10.1: Assembling a Continent

Klamath Clues to the Geologic Puzzle

In 1969, geologist Warren Hamilton of the U.S. Geological Survey laid out a bold new idea in his paper titled, “Mesozoic California and the Underflow of Pacific Mantle”. This new idea for how California came to be part of North America was largely based on work in the Klamath Mountains. For decades prior, geologists like Hamilton’s USGS colleague, William Porter Irwin, had been roaming the steep, loose hills of the Klamath range, forging through old-growth poison oak and a variety of other nasty underbrush, to uncover the secrets of the Klamath Mountains. At first geologists were in search of valuable resources, like gold (see 10.5: Uplifting a Mountain Range), copper, and chromium (see 10.3: The Josephine Ophiolite: A Little Slice of the Mantle), but later work sought to put together a bigger picture of Earth’s geologic history. Early geologists were puzzled by some things about the Klamath Mountains, such as the common presence of ultramafic rocks, typically formed in the mantle, in an alpine environment, but in the 1960’s a new idea called, “plate tectonics,” was taking hold in geology and reframing how geologists looked at just about everything. The idea that Hamiliton and Irwin described for the Klamath Mountains and other parts of California, would later come to be called, “terrane accretion.” This concept would eventually revolutionize how geologists thought about not only California, but all of western North America, as well as other mountain belts around the world. Today, this idea is the foundation of our understanding of how continents came to be in the first place, and how they have grown over time.

The word ‘terrane’ (not to be confused with the homonym, ‘terrain’), refers to a block of crust that has unique characteristics that are geologically distinct from the surrounding area. The terrane is interpreted as having formed elsewhere, and then moved to its current location by later tectonic processes. Terranes can include multiple stratigraphic units, but these units formed in the same geologic or tectonic context. For example, a single terrane might include volcanic rocks associated with an island arc, as well as sedimentary rocks associated with the coral reef that at one time surrounded the volcanic islands. Terranes can be transported by multiple tectonic processes, including transform plate boundaries like the San Andreas Fault zone (see 11.3: Evolution of the Coast Ranges). However, the term accreted terrane usually refers to terranes emplaced through the process of subduction.

Query \(\PageIndex{1}\)

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In general terms, accretion is a process by which an object grows larger through the gradual addition of material. At subduction zones, accretion usually occurs when any part of the subducting plate ends up in or on the overriding plate, rather than sinking into the mantle. In addition to terrane accretion, the word ‘accretion’ is also used to describe the process that occurs within an accretionary wedge. Both processes involve the accretion of material from the subducting plate to the overriding plate, but there are important distinctions between the two. In an accretionary wedge, material is removed from the top of the subducting plate and is highly altered and jumbled in the process. The resulting material is referred to as a mélange, a word of French origin, meaning “mixture”. This type of mélange contains a significant amount of serpentinite, a metamorphic rock formed in the deeper part of the accretionary wedge (see 2.6: Metamorphic Rocks). Terrane accretion differs from accretion in an accretionary wedge in that an entire block of lithosphere, not just the top of the plate, is accreted to the continent. Figure \(\PageIndex{1}\) illustrates both processes. The accretionary wedge is shown in purple and the accreted terrane is shown in green. Compared with a mélange, the original geologic structures in an accreted terrane remain relatively intact. Although, as one can imagine, the process of terrane accretion still results in quite a bit of folding, faulting and metamorphism.

Figure \(\PageIndex{1}\): The process of accretion at a subduction zone. "Plate accretion stages through time" by BZenith is licensed under CC BY-SA 4.0. Access a detailed description.

Figure \(\PageIndex{1}\) shows an example of an island arc being accreted onto a continent. Notice that by time 3 (Figure \(\PageIndex{1}\) C), both the mélange formed in the accretionary wedge (purple) and the former island arc (green) have become part of the continent. The purple mélange has been thrust above the former island arc along a fault that dips inland (toward the continent). If we were to continue the cartoon beyond time 3, a new accretionary wedge would form on the seaward side of the island arc (left side in Figure \(\PageIndex{1}\), and perhaps, another island arc might come along in time and it too would be accreted to the continent. The resulting pattern would be an alternating sequence of mélange and terranes, with each terrane getting progressively older moving inland, and separated from its neighbor by large thrust faults that dip towards the continent. This simplified description generally represents the geology of the Klamath Mountains province (Figure \(\PageIndex{2}\)).

Since the idea of terrane accretion was first invoked in the 1970s, geologists have sought to refine their understanding of exactly where these terranes came from. A healthy scientific debate has developed over whether the terranes were “exotic” island arcs, formed far from the continent, or whether they were home-grown arcs, that formed just offshore and remained close to the continent until finally accreting. Such home-grown island arcs are common elsewhere in the world today, particularly in the West Pacific (for example, the Japanese Islands) and Indian Ocean (for example, the Andaman Islands).

