Abstract

Interpretation of the geophysics, petrology, and structure of the Colorado Plateau indicates that it is a Theologically distinct element of the Cordillera. During the Phanerozoic Era, areas surrounding the Colorado Plateau have been subjected to repeated tectonism, including contraction, extension, and magmatism while the plateau has been little affected by these processes. Deformation and magmatism mostly wrap around the Colorado Plateau, suggesting the plateau is a rigid body that has often transmitted forces across itself. Compiled geophysical and petrologic evidence indicates that the lithosphere of the Colorado Plateau has a higher strength than regions to the east, south and west. Strength differences may be attributed to a mafic crustal composition and long-term lower crust and mantle geothermal gradients, especially relative to the Basin and Range Province. Estimates of crustal and lithospheric thickness indicate that the ratio of the thickness of mantle to crust in the northern Basin and Range Province ranges from 0.8 to 1, whereas the same value in the Colorado Plateau is about 1.2. Given that mantle rocks are stronger on average than crustal rocks, the ratio of crust to mantle and the greater total thickness of the Colorado Plateau lithosphere also make it inherently strong.

Some evidence suggests that fertile or hydrated mantle may exist beneath the Colorado Plateau. Rock strength data show that mafic rocks in the crust and high pyroxene and amphibole contents in the upper mantle may enhance lithospheric strength, or, at a minimum, provide no reason to presuppose that a fertile, hydrated mantle should be weak.

The Colorado Plateau, although inferred to be stronger than regions to the west, south, and east, may not be as strong as the Archean Wyoming Province to the north. Higher seismic velocities in the lower crust and upper mantle north of the Cheyenne belt imply a higher strength in the middle Rocky Mountains. In this context, the Uinta aulacogen, which separates the Wyoming Province and the Colorado Plateau, developed as a “pop up” structure during Mesozoic to early Tertiary contraction. Thus, in understanding the tectonic history of the western U.S., or any region for that matter, it is important to assess the relative compositional and thermal structure of the lower crust and upper mantle. These factors have exerted considerable control over the partitioning of strain and magmatism throughout the Cordillera in the Colorado Plateau region during the last 1 Ga. Similar factors play an important role in the architecture of mountain systems throughout the world.