6.5: Cenozoic California (66 Ma – Present)- Faulting, Uplift, and Basin Formation
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Thought it might be hard for residents of present day California to be believe it, by about 50 million years, the ancestral Sierra Nevada that had been uplifted in the Mesozoic had eroded down to relatively low-lying hills. Large rivers flowed from Nevada and gravels laden with gold (placer gold). This continued until around 40 million years ago when once again things began to change.
End of Subduction and the Birth of the San Andreas Fault
Throughout the Mesozoic Era, the interactions between the subducting Farallon Plate and the North American Plate shaped the tectonic evolution of western North America. As the Farallon Plate continued to subduct beneath the continent, the spreading ridge between the Farallon and Pacific plates—known as the East Pacific Rise—gradually approached the subduction zone (Figure 6.5.1).
Around 30 million years ago, the East Pacific Rise reached the margin and disrupted the subduction system. This event marked a significant tectonic shift, leading to the formation of a new plate boundary: the Proto-San Andreas Fault. As a result of this change, the Farallon Plate was split into the Nazca Plate to the south and the remaining Farallon Plate to the north. The Pacific Plate was now in direct contact with the North American Plate, initiating a transform plate boundary where the plates slide past one another, rather than converge.
This newly formed boundary gave rise to two migrating triple junctions—points where three tectonic plates meet. The Mendocino Triple Junction moved northward, while the Rivera Triple Junction moved southward. As the Mendocino Triple Junction progressed north, the margin transformed from a subduction zone to a transform fault system. The Pacific Plate, which was not being subducted, left a slab window—a gap where the subducting plate had been. This gap allowed asthenospheric mantle to rise and triggered a northward-migrating pulse of magmatism, which can be traced today by a series of volcanic rocks along California’s coast.
This tectonic reorganization also caused crustal rotation in Southern California. Several blocks, including the Transverse Ranges and California Borderland, rotated clockwise as a response to the new stress regime, a unique feature among North American mountain ranges.
This silent video animation presents a model showing how the plate boundary evolved over time and extended inland. A detailed text description is available on YouTube. As you watch, pay close attention to the changes in eastern California as the Basin and Range province begins to form.
Cascade Volcanism
In far northern California, the Cascade Range represents a continuation of active subduction-related volcanism. While most of California transitioned to a transform boundary with the development of the San Andreas Fault, a portion of the former Farallon Plate—the Gorda Plate—continues to subduct beneath the North American Plate offshore of northern California and southern Oregon.
This active subduction drives melting in the mantle above the subducting slab, generating the Cascade Volcanic Arc. Although the Cascade Range extends from British Columbia to Northern California, only two major Cascade volcanoes lie within California’s borders: Mount Shasta and Lassen Peak.
Subduction of the Gorda Plate continues today, making northern California volcanically active. However, seismic and geologic data suggest that the rate of subduction is slowing. If this trend continues, magmatism in the Cascades may diminish and eventually cease in the geologic future.
Basin and Range Extension and Crustal Thinning
The change from compressional subduction to lateral transform motion also altered the stress regime far into the continental interior. During subduction, crustal shortening had thickened the lithosphere, forming high-elevation regions. When subduction ceased, this thickened crust lost its tectonic support and began to collapse under its own weight, a process called gravitational collapse.
This collapse produced widespread extensional faulting across a broad swath of the western U.S., creating the Basin and Range Province. This geologic province, extending from eastern California through Nevada and into Utah, is characterized by alternating mountain ranges and valleys formed by normal faults. These faults accommodate the east-west extension and crustal thinning that is still ongoing today. Additionally, right-lateral strike-slip faulting—related to the San Andreas system—also contributes to the complex deformation patterns in this region.
This animation highlights the tectonic evolution of California from the Late Cretaceous through the early Cenozoic, emphasizing the transition from subduction to transform faulting around 20 million years ago.
Changes to the Great Valley and Coast Ranges
In the Cenozoic, especially after the onset of transform motion along the coast, the Great Valley gradually shifted from a primarily marine environment to a terrestrial one. As subduction slowed and eventually ceased in central California, the Sierra Nevada began to uplift more dramatically, increasing the rate of erosion and sediment delivery to the valley. Simultaneously, the Coast Ranges also rose, isolating the valley and enhancing its role as a sediment trap.
References
- Atwater, T., & Stock, J. (2010). Pacific-North America Plate Tectonics of the Neogene Southwestern United States: An Update. International Geology Review, 40(5), 375-402. https://doi.org/10.1080/00206819809465216
- Dickinson, W. R. (2004). Evolution of the North American Cordillera. Annual Review of Earth and Planetary Sciences, 32(1), 13-45. https://doi.org/10.1146/annurev.eart....101802.120257
- Luyendyk, B. P., Kamerling, M. J., & Terres, R. (1980). Geometric model for Neogene crustal rotations in southern California. Geological Society of America Bulletin, 91(4). https://doi.org/10.1130/0016-7606(1980)91<211:Gmfncr>2.0.Co;2
- McQuarrie, N., & Wernicke, B. P. (2005). An animated tectonic reconstruction of southwestern North America since 36 Ma. Geosphere, 1(3), 147-172. https://doi.org/10.1130/GES00016.1