10.3: The Josephine Ophiolite- A Little Slice of the Mantle

Obduction: A Rare but Important Event

When students first learn to classify plate boundaries, they learn that when oceanic lithosphere and continental lithosphere converge, it must always be the oceanic lithosphere that subducts. This is generally true, of course, but tectonic plates have been moving and converging for billions of years on Earth, and in such a great expanse of time, even unlikely and unusual events must sometimes occur. One such rare event is the process called obduction. The root “sub-” in “subduction” means “beneath” or below; “ob-”, by contrast means “toward,” “to,” “on,” “over,” or “against." Thus, in obduction, the oceanic lithosphere is pushed onto the continental lithosphere, rather than the usual subduction, in which it is pushed below. The resulting fragment of oceanic lithosphere, when eventually exposed at the surface, is known as an ophiolite.

California has had a long history of being a convergent plate boundary and so perhaps it should be no surprise that there are many ophiolites in California, including in the Coast Ranges (see 11.2: Lithology of the Coast Ranges), Sierra Nevada (see 9.2: Ancient Seas Form the Oldest Rocks) and the Klamath Mountains. There are multiple ophiolites in the Klamath Province, including the Trinity Ophiolite, Preston Peak Ophiolite and Josephine Ophiolite. This last is one of the most complete and best exposed ophiolites in North America, making a great case study for any student of plate tectonics. The Josephine Ophiolite is named for Josephine County, Oregon, but like many things in geology, ophiolites have no concern for arbitrary state boundaries. A large portion of the Josephine Ophiolite can also be found in Del Norte County, California (Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\): Location of the Josephine Ophiolite, after Shellnutt et al., 2021. "Josephine Ophiolite Location Map" by Emily Wright is licensed under CC BY-SA 4.0.

To understand the geologic history of the Josephine Ophiolite, you will need to be familiar with the geography of a subduction zone. Review your knowledge of important seduction zone terminology in Query \(\PageIndex{1}\).

Query \(\PageIndex{1}\)

Although there are many ways in which obduction can occur, the obduction of the Josephine Ophiolite is thought to be the result of the collapse of a back-arc basin. Prior to obduction, during the late Triassic Period into the Jurassic, the subduction zone exhibited Mariana type subduction. An oceanic island arc was separated from the mainland continent by a thin strip of oceanic lithosphere produced at a small spreading ridge resulting from back-arc extension ( Figure \(\PageIndex{2}\)). This might have looked like a mirror image of the east coast of modern Asia. In this analogy, the Josephine Ophiolite would be forming in the Sea of Japan (Figure \(\PageIndex{3}\)). Later, Mariana type subduction transitioned to Andean type subduction and back-arc extension ceased. The greater compressive stress associated with Andean type subduction caused buckling and collapse of the former back-arc basin. As the Japan-like island arc (the Rogue-Chetco arc) was accreted onto the mainland, the Josephine Ophiolite was caught up in the mix.

Figure \(\PageIndex{2}\): Illustration of an island arc with a back arc spreading center. The Josephine Ophiolite is thought to have formed in a back arc spreading center. View a detailed description. "Back Arc Spreading" by Emily Wright is licensed under CC BY-SA 4.0 / A derivative from the original work. Access a detailed description.

Figure \(\PageIndex{3}\): Japan is separated from mainland Asia by extension due to back arc spreading; a modern analogy to the formation of the Josephine Ophiolite. "Japan separation" by Oghmoir is licensed under CC BY-SA 3.0.

