Western Mesozoic Orogenies

Western Mesozoic Orogenies

At the end of this chapter, students should be able to:

  • Discuss changes to the tectonic setting along the western coast of North America throughout the Mesozoic
  • Describe individual similarities and differences between the Nevadan, Sevier, and Laramide orogenies
  • Evaluate changes in deformation style and location of deformation in these orogenies over time

Introduction

In the Paleozoic, after the breakup of Rodinia, Laurentia (the name for the continental core of North America) was an island continent near the equator. It was a time of great biological productivity, firstly in the shallow seas left behind by Rodinia’s breakup, and later on land as the first swamps emerged and spread across the continent. Most of the tectonic activity was found in lands that make up the modern-day Eastern Seaboard of the United States, with the Taconian, Acadian, Alleghanian, and Ouachita orogenies helped to build North America and assemble Pangaea. Below is a look at the Earth around the start of the Taconian Orogeny.

Early in the Paleozoic, the western side of North America was a relatively quiet passive margin. The enigmatic Antler orogeny occurred in the Devonian and Carboniferous, but the cause and extent of it are disputed. That was followed by the even more controversial Sonoma orogeny which may have occurred as late as the early Triassic. Evidence for both the Antler and Sonoma orogenies is only found in Nevada. The only other Paleozoic-aged tectonic activity in western North America is the uplift of the Ancestral Rockies, deep-seated uplifts that formed inland, in present-day Colorado and New Mexico. These may be far-reaching byproducts of other orogenies that were going on at the time (Ouachita or Sonoma orogenies).

Nevadan Orogeny

The island becomes a terrane as it attaches to the continental mainland.
An illustration of a terrane docking. As an island arc comes toward the main continent and collides (docks), the subduction zone switches to the other side of the island.
By Actualist via Wikimedia.

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.

They are in the northwestern corner of California and the southwestern corner of Oregon.
Location of the Klamath Mountains in California and Oregon.
By Alexrk2 via Wikimedia.

The Farrallon’s western boundary with the Pacific Plate was divergent: it was a spreading center (an oceanic ridge). The Farallon’s eastern boundary (with 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.


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.

Did I Get It? – Nevadan

Sevier Orogeny

It runs from the Mexican boarder and trends north, going through California, Nevada, Utah, Wyoming, Idaho, Montana, Alberta, British Columbia.
Generalized map of Sevier deformation.
Image by Quickdraw123 via Wikimedia.

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.

Sketch shows thin thrust faulting going toward the east.
W-E Sketch of general features of the Sevier Fold-and-Thrust Belt.
Drawing by Pinkcorundum via Wikimedia.

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.

Consider the nascent mountain-building evident in western North America in this paleogeographic depiction from 105 Ma:

Those mountains were driven by compression from the west. As new terranes accreted to the continent, they pushed the pre-existing sedimentary layers out of the way, so that direction was to the east. This process initiated during the Jurassic period of geologic time, but reached its zenith during the Cretaceous. Most of the Farallon Plate was subducted during that time, but 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.

Annotated photo of a simple felt model of a broken asymmetric anticline/syncline pair. Dozens of layers arch up in the middle (an anticline) that has its short limb on the left (east) and its long limb on the right (west). It's broken at its base along a thrust fault. Distinctive colored layers show that the offset direction is to the east, with the hanging wall moving up relative to the footwall. The anticline has climbed up and over its partner syncline.
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.

When 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 the typical structural ingredient in active and ancient compressional mountain belts around the world. This style of deformation is known as thin-skinned deformation. This is referring to these cover rocks, mostly sedimentary, that are involved in the deformation. Crystalline basement rocks are not typically involved in this process. A great example of this occurs in Glacier National Park.

Cartoon cross-section showing the structural geology of the a place like the Sawtooth Range at Sun River Canyon or Swift Dam: an imbricated stack of thrust sheets placing older Mississippian carbonate on top of younger Cretaceous siliciclastic strata. The thrust faults are youngest and shallowest-dipping in the east and oldest and most rotated into a steep orientation in the west. The Belt rock is relatively stiff and lurks far below, broken only by a single thrust fault.
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, with their significant lateral offsets, and their orientations so close to horizontal. The block or slab or rock above the thrust fault is sometimes called a thrust sheet, or by the term nappe.

