Geologic History of Western US: Text

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Precambrian

The interpretation of the earliest geologic history of Southwestern North America is very speculative. At approximately 1.7 Ga (1700 Ma), the region probably resembled modern SE Asia. As various tectonic blocks collided, orogenic (mountain-building) events were generated. This view shows what the region may have looked like during these collisional events.

About 1.1 Ga North America was part of the supercontinent Rodinia. The Southwest possibly lay adjacent to Australia and/or Antarctica or possibly Siberia. An arc complex lay off southern North America. Various sedimentary rocks were formed near the margins of North America.

The western margin of North America (NAM) was originally formed during the late Proterozoic (Precambrian), about 650 million years ago, during a major rifting event in which North America separated from the supercontinent Rodinia. The map shows the hypothetical pattern of rifting and the resulting patterns of late Proterozoic siliciclastic sedimentation. The rifted, attenuated (thinned and stretched) margin subsided and was the site of thick sequences of sandstone, mudstone, and limestone. The Cordilleran passive margin was born.

Cambrian

The hingeline marks the sharp, linear zone that separates thin sedimentary rocks of the cratonic interior from westward-thickening sedimentary rocks deposited on the more rapidly subsiding passive margin miogeocline. The hypothetical rifted microcontinent fragments apparently lingered off SW North America during much of the Paleozoic and were accreted to the southwest margin to form parts of Mexico and California in the Mesozoic. From the Late Cambrian through the Devonian, the region was dominated by carbonate deposits across the craton and miogeocline. Slope-rise and adjacent oceanic environments were the site of very fine-grained terrigenous mud and carbonate ooze deposition.

Ordovician

Cambrian depositional and tectonic patterns continued .

Silurian

Along the margin of North America, the above patterns continued. However, a volcanic arc approached the continent from the west.

Devonian

Sometime during the middle Paleozoic, a subduction zone developed off the coast of the western US. Some debate exists as to the orientation of this subduction; it is shown as initially dipping under the approaching Antler arc in this interpretation. The distance between the arc and NAM decreased through the Devonian. The hypothetical offshore blocks, if present, would have been the first elements of North America to collide with the approaching arc. During Late Devonian the encroaching arc began to effect and collide with the passive margin of western NAM.

Mississippian

As the arc approached, and then collided with NAM, neither the 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 (labeled as the reversal of arc polarity on the west map) and others with west-dipping subduction zones (the part of the arc that collided directly with North America).

The arc collision produced the Antler orogeny. The extent of this initial orogenic event of western NAM 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. As the forearc of the arc and distal passive margin of NAM converged, folding occurred.
  • 2. The forearc and parts of the distal passive margin were then thrust eastward over the miogeocline. This produced the Roberts Mountain thrust.
  • 3. The thrust belt added weight to the already subsiding miogeocline causing abrupt, rapid subsidence -- the Antler foreland basin. 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 NAM, the arc and associated thrust belt formed low highlands. However, when subduction was turned off or reversed and compression ceased, the uplift became cooler and subsided along with the general pattern of passive-margin subsidence.

    Pennsylvanian

    By Middle Pennsylvanian, the Antler orogenic belt was buried under a widespread overlap assemblage. Subduction was renewed an unknown distance off the west edge of the continent. This interpretation shows the Stikine fragment being transported northwest along a transform fault. This same fault may have been resposible for the late Paleozoic truncation of SW NAM. Northward, the Quesnell arc began its general approach. Meanwhile, the supercontinent Pangea was being assembled. Europe, Africa, and South America collided with eastern and southern NAM to form the Allegheny (Appalachian) orogeny. The southwest margin of that belt, the Ouachita-Marathon mountains, lay just to the SE margin of the map. The extreme and complicated collisions caused parts of cratonic North America to fail by faulting and the Ancestral Rockies orogenic belt was 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.

    Permian

    Pennsylvanian tectonic and sedimentation patterns continue. The Quesnell arc (Sonomia terrain) is closing on western NAM. Eastward-dipping subduction developed along the Gondwana western margin (south of California). Microcontinents accreted to this margin, especially during the ensuing Mesozoic. Left-lateral shearing and truncation of the southwest craton may have been active at this time. At the close of the Permian, various arc collisions (there is debate as to how many were involved) formed the Sonoman orogeny. The various accreted terrains are collectively known as Sonomia.

