The interpretation of the earliest geologic history of southwestern North America is very speculative. This view shows what the region may have looked like during the Paleoproterozoic. At this point in time, North America was called Laurentia, a freshly assembled supercraton including the Superior, Slave, Wyoming, and other small cratons. By 1800 Ma, Laurentia itself was part of the even larger Columbia supercontinent.

At ~1700 Ma across this portion of Laurentia/Columbia, various tectonic blocks collided and orogenic (mountain-building) events occurred, including the ductile deformation of the Yavapai Orogeny (1710-1680 Ma), and the more brittle style of deformation of the Mazatzal Orogeny (1680-1660 Ma). The region probably resembled modern SE Asia.

The Wyoming Province (a.k.a. the Wyoming Craton) was an Archean crustal block located to the north of an active ocean-continent margin. The Mojave terrane (here, called a provence (sic)) was likely an accreted crustal extension of the craton. A series of subduction zones, volcanic arcs, and continental fragments existed in the ocean basin south of the Laurentia. These tectonic fragments included the Yavapai and Mazatzal terranes.

In Arizona, the older Yavapai terrane is represented by the Yavapai Supergroup (1800-1740 Ma), whereas the Mazatzal terrane is composed largely of the metasedimentary Tonto Basin Supergroup (1720-1570 Ma), with numerous intrusions (1840-1660 Ma) exposed across the region.

Note - For reference, state boundaries are shown and north will be considered at the top of the image. Tan/brown represent land areas, light blue represent shallow marine areas, and dark blue represent deep marine areas.

The period from 1500 to 1400 Ma witnessed the break-up of supercontinent Columbia. In this portion of Laurentia, tectonism related to the Mesoproterozoic Picuris Orogeny occurred between 1485 to <1444 Ma, and formed most of the classic fold-and-thrust-style structures exposed in the Mazatzals. This overlapped with granitic plutonism that occurred mostly between 1490 to 1310 Ma in a broad belt trending from the southwestern United States, northeastward across the continent. Across Arizona, these intrusive rocks include the Dells Granite, Ak-Chin Granite, Komatke Granite, Ruin Granite, Quatermaster pluton, etc.

By ~1300 Ma, the assembly of the supercontinent Rodinia had begun with the Grenville Orogeny (1250-980 Ma), and by about 1100 Ma, Laurentia was part of the supercontinent Rodinia. Likely adjacent to the west were Australia and Antarctica, and an arc complex lay off to the south of this part of Laurentia/Rodinia.

Various sedimentary units were formed on or near the margins of the continent. In Arizona, this sedimentation included deposition of the Unkar Group (1254 to 1104 Ma) of the Grand Canyon Supergroup, as well as the Apache Group (1340-1050 Ma). The Pahrump Group (1200-800 Ma) was deposited farther west in the Death Valley area of California. Limited crustal extension occurred, which eventually gave rise to mafic volcanism (basalt) and intrusive activity (diabase) at ~1100 Ma. This coincided with more significant rift-related volcanism of the Keweenawan Supergroup in the mid-continent region.

Continental to shallow marine sedimentation occurred in across the region during the Neoproterozoic. From <800 to ~740 Ma, deposition of Nankoweap Formation and Chuar Group sedimentary units occurred in northern Arizona and correlate with the Beck Springs Dolomite (Death Valley area) and the Red Pine Shale (UT).

The rifting of the Rodinia supercontinent had likely begun by ~750 Ma and was well-underway at ~650 Ma (shown here).

As Australia and East Antarctica moved away from Laurentia, the rifted, thinned, and stretched western continental margin subsided and was the depositional site of thick sequences of sandstone, mudstone, and limestone across a broad continental shelf and slope. Thus, the passive western margin of Laurentia was formed.

A passive margin existed across this part of Laurentia during the Cambrian. Highlands of the western part of the Transcontinental Arch rose to the east, with the shoreline tracing across much of Arizona. A broad continental shelf extended westward, with sand, lime (calcium carbonate), and mud sediment deposition across the the shallow marine. A hingeline marked the zone separating the thin sedimentary cover over the craton from westward-thickening sediments deposited in the more rapidly subsiding part of the passive margin. Deeper water oceanic environments were the site of very fine-grained terrigenous mud and carbonate ooze deposition.

Hypothetical rifted microcontinent fragments apparently lingered off southwestern Laurentia during much of the Paleozoic and were accreted to the southwest margin to form parts of Mexico and California in the Mesozoic.