A cartoon diagram showing how the folding of a layer of sedimentary rock can create an asymmetric anticline/syncline pair with a short limb dipping in the direction of tectonic transport, which then becomes overturned and ruptured to make a thrust fault. The overlying mass of rock (the hanging wall, an overturned anticline) is now a "nappe."
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 laterally (i.e., from the side) and first buckle into asymmetric folds. The short “limb” of these anticlines and synclines forms in the transport direction (the direction toward which the strata are being thrust). Frequently, this short limb rotates back on itself and becomes tectonically inverted. It also frequently is the place where the strata rupture, and a new thrust fault initiates. Once the mechanical discontinuity of the fault exists, 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). 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. The result is that the older, more western thrust sheets get rotated to steeper dip angles as deformation proceeds. This whole “stack” of nappes is said to be “shingled,” or “imbricated.”

Animated GIF showing a sandbox deformation experiment. The view is a side view of a "fishtank" like apparatus with a transparent glass side. Inside the box are 10 layers of different-colored sand. The right wall is pushed inward (toward the left), compressing the layers. The layers first buckle adjacent to the wall, arching up and breaking, forming a thrust fault. Older, deeper sand layers climb up and to the left on top of younger, shallower layers. Then another fold/fault develops to the left of that, repeating the motion of the first. Finally, a third fold/fault pair develops left of the first too. The motion of the moving wall stops in the middle of the box.
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.

Watch the animated GIF here of a sandbox deformation experiment to get a better sense of this process. It’s useful to watch it many times, focusing on a different region each time and watching how it changes with increasing deformation. Imagine that the perspective here is the same as you would see if you were (a) able to go back in time and (b) hover above the crust and look down the axis of the range front, and (c) watch geological processes happen at much faster rates than they really do, and (d) had a magical pair of X-ray glasses that allowed you to peer into the Earth and see these complicated 3D structures evolve in a 2D cross-section.

Laramide Orogeny

A map and pair of cross-sectional cartoons to show details of Laramide mountain-building and to contrast it with Sevier mountain-building. On the left is a map of the western part of central North America. On it is shown a belt of north-northwest-trending thrust faults, with the sawteeth symbols (indicating the hanging wall) on the southwest side. In Utah and Nevada, the trend of these thrusts shifts to north-northeast, but it resumes the NNW trend in Mexico. The position of Glacier National Park is indicated on the eastern edge of this "Sevier fold and thrust belt" in northern Montana. In the central United States part of the map, but further to the east from the Sevier fold and thrust belt is a region of lumpy blobs marked "Laramide uplifts." The cross-sections appear to the right of these in the graphic. The upper cross-section shows the situation "During Laramide mountain-building," with a huge thrust fault breaking the Precambrian basement complex and shoving it upward and eastward. Two big layers of Paleozoic strata and Mesozoic strata atop the basement are arched upward over the leading tip of the fault, making an anticline. To the east, there is a matching down-dropped syncline. At the bottom, the post-erosion+deposition situation is shown, where the Mesozoic and Paleozoic strata have been removed by erosion to expose the basement core of the Laramide uplift. Meanwhile, the downdropped synclinal basin to the east has accumulated a lens-shaped deposit labeled "Cenozoic strata."
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 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, (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. These three variables allow the two orogenies to be distinguished.

The normal subduction is at about a 45° angle. The flat slab slides along the bottom of the overriding plate.
Normal (a) and flat-slab (b) subduction.
Picture by R. J. Lillie and Parks and Plates by the National Park Service.

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 started first and ended first.

Deformation style

Cartoon cross section across most of Wyoming, showing the Bighorn Basin on the left (west), the Bighorn Range in the middle, and the Powder River Basin to the right (east).
The Laramide Orogeny generated huge synclinal basins where sediments accumulated through the Cenozoic.

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.

Location

The mountain range is oval in shape.
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.

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.

Did I Get It? – Sevier and Laramide

Conclusion

Though the Nevadan, Sevier, and Laramide orogenies are long over, understanding their influence on the geology and topography of the western edge of North America is still crucial today. The Nevadan started as subduction allowed terranes to attach to Laurentia. The Sevier orogeny was an Andean-style subduction zone complete with back-arc thrust sheets. As subduction continued, the bouncy of the subducting plate caused the plate to descend at a shallow angle, causing a much-different Laramide orogeny. This then set the stage for the Basin and Range and the formation of the San Andreas fault.