Figure \(\PageIndex{2}\): Terrane map of the Klamath Mountains by Marli Bryant Miller, University of Oregon. Access a detailed description.

The Klamath Core

During the later part of the Paleozoic Era, Earth’s continents were all merged together into the supercontinent, Pangea, surrounded by the Panthalassa Ocean. What is now northeastern California and northern Nevada was the coastline of Pangea (see 6.3: Paleozoic Era (540 – 250 Ma)). Somewhere off the coast, in the Panthalassa Ocean, there was an ocean island arc, which would later become the Eastern Klamath Terrane. As island arcs go, this one must have been around for a long time, because the Eastern Klamath Terrane contains a long record of sedimentary rocks, from Silurian-Devonian time through the Jurassic.

The Eastern Klamath Terrane was already an accretionary backstop before it docked with the mainland of Pangea. The Central Metamorphic Terrane accreted to the Eastern Klamath Terrane before both accreted to Pangea during the Late Permian - Early Triassic.

The Eastern Klamath Terrane is the oldest of the Klamath terranes and consists of the Redding, Trinity, and Yreka subterranes (smaller terranes within a larger terrane group). The Eastern Klamath Terrane includes volcanic rocks, plutonic rocks, and sedimentary rocks that would be expected in an island arc setting, some of which have been metamorphosed. It also includes the Trinity Ophiolite, which is a little piece of the Panthalassan oceanic crust. Some of the sedimentary rocks have remained intact and unmetamorphosed enough to preserve fossils (see 10.2: The McCloud Limestone: An Ancient Coral Reef).

Mesozoic Expansion

After the docking of the Eastern Klamath and Central Metamorphic terranes, the rate of growth of the Klamath province slowed for a period of time. Then, in the late Triassic through the Jurassic, North America (which was then splitting away from the rest of Pangea), grew rapidly, with the addition of several terranes.

A mountain building event called the Siskiyou orogeny took place during the Jurassic Period and involved sequential accretion of three additional terranes, which had been forming offshore during the Late Paleozoic and Early Mesozoic. These are lumped together into the Western Paleozoic and Triassic Terrane in Figure \(\PageIndex{2}\). The first and most easterly is the Sawyers Bar terrane, which represents a Permian oceanic arc, associated sedimentary rocks from marine and coastal environments, plus some mélange from an accretionary wedge. Accretion of the Sawyers Bar terrane was followed by the accretion of two other terranes. The Western Hayfork terrane is composed of a younger oceanic arc (formed around 177–167 Ma). The Rattlesnake Creek terrane was, at one time, the oceanic crust on which the Sawyers Bar island arc formed, and thus contains mafic and ultramafic rocks, though they are mostly no longer neatly organized in the expected ophiolite sequence and many have been metamorphosed to serpentinite.

Following the accretion of the Rattlesnake Creek terrane, a back-arc basin, called the Josephine Basin opened, once again separating the island arc from the main continent. It was during this time that the Western Klamath terrane ( Figure \(\PageIndex{2}\) formed. The Josephine Ophiolite, which will be further discussed later in this chapter (see 10.3: The Josephine Ophiolite: A Little Slice of the Mantle), was the oceanic crust that formed in the back-arc spreading ridge, while the Galice Formation is composed of the sedimentary rocks that were deposited in the Josephine Basin. The rocks formed in the island arc itself are mostly confined to the Oregon part of the Western Klamath terrane.

During the Nevadan Orogeny, which took place in the Klamath Mountains around 157–150 Ma (see 6.4: Mesozoic Era (250 – 66 Ma)), the events of the Siskiyou orogeny in the Klamath province repeated, and the Western Klamath Terrane was accreted to North America. The pattern of accretion continues to the west of the Klamath Mountains, and into the later part of Mesozoic, with the accretion of the mélange of the Coast Range Franciscan Formation (see 11.2: Lithology of the Coast Ranges).

Sedimentary Evidence for Home-Grown Terranes

While fossil evidence suggests that the older, Eastern Klamath Terrane may be exotic in origin, evidence seems to be mounting in favor of a home-grown explanation for the younger, Mesozoic, terranes. One source of evidence comes from the ages of sediment grains found in the sedimentary rocks within these terranes. Because sedimentary depositional processes do not reset the radiometric dating clock (see 5.4: Absolute Dating of Geologic Materials), radiometric dating isn’t very helpful for determining the age of deposition of a sedimentary rock. Instead, it gives the age of the source rock, but this can also be very useful information. In this case, these ages can help distinguish between exotic and home-grown scenarios.

