11.3: The Coast Range Ophiolite

Where to Find the Coast Range Ophiolite

We find California’s Coast Range Ophiolite (CRO) in a relatively narrow sliver of rocks to the eastern side of the Coast Ranges, concentrated west of the Sacramento Valley, the Diablo Range, and in Lake and Sonoma counties. The Coast Range Ophiolite abuts the Great Valley Sequence to the east and the Franciscan Complex to the west, as shown in Figures \(\PageIndex{1}\) and \(\PageIndex{2}\):

Figure \(\PageIndex{1}\): Cross section of the Mesozoic active margin of California. Metamorphic ages generally become younger to the west; metamorphic grade becomes generally lower from west to east. "Franciscan Belts" by Allison Jones, a derivative of the original by Shervais et. al, is licensed under CC BY 4.0.

Figure \(\PageIndex{2}\): Geologic map of California showing main elements of the Coast Range geology. Eastern belt (blue), Central belt (yellow), and Coastal/Western belt (orange). The Great Valley Group (green) and Coast Range ophiolite (pink) are also shown. SF—San Francisco, SB—Santa Barbara, SAF—San Andreas fault, SNF—Sur-Nacimiento fault. Major Coast Range ophiolite localities (N-S): EC—Elder Creek, SFV— Stonyford (Black Diamond Ridge), DP—Del Puerto Canyon, Ll—Llanada, CR—Cuesta Ridge, PS—Point Sal. This work by Shervais et. al is licensed under CC BY. Access a detailed description.

The Coast Range Ophiolite is often left out of larger-scale maps, due to its small areal extent. Though Hopson et al. identify 23 separate Coast Range Ophiolite exposures spread over 700 km across the state, most of the exposures are relatively small compared to other groups, such as the Franciscan.

However, the Coast Range Ophiolite is important in two respects. First, it is the oldest rock unit in the Coast Ranges, the foundation of this story. Second, its very existence here speaks to the intense tectonic violence that shaped western California into the twisted, mangled, shattered wreckage that it is today.

What's an Ophiolite?

An ophiolite such as the Coast Range Ophiolite is a suite of rocks, a set of three distinct rock types that form together as part of a single process. The three rock types are pillow basalts, sheeted dikes, and gabbro. See also chapters on the Klamath Mountains and the Sierra Nevada for other examples of ophiolites in California.

Basalt is an iron-rich, silica-poor volcanic rock very familiar to visitors to the Hawaiian islands, because each island is composed of basalt and little else. This dark, volcanic rock begins life as lava, but in an ophiolite sequence there’s another twist: it forms under water, making distinct bulbous shapes that look like clusters of grapes. These clusters are strangely termed “pillow basalts,” though nothing about them is shaped like a pillow and they are certainly not something you’d want to rest your head on to sleep.

Figure \(\PageIndex{3}\): An illustration of pillow basalt formation, showing lava erupting underwater and cooling into bulbous shapes. This work by Callan Bentley is licensed under CC BY 4.0.

Beneath the pillow basalt, we find sheeted dikes. These tend to be a little harder to recognize, but often look like thin lines all roughly going in the same direction. One way to think about these linear features is that they are pipes or conduits bringing molten rock up to the surface. The rock that makes up these cooled conduits is dark like basalt, but often a little coarser grained; it gets the name diabase, which is intermediate in crystal size between basalt and gabbro. One way to distinguish diabase from basalt is that while basalt often has crystals so small that none are visible to the naked eye, diabase can display well-formed rectangular crystals of plagioclase feldspar.

Going further down in the ophiolite sequence, we find gabbro, is a phaneritic, dark, dense rock (see Minerals and Rocks: Igneous Rocks). Gabbro forms large crystals because it has plentiful time to cool; in this sense, it follows the same long formation time as granitic rocks, though they are chemically on opposite ends of the spectrum.

