11: Coast Ranges
- Page ID
- 20345
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The Coast Ranges are a series of mountains and valleys trending northwest-southeast along the western edge of North America. The California Coast Ranges begin near Santa Barbara and extend northward to the Oregon border, following the coastline of California. The Coast Ranges are a major division of the physiographic provinces of California, and abut the Klamath province and Great Valley to the east, and the Transverse province to the south. Although the Coast Ranges are not immediately adjacent to the Sierra Nevada, in some places they are geologically similar, as portions of the former Sierra Nevada have been amalgamated into the Coast Ranges through tectonic violence.
We can look across California from west to east and see the Coast Ranges descending into the plains of the Central Valley, then rising again into the Sierra Nevada, as shown in (Figure \(\PageIndex{1}\)):
Almost every major event in California’s geologically brief lifespan has left indelible imprints in the rocks of the Coast Ranges. Magma that cooled as the supercontinent of Pangaea began to break up? Check. Agonized seafloor rocks being forced underneath continents, even as a daring few escaped onto the continent? Yep. Bizarrely transformed metamorphic rocks thoroughly altered at temperatures less than your kitchen’s oven? Uh huh. The San Andreas ripping apart western California and implanting magma bodies which to this day feed hot springs and geysers? You bet.
In the 1994 Boomer nostalgia film Forrest Gump, the titular character, played by Bay Area native (and former community college student) Tom Hanks unwittingly intersects with major historical events such as the Vietnam War and Watergate, along the way meeting figures such as Elvis Presley, John Lennon, JFK, LBJ, and Nixon. The Coast Ranges are like Forrest Gump. They’ve unintentionally been in the right place to witness a lot of the important geologic events in California.
In this chapter we will divide the complexity of the Coast Ranges into broad geographic categories. We will talk about two regions:
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the Northern Coast Ranges and
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the Southern Coast ranges
The Northern Coast Ranges are primarily composed of sedimentary rocks that were deposited during the Mesozoic and Cenozoic Eras. The Southern Coast Ranges are generally younger, and are composed of sedimentary rocks that were deposited during the Cenozoic Era. For time ranges of these time periods it may be useful to review the timeline from the chapter Geologic Time.
We will also parse the major rock units in the Coast Ranges, including:
- The Coast Range Ophiolite
- The Franciscan Complex
- The Great Valley Sequence
- Quaternary rocks, including volcanics
For fun we will examine some of the most interesting features of the Coast Ranges, including:
- Geothermal fields of hot bubbling mud fed by the wake of the San Andreas fault
- A coal mining region, right here in California
- Metamorphic rocks so altered they could be described as Seussian
By the end of this chapter, you should be able to:
- Describe how plate tectonics influences the geology of the Coast Ranges.
- Explain our current understanding of the origins of the Coast Range Ophiolite, Great Valley Sequence, and Franciscan Complex.
- Explain the origins and organization of ophiolite sequences generally.
- Explain Coast Range geomorphic features associated with transpression and transtension.
- Describe San Francisco Bay during the last glacial maximum.