Mountain ranges of the western United States

Western North America is one of the most extensive mountainous regions on Earth, with mountainous topography  extending north-to-south for  thousands of miles (from northern Alaska to southern Mexico) and east-to-west  for roughly one thousand miles  at the latitude of Colorado. The western United States alone has hundreds of individual mountain ranges, too many to explore thoroughly in a single lifetime.  Uplift of these mountain ranges did not occur during a single geologic period, but instead occurred at different times in the geologic past in response to the ever-changing plate tectonic forces that were being exerted along the western edge of the North American plate.  Rocks in some mountain ranges were deformed by multiple plate tectonic events, giving rise to a relatively complex record of rock deformation that geoscientists have been working to decipher for more than 150 years.

As an introduction to mountains of the western United States, this lecture (and one subsequent lecture) provides basic information about the key formative events of several major geotectonic province of the western United States.  A "geotectonic" province is defined here as a geographic region whose mountain ranges (or more generally rock structures) share a common originating cause.  For example, active mountain building in western California is almost entirely attributable to active slip along the San Andreas fault and related faults, thereby leading geoscientists to define a geotectonic province associated with the San Andreas fault.  Mountain ranges within a particular geotectonic province are likely to have undergone uplift during roughly similar times, although such uplift could have occurred over periods of tens of millions of years.

 Orogeny refers to the process of mountain building.  Distinct  orogenic events are frequently assigned names by geoscientists to help organize information about the mountain building history of a particular region.  A particular orogeny refers to mountain building (or more generally, rock deformation) that occurred in a particular region during a particular period in the geologic past.  Mountains, once built, can be deformed by later orogenies that are caused by different sets of tectonic forces that act on the crust.

The goals of this lecture are threefold:
  1. Outline basic features of several major geomorphologic/tectonic provinces of the western United States
  2. Discuss the age that each of these areas last experienced major deformation (mountain building or volcanism)
  3. Show reconstructions of how the western United States looked at pivotal points in the past 100 million years and discuss how changes in the plate tectonic setting of the western United States have profoundly influenced its geologic history.
Let's first review the locations and ages of the major geotectonic provinces of the western  United States.

A list of most of the major provinces and their principal formative ages follows

Present-day tectonic and volcanic activity

Earthquakes, deformation, and volcanism in the western United States are largely focused along the San Andreas fault zone, and in Cascadia, Yellowstone, and the northern Basin-and-Range province.

Present mountain range morphology/physiography

The present physiographies of the mountain ranges and landscape in the major geologic provinces are not only profoundly influenced by their tectonic origins and properties of the rocks they are made of, but are shaped by erosional processes and the present and past regional climate. It is thus important to recognize that once the formative stages (orogenesis) of the mountain range are finished, much of their subsequent geologic evolution is dictated by non-tectonic factors. These processes are featured in later lectures.

The Past 100 Ma

The geologic history of a region as large as the western United States is not easily summarized for a period as long as 100 million years; however, there are several general comments and two diagrams that provide useful backdrops against which to view the major geologic events during this period. The key observation is that prior to the initiation of the San Andreas fault system at roughly 15 Ma, the plate tectonic setting of the western U.S. was dominated by subduction toward the east of one or more oceanic plates beneath the continent, the Farallon plate prominent amongst them, . The convergence of these oceanic plates with the western edge of the North American plate profoundly influenced volcanic and igneous activity west of and including Colorado, and caused mountain building (orogenesis) in the same region. The only major geomorphic features in the western United States that are NOT likely to be caused by subduction are the Columbia plateau volcanics, the Snake River plain, the Yellowstone hotspot, and the San Andreas fault zone. A hotspot in the mantle beneath the North American continent is the most probable cause of the Columbia plateau and Snake River plain volcanics, as well as geothermal activity and volcanism in and near Yellowstone National Park. Mountain building and faulting along the San Andreas fault zone are caused by transcurrent, northwest-directed slip of the Pacific plate past the North American plate in western California.