In Arizona, sedimentation during the Cambrian formed the Tapeats Sandstone (508-507 Ma), Bright Angel Shale (507-502 Ma), Muav Limestone (502-499 Ma), and Frenchman Mountain Dolostone (498-497 Ma). The Tapeats is correlative with the Bolsa Quartzite and Abrigo Formation (central and SE Arizona), as well as the Zabriskie Quartzite (NV), whereas the latter three units correlate with the Carrara and Bonanza King Formations (NV).

Along the passive margin of Laurentia, the Ordovician sedimentation patterns continued into the Silurian. Most of Arizona was likely above sea level, with the Trancontinental Arch to the east. However, a volcanic island arc (the Antler island arc) approached the continent from the west.

Along the eastern margin of Laurentia, the Caledonian Orogeny involved the collision of Laurentia with Baltica (a smaller craton) and Avalonia (a continental fragment). Thus, Laurentia evolved into a larger cratonic mass known as Euramerica.

The Devonian saw a return of the sea to this part of Laurentia (now Euramerica). While the Trancontinental Arch persisted to the east, shallow marine environments existed across much of Arizona during the Devonian Period. Lime (calcium carbonate) deposition was dominant in estuaries and across the continental shelf.

In Arizona, Devonian carbonate sedimentation is represented by the Temple Butte Formation (385-375 Ma), the Martin Formation, Chino Valley Formation, and Elbert Formation, which correlate with the Muddy Peak Limestone (southern NV).

Sometime in the early Paleozoic, a subduction zone developed off the coast of the western Euramerica. Some debate exists as to the orientation of this subduction; it is shown as initially dipping west under the approaching island arc (Antler arc) in this interpretation. The distance between the arc and western Euramerica decreased through the Devonian. The hypothetical offshore blocks, if present, would have been the first elements of Euramerica to collide with the approaching arc.

During Late Devonian, the encroaching arc began to collide with the passive margin of western Euramerica.

As the Antler island arc approached, and then collided with western Euramerica, neither the island arc nor the continent could be subducted. The collision likely temporarily stopped the subduction process. Most probably, a new subduction complex developed offshore. In the interpretation shown here, the arc fragmented into several pieces, some with east-dipping subduction zones and others with west-dipping subduction zones (the part of the arc that collided directly with Euramerica).

The Antler Orogeny was the result of this island arc collision. The extent of this initial orogenic event of western Euramerica is unknown because both the north and south margins have been removed or obliterated by younger tectonic processes. The orogeny developed in several phases:

1. Folding and thrust-faulting occurred as the eastern part of the island arc converged with the western edge of the Euramerica passive margin.

2. The forearc and parts of the passive margin were then thrust eastward over the continental shelf (miogeocline). This produced the Roberts Mountain thrust.

3. The Antler foreland basin formed as the thrust belt added weight to the already subsiding miogeocline, causing abrupt, rapid subsidence. In response to this, a broad, gentle forebulge (slight uplift) occurred east of the Four Corners region on the craton.

4. The foreland basin was filled with reworked chert gravels (from the uplifted cherty sediments) at its western margin, but most of the basin was the site of dark-colored, deep water mud deposition.

5. During thrusting and initial collision and accretion with Euramerica, the arc and associated thrust belt formed low highlands. However, when subduction was turned off or reversed and compression ceased, the uplift subsided along with the general pattern of passive-margin subsidence.

In the latest Mississippian, the supercontinent Pangea was forming as Gondwana collided with Euramerica in the Alleghenian Orogeny.

In Arizona, shallow marine Mississippian rocks include the Redwall Limestone (353-335 Ma) in central and northern Arizona and the Escabrosa Limestone in central and SE Arizona. These rocks also correlate with the Leadville Limestone (SW CO) and the Monte Cristo Group (SE NV). Later estuary-related deposition is represented by the slightly younger Surprise Canyon Formation (326-325 Ma) and other units across the region.

By Middle Pennsylvanian, the Antler orogenic belt was becoming buried. Subduction was renewed an unknown distance off the west edge of the continent.

The continent-continent collision between Euramerica and Gondwana resulted in the Alleghenian Orogeny (the third and final of the Appalachian Mountains orogenies). This produced the southern Appalachian & Ouachita mountains, whose E-W trend effectively separated southern Pangea (Gondwana) from northern Pangea (Laurasia).

