Text Box: Tectonic History of the Western United States

All of western United States from the Rocky Mountains to the Pacific coast, including much of Alaska, displays evidence of orogenic activity.  This portion of the country is part of the region called the western Cordillera (pronounced cor-dee-yair-a, or cor-dill-era), or simply the Cordilleran.  Although the name is Spanish for chain of mountains, the western Cordillera includes not only the mountains on the western part of the North American continent that extend from Alaska and Canada southward into Mexico, such as the Brooks Range in Alaska, Rockies, Cascades, Coast Ranges and the Sierra Nevada, but also includes the basins, valleys, plains and plateaus between the mountains. 
The origin and development of some features of the western Cordillera are fairly well understood.  For example, interactions among plates readily explain the location and height of the Sierra Nevada, the many batholiths beneath southern California and Idaho, and the linear chain of the Cascade volcanoes.  However, the developmental histories of many features, particularly the Rocky Mountains, Colorado Plateaus, and the Basin and Range province are less well understood. 

General Formation Factors
The western coast of North America from Alaska to Mexico is a collage of many terranes, created as the North American plate has advanced westward and encountered ocean floor, island arcs and microplates.  Perhaps more than 100 terranes comprise western North America; approximately 50 terranes have been identified within Alaska.  Some of these terranes originated in Asia and South America.  One terrane is estimated to have extended part of the continent westward about 125 miles.  Most of these terranes were accreted between about 180 million to 80 million years ago, although the accompanying folding and faulting that produced much of the orogeny in the western Cordillera continued for another 35 million years.  Continuing westward movement of the North American plate and associated subduction still instigate instability, ocean-floor accretion and volcanism along much of the western edge of the continent.  
Another major factor in the development of the western Cordillera is the formation and continuing movement of the San Andreas fault.  This fault is discussed in detail in the chapter on the Pacific Border province; only a brief discussion is presented here.  Presently the Pacific plate is sliding horizontally northwestward along the western edge of the North American plate.  The boundary of these two plates for more than 600 miles from the Gulf of California to Point Arena in northern California forms the San Andreas fault.  The fault came into existence about 30 million years ago when the Farallon plate split and produced the juxtaposition of the North American and Pacific plates.  It is a right-lateral strike-slip fault that is also classified as a transform plate boundary.  Continuing movement along this fault is a major cause of frequent earthquakes, and the movement contributes to instability, the slow rise of some previously-created mountains, and to crustal extension and accompanying igneous activity that have been significant for many tens of  millions of years over much of the southwestern portion of the United States.  
Hot spots have also influenced the development of parts of the western Cordillera.  A large hot spot presently beneath Yellowstone National Park is known to be very influential over large areas of the northwestern portion of the United States.  A hot spot (as yet undiscovered) has been suggested as one of the causes of crustal extension presently occurring in the Basin and Range province.  

Western Orogenies
Researchers have divided the development of the western Cordillera into six orogenies.  However, often the four main orogenies that affected the region within the last 250 million years–the Sonoma, Nevadan, Sevier and Laramide orogenies–are considered as four pulses of one extended orogeny, generally called the Cordilleran orogeny.  Each of the six orogenies  is briefly discussed here and further described in the chapters dealing with the provinces in which each orogeny was most influential.  

Antler Orogeny
One of the earliest documented orogenies of western United States was the Antler orogeny, which commenced about 400-380 million years ago, pre-dating the four mountain-building episodes that are sometimes considered the main pulses of the Cordilleran orogeny.  Much of the mountainous structure produced by the Antler orogeny has been destroyed or severely distorted by subsequent orogenies, allowing only conjectures to be stated about this event.  Although no single idea concerning the formation is accepted by all, many have stated that perhaps a microplate or volcanic island arc was thrust eastward over the continental shelf.  Thrust faulting and metamorphism produced the Antler orogenic belt; this belt represented the suture zone along the boundary of the continental plate (western edge of United States) and the accreted terrane.  The orogeny was probably most intense from east-central Nevada to western Montana (the orogeny was named for Antler Peak in Nevada), although there was little volcanism and the mountains did not rival the height of those produced in subsequent orogenies.  Near the end of the Antler orogeny, approximately 330 million years ago, the ancestral Rockies of Colorado began forming.  (These mountains are discussed in detail in the Southern Rocky Mountains province.)  

Sonoma Orogeny
The first of the four orogenies that affected western United States during the Mesozoic era, as well as the first of the pulses considered part of the Cordilleran orogeny, was the Sonoma(n).  It began about 245 (or perhaps 260) million years ago when the Sonoma terrane, consisting mostly of an island arc, thrust eastward onto the North American continent, in a somewhat similar fashion to the previous Antler orogeny.  Intense folding occurred in a large region of southwestern United States, especially in present-day west-central Nevada.  This was accompanied by extensive extrusive volcanism and minor development of plutons.  This accretion and orogeny, as with the Antler orogeny, extended the area of the North American plate westward.  This orogeny terminated about 210 million years ago.  

