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Earth Story: Franciscan Complex of the California Coast Range

Marin Headlands coastline
Marin Headlands, taken by Nicole Myers 2010

For those of you who live along the coast of Central and Northern California, you likely live in the California Coast Ranges and on top of the Franciscan Complex, a chaotic mix of rock that started to form when the dinosaurs were walking on Earth. What makes it complex is a large number of ingredients and the intense degree to which they have been mixed. The assemblage of this rocky chaos is a 200 million year story of tectonic collision, volcanic eruptions, the formation of the San Andreas Fault, and the rise of the California Coast Ranges.
California Geomorphic Provinces with Coast Ranges in green

California’s coastal mountains are not the Himalayas, the Rockies, or even the Sierra Nevada’s. They have not achieved those lofty elevations, and the rocks are not hard and capable of resisting the flow of wind and water. This weak rock is the stuff of the ocean floor that has accumulated over millions of years and then was uplifted above sea level to form the mountains and valleys of the California coast, through the Wine Country all the way to the Central Valley.
Simplified California Geologic Map with Franciscan Complex (aka assemblage) in blue

The Franciscan Complex records a long piece of the Coast Ranges’ story but it is jumbled and missing whole chapters, like a book that passed through a shredder and only portions can be pieced back together. The formation of the Franciscan is actually fairly simple, but it created complexity. As the first dinosaurs were walking on Earth, the oceanic Farallon Plate was sliding eastward and diving beneath the continental North American Plate, forming a convergent boundary, aka a subduction zone. As the Farallon Plate subducted into zones of higher temperature and pressure beneath North America, it caused rock to melt into magma. The magma rose and erupted through volcanos similar to the modern day Mount St. Helens and Mt. Shasta. Periodically the volcanos would explode, raining ash down on the land. Rivers carried some of the ash westward towards the Pacific Ocean where it settled to the ocean floor.
Simplified geologic cross-section of the subduction of the Farallon Plate

The ocean floor was deep beneath the waves, the depths of an oceanic trench that formed where the Farallon and North American Plates met. The trenches of today, like Mariana’s Trench, are 8-12km deep, the deepest places of the ocean. The volcanic ash transported to the trench by wind and rivers, was was joined by the broken pieces of rock and the skeletons of sea creatures that had fallen to the ocean floor out in the depths of the predecessor to the Pacific Ocean, the Panthalassic Ocean.
The tectonics of the Panthalassic Ocean 180 million years ago: FAR = Farallon Plate; Laurasia = North American Plate; Black lines = divergent boundary where adjacent plates are pulling apart; Red line with triangles = where to adjacent plate collide

At its western most point, the oceanic Farallon Plate pulled apart from the adjacent oceanic Pacific Plate. At this divergent boundary magma rose to the surface and cooled to form the youngest edges of the oceanic plates. The nascent Pacific Plate grew larger as North America slowly drifted towards it, and the Farallon Plate shrunk as it subducted, melted, and fueled volcanos on the edge of North America.
Farallon Plate subduction ~40 million years ago

At the Farallon-North American subduction zone trench, the top of the Farallon Plate scraped against the edge of the North America, and the loose rock and accumulated skeletons atop the plate were scrapped off. The scrapings built upon each other, folding and sliding on each other, like batter being scrapped off the edge of the mixing bowl with a spatula. It is a mixed up mess. Oceanic mud, skeletons from warm water creatures and cold water creatures, metals accumulated during volcanic eruptions, chucks of the oceanic floor erupted at the divergent boundary, remnants of volcanic islands, and sand and volcanic ash that washed into the trench after rivers carried them to the ocean. This accumulation is called an accretionary wedge. Accretion is the accumulation of small objects to form a larger mass, and these rocks accumulate in a wedge shape within the subduction trench. All these rocks get mixed together as the Farallon Plate subducts for over 150 million years, forming the Franciscan Complex accretionary wedge. Then the western edge of the Farallon Plate approached the trench, true chaos was coming.
Plate tectonics configuration ~40 million years ago

As the Farallon Plate moved southeast, the diverging Pacific Plate moved northwest. Approximately 30 million years ago the first edge of the Farallon Plate completely subducted, somewhere near modern day Los Angeles. The adjacent North American Plate was also moving northwest, though slower than the approaching Pacific Plate. As the two plates met they began to slide against each other, forming the tectonic transform boundary, which includes the San Andreas Fault. As each remaining piece of the Farallon Plate completed its dive beneath North America, the San Andreas transform boundary grew longer, stretching northwestward. The sliding plates also pressed into each other, creating enough compression to squeeze the Coast Ranges into existence.
Plate tectonics configuration ~20 million years ago

At least 315km of sliding occurred over 30 million years, creating a series of mountains and valleys that form as faults smear rocks northwest and cause unequal uplift. This transition from subduction to transform boundary tore the edges of the plates as they smeared against each other. Huge faults formed, rending gaps in the crust through which magma rose to form volcanos. The volcanos were torn in half, the pieces carried apart from each other by faults. These volcanic rocks are scattered through the Coast Ranges, including the Pinnacles Volcanic Field (Pinnacle National Park), Berkeley Hills, Sonoma Volcanics, and Clear Lake Volcanics, all of which erupted up through the rising Franciscan Complex and built volcanos atop the accretionary wedge.
Geologic map of volcanic rocks erupted as the transform boundary formed & continues to form

The San Andreas transform boundary continues the form beneath our feet. The Clear Volcanics are the northern most transform boundary volcanos and they are still active. Mt. Konocti will erupt again. The remnants of the Farallon Plate continue to subduct. The northern segment, the Juan de Fuca Plate, fuels the Cascade Volcanic Arc. The southern segment, the Cocos Plate, fuels the volcanos of Central America. The faults of the tectonic boundary continue to move, the mountains uplift, the rocks are carried northwest.
Illustration of San Francisco Bay Area Geology by Emily Underwood

The mountains of the Coast Ranges are just rocks that have risen higher than the valleys as the San Andreas transform boundary moves. The Franciscan underlies all of the mountains and valleys of the Coast Ranges, and is often exposed to the atmosphere and carved by surface waters. Where the Franciscan is exposed on the surface, the weak rocks scraped off the ocean floor are broken apart and are easily moved by rivers. The mountains cannot rise to the heights of the Sierra Nevada’s, the rocks are too weak. They are the remnants of the ocean floor, uplifted when a subduction zone ended and a transform boundary formed in its place, bringing the accretionary wedge that had formed far below the waves to meet the atmosphere and become the mountains and valleys of the California coast. We are only 30 million years into the ongoing story of California’s tectonic transform boundary and the formation of the Coast Ranges. Of course there is more to the story, there always is. Each chapter took millions of years to record, many decades to read, and yet humans strive to tell the story we recently learned in mere minutes to hours. A story of such complexity requires years to unravel.
Terry Wright geologic Cross Section of Sonoma County with Franciscan Formation in teal-green


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