Tectonics of Western North America -- Narrative

Tectonic Evolution of Western North America
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Narrative of Hypothesized Events

These cartoons illustrate an hypothesized sequence of events in Western North America's recent geologic history. Interpret them with a relaxed mind and several grains of salt. (Preferably on the rim of a Margarita.) Inevitably, they are incomplete and distorted. I intend them to illustrate ideas, not exact details.

Main ideas:

North America overrode the Pacific spreading center about 30 million years ago, and continued moving west relative to the zone of upwelling mantle which drove the spreading center. That upwelling zone is still active beneath the North American continent.
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Thin crust of the Basin and Range province fractured as it passed over the ridge of upwelling mantle. Fractures and block axes trend north-south, following the mantle ridge axis.
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The Basin and Range formed in two phases: first, it fractured as described above; later, east-west tensional forces released confining pressure on the fractured blocks, allowing them to rotate into today's ranges and valleys. Extension alone is not responsible for Basin and Range fracturing.
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Thicker crust of the Colorado Plateau and the Rockies platform now sits above the upwelling zone, which has uplifted them.
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Where Basin and Range faulting terminates northward, detachment of Basin and Range blocks from unfractured northerly crust may have resulted in release of Columbia Plateau flood basalts. The same may also be true at the southern edge of the Colorado Plateau.
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Apparent tracks of hot spots leading to Yellowstone and northern New Mexico do not reflect discrete mantle plumes. Instead -- occurring at rifts and transitions between fractured and unfractured crust -- they outline passage of North America over the mantle ridge.
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East-west bands of disruption in coastal continental crust align with major oceanic fracture zones. These bands locate critical points in the San Andreas fault, and major coastal promontories.
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The San Andreas fault began to form about 10 million years ago at the mouth of the Gulf of California, propagating northward to its termination at Cape Mendocino. The fault steps westward at a band of disrupted crust south of the Sierra Nevada, and again at Cape Mendocino.
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Total northwestward displacement on the San Andreas fault equals displacement of Baja California from its origin in Mexico. "Exotic" blocks on the Pacific side of the San Andreas fault are fragments of northern Baja California or the southern Sierra Nevada.
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Northwestward movement on the San Andreas fault since about 10 million years ago has exerted tensional force on the Basin and Range, releasing previously fractured blocks.
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It appears that Baja California and the San Andreas fault lie on the eastern edge of a slab of oceanic crust being driven northward relative to both North America and the remainder of the Pacific Plate.

San Andreas Fault and Associated Features	.		Basin and Range, Colorado Plateau

		Until approximately 30 million years ago (MYA), the Pacific Spreading Center operated as a continuous north-south zone, creating crust of the Pacific Plate (moving west) and the Farallon Plate (moving east). The oceanic Farallon Plate subducted under the western margin of the continental North American Plate, creating a coastal volcanic/intrusive belt similar to today's Andes or Cascades, wherein solidified granites of the Sierra Nevada, Klamath, and Peninsular Ranges. Where major oceanic fracture zones intersected the North American coast, they propagated into -- and weakened -- continental crust.	.

		Approximately 30 MYA, the North American Plate began to overtake segments of the Pacific Spreading Center. First contact probably occurred near today's Cape Mendocino or Point Arena, California.	.

		Bounded north and south by east-trending strips of weakened crust (due to fracture zones), the northern Sierra Nevada block was pushed eastward by contact with a segment of Pacific Plate between the Mendocino and Murray fracture zones.	.

		By approximately 25 MYA, North America overrode most Farallon Plate crust between the Mendocino and Murray fracture zones, although spreading centers in the southern part of this region were active later than this time. South of the Murray fracture zone, crust generation and subduction continued into Miocene time, and farther south into recent time.	.

.		Between 25 and 10 MYA, as the North American Plate passed over the former Pacific Spreading Center, relatively thin crust of the Basin and Range was uplifted by upwelling mantle, and fractured along north-south trends. Basin and Range fracturing ends northward in an area covered by flood basalts.

		As North America progressively overrode more segments of the Pacific Spreading Center, subduction of oceanic crust under North America diminished; the North American and Pacific Plates coupled and moved westward together. Approaching 10 MYA, the triple junction of North American, Pacific, and Farallon Plates had moved south near Jalisco, Mexico. Western North America passed westward over the presumed stationary mantle upwelling which drives the Pacific Spreading Center.	.

.		Upwelling along a generally north-south line, the Pacific Spreading Center's mantle ridge uplifted and fractured Basin and Range crust progressively from west to east as the North American Plate moved west.	$Uplift and fracturing of the Basin and Range$

.		As the edge of North America passed over the Pacific Spreading Center, strong granites of the Sierra Nevada lifted as a block, but did not fracture like thinner Basin and Range crust.

.		The Sierra Nevada remained tilted westward as North America continued west over mantle upwelling of the Pacific Spreading Center. No longer dragged downward by the subducting Farallon Plate, the edge of North America rebounded isostatically, lifting the Coast Ranges. As thinner crust east of the Sierra Nevada passed over the zone of upwelling mantle, the crust broke into north-trending blocks.	$Progressive fracturing of the Basin and Range$

		Approximately 10 MYA, the San Andreas fault began to form as North America's coast passed over the Pacific Spreading Center near Jalisco, Mexico, where North American continental crust was weakened by the Clarion fracture zone.	.

		Propagating along the east side of the Peninsular Ranges, the San Andreas fault moved north. Reaching the southern boundary of the Sierra Nevada block, the San Andreas fault stepped westward at a zone of crust weakened by the Murray fracture zone. West of the Sierra Nevada, the fault propagated north until reaching a similar zone of crust weakened by the Mendocino fracture zone, where the fault again stepped westward. [Although this cartoon shows the entire course of the San Andreas fault -- implying a fully formed fault -- the fault probably formed progressively as the Gulf of California opened and Baja California moved north.]	.

		At the southern terminus of the San Andreas fault, spreading centers formed; southern Baja California began to move north, opening the Gulf of California. The northern end of southern Baja California sheared west at a zone of crust weakened by the Molokai/Shirley fracture zone, forming a westward-pointing cape.	.

.		When thicker crust of the Colorado Plateau and the Rockies platform reached the zone of upwelling mantle (approximately 10 MYA?), it responded by uplifting rather than fracturing.

		Between 10 MYA and recent time, rifts have continued to open in the Gulf of California. Baja California has moved northwest about 500 km, bending the Sierra Nevada's southern tail westward. Baja California's northern tip has fractured and sheared toward the west, forming the San Gabriel mountains. Fragments of Baja California have traveled north along the San Andreas fault as exotic blocks such as the Salinian Block. Pushed northwest by Baja California, the Sierra Nevada block has distorted the Klamath block, and opened a gap between the Sierra Nevada and Klamath mountains at the Central Valley's northern end.	.

.		Northwestward movement of Baja California, with accompanying distortion of the Sierra Nevada and Klamath blocks, exerted tensional stress on the Basin and Range province between 10 MYA and recent time. Twisted by northward pressure, the Sierra Nevada tilted more to the west. Released from horizontal confining forces, previously fractured Basin and Range blocks rotated to form ranges and valleys. At the northern extreme of the Basin and Range, Columbia Plateau flood basalts were released, possibly where Basin and Range blocks detached from relatively unfractured crust.