Nevadan Orogeny
The relatively short-lived Nevadan orogeny probably began between 175 and 155 million years ago and terminated about 140 million years before the present.  This was one of the two most intense of the Cordilleran orogenies, with widespread deformation and volcanism.  Subduction, possibly sometimes from opposite directions, collision and significant accretion occurred at this time, initiated by subduction of the Farallon plate beneath the North American plate.  Several researchers have suggested that an exceptional rate or steep angle of subduction produced the massive batholiths in Idaho, southern California, and the numerous ones that coalesced and eventually have been faulted upward to yield the Sierra Nevada (for which this orogeny has been named), accompanied by extrusive igneous activity and significant uplift.  Others have noted the possibility that a microcontinent or island arcs moved eastward while the North American plate was traveling toward the west, both movements narrowing a small marginal sea that existed between the advancing landmasses.  The sea was closed when the masses collided during the peak of the orogeny, raising mountains and extending the United States 125 miles westward.  (This orogeny is discussed further in the chapter on the Sierra Nevada province.)  

Sevier Orogeny
The next major orogeny, the Sevier (pronounced severe), has several suggested beginning and ending dates.  Dates for the beginning of the orogeny range from 165 million to 120 million years ago, and termination times have been stated to be about 80 million to 50 million years before the present.  Some of the most recent research suggests that it occurred between about 150 million and 65 million years ago.  A few researchers restrict the Sevier orogeny to deformational activity in the Nevada-Utah area (partially explaining the different ages applied to the onset and termination of the orogeny).  Also, the most significant changes may have occurred in just a 20-million-year span within the orogeny, adding another complexity to the determination of the timing of this event.  Changes were most pronounced along the eastern edge of the Great Basin in Utah; the Sevier River area in central Utah is the namesake of this orogeny.  The major activity was mostly east of the main effects of the preceding Nevadan orogeny and west of the primary region of the Laramide orogeny, although there was some overlap in location and time of the Sevier and Laramide orogenies.  Accretion of terranes, mostly microcontinents and island arcs along the west coast produced folding and multiple major low-angle eastward thrust faults.  These faults and folds involved a relatively thin part of the sedimentary portion of the crust, not extending into the igneous and metamorphic basement rocks, and thus this has been called a thin-skinned deformation.  The mountains produced were not nearly as high as those created by the Laramide orogeny, and intrusive igneous activity was relatively minor.  

Laramide Orogeny
The Laramide orogeny occurred from about 80 million years to approximately 55 million years ago (some earlier research suggesting it may have continued until about 40 million years before the present).  It overlapped the late part of the Sevier orogeny.  The Laramide was a complex series of tectonic events, affecting especially the eastern side of the western Cordillera, and was responsible for significant uplift and folding of much of the Rocky Mountains.  Although the orogeny was named for the Laramie Formation in Colorado and Wyoming (and the Laramie Range in Wyoming), sometimes all of the Rockies are referred to as the Laramide Mountains or Laramides.  This orogeny was one of the two most significant mountain-building episodes of the Mesozoic that affected western United States.  The series of mountain-making, as well as basin-producing events involved extensive eastward thrust faulting, often including terranes that had earlier accreted to the western coast.  Some parts of the crust were carried more than 100 miles eastward, yielding crustal shortening of 50-150 percent in places.  The thrust faulting was accompanied by block faulting and metamorphism but only minor and sporadic extrusive volcanism and batholith formation.  (This orogeny is discussed further in the chapters on the Southern Rocky Mountains province and the Colorado Plateaus province.)  

Cascadian Orogeny
Approximately 37 million years ago (or possibly somewhat earlier), a few million years after the end of the peak of the primary pulses of the Cordilleran orogeny, a north-south line of volcanoes began developing near the coast extending from northern California to southwestern Canada, produced by subduction of the Juan de Fuca and Gorda plates.  This initial volcanism lasted almost 20 million years and developed the Western Cascades.  A renewal of volcano-producing activity beginning about 13-16 million years ago has produced the High Cascades, consisting of a nearly straight line of 15 stratovolcanoes 50 miles east of the now-subdued Western Cascades in California, Oregon and Washington.  Subduction is still occurring and has produced recent volcanism, including the eruption of Mt. St. Helens in Washington in 1980 and subsequent activity of the volcano (see further discussion in the Middle Cascades division in the Cascade Mountains province).  These volcanoes produced by the Cascadian orogeny are part of the Pacific Ring of Fire.  (This orogeny is discussed further in the Cascade Mountains province.)  

Continuing Change
Western North America remains a very dynamically active region.  The main formation factors of the western Cordillera, including subduction, faulting and hot spots, continue to influence the region.  Some mountains and plateaus are still slowly rising, such as the Sierra Nevada, several ranges in Alaska, and the Colorado Plateaus.  Recent volcanism in Washington and Alaska attests to continuing subduction, and frequent earthquakes indicate continuing movement along the San Andreas and many other western faults.  Parts of the crust of southwestern United States are being stretched by extensional forces, including the Basin and Range province; stretching rates of 0.5  inch per year have been measured in some sections.  Additionally, active hot spots and magma bodies keep several regions geologically active.  The potential future arrival and accretion of additional significant terranes, perhaps coupled with other causes of orogenesis, may further de-stabilize the western edge of the continent and once again fold and fault vast areas of the western Cordillera into rapidly rising mountains.