The southwest margin of the Alleghenian Highlands, the Ouachita-Marathon mountains, lay just to the SE margin of the map. The extreme and complicated collisions caused parts of cratonic Laurasia (North America) to fail by faulting and the Ancestral Rockies were formed.

Note the complex relations of sedimentation to tectonics. Sand and gravel were deposited adjacent to the mountains while lime formed in clear, shallow seas. Eolian dune deposits and evaporites confirm an arid climate.

Various Pennsylvanian rock units were deposited across the region, and include the varied formations of the Supai Group (323-290 Ma), Hermosa Group (NE Arizona), Earp Formation (SE Arizona), Callville Limestone (Nevada), Halgaito Formation (AZ/UT), Cedar Mesa Sandstone (AZ/UT), etc., etc.

Pennsylvanian tectonic and sedimentation patterns continued during the Permian Period. As erosion progressively lowered the Ancestral Rockies, a large sand sheet (erg) developed across the continental margin to the west.

As in the Pennsylvanian, various early Permian rock units were deposited across the region. In northern Arizona southern Utah, the erg is represented by the Esplanade Sandstone (299-290 Ma) of the Supai Group, the Queantoweap Sandstone, the Cedar Mesa Sandstone, the Coconino Sandstone (284-272 Ma), the White RIm Sandstone, and the De Chelly Sandstone. Early Permian flood plain environments in this area are represented by the Hermit Formation (292-284 Ma) and the Halgaito Formation. Extensive evaporite deposits are contained in the Hermosa Group, including the Pardox Formation. Shallow marine shelf deposition is represented by the Pakoon Limestone (299-295 Ma) of the Supai Group and the Toroweap Formation (284-272 Ma).

In the late Permian, various island arc / microcontinent terranes closed on western Laurasia, and eventually accreted to this margin, especially during the ensuing Mesozoic. One such terrane assemblage was Sonomia. At the close of the Permian, various island arcs (Sonomia) collided with western Laurasia and formed the Sonoman Orogeny. Left-lateral shearing and truncation of the southwest Laurasian craton may have been active at this time.

Before the final retreat of the sea, the last marine environments are represented by the Kaibab Formation (272-269 Ma), the Concha Limestone and Rain Valley Formation (SE AZ), the Plympton Formation (UT), the Garden Valley Formation (NV), and the Road Canyon Formation (TX).

The collision of Sonomia during the late Permian-early Triassic Sonoman Orogeny was very similar to that of the older Antler event. Sonomia was fused to western Laurasia and the back-arc basin was thrust eastward over the western margin of Laurasia. This is the classic Sonoman orogeny.

The McCloud arc is part of the Sonomia terrain and contains blocks and fragments of late Paleozoic limestone that have fossils more closely related to Japan and China than Laurasia, suggesting that part or all of the arc originated far from Laurasia.

The Cache Creek interarc basin was trapped between the two elements of Sonomia (the McCloud arc). The trench-forearc along western Laurasia developed into an accretionary wedge prism.

Fluvial (stream) mudstone and sandstone of the Moenkopi Formation (247-241 Ma) represent northwest-flowing streams that flowed across the eroding, arid coastal plain and into shallow marine waters.

By Late Triassic, the new, west-facing Cordilleran continental volcanic arc was well developed along most of western Laurasia. The earliest plutons of the Sierra Nevada batholith were emplaced during this time. A foreland basin developed between the arc and the craton, over the site of the older Antler and Sonoman orogenies.

Northwest-flowing streams transported fluvial sediments of the Chinle Formation (227-202 Ma) and Dolores Formation (CO) from the eroding remnants of the Ouachita highlands to the southeast.

With the progressive rifting of Pangea, Laurasia was split up, forming North America and Eurasia. On North America's western sie, a major shift in plate motions resulted in highly oblique convergence between the ancestral Pacific plate (Farallon). The active arc trended across the entire region and part was built on the North American craton in Arizona. The oblique intersection resulted in complex compressional, tensional, and transtensional forces.

Fluvial (stream) and eolian (wind) sediments of the Glen Canyon Group were deposited across Utah and Arizona at this time. This includes the Wingate Sandstone, Moenave Formation, Kayenta Formation, and Navajo Sandstone. The Navajo Sandstone and related units represent a huge erg (dune field / sand sea) that developed behind the arc. In Arizona, California, and Nevada, dune deposits are interbedded with arc volcanics.

