6.4: Mesozoic Era (250 – 66 Ma)

Mesozoic History

The Mesozoic is when most of the rocks that make up California are added onto North America. These Mesozoic rocks are dominantly subduction related: igneous intrusions, forearc sediments, arc volcanics, and accreted terranes. The exact geometry, orientation, and even number of subduction zones that created the features we see today is an area of active research and debate among geologists. There is resounding agreement, however, that the Mesozoic margin was shaped by long lived subduction processes.

The Sierra Nevada, the Peninsular Ranges, as well as numerous smaller ranges throughout eastern California are made of predominantly felsic plutonic rocks that were generated through flux melting associated with this subduction (Figure 6.4.1). These plutons essentially stitch together the North American continent, accreted island arcs, and sediments from the ocean basins that once separated them.

Figure \(\PageIndex{1}\): implified Geologic map of California highlighting the distribution of Mesozoic granitoids. "Mesozoic granites" by Skinner is licensed under CC BY-NC 4.0 / A derivative from the original work

Along the coast of California are vast exposures of mélange, or sedimentary packages that are too deformed to easily interpret. The rocks of the Franciscan Complex are one of these mélanges and represent the location of one of the Mesozoic subduction zones (Figure 6.4.2). Sediment from the marine shelf became unstable and traveled underwater as submarine landslides. These sediments are then thrust under the overriding plate as multiple slices. It is no wonder geologists have had a hard time unraveling these packages of sediment that have had at least two opportunities to get mixed up in a sedimentary and then a tectonic washing machine. In some special cases like Ring Mountain near San Francisco, the minerals present and their growth relationships show us that some of these rocks have traversed the subduction zone multiple times, as evidenced by metamorphic minerals formed by different regimes of pressure and temperature occurring together in the same highly-altered rocks. The Cretaceous sediments are not all a tumble jumble though, in the Coast Range the Great Valley group is also preserved (Figure 6.4.3). The Great Valley group records giant submarine fans that were fed by rivers draining the highlands of the uplifting Sierra Nevada.

Figure \(\PageIndex{2}\): Simplified Geologic map of California highlighting the distribution of Mesozoic Franciscan Complex. "Mesozoic Franciscan" by Skinner is licensed under CC BY-NC 4.0 / A derivative from the original work

Figure \(\PageIndex{3}\): Simplified Geologic map of California highlighting the distribution of Mesozoic Great Valley group. "Mesozoic Great Valley" by Skinner is licensed under CC BY-NC 4.0 / A derivative from the original work.

Three Mesozoic mountain-building episodes were particularly important in the formation of the western United States and ultimately California. The Nevadan, Sevier, and Laramide orogenies took place one after another (with some overlap) from the Jurassic to the Paleogene, between about 180 and 50 million years ago. These orogenies were caused by the subduction of the Farallon Plate under the North American Plate.

During these events, subduction was occurring along the western margin of California. The topography and geology of the state thus reflect the features of a subduction zone. The California Coast Ranges formed as an accretionary wedge. In some places, mafic rocks (rocks rich in iron and magnesium) can be found in the Coast Ranges; these rocks came from ocean crust or the Earth's mantle. They were scraped off the subducting Farallon Plate and accreted to the edge of the overriding North American Plate, forming ophiolites. Serpentinite, the state rock of California, is a type of rock formed when ophiolite is metamorphosed.

Nevadan Orogeny

The first major orogenic event in the Mesozoic of North America was the Nevadan orogeny. This occurred on the other side of North America from the breakup of Pangaea and started later, in the mid-Jurassic. At some point prior to the Nevadan deformation, an active margin had developed with a volcanic arc along the west coast. These volcanic rocks and related sediments were deformed with the accretion of numerous terranes from the ancient Pacific Ocean, brought eastward as the Farallon Plate (along with the Kula plate) subducted beneath the North American Plate. The Farallon Plate used to exist between the Pacific Plate and the North American Plate.

The Farrallon’s western boundary with the Pacific Plate was divergent: it was a spreading center (an oceanic ridge). The Farallon’s eastern boundary adjacent to the North American Plate was convergent: a subduction zone. There, the Farallon was recycled into the mantle. Any island arcs, microcontinents, or oceanic plateaus it was carrying were too thick, buoyant, or protruding too much to subduct. When they reached the subduction zone, these terranes were scraped off and pushed onto the North American continent, adding to its volume. As they were pushed eastward, they acted like a bulldozer, scraping and shoving the volcanic and sedimentary rocks that were layered atop the continental crust. The deformation of the Nevadan orogeny is most easily seen in the Sierra Nevada mountains of California and the Klamath mountains along the coast of California and Oregon.

Sevier Orogeny

The next orogeny to occur in western North America was the Sevier orogeny. While terranes still played a factor in the deformation, the Sevier orogeny occurred much further inland and was more widespread than the Nevadan orogeny. The earliest events associated with the Sevier orogeny are as early as 160 Ma (even before the Nevadan) and the latest deformation was 50 million years ago. However, the majority of this deformation occurred between 120 and 80 million years ago. Subduction at this time changed due to the fact that the Farallon plate was hot and young, causing a Chilean-style subduction zone.

This means the resistance to subduction caused extra convergent forces in the overriding plate, mainly in the form of a fold-and-thrust belt, which extended inland. Much like the modern Andes, this created a two-ridged mountain chain, with the volcanic arc positioned closer to the trench, and a back-arc mountain range formed from compressional, low-angle thrust faulting. Between these two ridges today is a high plain, called the Altiplano in the Andes. While the exact geography of the mountains during Sevier time is unknown, it can be assumed that it was similar. Igneous rocks, closer to the Sierra Nevada, and shoved stacks of sedimentary rocks far from their original positions deep in the crust and further to the west back up this assertion. These displaced layers of rock arched upward and broke, with the slabs of older rock traveling along thrust faults up on top of younger rocks.

