11.5: The Franciscan Complex

What the Franciscan Is

If you look at older textbooks or journal papers, you can find the Franciscan called the Franciscan Formation. Or the Franciscan Group. Or the Franciscan terrane. Or the Franciscan Assemblage. Today, perhaps due to general exhaustion over the issue, geologists have settled on calling this vast part of western California the Franciscan Complex. In the future it may yet be rebranded as something else.

In any event, the name is inconsequential compared to what the Franciscan Complex is and the story it tells about California’s geologic development.

The rocks of the Franciscan are, as you might imagine, varied and quite complicated. Sedimentary, metamorphic, and igneous rocks are all represented within the formation and it is not uncommon to glimpse 4-5 different rock types of the Franciscan Complex within the span of a single photograph (Figure \(\PageIndex{1}\)).

Figure \(\PageIndex{1}\): An outcrop of representative Franciscan rocks at Baker Beach, San Francisco. Green-blue rocks in the foreground are melánge with serpentinite and argillite. Behind them, red radiolarian chert outcrops. Behind the chert is greenstone and graywacke sandstone. This work by the National Park Service is in the public domain.

The Franciscan contains a lot of ocean sediment and a lot of oceanic volcanic rocks. But most everything has been metamorphosed to some degree. And some of the Franciscan rocks have been greatly metamorphosed, altered beyond all recognition so thoroughly that it is hard to imagine the immensity of heat and pressure that have shaped them.

The Franciscan contains sedimentary rocks: greywackes, a dark, dirty-looking sandstone filled with fragments of other rocks, suggesting sediment raining down from eroding surfaces. Research suggests that the source of lithic fragments in Franciscan greywacke derived from the same source as the Great Valley Sequence, in other words, from the now missing volcanoes that once sat above the Sierra Nevada Batholith.

Franciscan chert is a glassy burgundy rock consisting almost entirely of the microscopic glassy shells of single-celled organisms called radiolaria, now stacked in fine layers hundred of meters thick.

The Franciscan has igneous rocks: pillow basalts, rounded remnants of volcanic eruptions underwater. Much of this basalt has been metamorphosed to greenstone, but the pillow structures are still visible.

Video \(\PageIndex{1}\): Virtual Field Trip: Pillow lava in the Marin Headlands

The following video describes pillow lavas found at the Marin Headlands in the northern Coast Ranges.

The Franciscan also has metamorphics, such as serpentinite, California’s state rock, with its elegant, slippery, brilliant green surfaces. There is also blueschist, a very rare and highly altered metamorphic rock that comes to us by the strange combination of incredible pressure and cool temperatures. The Franciscan Complex has all of these and more.

Query \(\PageIndex{1}\)

Forming the Franciscan

The story begins with the breakup of the supercontinent Pangaea, about 200 million years ago. As the separating continents moved apart, giant subduction trenches formed along their advancing edges. Along the west coast of North America, this was one in a series of such subduction events which, through terrane accretion, assembled what is now California (see A Brief Geologic History of California) Because Earth’s size is virtually unchanging, if you create new crust–for example, where the expanding proto-Atlantic grew–you have to remove old crust at the same rate. Subduction zones accomplish this by having denser, colder crustal material slide down into the deep mantle, where ancient gigantic slabs surround the Earth’s core, phantoms of a lost world. These subduction zones also feed water-rich rock into the trenches, whereupon this aqueous bounty aids the distribution of heat as the slabs descend, allowing magma to form in a process called flux melting (see Cascades).

Meanwhile, as the seafloor of the proto-Pacific advanced toward the bleeding edge of North America, the Farallon trench awaited to consume it. Had this process proceeded smoothly, we would not have the Franciscan Complex, but rather a deep offshore trench, not unlike what currently exists off of modern day Chile and Peru. But a significant portion of this seafloor crust would jam in the trench, scrape off, become part of North America - repeatedly. That’s the Franciscan Complex.

The Franciscan, much like the Sierra Nevada metamorphic belts, can be divided into at least three accreted terranes, but all three formed through dominantly mélange style accretion (see Klamath Chapter, section 10.2) For the sake of this text, we’ll refer to the Franciscan as a whole.

When did this happen? Wakabayashi (2015) finds evidence for subduction beginning at 165 Ma, and lasting until 25 Ma. Mulcahy et al. (2018) estimate that Franciscan subduction was occurring no later than 180 Ma, a date significantly older than much previous research. In any event, this began in the Jurassic and continued emplacing material until quite recently, about 25 Ma.