The subduction-related Nevadan Orogeny began and resulted in a broad, plateau uplift in Nevada. The foreland basin shifted eastward and was flooded by the Carmel Sea.

Continental arc volcanism continued, partly represented by Jurassic plutons of the Sierra Nevada batholith. Complex margins like the Mesozoic/Cenozoic Cordilleran margin of the western US can have arcs build on a variety of geologic materials, including oceanic crust, continental crust, and various accretionary materials such as older arcs, oceanic plateaus, and accretionary wedges. All of the above were sites of arc building at one time or the other along the Cordilleran margin of the western US.

Dominantly fluvial (stream) and eolian (wind) sediments of the San Rafael Group were deposited across Utah and Arizona at this time. This includes the Page Sandstone, Carmel Formation, Entrada Sandstone, Curtis Formation, and Summerville Formation.

Subduction-related folding, thrusting, & uplift occurred as the Nevadan Orogeny continued.

Volcanic arc activity began to subside, perhaps in response to subduction zones jumping westward past newly accreted terrane. Continuing offset of extreme SW North America formed the Mojave-Sonoran megashear. Rifting continued inboard of the megashear. Some of these along SW North America are related to the opening of the Gulf of Mexico.

Continental sedimentation continued across Utah and Arizona. The Sundance Sea filled the large foreland basin to the north.

By Late Jurassic, the Nevadan orogeny was fully active. Terrane suturing, thrusting, lateral offset, and rifting occurred simultaneously in different places. North America abruptly changed to northwestward drift. The main arc was temporarily shut down.

The Franciscan Melange (accretionary wedge) and the Great Valley Sequence (forearc basin) were deposited in the arc-trench area. Northern California and Oregon contain a number of different ophiolites. Ophiolites are fragments of ancient oceans later trapped in continental crusts. Most ophiolites represent suture zones between collided continental blocks or between collided arcs and continents.

Wrangellia is considered an exotic terrane, although considerable debate exists as to when and where Wrangellia first collided with western North America. There is also considerable debate as to whether the Wrangellia collision was related to major orogeny. The collision has been related to the Nevadan (Late Jurassic), Sevier (Cretaceous), and Laramide (Early Tertiary) orogenies; perhaps it's not closely related to any of them.

Broad basins formed in the foreland basin region. With the regression of the Sundance Sea, major rivers drained northeastward across the Western Interior. Fluvial (stream) and lacustrine (lake) sediments were deposited and formed the Morrison Formation.

The North American and paleo-Pacific plates collided nearly head-on. Continued rapid subduction rebuilt a major continental volcanic arc complex.

Thrusting and uplift signify the beginning of the Sevier Orogeny, which created highlands across much of the Nevada region.

The shallow marine environment of the Western Interior Seaway (or North American Interior Seaway) covers Colorado and extends into Arizona.

At times, the Western Interior seaway extended from the Gulf of Mexico to the Arctic region, and is represented by rocks of the Trinity and Edwards Groups (TX). Streams spawned by the Sevier uplift deposited terrigenous sediment into the seaway.

Folding and thrust faulting of the Sevier Orogeny continues, accompanied by intense volcanism and intrusion of major batholithic complexes. In California, a large portion of the Sierra Nevada batholith was emplaced between 105-84 Ma.

The Mogollon Highlands form in Arizona, and streams drained to the north across a broad coastal plain.

The shallow marine environment of the Western Interior Seaway covers Colorado (and the Great Plains) and extends into northeastern Arizona. This includes deposition of the nearshore Dakota Sandstone (100-90 Ma) and the deeper marine Mancos Shale (95-80 Ma).

The large landmass on the western side of the seaway is sometimes referred to as Laramidia, and the one on the eastern side has been called Appalachia.

Extended periods of rapid subduction of the oceanic Farallon plate caused the continental arc to shift somewhat eastward and produced the culmination of the Sevier Orogeny. Right-lateral slip inboard of the subduction zone resulted in major northward displacement of previously accreted terranes and accretionary wedge complexes.

The continuation of the continental arc eastward into Arizona began to produce porphyry copper deposits in the Mogollon Highlands area.

Western Interior Seaway deposition is represented by the carbonate rocks of the Niobrara Group (87-82 Ma), as well as the mudstones of the Eagle Ford Group (TX) and the Pierre Shale.