Mid Ocean Ridges and the Ophiolite Sequence

Most of the earth is covered by oceanic lithosphere and even that makes up only a tiny fraction of the oceanic lithosphere that has ever existed on Earth. Unlike continental lithosphere, oceanic lithosphere is constantly being created and destroyed. Only very occasionally do small bits of this very common earth material get obducted to another plate. This fact makes ophiolites incredibly valuable to geologists. Not only do they help tell the story of terrane accretion, but they also provide a rare glimpse into the inner workings of a far more common process, the creation of oceanic crust at mid ocean ridges (Figure \(\PageIndex{4}\)). There is a specific stratigraphy or sequence of layers present in all complete ophiolites. The Smith River cuts through all of these layers of the Josephine Ophiolite, just east of Crescent City, California (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{4}\): Sea Floor Spreading at a Mid Ocean Ridge produces the layers of the ophiolite sequence. This work by Steven Earle is licensed under CC BY 4.0. Access accessibility guidance for this image.

Figure \(\PageIndex{5}\): The middle fork of the wild and scenic Smith River tumbles through the basalt of the Josephine Ophiolite. "Basalt Outcrop Smith River" by David Bazard is licensed under CC BY-NC-SA 2.0.

Query \(\PageIndex{2}\)

Peridotite

The lowest layer in the ophiolite sequence is a layer of peridotite. Peridotite is the rock of the mantle. The brittle lithosphere that makes up tectonic plates includes a little bit of the top of the mantle. Therefore, when a piece of a plate is obducted, a little of the mantle comes along. An excellent place to see this mantle rock exposed is at the confluence of the South Fork Smith River with the main Smith River. At first glance the rock appears a light orange color, but when a fresh surface is exposed with a rock hammer, the inside is a dark shade of green. Because peridotite is so rich in iron, the surface of the rock is easily oxidized once exposed, leaving a rust colored weathering rind (Figure \(\PageIndex{6}\)).

Figure \(\PageIndex{6}\): Peridotite outcrop at the confluence of the middle and south forks of the Smith River, northern California. A rock an knife are used for scale. The weathered surface of the rock appears orange due to the oxidation of iron, but a closer view shows freshly exposed dark green rock (e.g. near the tip of the knife). "Peridotite Outcrop" and "Peridotite Serpentinized Outrcrop" by David Bazard are licensed under CC BY-NC-SA 2.0.

Layered Gabbro

The lava that erupts at mid ocean ridges is fed by a magma chamber near the base of the crust. Not all of the magma in this chamber ultimately erupts at the surface though; some cools and solidifies at the edges of the chamber as it moves with the tectonic plate away from the mid ocean ridge. Partial melting of this mantle produces mafic magma, which forms gabbro when cooled intrusively. In the Josephine Ophiolite, the gabbro has a layer structure, which is thought to have formed as a result of episodes of fractionation occurring within the chamber and producing layers of settled early formed crystals ( Figure \(\PageIndex{7}\)). For further explanation of fractionation see 7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc.

Figure \(\PageIndex{7}\): Crystal settling at the base of the magma chamber creates layering in the resulting intrusive igneous rock. This layering was originally horizontal, but has been titled in the process of obduction. "Layered Gabbro" and "Layered Gabbro Close Up" by David Bazard are licensed under CC BY-NC-SA 2.0.

Sheeted Dikes

At a mid ocean ridge, magma moves upward from the magma chamber to erupt at the surface through cracks in the rock that overly the magma chamber. The magma that cools within these cracks forms basalt dikes. Although technically intrusive, this magma cools quickly enough to form an aphanitic texture and is therefore called basalt, rather than gabbro. Both the layered gabbro and the sheeted dikes can be seen along the main fork of the Smith River (Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{8}\): Fine-grained mafic (basaltic) dikes intrude into the lighter-colored, coarse-grained gabbro. The contact between the basaltic (mafic) dike and the gabbro is at the tip of the knife blade. "Sheeted Mafic Dikes" and "Mafic Dike Margin" by David Bazard are licensed under CC BY-NC-SA 2.0.

Pillow Basalt

When lava erupts underwater, it cools very quickly forming a glassy outer shell, as more lava forces its way into this shell, the shell cracks and a new shell forms around the expanding lava. The result is a blobby or pillowy structure that forms as these basalt “pillows” successively pile on top of each other. You can see video footage of this process in the video “Pillow Lava”. The pillow basalt layer can be seen further upstream, along the middle fork of the Smith River ( Figure \(\PageIndex{9}\)).

