Abstract

The Paleoproterozoic lithosphere of the region that includes the Colorado Plateau (CP) and the Southern Rocky Mountains (SRM) does not appear to have had an inherent Proterozoic structural demarcation of the CP and SRM. Following its accretion to the Wyoming craton, it had relative periods of stability and sedimentation, followed by short periods of widespread faulting and disruption of the sedimentary rocks previously deposited. First demarcation of the SRM was during late Paleozoic deformation of the Ancestral Rocky Mountains in which deformation was focused in Texas, Oklahoma, and probably the SRM by plate boundary forces. When more regional stresses of the Cordilleran orogenies compressed the region, the SRM appear to have been reactivated because their lithosphere was weak rather than because the CP lithosphere was strong, and thus the CP and SRM were delineated by events rather than by inherited lithospheric properties. The late Mesozoic and Cenozoic uplift and erosion histories of the CP and SRM are difficult to constrain: Both were submerged or were close to sea level by a Late Cretaceous high sea level stand just prior to Laramide deformation, and we know their present elevations. There is little consensus on elevations between these two end points. The SRM were uplifted during Laramide deformation, but the elevation of a late Eocene erosion surface in the region is less certain. Evidence for a component of late Cenozoic uplift of the SRM is provided by geophysical data that indicate modern crustal and upper mantle anomalies consistent with the cause of recent uplift. Data concerning uplift age(s) for the CP are currently under debate, but xenolith data provide useful constraints on models of uplift. These data preclude any model that requires significant heating or replacement of the mantle lithosphere above a depth of 140 km by mechanisms such as low-angle subducting slabs, delamination, or asthenospheric heating, prior to ~25 Ma, and any model of complete replacement of the mantle lithosphere beneath the CP prior to as recently as ~1 Ma. Mid-crustal flow models are precluded primarily on calculations of cooling with reasonable CP geotherms and arguments based on uniformitarianism. A new paleo-altimeter, based on basalt vesicle size, indicates about 1 km of uplift during the past 5 Ma, a number consistent with previous geological estimates. The preferred mechanism for uplift, consistent with the xenolith data, and the new basalt paleo-altimeter data, is a phase change from dense eclogite to less-dense garnet granulite in mafic rock close to the crust-mantle boundary. The model is consistent also with the relatively thick crust and gradational Moho measured by seismic experiments in the southwestern CP.