The Western Interior Seaway retreats for the last time as uplift occurs related to the beginning of the Latest Cretaceous Laramide Orogeny. Deformation and volcanism migrate eastward as the Farallon subduction angle decreases. The Rocky Mountains begin to form. Basement-cored uplifts began to form in the southern Rockies (Colorado and New Mexico) and Arizona, resulting in monoclinal folding and flatirons. Fold-and-thrust style deformation occurred in the Northern Rockies and Canadian Rockies, as well as in the Sierra Madre region to the south.

The regression of the Western Interior Seaway is represented by the Mesa Verde Group in the Four Corners region.

Although some Cretaceous trends continued into the Paleogene, major changes occured across the region. This map highlights the landscape formed during the peak of the Laramide Orogeny.

The subduction angle of the Farallon oceanic plate decreased to its minimum ("flat" subduction), which had two major effects:

1) deformation propagated to its furthest eastward extent

2) continental arc magmatic activity dramatically decreased

Basement-cored uplifts continued to occur in the southern Rocky Mountains, and produced monoclinal folds across the Colorado Plateau (Coconino Point, Grandview, East Kaibab, Echo Cliffs, Organ Rock, etc.). Fold and thrust belt style deformation was dominant in the northern Rockies. Arc magmatism resulted in the formation of major porphyry copper deposits across the region, including at Morenci, Bingham Canyon, etc.

As the Rocky Mountains rose during the early Paleogene, stream drainages were diverted or cut off, and several large intermontane lake basins were created as a result (Green River, Uinta, Piceance, etc.). The Eocene Green River Formation (54-48 Ma) of Wyoming includes a significant lacustrine sedimentary record that includes oil shale. Similar-aged fluvial & lacustrine sedimentation of the Eoene Claron Formation occurred in and around Lake Claron of southwestern Utah formed the Pink Cliffs of the Grand Staircase National Monument, Bryce Canyon National Park, and Cedar Breaks National Monument.

The Laramide Orogeny waned in the Eocene, resulting in a period of erosion across the region. In Arizona, the Rim Gravels were deposited along the Mogollon Rim by north-flowing streams.

By the Oligocene, the Laramide Orogeny had finished, but the Cordilleran region remained high. Erosion produced sediments that were shed eastward over the Great Plains.

Farallon subduction angle was increasing again, causing continental arc magmatism to slowly migrate back to the west. During this time, felsic, caldera-style magmatism expanded across region and produced the "Mid-Tertiary Ignimbrite flare-up". Many large volcanic fields formed as a result, including the San Juan Mountains (30-26 Ma), Mogollon-Datil (36-24 Ma), Bootheel (35-27 Ma), Marysvale (32-22 Ma), etc. Contemporaneous intrusive centers (called laccoliths) are present across southeastern Utah, and form the Abajo (29-22 Ma), Henry (31-23 Ma), and La Sal Mountains (28-25 Ma). Over 80 former volcanic/intrusive centers of the Navajo volcanic field (27-24 Ma) are exposed across the Four Corners region, and include iconic Shiprock (~27 Ma), Agathla Peak, and many others.

Rapid subduction of the Farallon plate was bringing its mid-ocean ridge (the East Pacific Rise) closer and closer to North America. In the Late Oligocene, the San Andreas transform boundary began to form with the subduction of the first parts of the East Pacific Rise (~28 Ma). For western North America, this signaled the beginning of the change from a subduction-style margin to a transform-style one. The Mendocino fracture zone also intercepted the subduction zone. A triple junction resulted and the Farallon eventually became two separate plates.

During the Miocene, the San Andreas transform margin lengthened as more of the Farallon mid-ocean ridge intercepted western North America. Subduction continued to the north (Cascade continental arc) and to the south in central America.

Caldera-style volcanism continued in the Early Miocene, forming the various felsic volcanic units in the Superstition Mountains (21-18 Ma) in central Arizona, as well as the Peach Springs Tuff (19 Ma) in northwestern Arizona. Subduction eventually ceased, effectively ending widespread continenal arc volcanism.

With the change from compressional subduction-related boundary to a transform boundary, the region experienced a period of extensional tectonism and the Basin and Range orogeny developed.

The combination of a lack of significant tectonic compression, Laramide-thickened crust, and high heat flow allowed rapid extension and thinning, forming the Basin and Range physiographic province. This included the formation of several metamorphic core complexes across southern Arizona to southeastern California (e.g., Santa Catalina-Rincon, South Mountains, White Tank, Harcuvar, Harquahala, Buckskin-Rawhide, Whipple, etc.).