Imagine a grain of sand on the beach of an ocean island located far from a continent. This grain of sand must have been eroded from the rocks on the island, transported by rivers and deposited on the beach. Its age would therefore have to match the age of the volcanic rocks on the island. This would hold true for the detrital sedimentary rocks in deeper marine environments near this island arc as well. Since there are no other sources for detrital sediment, the grains would have to come from the arc itself. By contrast, sand found in a sea between a home-grown island arc and the main continent would contain some grains from the island arc and some grains from the continent. Since the continent contains rocks that were much older, even at the time, the ages of sedimentary grains in this environment would be highly variable and contain some very old grains, older than the island arc. This later scenario is what geologists have found for the Galice Formation and other sedimentary rocks of the Mesozoic Klamath terranes. The Mesozoic terranes of the Klamath Mountains, therefore, may not represent island arcs brought in from afar, but rather, tell the story of a long-lived subduction zone, which alternates between different styles of behavior. These subduction styles are sometimes referred to as Mariana type (after the Mariana islands) and Chilean or Andean type (after the Andes mountains in Chile). Mariana type subduction includes back arc spreading behind an ocean island arc and is also sometimes referred to as ocean-ocean subduction. Andean style subduction is associated with a shallower angle of subduction and a volcanic arc mountain range located on the main continent. The modern Cascade Range is an example of Andean type volcanic arc mountain range (see 7.1: The Cascadia Subduction Zone and the Cascade Continental Volcanic Arc).

Query \(\PageIndex{2}\)

Connection to the Sierra Nevada

Geologists have noticed similarities between the geology of the terranes in the Klamath Mountains and the geology of other accreted terranes in western North America, most notably those found in the Sierra Nevada to the southeast and Oregon’s Blue Mountains to the northeast (figure \(\PageIndex{3}\)). This correlation implies that the changes between Andean and Mariana type subduction that drove the creation and accretion of these terranes occurred across continental scales. The same terrane sequence likely underlies much of western North America, but in most places the terranes have been covered by younger geology. These terranes are exposed in places where younger uplift events have returned these rocks to the surface, as discussed in 10.5: Uplifting a Mountain Range.

A quick glance at a map of the Sierra Nevada, Klamath and Blue Mountains, shows that the Klamath Mountains lie considerably west of the both adjacent ranges. This is due to two factors. The first is the shape of the coastline during accretion; the Klamath province was probably a cape or promontory along the coast at the time. However, this only explains part of the separation. The Klamath mountains were also moved further west by tectonic events that occurred after accretion.

Figure \(\PageIndex{3}\): Map illustrating the correlation between terranes of the Sierra Nevada, Klamath and Blue Mountains. Dark purple and pale purple striped areas (C and D in the legend) represent Mesozoic accreted terranes. Brown with cross hatching represents Paleozoic accreted terranes and red with “v” marks represents rocks equivalent to the oldest rocks of the Eastern Klamath Block. Dotted lines project the probable locations of the accreted terrane sequence beneath Cenozoic Cover. This figure by William R. Dickinson was originally published in Geosphere, which is published under the terms of the CC-BY-NC license. Access a detailed description.

References

1. Chapman, A. D., Grischuk, J., Klapper, M., Schmidt, W., & LaMaskin, T. (2024). Middle Jurassic to Early Cretaceous orogenesis in the Klamath Mountains Province (Northern California–southern Oregon, USA) occurred by tectonic switching: Insights from detrital zircon U-Pb geochronology of the Condrey Mountain schist. Geosphere, 20(3), 749–777. https://doi.org/10.1130/GES02709.1
2. Hamilton, W. (1969). Mesozoic California and the underflow of Pacific mantle. Geological Society of America Bulletin, 80(12), 2409–2430.
3. Harden, D. (2003). California Geology (2nd edition). Pearson.
4. Irwin, W. P. (2003). Correlation of the Klamath Mountains and Sierra Nevada. In Open-File Report (2002–490). U.S. Geological Survey. https://doi.org/10.3133/ofr02490
5. Kauffmann, M. E., & Garwood, J. (2022). The Klamath Mountains: A Natural History. Backcountry Press.
6. LaMaskin, T. A., Rivas, J. A., Barbeau, D. L., , Jr., Schwartz, J. J., Russell, J. A., & Chapman, A. D. (2021). A crucial geologic test of Late Jurassic exotic collision versus endemic re-accretion in the Klamath Mountains Province, western United States, with implications for the assembly of western North America. GSA Bulletin, 134(3–4), 965–988. https://doi.org/10.1130/B35981.1
7. Snoke, A. W., & Barnes, C. G. (2006). The development of tectonic concepts for the Klamath Mountains province, California and Oregon. https://pubs.geoscienceworld.org/gsa/books/book/574/chapter/3803324/The-development-of-tectonic-concepts-for-the
8. 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