    A second interpretaion is also shown. In this model, the McCloud arc is a single arc, probably related to North America and possibly a rejuvenation of the older Antler arc. Its collapse (by telescoping the back-arc basin between the arc and NAM) created the Sonoman orogeny. The Stikine arc is well off shore and is interpreted as exotic to NAM, possibly originating in the Tethys Ocean near China.

    Triassic

    Note that the collision of Sonomia was very similar to that of the older Antler event. The McCloud arc is part of the Sonomia terrain. It contains blocks and fragments of late Paleozoic limestone that contains fossils more closely related to Japan and China than NAM. This suggests that part or all of the arc originated far from NAM. The two somewhat different interpretations shown here explain this fact in different ways.

    Sonomia (or McCloud arc) was fused to NAM. The back-arc basin was thrust eastward over the western margin of NAM. This is the classic Sonoman orogeny. The Cache Creek interarc basin was trapped between the two elements of the McCloud arc. The trench-forearc along western NAM developed into an accretionary wedge prism.

    By Late Triassic, the new, west-facing Cordilleran arc was now well developed along most of SW NAM. A backarc basin developed between the arc and the craton over the site of the older Antler and Sonoman orogenies. Remnants of the Ouachita systems fed northwest-flowing streams of the Upper Triassic Chinle Formation.

    The "exotic interpretation" requires only a single McCloud arc. The basic mechanics of the Sonoman orogeny, at least along the margin of NAM, are similar to the interpretation shown above; however, a new exotic arc, Stikinia, approaches from the west. The dip of the subduction zone under the exotic arc is westward. Therefore, an ocean basin between the Stikine and NAM subduction zones is subducted under both; as this occurs, the ocean basin shrinks and Stikinia moves towards NAM.

    Jurassic

    A major shift in plate motions resulted in highly oblique convergence between the ancestral Pacific plate and NAM. The active arc trended across the entire region and part was built on the craton in Arizona. The oblique intersection resulted in complex compressional, tensional, and transtensional forces. A huge dune field (Navajo Sandstone and related units) developed behind the arc. In Arizona, California, and Nevada, dune deposits are interbedded with arc volcanics.

    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, including the craton, 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.

    Areas of extension resulted in rifts. Some of these along SW NAM are related to the opening of the Gulf of Mexico. Continued subduction caused broad, plateau uplift in Nevada. A large oceanic plateau, Wrangellia (or Insular superterrane) came into contact with SW NAM. Fossils and paleomagnetic patterns suggest that it traveled from far to the southwest before encountering NAM. Lateral offset displaced major pieces of previously accreted terrane including parts of Wrangellia. Several pieces collided violently in the Pacific northwest. Thrusting and uplift began in Nevada as the Nevadan orogeny initiated.

    Volcanic arc activity began to subside, perhaps in response to subduction zones jumping westward past newly accreted terrane. Continuing offset of extreme SW NAM formed the Mojave-Sonoran megashear. Rifting continued inboard of the megashear.

    By Late Jurassic, the Nevadan orogeny was fully active. Terrane suturing, thrusting, lateral offset, and rifting occurred simultaneously in different places. NAM abruptly changed to northwestward drift. The main arc was temporarily shut down. Broad basins formed in the forearc region. Major rivers drained northeastward across the Western Interior and deposited the Morrison Formation.

    During the Jurassic, the differences between the "exotoc" model and the one presented above are very obvious. 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. The first model presented above mainly uses transform processes and interarc basins to form the ophiolites. Although some exotic material is accreted, most of the terrains were once part of NAM. Lateral transform movement transposed the blocks and trapped ophiolites as slivers between them. Blocks are doubled up and then collide trapping the ophiolites between them. The modern Adaman Sea south of Burma may be an example; the sea is along a transform margin created by oblique collision between the Indian Plate and western SE Asia. No exotic continental block is involved in the tectonic sequences.