The Colorado Plateau physiographic province also started to form during this time, evidenced by a regional drainage reversal as the southern Arizona crust thinned, lowering the surface elevations there.

The plate tectonic changes also produced a switch to mafic-dominated, extension-related volcanism across most of western North America. The Hickey Formation (16-10 Ma) in central Arizona represents this basalt-dominated volcanism.

North of the Mendocino triple junction, normal subduction continued and the Cascade continental volcanic arc remained active.

Volcanism continued over wide areas. The bimodal volcanics (basalt & rhyolite) of the Snake River Plain began erupting at ~12 Ma and cut across the Rockies in southern Idaho and culminated in the Yellowstone area. This is the volcanic hot spot trace of the mantle plume currently located beneath the Yellowstone area.

On the Colorado Plateau, lake deposits of the Bidahochi Formation (16-5.9 Ma) represent an internally-drained area likely fed by the ancestral Colorado River. Numerous intrusive and volcanic centers of the Hopi Buttes volcanic field (8.7-4.2 Ma) also dot this area. Eruptions at other volcanic fields also occurred across the region (e.g., San Francisco, Mormon Mountain, Mount Floyd, Burro Creek, Springerville, and many others).

The Sierra Nevada Mountains began to rise in California as Basin and Range extension spread northward, waning to the south. And at ~6 Ma, an integrated, through-flowing Colorado River began cutting the modern Grand Canyon.

Most of western North America was high topographically, but lower basins received locally thick sediments. As the Gulf of California opened, a large granitic block of southwestern North America (Salinia or Salinian terrane) slid northwestward on the Pacific Plate along the San Andreas Fault.

Most of the physiographic features of the western United States were well expressed in the landscape by the Pleistocene. The Quaternary Ice Age included alpine glaciers in the Sierra Nevada and Rocky Mountain areas, which carved distinctive glacial terrains like U-shaped valleys, etc. The Colorado River continued to cut into the Colorado Plateau, widening and deepening the Grand Canyon during the Pleistocene.

Large pluvial lake systems that spread across the northern Basin and Range, including Lake Lahonton and Lake Bonneville. Other basins fringing the Sierra Nevada Mountains filled with smaller lakes including Pyramid Lake, Lake Tahoe, Mono Lake, Lake Manly (Death Valley), Searles Lake, etc. These lakes progressively shrank, leaving immense playa salt deposits behind (Bonneville salt flats, Death Valley). Remnants of some of the larger lakes still persist to this day.

The North American plate continued to move over the Yellowstone mantle plume producing several large caldera-style eruptions (2.1 Ma, 1.3 Ma, & 0.64 Ma). Other major caldera eruptions were located at the Long Valley caldera in California (Bishop Tuff / 767 Ka ) and the Valles Caldera in New Mexico (1.2 Ma to 50 Ka). Basalt-dominated volcanism occurred across the region at volcanic fields like Sentinel, Uinkaret, San Francisco, Springerville, San Bernardino, Portrillo, Zuni-Bandera, etc.

Several major physiographic provinces dominate the landscape of the western U.S. today.

The North American Cordillera covers an extensive area from Mexico (Sierra Madre) through New Mexico and Colorado (southern Rockies) into Wyoming, Idaho, and Montana (northern Rockies) and finally into Canada (Canadian Rockies) and Alaska. The Colorado Plateau is a topographically elevated area centered on the Four Corners region, more or less. The Rio Grande Rift trends N-S through the middle of New Mexico into southern Colorado, and separates part of the southern Rockies from the Colorado Plateau.

The Basin and Range extends northward from Mexico into New Mexico and Arizona and as far north as southern Idaho. This is region is home to several mid-latitude desert areas that each develop at slightly different elevations. The lowest is the Mojave Desert in southern California, followed by the Sonoran, Chihuahuan, and finally the Great Basin Desert, which is the northernmost and highest of these desert areas.

The San Andreas Fault system is comprised of a multitude of right-lateral strike-slip faults. Collectively, these faults form an active transform plate boundary along the western edge of the North American plate.

Farther north, the North American continental plate continues to override the Juan de Fuca oceanic plate along the Cascadia subduction zone. The Cascade Mountains are the active continental volcanic arc built above this subduction zone.