I say again: quite possibly, none of this is true. I would be quite surprised if it is all true.
Virtues of this model:
If true, this model relates many recent geologic events and features:
Rates of Plate Movement; Timing of Events
For westward movement of the North American and Pacific Plates, I have estimated a value of 5 cm/yr. This is a derived rate; that is, the approximate mean rate needed to move the North American plate surface from the California coast to the Colorado Plateau across an assumed stationary zone of upwelling mantle in a period of 30 million years. 5 cm/yr is a reasonable value in the context of crustal plate velocities; however, its use is based on necessity, not observation.
Relative movement of two plates at a point on a plate boundary should equal the vector difference of the plates' individual motions at that point. Because plates may deform internally, all points on a plate may not have the same motion; however, intra-plate movement should be small relative to inter-plate movement.
Estimates of plate velocities vary. According to one current estimate, the Pacific Plate moves about 40 degrees west of north at a speed of about 10 centimeters/year, while the North American Plate moves slightly south of west at about 2 centimeters/year. Based on these estimates, I calculate that the Pacific Plate should be moving westward at about 4-1/2 centimeters/year and northward at about 8 centimeters/year relative to the North American Plate. In other units, the Pacific Plate -- including Hawaii -- should be moving westward away from North America at approximately 2 inches/year, while sliding relatively northward at nearly double that rate. However, displacement on this scale does not seem to be taking place, either as movement on the San Andreas fault system or as deformation in the Basin and Range. Motion of the Pacific Plate probably is well established relative to the assumed stationary Hawaii plume; therefore it is reasonable to suspect that our estimate of North American Plate motion is in error. Although I have not yet found a definitive reference, I suspect that estimates of North American Plate velocity may be based in part on evidence of perceived hot spot tracks approaching Yellowstone and northern New Mexico.
Approximate timing implied by this model:
What's driving this?
I don't know.
My current best guess is that Baja California is not being driven north by spreading centers in the Gulf of California. Rather, these spreading centers are opportunistic tensional features, resulting from rather than causing Baja California's movement.
Also, I don't think that the entire Pacific Plate is moving northwest relative to the North American Plate at the speed of displacement on the San Andreas fault. I suspect instead that an area of Pacific Plate crust extending 16-20 degrees west of California's coast is being pushed north, including Baja California. What's pushing? Good question. Presumably (an)other spreading center(s). Much of the southern hemisphere seems to be pushing north, causing earthquakes in Turkey and volcanoes in Italy, and narrowing both the Mediterranean and the Caribbean, as evidenced by the S-shaped Central American isthmus and arcuate (concave north) fracture zones south of the Gulf of California's mouth.
Probably, plates move because of viscous drag against the plate underside, caused by horizontal flow of mantle away from upwelling zones. Therefore generally it is misleading to speak of spreading centers "pushing" a plate. Also, oceanic crust probably is too thin to transmit tectonic-scale compressive forces over long distances.
Problems with this model:
Current estimates of rate of movement of the North American Plate do not match the rate required. See above.
The Hawaii/Emperor hot spot track shows no clear record of velocity change around 30 MYA.
If the mantle source for the former Pacific spreading center now lies under the Colorado Plateau, then mantle to feed the Juan de Fuca and Gorda spreading centers appears to be flowing laterally westward under the continental crust of Montana, Idaho, Washington, and Oregon.
Why did Basin and Range faulting terminate northward in Oregon and Idaho?
It is suspiciously convenient that the proposed confining pressure on Basin and Range blocks was just sufficient to prevent large-scale eruptions, yet not so great as to buckle the lateral "stack" of already-broken blocks.
Miscellaneous Notes:
Estimates of the lateral (east-west) magnitude of Basin and Range extension vary from 50 to 200 miles. I suspect that truth probably lies closer to the lower estimate, because I think that large extension ratios (estimated near 100% by some authors) would separate Basin and Range blocks sufficiently to cause massive and ongoing volcanic activity, perhaps on the scale of Columbia flood basalts. I suspect that Columbia-style flood volcanism did not occur because Basin and Range blocks lie snugly against each other.