    The more exotic models create ophiolites in back arc basins, mostly associated with exotic arcs that approach western NAM from far to the west. Back arc basins are commonly formed on fast-moving oceanic plates when the arc moves faster than the plate as a whole and "pulls away" creating a back arc basin. The numerous arcs west, north, and east of New Guinea are modern examples of this model. Many back arc and interarc basins dot the region. When (and if) these back arc basins are trapped in collisions between New Guinea-Australia and another continent, numerous ophiolites will undoubetly be formed. Note the various back arc basins and collisional epidodes that occur on the exotic model for the Middle and Late Jurassic along western NAM. Each time an arc pulls away to form a back arc basin, another potential ophiolite setting is created.

    Both models consider Wrangellia exotic, although considerable debate exists as to when and where Wrangellia first collided with western NAM. 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.

    Cretaceous

    The NAM and paleo-Pacific plates collided nearly head-on. The arc was rebuilt. Continued rapid subduction built a major arc complex. Thrusting and uplift signify the beginning of the Sevier orogeny. At times, the Western Interior seaway extended from the Gulf of Mexico to the Arctic region. Streams spawned by the Sevier uplift deposited terrigenous sediment into the seaway.

    Extended periods of rapid subduction led to intense volcanism and intrusion of major batholithic complexes. The arc also shifted somewhat eastward and the paleo-Pacific plate moved northeastward, oblique to the movement of NAM. These events 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.

    Tertiary

    Although some Cretaceous trends continued into the Tertiary, major changes, especially in arc tectonics and in locations of arc elements, occured across the region. The following maps show only broad tectonic and sedimentary patterns. The first two maps illustrate the Early Tertiary. The first (Eocene) shows the highlights of the Laramide orogeny. The second (Oligocene) illustrates the great expanse of Tertiary volcanism that followed the uplift of the Rocky Mountain Region. Subduction ceased across much of SW NAM. Two hypotheses have been evoked to explain these events: 1)The Baja British Columbia hypothesis emphasizes extreme right lateral juxtaposition of terranes; these terrains either jammed up the subduction zone or forced it to migrate elsewhere. 2) The flat slab hypothesis relates Laramide (Rocky Mtn) events to a very shallow subduction zone along western North America.

    The Baja BC hypothesis suggests that much of modern British Columbia was situated in the Baja region during late Mesozoic and was then transferred north along major right lateral strike slip faults to present positions. As these blocks moved northward between the previous arc (eg. Sierra Nevada) and the westward subduction zone, the arc was built on the Baja BC blocks and the inboard Mesozoic arc was shut down.

    The development of the Rocky Mountains and related structures in western North America is one of the most puzzling of tectonic events. Why were the Rockies built so far inland? The shallow slab hypothesis suggests that a shallowing of the angle of the Cordilleran subduction zone caused inboard migration of arc activity into the Rocky Mountain region. Are both the Baja BC and shallow subduction hypoyheses workable at the same time and place? This question remains to be resolved.

    By the Oligocene, the old elements of the Cordilleran arc remain inactive and magmatism expanded in the Rocky Mtn region. The Farallon plate was rapidly subducted and the spreading center (East Pacific Rise) closed on North America. The Laramide orogeny waned and the Cordilleran region remained high. Sediments shed eastward from both regions were spread over the Great Plains. Magmatism slowly retreated back to the west into a more normal position. A new arc formed along the Cordilleran margin and forearc deposits formed along the Pacific margin. The Pacific spreading center neared North America; a major transform fault, the Mendocino fracture zone, intercepted the NAM continent. A triple junction resulted and the Farallon became two separate plates.

    During the Miocene, more of the Mendocino FZ intercepted western NAM and a long transform margin was established. Subduction continued to the north (Cascade Arc) and south. Along the transform margin, a strike slip rather than a convergent plate boundary formed as both the Pacific and North American plates had strong westward components of movement. The landscape behind this zone became extended and the Basin and Range orogeny developed. Volcanism enveloped large areas of western NAM.

    North of the Mendocino triple junction normal subduction continued and the Cascade arc remained active. Southward, extension spread northward and waned to the south. Volcanism continued over wide areas. Most of western NAM was high topographically but lower basins received locally thick sediments. As the Sea of Cortez (Gulf of Cal.) opened, a large block of SW NAM slid northwestward on the Pacific Plate along the San Andreas Fault.


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