13.1: Paleography and Tectonics of the Mesozoic

Breakup of Pangea

Early in the Mesozoic, Pangaea was fully assembled, but by the late Triassic, around 210 million years ago, Pangaea began to break up. Evidence for this process includes the age of the sediments in the Newark Supergroup, which was deposited in rift valleys formed during continental rifting. Additional evidence is the ages of the Palisades sill and of the eastern part of North America and the Atlantic Ocean floor.

Figure \(\PageIndex{1}\): Animation showing Pangea breaking up

Due to seafloor spreading, the oldest rocks on the Atlantic floor are along the coasts of northern Africa and the east coast of North America, while the youngest are along the mid-ocean ridge. The age of these oldest basalts supports a timing for the breakup around 210 Ma.

Figure \(\PageIndex{2}\): Age of oceanic lithosphere, in millions of years. Notice the differences in the Atlantic Ocean along the coasts of the continents.

This age pattern shows how the Atlantic Ocean opened as the young Mid-Atlantic Ridge began to create the seafloor. This means that the Atlantic Ocean began to form and open up here. The southern Atlantic opened next, with South America separating from central and southern Africa. Last (happening after the Mesozoic ended) was the northernmost Atlantic, with Greenland and Scandinavia parting ways. The rifted plate margins eventually became the passive plate boundaries along the east coast of the Americas today. Here is a video by Tanya Atwater of Pangea breaking apart.

During the Mesozoic, the western margin of North America was active as it moved away from Pangaea, and the eastern margin became passive after the rifting that broke the continent free from Eurasia subsided. The convergence of North America with the ancient oceanic Farallon Plate drove subduction and the accretion of terranes in the west during four orogenic episodes: the Sonoma (continuing from the late Paleozoic), Nevada, Sevier, and Laramide orogenies. These tectonic events built up the mountain ranges that make up the North American Cordilleran and added considerable mass to the continent through the accretion of terranes.

Figure \(\PageIndex{3}\): Map of the American Cordillera. Image from Knightoftheswords281, CC BY 4.0 <https://creativecommons.org/licenses/by/4.0>, via Wikimedia Commons.

In response to the mountain building, basins developed into which sediment was deposited from the adjacent highlands. One was the Cretaceous Western Interior Foreland Basin, which flooded during high sea levels, forming the Cretaceous Interior Seaway. This created a shallow epicontinental seaway that extended from the Gulf of Mexico to the Arctic Ocean, dividing North America into two separate land masses: Laramidia to the west and Appalachia to the east, for approximately 25 million years.

Figure \(\PageIndex{4}\): The Cretaceous Interior Seaway in the mid-Cretaceous.

The Sonoma Orogeny

During the Sonoma Orogeny, which spanned from the late Permian to the Triassic, mountains were formed through the accretion of island arcs and other fragments of continental crust. This event also thrust seafloor sediments onto the continent, adding considerable volume to its western margin.

The Navadan Orogeny

Like the Sonoma Orogeny, the Nevadan Orogeny occurred approximately 155 Ma to 145 Ma (Schweikert, R. et al., 1984)* during the Late Jurassic to Early Cretaceous period, when North America collided with an island arc that had formed offshore. This resulted in the accretion of more material onto the continent, creating a large Andes Mountains-style volcanic arc and a belt of thrust faults (thrust belt), and generated magma that crystallized into rock that forms the core of mountain ranges like the Sierra Nevadas, the San Gabriels, and the San Bernardinos in California.

Figure \(\PageIndex{5}\): Andes Mountains-style subduction zone. Image from MichaelZuo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

The deformation of the Nevadan orogeny is most easily observed in the Sierra Nevada Mountains of California and the Klamath Mountains along the coast of California and Oregon.

Figure \(\PageIndex{6}\): A map of California's geomorphic provinces, including the Klamath and Sierra Nevada Mountains. Image from the California Department of Conservation: https://www.conservation.ca.gov/cgs/...fornia-geotour.

This deformation is preserved as folded and faulted rock.