Volcanic history

The above diagram, which is taken from Atwater (1989), shows the ages and locations of volcanic and igneous activity in the western U.S. over the past 100 Ma and how the location of volcanic activity has changed location through time. Each dot in the left hand plot shows the absolute age of a volcanic or igneous rock that is located within the broad east-west zone that is depicted in the panel at the right. An absence of age dates for a particular time and geographic area implies an absence of volcanic and igneous activity. The left-hand plot shows that from 100-80 Ma, volcanic and igneous activity within an east-west belt south of Idaho and north of central Arizona was limited to the Sierra Nevada belt of eastern California and western Nevada. At the beginning of the Laramide orogeny (roughly 70 Ma), volcanic activity in this belt suddenly ceased and jumped far eastward into the continent, to Colorado. At the same time, the Laramide uplifts of northern Colorado, Wyoming and Montana, starting going up. These ranges are/were located a remarkably long distance from the then-western coast of the North American continent, where subduction was occurring, as was the volcanic and igneous activity in central/southern Colorado during this period. One plausible hypothesis for the jump of volcanic and orogenic activity far into the continent during this period is that the dip of the oceanic plate then subducting off the west coast of North America became much shallower than during the previous tens of millions of years, thereby pushing the locus of active faulting and volcano-igneous activity far into the continent.

After 50 Ma, volcanic and igneous activity began migrating slowly back toward the west coast and by 25 Ma, the vast majority of the volcanic and igneous activity in the Colorado Rockies ceased. Uplift of the Laramide mountain ranges also ceased by roughly 40-30 Ma. The coastward migration of the volcano-igneous activity and cessation of Laramide uplift both suggest that subduction of the oceanic plate resumed at a steeper dip after roughly 40-30 Ma. It is unclear why the elevations of the Colorado Plateau and Basin-and-Range province began to increase in the past 15-30 million years. This uplift might be an after-effect of the long period of unusually shallow subduction during the Laramide orogeny, or could be related to the change at 20-15 Ma in the nature of the plate boundary off the west coast from a subducting (convergent) plate boundary (Farallon-North America) to a transcurrent plate boundary (Pacific-North America). Regular subduction of oceanic lithosphere continues to the present-day off the coasts of northern California, Oregon, and Washington, where the Juan de Fuca oceanic plate subducts and gives rise to the Cascade volcanic arc.

Plate tectonic history

The figure below, taken from Atwater (1989), shows the reconstructed positions of the North American plate and oceanic plates in the Pacific basin over the past 110 million years. The bold lines show the positions of seafloor spreading centers. The oceanic plates adjacent to the west coast were subducting to the east until roughly 15 Ma (not shown). Subduction of these plates gave rise to the extensive and long-lasting volcanic and igneous activity in the western United States, as well as the multiple phases of mountain building evidenced by the numerous mountain ranges of the western U.S. At roughly 20-15 Ma, the seafloor spreading center just west of North America that is shown in the 37 Ma "panel" below intersected the western margin of North America. Some of the seafloor spreading center subducted; some "froze" offshore and thus became part of the Pacific plate, and some of it is still active (the segment that separates the Pacific and Juan de Fuca plates). Extinction and subduction of large segments of this seafloor spreading center brought the Pacific plate into contact with North America along the present San Andreas fault system, thereby giving birth to the latest phase of mountain building and tectonic activity in the western United States.

Reconstructions of the paleotopography and paleogeography of the western United States

Professor Ron Blakey of Northern Arizona University has reconstructed how the western United States looked at different times in the remote geologic past by synthesizing a huge amount of geologic and other data collected by himself and numerous other researchers.  Below, I have selected a series of graphics from his Web site for the last 100 million years and have written some text to accompany each graphic (freely borrowing from Prof. Blake's text, too). These reconstructions are subject to significant uncertainties, but give you a flavor for how much change has occurred in the western United States.  Remember that the last 100 million years is just the most recent 2.5% of Earth history.....

Western U.S. at 90 million years before present (below). Ninety million years ago (90 Ma), the western U.S. didn't look much like it does today.  Relative to U.S. state borders (dashed gray lines), there was almost no continent west of the California-Nevada border.  A subduction zone (trench) and  sediments being scraped off the subducting Farallon plate onto the edge of the continent were instead located where California is now found.  In addition, one of the Earth's largest ever intra-continental seas (a shallow sea located within a continent) covered the central United States from central Utah all the way to central/eastern North America.   Thrust faulting, convergence, and crustal uplift (orogenesis) were occurring in the Sevier fold-and-thrust belt in response to subduction of the Farallon oceanic plate west of the continent.