Figure \(\PageIndex{4}\): Illustration of ophiolite sequence, with marine sediment underlain by pillow basalts, sheeted dikes, and gabbro. Such sequences are typically around 10 km (6 mi) thick. "Ophiolite-cross-section.jpg" by Dexter Perkins, Open Petrology is licensed under CC BY-NC-SA 4.0.

The ophiolite suite is pillow basalt, sheeted dikes, gabbro. Okay… who cares? So what?

Let’s do a little thought experiment. Off the top of your head, think of the most common type of rock on the planet. Some might say granite. Others will think sandstone. Some may suspect it’s a trick question and say clay. Wrong, all wrong.

If you take a research drilling vessel (such as the recently retired JOIDES Resolution or the Chikyu) anywhere in the ocean and start drilling, you will hit some sort of mud and unconsolidated “ooze” made of radiolaria, or foraminifera, or diatoms. But that’s just a relatively thin veneer of sediment on the ocean floor. Drilling deeper, and you hit–insert drum roll!--pillow basalts. Then sheeted dikes. Then gabbro. Everywhere in the ocean. Everywhere you drill. Always in this order. The entire ocean crust is an ophiolite suite.

Because the oceans constitute almost exactly 70% of the Earth’s surface, if you randomly start drilling on Earth, you have a 7/10 chance of immediately hitting an ophiolite sequence. Although we humans have a bias toward thinking of our cities and forests and deserts and such to be very important and consequential, the humbling truth is that the Earth could shrug off its mountains and grass plains and continents and keep going along without even noticing. We don’t count for much; ophiolites do.

If you’ve been reading carefully, a puzzling thought should have formed by now. Ophiolites are seafloor crust. And they’re really common around the world. But what business does seafloor crust have being part of the California Coast Ranges? How could this deep oceanic rock have ended up so far from the ocean?

Query \(\PageIndex{1}\)

How might the Coast Range Ophiolite have formed?

We know a few things, but the complete picture will require much more research. One thing we know: because the Coast Range Ophiolite is igneous, its rocks are amenable to radiometric dating techniques, which have established dates of mid to late-Jurassic, from about 166 to 153 Ma, making it roughly coeval with the Smartville Ophiolite in the Sierra Nevada, the Josephine Ophiolite in the Klamath Mountains and other ophiolites in southern Oregon.

Another thing we know: at the time the Coast Range Ophiolite was moving toward California, there existed a giant subduction trench all along the coast of western North America. Much further to the east we had the Sierran magmatic arc, which at this time involved a series of volcanoes similar to the modern Cascades; flux melting from this subduction zone would eventually generate the granitic material that would one day become the Sierra Nevada.

In the most rudimentary sense, what happened to form the Coast Range Ophiolite was some oceanic lithosphere was coughed up–for reasons we don’t fully understand–rather than being pushed down into the mantle, the far more common fate of oceanic lithosphere. Geologists have a fancy term for this: obduction (see also Klamaths chapter - Josephine ophiolite section). This obduction process would have involved dismembering whatever coherence these rocks had, and they were separated and twisted. These obducted ocean crust fragments then became sutured on the western edge of North America.

Figure \(\PageIndex{5}\): An illustration of subduction and obduction, showing a descending plate either going underneath a converging plate, or having portions detaching from the subducting plate. "Obduction and subduction" by Dexter Perkins et. al, A derivative from the original, is licensed under CC BY-SA 4.0.

Now that we have established the basement in this region, we’re going to add more on top of it.

References

1. Dickinson, William R., Clifford A. Hopson, and Jason Saleeby. 1996. “Alternate Origins of the Coast Range Ophiolite (California); Introduction and Implications”. GSA Today 6 (2): 1–10.
2. Hopson, C., Mattinson, J, Pessango, E, 1981. “Coast Range Ophiolite, Western California,” in Ernst, W.G. (ed.), The Geotectonic Development of California. Prentice-Hall, Englewood Cliffs, NJ.