siesta Valley Syncline - and thus are repeated from west to east.

Eighty miles south of the Berkeley volcanics and on the opposite side of the Hayward/Calaveras Fault system is the Quien Sabe volcanic field. Some research suggests that these two fields are of the same origin. Mount Burdell north of Novato in Marin County is also home to similar volcanic rocks suggesting that it may also be similarly displaced Quien Sabe volcanics.

The Sonoma/Napa volcanics are mostly Pliocene age. They are highly variable in their composition containing andesite, basalt, rhyolite flows, pyroclastic tuff, and breccia, diatomaceous mud, silt, and sand. There is also some obsidian and perlitic glass. Notably, the Sonoma/Napa volcanics supply the Sonoma and Napa valleys with rich soils that contribute to the region’s world-renowned viticulture. Older parts of the Sonoma Volcanics have been displaced 28 or more km along the Rodgers Creek fault in the last 7 million years.

The Clear Lake volcanic field is the youngest and the northernmost of the young volcanic fields in the Coast Ranges of California. It has been active through the Pleistocene, with the most recent eruptions occurring about 11,000 years ago. The Clear Lake Volcanic Field is now home to the world's most productive geothermal energy power plants, which supply enough electricity for about 850,000 homes in the region.The Geysers geothermal field south of Clear Lake along Geyser Creek is currently the world’s largest complex of geothermal power plants, containing 22 plants that generate an average of 955 MW (Figure \(\PageIndex{8}\)). Here, groundwater percolates down through fractures in the permeable Franciscan Complex greywacke where it is heated by shallow magma. The greywacke here is overlain by a relatively impermeable layer which traps steam. Wells have been drilled down to extract the steam which is in turn converted to electricity by turbines. The Geysers Geothermal field has been used for electric power since 1960.

Figure \(\PageIndex{8}\): Sonoma Calpine 3 plant at The Geysers geothermal power complex, Sonoma County, California. This work by Stepheng3 via Wikimedia Commons is licensed under CC0 1.0.

Transpression and Transtension

Another symptom of San Andreas faulting in the Coast Ranges is their elevation. Approximately 3.5 million years ago, the Pacific and North American Plates went from sliding alongside each other in true strike-slip fashion, to moving at a slight angle to one another. This small shift had huge consequences for the topography of the Coast Ranges.

While most of the plate motion of the San Andreas fault today is strike slip (about 90% of it) the remaining 10% is actually compressional. It was only about 1-2 million years ago that uplift of the Coast Ranges really began in earnest. They continue to rise today by about 1-2 mm (1/16 of an inch) per year. This combination of strike slip motion and compression is known as transpression. It can thicken the crust in two ways: by faulting it or by folding it. Both deformation styles are present in the Coast Ranges.

This crustal deformation is even more dramatic where a fault bends. In a right-lateral fault like the San Andreas and its ancillary faults, a bend to the left will result in an even more locally shortened and thickened area. This is known as a restraining bend (Figure \(\PageIndex{9}\)). Should the same right-lateral fault instead take a bend to the right, it will instead experience stretching and thinning forming down-dropped features like sag ponds. This is known as transtension and the resulting feature is called a releasing bend. Both releasing and restraining bends are common all along the SAF. The aforementioned Clear Lake Volcanic Field is a transtensional basin.

Figure \(\PageIndex{9}\): Left: A releasing bend forms when a fault bends creating localized areas of extension. This can form locally down-dropped blocks and features like pull-apart basins and sag ponds. Right: A restraining bend forms when a fault bends creating an area of localized compression. This can form locally uplifted blocks and features like mountains, reverse faults, thrust faults, and anticlines. This work by John Waldron & Morgan Snyder, Open Education Alberta, is licensed under CC BY-NC.

Video: Virtual Field Trip: A sag pond along the San Andreas Fault

In the following video, Dr. Kim Blisniuk (San Jose State University) describes how a sag pond has formed along the San Andreas Fault in Sanborn County Park.

Box: Mount Diablo

One of the most notable areas of recent uplift in the Coast Ranges is Mount Diablo, the highest peak in the Diablo Range (Box Figure \(\PageIndex{1}\)). Rising 1,173 meters (3,849 feet) above sea level, Mount Diablo’s south peak was selected as the initial point (where the Mount Diablo Base and Mount Diablo Meridian lines intersect) for cadastral surveys. It has since been used as the reference point for much of California and all of Nevada.

Box Figure \(\PageIndex{1}\): View of Mount Diablo in Contra Costa County looking south. “Mount Diablo” by Wayne Hsieh via Flickr is licensed under CC BY-NC.

Mount Diablo consists of typical Coast Range rocks: Franciscan Complex, Coast Range Ophiolite, and Great Valley Sequence rocks are all present. While these rocks initially formed below the surface of the ocean, today they form Mount Diablo’s two peaks.