Western U.S. at 75 million years before present (below)
. Fifteen million years after the prior scene, the interior seaway had shrunk substantially, leaving enormous low areas of sea-bottom and river-transported sand and mud in its wake.  Some of the world's greatest dinosaur remains are found in these deposits. Mountain building associated with the Sevier orogeny reached its peak .  Along the "West Coast" subduction zone, the Farallon plate was converging at a low angle with the coast, causing pieces of North America and other oceanic crustal fragments to slide northwards along the western edge of the continent.  Sediments being scraped off from the subducting Farallon plate were complexly deformed and accreted to the continent's edge.  At least some of these complexly deformed marine rocks are commonly exposed in the San Francisco region (and many other areas of western California).

Western U.S. at 65 Ma (below). Between 75 Ma and 65 Ma, there were enormous geotectonic changes in western North America. The continental sea withdrew completely, and the Laramide orogeny began.  With the formation of the Laramide uplifts (discussed in a later lecture), many of today's most beloved and recognized western U.S. mountain ranges were added to the landscape. Volcanism in the Colorado Rockies also began during this period (see figure farther above in this lecture), representing a huge eastward shift in the location of volcanism prior to 75 Ma.  A shallowing of the angle of subduction of the Farallon oceanic plate under western North America is presumably responsible for shifting subduction-related volcanism far eastward into the Rocky Mountain region.  Some evidence suggests that the collision of a large and buoyant oceanic plateau on the Farallon plate with the subduction zone resulted in Laramide-age mountain building and volcanism.

Western U.S. at 50 Ma (below). Laramide orogenesis continued during this period. Large lakes formed in the basins located between the major mountain ranges. As sediments eroding off the neighboring ranges poured into these lakes, the basins subsided further, leading to very deep sedimentary fill. Subduction of the Farallon plate continued during this period.

Western U.S. at 35 Ma (below). During this period, widespread volcanic activity occurred (see the "volcanic history" figure earlier in this lecture), predominantly due to the continued influence of Farallon plate subduction along the west coast. As the subducting Farallon plate grew ever narrower, the Pacific-Farallon seafloor spreading center neared the West Coast and brought Pacific plate seafloor ever closer to contact with the western edge of North America.

Western U.S. at 20 Ma (below). First contact of the Pacific and North American plates occurred around 15 Ma because the Farallon plate had, by this time,  largely subducted into the mantle in the vicinity of California. This change fundamentally altered the forces that were acting along the plate boundary on the western edge of North America, with east-directed convergence that prevailed during Farallon plate subduction changing to northwest-directed Pacific plate movement parallel to the West Coast. The transcurrent San Andreas fault of western California was born at about 15-12 Ma - northwestward movement of the Pacific plate was accommodated in part by slip along the San Andreas fault. Relaxation of the convergent stress regime associated with Farallon plate subduction lead to widespread extension in much of western North America, giving rise to Basin-and-Range uplift. Drainage systems of the Western Interior were still not well integrated. Note the absence of a through going stream on the Colorado Plateau.  Subduction continued in northern California, Oregon, and Washington, where the Juan de Fuca plate (a remnant of the Farallon plate) continued subducting eastward beneath the coast.  Interestingly, there is little or no evidence from this time for the existence of the deep canyons carved by the Colorado River. Notice that the Great Valley of central California (just west of the Sierra Nevada Mountain Range) was an inland sea at this time.

Western U.S. at 5-10 Ma (below). Basin-and-Range extension continued during this period, as did volcanic activity in some areas. Significant downward erosion of rivers into the Colorado Plateau began as the plateau experienced net uplift of 4000-6000 feet.
The transition to an extension-dominated tectonic regime with an ever-decreasing amount of volcanic activity was well underway, reflecting the change from subduction-dominated to strike-slip dominated plate boundary motion along the West Coast.

Today's western U.S.(below). Based on the images and descriptions given above, I hope that you now see the present topography of the western U.S. as the product of a long and complex geotectonic past, rather than an unchanging, fixed landscape. Assembling the details of the geologic history of the western United States from an eroded, multiply deformed, and largely buried rock record is a difficult task, one that will undoubtedly continue for many decades..