Figure \(\PageIndex{4}\): Diagram of the subduction of the Farallon plate during the Cretaceous Sevier Orogeny. The Wester Interior Seaway formed when the continental crust was downwarped in the central region of the North American continent. To the west, underground magma formed and the Sierra Nevada experienced volcanism. An overthrust belt occurred between the Sierra Nevada and the Western Interior Seaway. Diagram by Wade Greenberg-Brand, adapted from an image by the New Mexico Institute of Mining and Technology, originally published in the Teacher Friendly Guide to Earth Science series. Modified for Earth@Home.

Laramide Orogeny

Near the end of the Mesozoic, the long-lived subduction related volcanic arc was disrupted by the tectonic forces that led to the Laramide Orogeny. The Laramide Orogeny, named for a range in Wyoming, might seem like an odd subject to cover under the title of California Geology, however, the forces that enabled the deformation well into the interior of North America were exerted along the California subduction zone. There are multiple models for transmitting compression deep into the continent. One model calls upon the collision of a series of island arcs and microcontinents, that are then translated north along transform faults and by oblique convergence at the subduction zone. Another model for the Laramide deformation is a change in the geometry of the subducting slab. A flat slab geometry, where the downgoing plate rides just beneath the overriding plate, rather than immediately descending into the mantle, allows for deformation in the interior of the continent because the area where the two plates are in contact extends well into the interior as the continent rides over the shallow dipping subducted slab. The flat slab model has been used to explain the progressive northward shutoff of magmatism in the Mesozoic arc, the removal of mantle lithosphere beneath southern California, as well as the exhumation of the southern Sierra Nevada.

Figure \(\PageIndex{5}\): Paleogeographic maps illustrate the deformation taking place during the Sevier Orogeny (left) at ~75 Ma and the Laramide Orogeny (Right) at ~65 Ma. The inland sea that had been present during the Sevier disappeared as the interior United States was uplifted. These maps by Gates, et al. are a derivative of the original maps by Ron Blakey. They are licensed under CC BY 4.0.

The third mountain-building event that took place in the western US during the late Mesozoic and early Cenozoic, overlapping with but distinct from the Sevier orogeny, is the Laramide orogeny. Ultimately, the Laramide, too, was caused by the subduction of the Farallon Plate. Compared to the Sevier, however, Laramide orogeny deformation:

• was typically more recent
• took place further east
• was deeper in the crust, involving the crystalline basement

These three characteristics allow the two orogenies to be distinguished.

The Laramide Orogeny was driven from beneath. Continuing the trend from the Sevier orogeny, the subducting plate was even younger and hotter, causing the subduction angle to become more shallow over time, eventually getting so small that the slab was dragging along the base of the overriding plate. This is known as flat-slab subduction. This subducting but dragging slab is believed to put pressure on the overriding plate, causing warps in the crust to develop. The lack of a mantle wedge below the overriding plate and above the subducting plate also meant that no magmatism occurred during the Laramide orogeny, or at least none related to subduction as it had occurred in the previous orogenies. The onset of Laramide mountain-building was in the late Cretaceous, 80 to 70 Ma, with the activity wrapping up in the window of 55 to 35 Ma. So the two orogenies overlapped in time, but the Sevier began and ended first.

Laramide Deformation Style

The Laramide orogeny’s style of deformation is thick-skinned, which is a way of noting that the crystalline rocks deep in the crust are very much involved. Laramide deformation is characterized by very large anticlines and synclines that involve basement rock. The anticlines generated highlands and mountain ranges, which were weathered and eroded. The synclines were so significant that they warped the surface of the Earth, generating lowlands. These sedimentary basins that were filled with huge amounts of cast-off sedimentary detritus from the eroding mountains nearby, making new Cenozoic-aged sedimentary deposits that filled the depressions. In some cases, these young sedimentary layers reach thicknesses of 5000 meters! The economically important coal and natural gas deposits of Wyoming formed in Laramide basins, as did the fossil-rich lake deposits of the Green River Formation.

Laramide deformation does not geographically overlap Sevier deformation. Generally, Laramide deformation is further east. The Black Hills of South Dakota, the Bighorn Mountains of Wyoming, and the Colorado Front Range are all examples of basement-cored Laramide deformation. In contrast, the Sevier fold and thrust belt is further west, in a sinuous arc that runs from the Alberta/British Columbia border through western Montana, Wyoming, Utah, and Nevada, and all the way to Mexico. The Sevier orogen is a much longer feature, too, running twice as long from Canada through the U.S. to Mexico, while the Laramide uplifts are all clustered in the mountain west of the United States alone.

Figure \(\PageIndex{6}\): Diagram of the subduction of the Farallon plate during the Late Cretaceous to Paleogene Laramide Orogeny. During this orogeny, the Western Interior Seaway closed and the Rocky Mountains were uplifted. The Sierra Nevada was no longer volcanically active. Diagram by Wade Greenberg-Brand, adapted from an image by the New Mexico Institute of Mining and Technology, originally published in the Teacher Friendly Guide to Earth Science series. Modified for Earth@Home. (Copyright; author via source)

Figure \(\PageIndex{7}\): The Sevier fold and thrust belt can be seen in part of the western U.S. and Canada including Glacier National Park/B . The Laramide uplifts are present further east.

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