The Colorado Plateau (CP) is a relatively coherent block surrounded on three sides by the extensional block faulted regime of the Basin and Range Province (BRP) and the Rio Grande Rift (RGR). The CP appears to be part of an inter-related system including the Sierra Nevada, BRP, and RGR which has undergone major uplift and extension during the last 20 m.y. The final elevation in any area probably depends upon which processes dominate there. In most geophysical properties the CP is intermediate between the BRP/RGR and the stable platform of the southern Great Plains. However, many BRP/RGR geophysical characteristics appear to extend well inward of the classical Plateau physiographic boundary. Geologically these 50-100 km wide zones of transition are marked by normal faulting and late Tertiary and Quaternary volcanism. The interior of the Plateau is characterized by a 40 km-thick crust, a Pn velocity of about 7.85 km/sec, and an average heat flow of 1.5-1.6 HFU. Available data on the modern stress field in the Plateau interior indicate high horizontal stresses and a stress field oriented differently from that in the surrounding BRP/RGR, inconsistent with the theory that the Plateau is merely an inherited, more coherent subplate subjected to the same stresses as its surroundings. Free-air gravity anomalies on the CP average near zero and imply isostatic equilibrium; however, crustal thickness is insufficient to explain all the elevation, and low density mantle material must be involved. P wave velocity, gravity, and electrical data are compatible with a depth of approximately 80 km to low density, low velocity, hot conductive mantle (= asthenopshere?). These data also suggest a lithospheric thickness of ~120 km for the southern Great Plains, while surface wave data for the BRP suggest an approximately 60 km-thick lithosphere. Low angle subduction in mid-Cenozoic time has been inferred from calc-alkalic magmatic activity throughout a broad region of the western U.S. Subsequent restriction of this activity to the Sierras and westernmost BRP suggest an abrupt switch to steep subduction which occurred by about 20 m.y. ago, probably with rupturing of the low angle slab. Gradual warming and expansion (with phase changes) of the cutoff stagnant shallow slab is suggested as a possible mechanism for regional uplift. © 1979.