The actual uplift of Mount Diablo is geologically very recent, occurring only in the last 2 million years. Right lateral strike slip motion on the Calaveras and Clayton-Marsh Creek-Greenville faults on either side of the mountain contributed to the formation of a thrust fault beneath the mountain that continues to raise the mountain ever higher. The so-called Mount Diablo Thrust Fault also folded the layers above it into a large anticline.

Also unique to the vicinity of Mount Diablo is a very rare-in-California appearance of the organic sedimentary rock coal, which is found on the northern flanks of the mountain and was deposited between the Paleocene and Miocene (Box Figure \(\PageIndex{2}\)). During this time, multiple changes in sea level occurred. During the third of these cycles, tectonic uplift caused the continental shelf to rise, forming shallow marshes. These marshes, in addition to the hot and humid climate of the time, led to the growth of marsh flora that would eventually be buried to form coal. This coal was discovered in 1859 and coal mining operations quickly began to supply coal to nearby San Francisco. These coal mines operated until the early 1900s until the small coal seams were depleted. The Black Diamond mines, named after the “black diamond” coal, were to this day the largest producer of coal in the state of California producing more than 400 million short tons of coal.

Box Figure \(\PageIndex{2}\): A dark seam of coal cutting through a mining shaft at the Hazel-Atlas mine. This work by Allison Jones is licensed under CC BY 4.0.

The coal here is interbedded with silica-rich sandstone. In the 1920s, a silica mine was started and sold to the Hazel-Atlas Glass Company which produced glass jars, bottles, and other glass items in Oakland. 1.8x 10⁶ short tons of sand was recovered between the 1920s and the 1940s. Today, the Hazel-Atlas mine is maintained by East Bay Area Parks staff and can be visited on guided tours.

Not all of the Coast Ranges are so heavily influenced by the transform boundary. On its northward migration the Mendocino Triple Junction has yet to reach the Oregon border, leaving about 170 km of California Coast Ranges that have yet to be twisted and mangled by the San Andreas. The furthest northern Coast Ranges are part of the forearc of the Cascadia Subduction Zone, and as such have experienced somewhat different uplift processes, discussed in more detail in the chapter on the Klamath Mountains. But the grasp of the San Andreas is far reaching; in geologic time, the fight between ridge and trench is still ongoing, and the front seats to watch the action are now in Humboldt County.

Northern Coast Range Folding and Faulting

Near the Mendocino Junction, the Coast Range has been shaped by both convergent and transform plate motions. As the Juan de Fuca Plate (and the Gorda Plate) subducts beneath the North American Plate, it pushes against it, creating east-west compressional stress. Meanwhile, further south, the Pacific Plate slides against the North American Plate along the San Andreas fault. As it does so, it drags the edge of the North American Plate with it. This northward motion of southern and central California pushes against the northern part of the state, creating north-south compression. These two sources of compression combine, resulting in net northeast-southwest compression. This stress has caused the crust to crumple in a combination of folding and thrust faulting (see Geologic Structures and Seismology). The northwest-southeast striking thrust faults can be seen on the geologic map. The crest of the anticlines frequently form ridges and the synclines form valleys. On the coast, the uplifting anticlines are associated with features like sea cliffs, typical of an uplifting coastline, whereas the subsiding synclines are host to estuaries, lagoons and bays (see California’s Coastline). These include the several Lagoons near Redwood National Park and the second largest bay in California, Humboldt Bay. Because it is one of the few parts of California that is tectonically subsiding (even as the surrounding region uplifts), Humboldt Bay is experiencing the fastest rate of sea level rise in the state.

Faulting in this region has not only contributed to the uplift of the anticlines and shaped the topography of the landscape, but also poses earthquake hazards. Although the biggest earthquake threat in this region lies offshore, crustal thrust faults within the North American Plate pose their own set of issues. The College of the Redwoods Eureka Campus was built in 1968, right along the Little Salmon Fault (before the fault was mapped by geologists). Many of the original buildings have now been demolished and new ones built in locations that comply with state laws regarding active faults (see California's Earthquakes).

References

1. 2006 Humboldt | Friends of the Pleistocene. (n.d.). Retrieved September 13, 2024, from https://www.fop.cascadiageo.org/field-trips/pacific-cell-field-trips/2006-humboldt/
2. Ball, J. L. (2022). Stratigraphy and eruption history of maars in the Clear Lake Volcanic Field, California. Frontiers in Earth Science, 10, 911129. https://doi.org/10.3389/feart.2022.911129
3. Coffey, G. L., Savage, H. M., Polissar, P. J., Cox, S. E., Hemming, S. R., Winckler, G., & Bradbury, K. K. (2022). History of earthquakes along the creeping section of the San Andreas fault, California, USA. Geology, 50(4), 516–521. https://doi.org/10.1130/G49451.1
4. Patton, J., Williams, T., Anderson, J., & Leroy, T. (n.d.). Tectonic land level changes and their contribution to sea-level rise, Humboldt Bay region, Northern California.