Why is simple extension not a likely cause for Basin and Range faulting? Because the Basin and Range has too many faults. Cold rock rarely stretches. When a brittle material such as rock fails in tension, it is likely to fail at one major break, not in a multitude of parallel breaks. Shear can produce parallel faults en echelon; however, shear of such magnitude and direction does not appear to be operating in the Basin and Range.
Pinched between Baja California and the Sierra Nevada, a westward-pointing wedge of territory (I'll call it the Mojave Triangle) appears to have slid northward as Baja California has pushed against the Sierra Nevada's southern tail. Bounded southward by the San Andreas fault and northward by the Garlock fault, this region does exhibit something like en echelon faults oriented northwest-southeast, parallel to its eastern boundary. Anatolia, in Turkey, shows a somewhat similar tectonic structure where northward movement of the Arabian Plate is compressing Anatolia against the Eurasian Plate. Whereas Anatolia is squeezing westward like a watermelon seed pinched between God's fingers, the Mojave Triangle -- buttressed eastward by North America -- grinds north while Baja California and the Sierra Nevada slide westward around it. Perhaps the Mojave Triangle acted as a north-moving wedge to peel the Sierra Nevada westward, or perhaps it moved north opportunistically following the Sierra Nevada's westward deflection. The region just north of the Mojave Triangle probably has experienced greater extension and tectonic dislocation than most other parts of the Basin and Range. (This region encompasses both the highest and lowest points in the 48 contiguous states.) The Garlock fault may have begun as the continental trace of the Murray fracture zone.
Without question:
Questions and avenues of research:
On closer study, how well does event timing fit the model?
If we accept that Yellowstone and New Mexico "hot spots" may not indicate discrete mantle plumes, and discard their evidence of North American Plate velocity, then do other indicators of velocity -- relative to plumes and spreading centers -- suggest that North America has moved west at about 5 cm/yr?
Total displacement along the San Andreas fault should approximately equal the displacement of Baja California from Mexico, taking into consideration Mexico's probable distortion.
The Salinian block should prove not a southern exotic, but a fragment of the northern Peninsular Ranges. Paleomagnetic latitude of Salinian granite should be only a little farther south than Sierra Nevada granite, and north of the Peninsular Ranges. If all western granites formed close together well south of their current latitude, this implies that all moved north together as part of the North American continent.
Northward termination of Basin and Range faulting may relate to release of Columbia Plateau flood basalts, possibly because faults could not propagate into the Plateau's thicker crust, causing Basin and Range blocks to separate from the Plateau. Is crust thicker north of Basin and Range? Does thicker crust also permit lateral westward flow of magma from the upwelling zone to the Gorda / Juan de Fuca spreading centers?
What is the Snake River Plain, if not a hot spot track? Probably a rift propagating northeast. What kind of rift, and how formed? Possibly:
How do events in this model relate to the record of Pacific Plate movement shown in the Hawaii/Emperor hot spot track? When North America overrode the Pacific spreading center, how did this event affect Pacific Plate velocity? Little velocity change might be expected if the North American Plate had about the same westward velocity as the Pacific Plate, resulting in a "soft" collision.
Some authors suggest that Basin-and-Range-type fracturing and deformation differ in style and timing north vs. south of the Murray Fracture Zone (Mojave / Los Angeles). I propose that:
Other Wild Ideas:
In a closed convection system, what flows up must flow down. If hot convecting mantle upwells at a spreading center, then somewhere between two spreading centers, cooler mantle descends. If we accept the above proposition that the Pacific Spreading Center's upwelling zone lies under the Colorado Plateau, then where is the downflow zone between that upwelling and the Mid-Atlantic Ridge? An eyeball estimate suggests that the downflow zone lies beneath the eastern United States, possibly under the Appalachians.
If an upwelling zone lifts crust, does a downflow zone tend to depress crust above it? Does downflow under oceanic crust create a likely zone of downward buckling, and a starting point for subduction? This line of speculation -- admittedly tenuous -- suggests that closure of the Atlantic may begin when the North America moves farther west so that the between-ridge downflow zone lies offshore east of the southeastern United States' Atlantic continental shelf. Look for volcanos in Florida within 20 million years.