10.5: Uplifting a Mountain Range

From Continental Shelf to Alpine Peaks

When first learning about the idea of terrane accretion, it is easy to imagine that as a terrane gets accreted to a continent, it is thrust up into a mountain range and has remained a high mountain range until the present day. Indeed, this is roughly what is shown in Figure 10.1.1. As is often the case in geology, however, reality is a bit more complicated. In hundreds of millions of years time, mountain ranges tend to come and go. For example, there is evidence that, by the Late Cretaceous, after all the Klamath Terranes had been accreted, the Klamath province was part of the forearc basin of the Sierra Nevada arc, along with the Great Valley to the south (see 12.2: Geology of the Great Valley). What is now a high mountain range was, at that time, below sea level and accumulating marine sediment. In the intervening time, both the Klamath Range and the Great Valley have been uplifted above sea level, but while the Great Valley remains relatively low and flat, the Klamath Range has become mountainous.

Exactly when, why and how the Klamath province went from being a low, flat continental shelf, to high mountain peaks is a question of great interest to geologists. The challenge with understanding mountain uplifts is that uplifting mountains are erosional environments. Not only does the process of erosion leave nothing in the rock record, but erosion can also erase the rock record of earlier events as sediments are stripped away. Very little of the once thick Cretaceous sediments remain anywhere in the Klamath Province, and younger Tertiary sediments are not common either.

Extensional Faulting

Although Cenozoic faulting is not common in the Klamath Mountains province, there are a few extensional faults in the southern part of the Klamath Mountains. These extensional faults cut across the older thrust faults that formed during Mesozoic accretion, and therefore, according to the Principle of Cross Cutting (see 5.1: Relative Dating), must represent a younger period of tectonic evolution. One major extensional structure in the southern Klamath Mountains is the La Grange Fault, a large low angle normal fault, called a detachment fault. This fault was first discovered at the LaGrange Mine (see Box \(\PageIndex{1}\): La Grange Mine), when miners stripped away the gravels of the Weaverville Formation and uncovered the fault surface. The fault surface is composed of cataclasite, a type of rock that is formed as a result of the sliding and grinding that occurs during fault motion and has distinct slickenlines ( Figure \(\PageIndex{1}\)) indicating the direction of fault motion. The strike of the La Grange Fault is Northeast and the slickensides indicate a south to south eastward motion for the hanging wall. Based on the displacement of specific rock units, geologists estimate that total displacement was about 60 km (roughly 40 miles). This normal fault motion also uplifted the footwall block on the northern side of the fault, bringing deeper sections of basement rock to the surface. Although younger than the more common Mesozoic compressional faults in the Klamath Mountains, the La Grange fault is no longer active. Since it is a low angle fault that fault has also been eroded in places, leaving a rather circuitous trace on a map ( Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{1}\): Surface of the La Grange Fault near the La Grange Mine. "LaGrange Fault Surface" was taken by Susan Cashman in 2014 and used here with permission.

Figure \(\PageIndex{2}\): Simplified lithologic map showing select provinces and Cretaceous–Neogene geologic formations overlain on a hillshade digital elevation model BMF—Browns Meadow fault, LGF—La Grange fault, HB—Hayfork basin, DGF—Democrat Gulch fault. Yellow box shows location of Figure 3 in Michalak et al. (2023). The yellow box and the letter A are not relevant to this publication. This figure by Michalak et al. was originally published in Geosphere, which is published under the terms of the CC-BY-NC license. Access a detailed description.

South of the La Grange fault, there are several other normal faults of similar age and similar strike. Some of these faults form grabens (see 8.2: Rifting in the Basin and Range), which form one of the few depositional environments where sediment could accumulate during the uplift of the Klamath Mountains.

Insights from the Weaverville Formation

As the Klamath Mountains uplifted, very little of the eroded material stayed in the area and accumulated as sediment. The most notable exception is the Weaverville Formation ( Figure \(\PageIndex{2}\), which accumulated in small grabens and basins formed by extensional faulting. The lower Weaverville Formation is composed of sandstone, mudstone, and claystone with minor interbedded coal and tuff, which are interpreted to have formed in lake environments. These transition to pebble and cobble conglomerate in the upper Weaverville formation, suggesting a transition from the lake environment to one with rivers or streams. Deposition of the Weaverville Formation began in the early to middle Miocene and possibly continued into the late Miocene.

