6.5: Cenozoic California (66 Ma – Present)- Faulting, Uplift, and Basin Formation

End of Subduction and the Birth of the San Andreas Fault (~30 Ma)

Throughout the Mesozoic Era, the interactions between the subducting Farallon Plate and the North American Plate drove the tectonic development of the margin. Through time, as the Farallon plate was consumed beneath North America, the spreading ridge between the Pacific and Farallon plates eventually approached the subduction zone (Figure 6.5.1). Around 30 Ma, the East Pacific Rise intersected the California subduction zone and created a new plate boundary, the Proto-San Andreas Fault. This interaction separated the Farallon plate into a northern plate (Farallon) and a southern plate (Nazca). It also brought the Pacific plate into contact with North America and drastically changed the forces at the plate boundary. The relative motion between the Pacific and North American plates was much different than the Farallon and North American relative motion. Farallon and North American motion was toward one another and created a convergent margin while Pacific and North American motion created a transform boundary. As the Nazca and Farallon plates continued to subduct, the portion of the North American plate in contact with the Pacific lengthened. A meeting of three plate boundaries, or triple junction, was formed where Farallon-Pacific-North American plates and Nazca-Pacific-North American plates intersected. The northern triple junction (Mendocino) migrated north and the southern triple junction (Rivera) migrated south. The progressive northward movement of the Mendocino Triple Junction brought with it a change from convergent to transform motion. Because the Pacific plate was not subducting beneath North America, a gap in the downgoing plate or slab window formed and migrated north with the triple junction. The gap in the downgoing plate created a pulse of magmatism that also migrated north along the margin and left behind a trail of volcanic rocks recording its passage. The new translational boundary also caused rotation, about vertical axes, of several blocks in coastal southern California, including the Transverse Ranges and Borderland.

Figure \(\PageIndex{1}\): Evolution of the tectonic boundary from a subduction zone to a transform margin. "Evolution of the San Andreas Fault" by USGS is in the Public Domain, CC0

Video \(\PageIndex{1}\): Cenozoic Plate Boundary Evolution

The video animation that follows presents a model for how this region has extended as the plate boundary to the west developed. There is no narration for this video, but a detailed text description can be found in the show notes on YouTube. As you review this video, pay attention to what happens to the region of eastern California as the Basin and Range forms!

Basin and Range Extension and Crustal Thinning

The change from a convergent margin to a transform boundary also influenced deformation well into the continent. With the end of subduction, the compressional forces that had built and sustained a thickened crust, also came to an end. Without the subduction margin to support it, the thickened high-elevation crust became susceptible to gravitational collapse. From the Wasatch Fault in Utah to the eastern escarpment of the Sierra Nevada, the Basin and Range province formed with normal faults accommodating the extension and thinning of the continental interior. The patterns on the geologic map are heavily overprinted by this latest and ongoing episode of deformation that includes east-west extension and right-lateral transform motion.

 

Video \(\PageIndex{2}\): Changing Plates: The Tectonic Story (85 to 20 million years ago)

The following video details the evolution of California from 85 to 20 million years ago. Notice the transition from subduction to transform around 20 million years ago.

Formation and Evolution of the Central Valley

 

Cascade Volcanism: Mount Shasta, Lassen Peak, and Other Features

"Like the mountain ranges further south in California, the mountain ranges found from southern British Columbia, Canada, to northern California reflect the standard features of a subduction zone. The Coast Ranges and the Olympic Mountains are part of an accretionary wedge; the Willamette Valley and Puget Sound are parts of a forearc basin; and the Cascades are inland volcanoes.

In the early Cenozoic, the Farallon plate was being subducted beneath the North American plate in this region. As this plate continued to subduct, in broke into smaller plates, which were each given their own names. Today, the plate being subducted under the North American plate in the region spanning southern British Columbia to northern California is called the Juan de Fuca plate (sometimes itself considered three small plates: the Explorer, Juan de Fuca, and Gorda plates).

The present-day Cascade Range is made up of a series of volcanoes that have built up a large platform of volcanic debris. These volcanoes, the Cascade Volcanic Arc, began to arise 36 million years ago (in the late Eocene). However, the major volcanic peaks that make up the High Cascades formed more recently, within the Pleistocene.

Active subduction still occurs off the west coast of northern California, Oregon, Washington, and southern British Columbia. As long as subduction continues, the Cascades will remain volcanically active; there is evidence that the rate of subduction is slowing, and, as a result, volcanism in the Cascades will eventually cease."

References

1. 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
2. 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
3. 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
4. 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