Marbles of the Boyden Cave Roof Pendant show the results of deformation culminating with batholith emplacement. In addition to the intricate folding displayed here, don’t forget to appreciate the patterns that recent weathering has superimposed on the exposure.

The Sevier Orogeny

The next orogeny of western North America was the Sevier Orogeny that spanned 160 million years (Ma) ago to around 50 Ma (Giallorenzo, M.A., et al., 2018)**. While terranes still caused crustal deformation, the Sevier orogeny occurred further inland and was more widespread than the Nevadan orogeny.

Figure \(\PageIndex{7}\): The Sevier fold and thrust belt. Image from Quickdraw123, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.

The earliest events associated with the Sevier orogeny are as early as 160 Ma, which predates the Nevadan Orogeny. The latest deformation occurred approximately 50 million years ago, during the Cenozoic Era, while most deformation events took place between 120 and 80 million years ago, during the Cretaceous Period.

Figure \(\PageIndex{8}\): Paleogeography of Earth around 105 Ma.

The most significant aspect of the tectonic event was that the angle of the subducting plate decreased. This occurred as the spreading center between the Farallon and Pacific plates approached the continental margin, meaning the ocean floor of the Farallon plate was relatively young and warm, making it more buoyant.

Figure \(\PageIndex{9}\): Sketch of a subduction zone and fold and thrust belt.

This caused more resistance to subduction and additional convergent forces in the overriding North American plate, resulting in a fold-and-thrust belt that extends far 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. Evidence for this includes igneous and sedimentary rocks that were thrust and folded upward from deeper in the crust, along faults, placing older rock atop younger rock. As new terranes accreted to the continent from the west, they pushed the pre-existing sedimentary layers out of the way towards the east. While most of the Farallon Plate was subducted during this event, two remnants persist into the present day: the Cocos Plate, currently being subducted beneath Central America, and the Juan de Fuca Plate, currently being subducted beneath the Pacific Northwest states of Washington, Oregon, and northern California.

Figure \(\PageIndex{10}\): This felt model of Turtle Mountain, Alberta is on display at the Frank Slide Visitor Center. The structure shown is very similar to the structure of many faults and folds in the Rockies caused by Sevier deformation.

As the sedimentary layers were compressed, they buckled and broke, shearing to the east as they deformed. The result was an immense number of asymmetric folds with the short limb on the east and the long limb on the west, frequently breaking at the hinge of the fold, and initiating a fault. These distinctive structures are typical of active and ancient compressional mountain belts worldwide. This style of deformation is known as thin-skinned deformation.

Figure \(\PageIndex{11}\): Cartoon cross-section through an imbricate stack of thrust sheets, as seen at Sun River Canyon, near Glacier National Park.

In general with faults, the footwall rocks are those below the fault, while the rocks above the fault are called the hanging wall. However, additional terminology is used when it comes to thrust faults when their orientations are so close to horizontal. The rock body above the thrust fault is sometimes referred to as a thrust sheet or nappe.

Figure \(\PageIndex{12}\): A cartoon cross-section showing 5 steps in the folding and thrust faulting of an asymmetric anticline to make a “nappe.”

Nappes form when horizontal rock layers are compressed, buckling into asymmetric folds and thrust faults. Once a fault forms, it’s the “weakest link,” and further deformation is accommodated mainly through sliding along that fault surface, not additional folding. Eventually, the fault stops moving, and a new fault forms “outboard” of the first (i.e., further away from the source of the compression) where the crust is weaker. As deformation proceeds, the older thrust sheets get shoved backward and out of the way as new thrusts form beneath them, rising up to the surface. As deformation proceeds, the older, more western thrust sheets get rotated to steeper dip angles. This whole “stack” of nappes is said to be “shingled,” or “imbricated.” The animated GIF below illustrates these processes, although in reality, they occur over tens of thousands of years.

Figure \(\PageIndex{13}\): Sandbox deformation experiment showing how layers of sand fold and fault due to lateral compression. This closely mimics the evolution of fold-and-thrust faults in the Sevier orogeny (except in this model, compression comes in from the east instead of the west).
Model and source video by Marco Martins-Ferreira.

