Based on AAPG's Tectonic Map of North America (1992)
and other sources
C. G. Cochran
December, 1997
To smooth this narrative, I have phrased it with indefensible certainty. I should precede everything that follows with "it appears that" or "probably":
As other authors propose, volcanic activity in the Yellowstone area and in New Mexico, and elevation of the Colorado Plateau, are effects of mantle upwelling. However, Yellowstone and New Mexico are not independent hot-spots; instead, they are expressions of a north-trending curtain of upwelling mantle rising beneath -- and uplifting -- the Colorado Plateau. This upwelling once drove the Pacific spreading center. The western coast of North America overrode the Pacific spreading center about 30 million years ago. Since that time, North America has continued moving west at approximately two inches (five centimeters) per year. The Pacific upwelling zone has passed under North America from its west coast to the Colorado Plateau, tilting the Sierra Nevada block, and progressively uplifting and fracturing the Basin and Range in subparallel north-trending breaks. Later tectonic events (see below) drew this broken terrain into its current form. Because crust under the Colorado Plateau is thicker than crust under the Basin and Range, the Colorado Plateau lifted instead of fracturing.
Ocean crust age patterns show that the Pacific spreading center segment between the Mendocino and Murray fracture zones lay farther east than segments north and south of this band. In consequence, the North American coast first contacted the Pacific spreading center between the Mendocino and Murray fracture zones, probably near Point Arena. This advance contact, or the forces producing eastward offset of the Mendocino-Murray spreading center segment, displaced the Sierra Nevada block eastward relative to the Klamath and Peninsular blocks. The Sierra Nevada block moved independently of neighboring blocks because it is bounded north and south by east-trending strips of crust weakened by the Mendocino and Murray fracture zones. A wide zone of crust at the Murray break may have been disrupted further by northward displacement of the Murray fracture zone during the period when North America approached the Pacific spreading center, possibly beginning as early as late Cretaceous time.
In late Miocene time (ca. 10 million years ago?), the Gulf of California began to open near Jalisco, Mexico, when westward movement of North America carried a weak zone in coastal crust over the Pacific spreading center. Continental crust in this area was disrupted by the Clarion fracture zone. The Gulf of California rift propagated northwest through the continent's edge, east of the Peninsular Ranges batholiths, until reaching an east-trending band of continental crust weakened by the Murray fracture zone. The rift stepped westward at the Murray break, passing west of the displaced Sierra Nevada block to become today's San Andreas fault. When the rift reached the Mendocino fracture zone, it stepped westward again; it extinguishes in a skewed subduction zone off Cape Mendocino. Spreading centers in the Gulf of California drove Baja California northwest, fracturing its northern end at the Murray break, and transporting fragments northward as the Salinian block and others. Many block fragments never reach Mendocino; they are carried seaward on splay faults between the Murray break and Point Arena. The slab of crust carrying Baja California has moved northwest relative to both North America and the Pacific plate, taking Baja California about 500 kilometers from its original location.
Pushing northward against the Sierra Nevada block at the Murray break, Baja California bent the Sierra Nevada's southern "tail" westward. The east-shifted Sierra Nevada block pushed northward against the Klamath block, distorting the Klamath block and displacing it westward. Northwestward movement of the Sierra Nevada and Klamath blocks applied tensional stress to the Basin and Range, allowing previously fractured blocks (see above) to drop and rotate into north-trending ranges and valleys. North of the Basin and Range, deep fractures released Columbia flood basalts.
Similarly fractured, much of Mexico has been displaced eastward by tectonic thrust from the Pacific. South of the Gulf of California, arcuate transform faults show that southern Mexico has rotated north and east(counterclockwise) since Miocene time, implying that Yucatan lay south and west of its current position when the Chicxulub object fell. The sigmoid shape of Central America's isthmus suggests that South America has moved north since the isthmus formed, narrowing the Caribbean.
I did not originally write this for WWW publication, so it has no links or visuals. I'll work on it.
Caveat lector. I don't think you will find this theory in professional papers or textbooks; this is my own speculative product. If you quote this, real geologists will probably laugh at you.