DEFINING THE GEOLOGIC AND TECTONIC COLORADO PLATEAU IN SPACE AND TIME BY EXTENSION

Fenneman (1916) defined the Colorado Plateau (CP) as one of three Intermontane Plateau physiographic provinces in the United States. He called the province the Colorado Plateaus and subdivided it into six sub-provinces. Although Fenneman and other pioneers of Western US geology made remarks on the apparent youth of the topography and uplift of the province, it was defined by physiography, not geology. During the past several decades, geophysical characteristics of the CP lithosphere, and its apparent lack of deformation with respect to adjacent provinces, have led many researchers, including me, to speculate that the pre-Cretaceous CP lithosphere was different from adjacent lithosphere; i.e., that there was a pre-Cretaceous precursor in the lithosphere that preserved the CP from major deformation and perhaps was a factor in its uplift. As reported by Vince Matthews (Whither the Rio Grande Rift? 2013), however, the geologic/tectonic margins of the CP were established by Neogene extension. From a review of the Phanerozoic geologic history of the CP region the following conclusions may be made: 1) The geologic/tectonic Colorado Plateau (gtCP) is larger than the physiographic CP and its margins are defined by extensional tectonics; 2) The gtCP only became a distinct province after being separated from adjacent provinces by Neogene extensional tectonics; 3) Early to mid-Tertiary magmatic activity and Laramide deformation were not limited by the margins of the gtCP; 3) Thickening of the crust/lithosphere during Cretaceous compressional events and possibly heating by Paleogene magmatic activity may have resulted in regional foci for later Basin and Range and Rio Grande rift extension; and 4) There were no pre-Cretaceous tectonic or magmatic precursors to the margins of the gtCP. Prior to Rio Grande rift extension, a large area, including the CP, southern Rocky Mountains and westernmost Great Plains, could have been considered to be a single geologic/tectonic province characterized by Laramide deformation of varying degrees of intensity.