Discerning students might frown about these dates, remembering that the previous section had given a Coast Range Ophiolite date of 166 Ma–not much different than the Wakabayashi date, and earlier than the Mulcahy date. Part of the reasoning for putting the Coast Range Ophiolite as the foundation is that the Franciscan kept going for over a hundred million years, while the Coast Range Ophiolite had a briefer span. Another reason is that the structural arrangement in the eastern Coast Ranges suggests the Coast Range Ophiolite lies underneath the Great Valley Sequence, which is contemporaneous with the Franciscan. So because the Coast Range Ophiolite is beneath the Great Valley Sequence, and the Franciscan and Great Valley Sequence were coeval, then it reasons that the Coast Range Ophiolite is older than the Franciscan, except in those places where it’s about the same age. Remember earlier when we said this story was complicated?

The phrase you encounter when reading about the Franciscan is “accretionary wedge,” which is not particularly useful unless you have a diagram of what this means. To wit, Figure \(\PageIndex{2}\):

Figure \(\PageIndex{1}\): Cross section of the West Coast of North America at about 100 Ma, when the Franciscan complex was forming. As the ocean crust was thrust under the North American plate, pillow basalts, chert, and limestone-capped seamounts were scraped off and mixed with graywacke sandstone and shale shed from the continent, and serpentinite formed at the upper mantle, to create the Franciscan Complex.

Accessibility Guidance: The process illustrated in this diagram is described within the text. However, the process is difficult to understand without a diagram. Some users may therefore benefit from the tactile image, Continental Subduction Zone, which includes an accretionary wedge. This work by the National Park Service is in the public domain.

In this diagram we find the area indicated as the Franciscan Complex wedged in between mighty tectonic forces–to the west, the relentless movement of the Farallon plate crashing into North America, while to the east, the immovable North American continent acting as a doorstop. This wedge of Fraciscan is accreted, or stuck onto, North America–hence the name accretionary wedge.

The rocks of Franciscan are caught in the middle and transferred from the Farallon plate to the North American plate. They are also crushed, squeezed, torqued, and shattered to a degree difficult to imagine. This is why Franciscan rocks are so hard to map or describe–they are a jumbled mess of different rocks all swirled together, as if someone put a petrology book in a blender.

One important thing to notice in Figure \(\PageIndex{2}\) is the location of sea level. Both the Franciscan Complex and the Great Valley Sequence were forming offshore in the Mesozoic. The tectonic environment of the Coast Ranges is quite different today; a transform boundary has replaced subduction. However, even north of the Mendocino Triple Junction, where subduction continues to this day, the outcrops of the Franciscan Complex are very far from the modern accretionary wedge. In both cases, the rocks have been moved to their current location by later events of the Cenozoic.

References

1. Berkland, J.O., Raymond, L.A., Kramer, J.C., Moores, E.M., O’day, M.; What is Franciscan?. AAPG Bulletin 1972;; 56 (12): 2295a–2302. doi: https://doi.org/10.1306/819A421A-16C5-11D7-8645000102C1865D
2. Blake, M.C., Jr., and D.L. Jones, 1981, The Franciscan assemblage and related rocks in northern California: a reinterpretation, pp. 306-329, in Ernst, W.G. (ed.), The Geotectonic Development of California. Prentice-Hall, Englewood Cliffs, NJ.
3. Dickinson, W.R., Ingersoll, R.V., Cowan, D.S., Helmold, K.P., Suczek, C.A., 1982, Provenance of Franciscan graywackes in coastal California. GSA Bulletin (1982) 93 (2): 95–107. https://doi.org/10.1130/0016-7606(1982)93<95:POFGIC>2.0.CO;2
4. Goodstein, David L. States of Matter. Dover Publications, 2014. ISBN 0-486-49506-X (p. 1). https://www.amazon.com/States-Matter-Dover-Books-Physics-ebook/dp/B00LV86COS/
5. Mulcahy, S. R., Starnes, J. K., Day, H. W., Coble, M. A., & Vervoort, J. D. (2018). Early onset of Franciscan subduction. Tectonics, 37, 1194–1209. https://doi. org/10.1029/2017TC004753
6. Wakabayashi, J. Anatomy of a subduction complex: architecture of the Franciscan Complex, California, at multiple length and time scales. International Geology Review, 2015. Vol. 57, Nos. 5–8, 669–746, http://dx.doi.org/10.1080/00206814.2014.998728