Southwest North America

The paleogeographic maps of Southwestern North America were completed in February 2023 and are expanded and modified from the former Colorado Plateau series. Their detail and image size make them some of our most detailed maps. The maps were compiled from many sources (see references) and do not follow any single model (I am unaware of any model that details the history of the region over 550 Ma). Rather, the maps represent compilations and compromises from the geologic literature. Particular attention was paid to making sure that tectonic and geologic events between adjacent time slices made geologic, geographic (space issues), and tectonic sense.

All of the features on the maps are palinspastically restored — that is, the positions of various terranes and blocks are shown in their presumed location during a given time slice. Considerable offset of these elements occurred west of a line from Western Wyoming, through the Salt Lake City area, south towards Las Vegas, and SE towards SE Arizona. Parts of this line are known as the Wasatch Line or Cordilleran hingeline. Displacement of elements west and south of this line ranged from tens to hundreds of kms to a thousand or more kms for terranes along the West Coast. Many of these motions can be followed by comparing various successive sed-tectonic-plate maps. Note that state lines are not deformed, a common method of showing palinspastic restorations in many publications; rather state-county lines are minimized (grayed) until the time slice in which the terrane arrives at it present location for a given area and then they are darkened. Terrane-block motion is shown (restored) relative to areas east of the Wasatch Line. By not deforming state lines, the paleogeography shows more clearly and without confusing lines. No attempt is made to show minor Cenozoic rotations of the Colorado Plateau.

Although the time slices are given as a numerical (absolute) single age, it is realized that over an area this large, errors in correlation across the map is inevitable. Therefore, an age range is given in parentheses behind the time slice age. The range is a best estimate and varies, generally longer for older time slices and shorter for more recent time slices. Period names from the relative time scale are subdivided into epoch and sometimes age; correlation between the numerical and relative time scale follow Gradstein and Ogg, 2004.

Highlights of the geologic history

• Early Paleozoic passive margin of Western North America — the edge of the continent ran north-south through central Nevada; ocean crust lay to the west
• Devonian incursion of fast-moving arc systems, possibly from both the SW and NW; these arcs transported exotic terranes originally sourced from Gondwana, Baltica, and the Caledonian region of the Iapetus Ocean.
• Late Devonian-Mississippian terrane accretion (Antler orogen) and evolution of Western North America into an active tectonic margin
• During the late Paleozoic and early Mesozoic, a series of arcs, probably both west- and east-facing, were along the western margin of the continent; some of these arcs, with both exotic and peri-North American terranes, accreted to the western margin during the Permo-Triassic Sonoman orogen
• Triassic establishment of Cordilleran magmatic arc — along the southern margin of the continent, the arc was built on Proterozoic North American crust — an Andean-style arc. Farther north, the arc was built on fringing terranes as one or more complex island arcs
• The exotic greater Wrangellia terrane (aka Insular Superterrane) drifted towards North America from the west and initially collided and amalgamated with the fringing island arcs in the Middle and Late Jurassic. The resulting block, commonly referred to as Baja BC, then accreted to the western margin of the continent in the Late Jurassic and Early Cretaceous. Some of these events were responsible for the Late Jurassic Nevadan orogen and perhaps initiated the Cretaceous Sevier Orogen.
• The ensuing plate reorganization and renewed subduction beneath western North America resulted in the oblique, left-lateral subduction of the oceanic Farallon Plate. The left-lateral transpression drove Baja BC southward along the Pacific margin. The model presented in these maps follows a moderate translation interpretation in which the southern margin of Baja BC reached the approximate lattitude of Central California by the Late Cretaceous.
• During the Cretaceous, huge portions of the Farallon Plate were subducted below SW North America. The great batholiths of the Peninsular Ranges, Sierra Nevada, Idaho Batholith, and Coast Plutonic Complex were generated. Resulting compression formed the Sevier Thrust Belt and caused significant compression of the western continent
• Between 85 Ma and 70 Ma, a large fragment of an inactive ocean ridge was subducted beneath the southern portion of the map region. The resulting reorganized subduction shifted to right-latteral transpression. The subducted and thickened ocean crust wrecked havoc with SW North America and eventually shut down normal Cordilleran subduction and replaced it with shallow subduction, generated widespread regional metamorphism and uplift, and caused uplift of the Central and Southern Rocky Mountains and Colorado Plateau. These events persisted well into the Neogene.
• In the mid Cenozoic, the western margin of the Farallon Plate, the East Pacific Rise, drifted towards SW North America. As collision and subduction occurred, the resulting shift in plate dynamics caused right-lateral transform faulting and extension of SW North America. During the Neogene, these complicated events generated the Basin and Range, resulted in widespread volcanism, created the San Andreas Fault system, and resulted in capture and NW translation of parts of the margin of the continent by the Pacific Plate. These tectonic conditions persist into the present.