Laramide Orogeny

The fourth mountain-building event that occurred in the western US during the late Mesozoic and early Cenozoic, spanning approximately 80 to 35 million years ago, was the Laramide Orogeny. Its timing overlaps the Sevier orogeny and was also caused by the subduction of the Farallon Plate. Still, it was distinct from the Sevier, due to several factors: (1) the deformation style was different, (2) the deformation was typically more recent, (3) the deformation took place further to the east, and (4) the deformation was deeper in the crust, involving the crystalline basement.

Figure \(\PageIndex{14}\): Map showing the geography of the Sevier fold and thrust belt relative to Laramide uplifts. Cross-sectional views show the structure of the basement-cored Laramide uplifts.

The subducting plate was younger and hotter than during the Sevier orogeny, resulting in the subduction angle becoming increasingly shallow over time. Eventually, it became so small that the slab was dragging along the base of the overriding plate. This is known as flat-slab subduction. This put pressure on the overriding plate, causing warps in the crust to develop from beneath, affecting the overlying continent hundreds of miles east of the continental margin, where it built high mountains. The very low angle also eliminated the lithospheric mantle wedge as the ocean plate was in direct contact with the crust, which prevented magma from forming. Consequently, there was no magmatism during the Laramide orogeny, or at least none related to subduction as it had occurred in the previous orogenies.

Figure \(\PageIndex{15}\): Normal (a) and flat-slab (b) subduction. Picture by R. J. Lillie and Parks and Plates by the National Park Service.

Deformation style

The Laramide orogeny’s style of deformation is thick-skinned, meaning that the crystalline rocks deep in the crust were deformed, resulting in 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 Earth's surface, generating lowlands. These sedimentary basins were filled with large volumes of detrital sediment from nearby eroding mountains, creating new Cenozoic-aged 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.

Figure \(\PageIndex{16}\): The Laramide Orogeny generated huge synclinal basins where sediments accumulated through the Cenozoic.

Location

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 (deep crystalline rock) Laramide deformation. In contrast, the Sevier fold and thrust belt is longer and further west, in a sinuous arc that runs from the Alberta/British Columbia border through western Montana, Wyoming, Utah, and Nevada, and to Mexico.

Figure \(\PageIndex{17}\): Shaded relief map of the Black Hills of South Dakota. This bulging dome-shaped uplift is common in the Laramide, in contrast to the much more linear Sevier orogeny uplifts. Image by nationalatlas.gov in the public domain.

Key Terms

• Laramide Orogeny - a major mountain‑building event that affected western North America from roughly the Late Cretaceous to the early Paleogene (about 80–40 million years ago.
• Nevada Orogeny - a Middle to Late Jurassic (roughly 170–140 million years ago) compressional event along the western margin of North Americ
• Sevier Orogeny - a major compressional mountain‑building episode that occurred from the Late Jurassic through the Late Cretaceous (approximately 140–50 million years ago)
• Sonoma Orogeny - an Early Triassic (about 250–230 million years ago) mountain‑building event along the western margin of North America

* Schweikert, Richard; Bogan, Nicholas L.; Girty, Gary H.; Hanson, Richard E.; Merguerian, Charles (1984). "Timing and Structural Expression of the Nevadan Orogeny, Sierra Nevada, California". Geological Society of America Bulletin. 95 (8): 967–979. doi:10.1130/0016-7606(1984)95<967:taseot>2.0.co;2.

** Giallorenzo, M.A.; Wells, M.L.; Yonkee, W.A.; Stockli, D.F.; Wernicke, B.P. (2018-03-01). "Timing of exhumation, Wheeler Pass thrust sheet, southern Nevada and California: Late Jurassic to middle Cretaceous evolution of the southern Sevier fold-and-thrust belt". GSA Bulletin. 130 (3–4): 558–579. doi:10.1130/B31777.1. ISSN 0016-7606.