Figure \(\PageIndex{9}\): Pillow basalt in the Josephine Ophiolite. In some places the matrix rock between the pillows has been weathered away. In others, the matrix forms light-colored, resistant rock surrounding the dark pillows. "Pillow Basalt Detail" and "Pillow 'Breccia'" by David Bazard are licensed under CC BY-NC-SA 2.0.

Video \(\PageIndex{1}\): Pillow Lava

Watch pillow basalt forming in the video “Pillow Lava”. There is no narration; audio includes crackling and hissing sounds.

Marine Sedimentary Rocks

As the oceanic crust moves away from the mid ocean ridge, marine sediments settle on top of the pillow basalts. In a large ocean basin, these sediments would be mostly biochemical sedimentary rocks such as limestone and chert, which form from the accumulation of the mineralized hard exterior parts of dead plankton. But since the Josephine Ophiolite formed in a back arc basin, close to a continental volcanic arc, there was a large input of sediment eroded from the continent. The Galice Formation, which overlies the Josephine Ophiolite, consists of detrital sedimentary rocks, with sediment sourced in the Klamath Mountains and the active Sierra Nevada volcanic arc. Figure \(\PageIndex{10}\) shows the contact between the basalt and the Galice Formation. The contact represents the Jurassic ocean floor where sediments accumulated at about 160 Ma. The contact has now been tilted to near vertical during the process of obduction.

Figure \(\PageIndex{10}\): Contact of the oceanic basalt (greenish rock to the right) and the overlying pelagic sediments (layered rock on the left). "Contact Pillow Basalt and Pelagic Seds" by David Bazard is licensed under CC BY-NC-SA 2.0.

Query \(\PageIndex{3}\)

You may find a review of Chapter 10: Klamath Mountains (Introduction section) helpful in completing this activity.

Turquoise Waters, Ultramafic Soils and Carnivorous Plants

In the whole earth, ultramafic rock is the most common type of rock there is, because it makes up Earth’s mantle, which is the largest of Earth’s layers by volume (84%). However, the density of ultramafic rock usually prevents it from rising into the crust, thus, this iron and magnesium rich chemistry is relatively rare in rocks exposed at the surface. The result is a soil chemistry unique to regions with ophiolites, known as ultramafic soil, or sometimes serpentine soil. Serpentine minerals are the result of metamorphism of ultramafic rock. Because the parent material for the soil is rich in heavy metals, the soil is also relatively poor in lighter elements that are essential nutrients for plant life. In the region of the Josephine Ophiolite, the effect of the soil chemistry on the ecology is striking. Just outside the bounds of the ophiolite itself, the plant kingdom thrives. Healthy redwood stands, like those found in Jedediah Smith Redwoods State Park (part of the Redwood National and State Park system) rank among the highest known biomass of standing vegetation on earth, but the forest within the Josephine Ophiolite is very different in character. Although this is the wettest part of California, the forest in some ways resembles that of a much drier climate, although growth is limited by nutrients, not by water. Some plants that do take root in the inhospitable ultramafic soil have unique adaptations to deal with the shortage of nutrients. The California pitcher plant (Darlingtonia californica), receives some of its nutrients from insects that it traps within its “pitcher” Figure \(\PageIndex{10}\). Other carnivorous plants found on serpentine soils are Horned butterwort (Pinguicula macroceras) and Round-leaved sundew (Drosera rotundifolia).

Figure \(\PageIndex{11}\): The California pitcher plant (Darlingtonia californica) has adapted to the low-nutrient ultramafic soils of the Josephine Ophiolite by luring and trapping insects for nutrients. "Darlingtonia californica" by Noah Elhardt is licensed under CC BY-SA 3.0.