The source rocks for the Weaverville Formation are the rocks of the Terranes and Plutons of the Klamath province. Some of these rocks are gold bearing, much like those of the Sierra Nevada (see 9.4: Gold of the Sierra Nevada), making the Weaverville Formation a productive Placer deposit (see container box). Importantly, the Weaverville Formation seems to be sourced entirely within the Klamath Province. This is significant because it implies that by the time the Weaverville Formation was being deposited, there was already enough elevation in the Klamath Mountains to isolate the grabens and basins from rivers draining more distant sources. The older Cretaceous Hornbrook Formation and Eocene Tyee Formation of the Oregon Coast Ranges ( Figure \(\PageIndex{2}\) contain grains that have been identified as coming from sources as far away as the Sierra Nevada and the Idaho Batholith (in Idaho). This would imply that, up until at least Eocene time, the area that is now the Klamath Mountains was flat enough to host a river system fed by distant tributaries, but that by the Miocene this was no longer the case. Based on studies of the Weaverville Formation, combined with other geologic evidence, geologists have identified a major pulse of mountain uplift during the Miocene Epoch.

Box \(\PageIndex{1}\): La Grange Mine

The Klamath Mountains province ranks second in gold abundance of all the California geomorphic provinces. Not long after the California gold rush began, prospectors realized that the Klamath Mountains bore similarities to the northern Sierra Nevada. In 1848 Major Pearson B. Reading discovered gold in the Trinity River and people came from all over the world to make their fortune in the Klamath Mountains. The La Grange Mine was among the largest and most successful mines in the Klamath Mountains. Between 1893 to 1915, the La Grange mine was the largest hydraulic mine in California. The La Grange Mine owed its success in part to the Sawyer Decision of 1884, which significantly restricted mining of placer deposits in the Sierra Nevada because hydraulic mining was affecting agriculture downstream in the Central Valley (see 9.4: Gold of the Sierra Nevada). The Sawyer Decision did not affect hydraulicing in the Trinity River, however, because the Trinity River, a tributary of the Klamath River, flows directly from the mountainous terrain of the Klamath Mountains into the Pacific Ocean without passing through agricultural lands. The La Grange Mine continued to operate until 1918, when economic issues surrounding World War I made the cost of mining too high to be profitable. Today the mine is recognized as a historical landmark with a roadside marker along Highway 299.

Figure Box \(\PageIndex{1}\): “Hydraulic gold mining in Trinity County, Calif.” by Jervie Henry Eastman is In Copyright - Non-Commercial Use Permitted.

The Klamath Mountains in the Quaternary

Though the Klamath Mountains were likely already a topographically elevated region in the Neogene, they were not fully shaped into their current configuration until more recently. Like the Cascade Range (see 7.8: Glaciers in the Cascades) and the Sierra Nevada (see 9.6: Ice Shapes the Landscape), the Klamath Mountains have been heavily shaped by glaciers during the Pleistocene Epoch. One effect of this glacial erosion is the formation of high alpine lakes, which make places like the Trinity Alps popular backpacking destinations.

Figure \(\PageIndex{3}\): Sapphire Lake in the Trinity Alps is an example of an alpine lake in glacially carved terrain. "Sapphire Lake -- Suicide Ridge -- Trinity Alps" by Gary Robertson is licensed under CC BY-NC 2.0.

Until quite recently, the Trinity Alps also hosted the lowest elevation glaciers in California: Salmon Glacier and Grizzly Glacier. These glaciers sat at around 2500 meters (8200 feet), about 520 meters (1,700 feet) below other California Glaciers. Their low elevation is made possible by high rates of precipitation. However, these glaciers experienced significant retreat in recent decades. Salmon Glacier disappeared in 2015 and by 2021, Grizzly Glacier, the last remaining glacier in the Klamath Mountains, had retreated and broken apart to the point that it is no longer classified as a glacier.

In this chapter, you have learned about several geologic events that shaped the Klamath Mountains. Query \(\PageIndex{1}\) will help you place these events into the context of geologic time.

Query \(\PageIndex{1}\)

Acknowledgements:

Special thanks to Dr. Melanie Michalak for her help with the research and editing of this chapter.

References:

1. Cashman, S. M., & Cashman, K. V. (2006). Cataclastic textures in La Grange fault rocks, Klamath Mountains, California. In A. W. Snoke & C. G. Barnes (Eds.), Geological Studies in the Klamath Mountains Province, California and Oregon: A volume in honor of William P. Irwin (Vol. 410, p. 0). Geological Society of America. https://doi.org/10.1130/2006.2410(21)
2. Christensen, D. (2021). Evidence for middle Miocene elevated topography isolating the southern Klamath mountains province: A u-pb & lu-hf detrital zircon study of the Weaverville formation. Cal Poly Humboldt Theses and Projects. https://digitalcommons.humboldt.edu/etd/529
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