The unique geology of the Josephine Ophiolite, also gives an inviting appearance to the rivers that run through it. The Smith River is one of the cleanest rivers in the contiguous United States. On sunny days the sunlight penetrates the clear water and reflects off the serpentinite below, giving the water a picturesque hue of turquoise (Figure \(\PageIndex{12}\)).

Figure \(\PageIndex{12}\): An X post by Redwood National and State Parks celebrating the turquoise color of the Smith River. View Detailed Description. "Post" by Redwood National and State Parks is in the Public Domain. Access a detailed description.

Box \(\PageIndex{1}\): Mantle Magma Chambers Fuel the War Machine

The two main minerals in peridotite are olivine and pyroxene and the ultramafic chemistry is usually described as being rich in iron and magnesium. However, the mantle is also relatively rich in other heavy metals when compared with the crust. One such element is chromium, which can be found in the mineral Chromite (Fe₂CrO₄). California's ultramafic rocks, including those of the Josephine Ophiolite, contain pods and slivers of chromite. These small pockets are not concentrated enough to make chromite mining economically viable in today’s global economy. Since 1961, the United States has imported all of its chromite. But chromite is considered a strategic mineral, meaning that it is necessary for military or industrial use during periods of national emergency. During World War II, when the global supply of chromite was cut off and it was needed for production of steel for the war effort, chromite mines operated throughout the Klamath Mountains, Coast Ranges and the Sierra Nevada foothills. The search for chromite motivated geological mapping of the ultramafic rocks in the Klamath Mountains during and after the war. This mapping effort in turn helped lead geologists to important conclusions about terrane accretion and plate tectonics.

Acknowledgements

For this section, I’d like to give a special thanks to Dr. David Bazard, who has kindly shared his photos in a public platform for the benefit of this project. If you are interested in taking your own journey into the mantle, you will appreciate Dave Bazard’s Josephine Ophiolite map.

References

1. California Division of Mines and Geology. (1957). Mineral commodities of California: Geologic occurrence, economic development, and utilization of the state’s mineral resources (Lauren A. Wright, Ed.).
2. Casteran, R. (2022, September 22). Chromite mining. Oregon Encyclopedia. https://www.oregonencyclopedia.org/articles/chromite_mining/
3. Dengler, L. (2023, June 18). Exploring the Josephine Ophiolite which shows mantle. Eureka Times Standard. https://www.times-standard.com/2023/06/18/lori-dengler-exploring-the-josephine-ophiolite-which-shows-mantle/
4. Harden, D. (2003). California Geology (2nd edition). Pearson.
5. Harper, G. (1984). The Josephine Ophiolite, northwestern California. Geological Society of America Bulletin - GEOL SOC AMER BULL, 95. https://doi.org/10.1130/0016-7606(1984)95<1009:TJONC>2.0.CO;2
6. Serpentine soil. (2024). In Wikipedia. https://en.Wikipedia.org/w/index.php?title=Serpentine_soil&oldid=1225226416
7. Shellnutt, J. G., Dostal, J., & Lee, T.-Y. (2021). Linking the Wrangellia flood basalts to the Galápagos hotspot. Scientific Reports, 11(1), 8579. https://doi.org/10.1038/s41598-021-88098-7
8. Smith River. (n.d.). National Wild and Scenic River System. Retrieved July 1, 2024, from https://www.rivers.gov/river/smith
9. Strickler, M. D. (n.d.). Josephine Ophiolite. Retrieved July 1, 2024, from http://homework.uoregon.edu/mstrick/GeoTours/Josephine%20Ophiolite/JoOphiolite.html
10. Surpless, K. D., Alford, R. W., Barnes, C., Yoshinobu, A., & Weis, N. E. (2023). Late Jurassic paleogeography of the U.S. Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA. GSA Bulletin, 136(3–4), 1488–1510. https://doi.org/10.1130/B36810.1
11. U.S. Forest Service. (n.d.). Serpentine Soils and Plant Adaptations. Retrieved July 1, 2024, from https://www.fs.usda.gov/wildflowers/beauty/serpentines/adaptations.shtml