Isostatic surface uplift of large continental regions lacking deformation remains largely unexplained. Evidence from the eastern parts of the Cordilleran orogen in the western United States suggests that increased buoyancy in the lower crust supports the elevations of the High Plains and Wyoming craton. We suggest that hydration of the lower crust associated with the Laramide orogeny produced surface uplift by replacing dense mineral phases such as garnet with less dense phases such as amphibole and mica. Seismic and petrologic evidence from Wyoming and Montana is consistent with such changes. Comparable hydration in the Colorado Plateau is dated to the early Tertiary. Beyond establishing a newly recognized mechanism for broad continental uplift, such hydration suggests that interactions of subduction-derived fluids and the lithosphere can be more profound than previously envisioned.

No abstract

Subduction at plate boundaries can have thermal, chemical, and physical impacts on broad regions of the continental interior, but these interactions are not as readily obvious as deformation near the continental margin. Such cryptic alteration has produced surface uplift in the Colorado Plateau and western Great Plains of North America, which have risen-largely undeformed-1.6 and 1.3 km, respectively, relative to the eastern Great Plains during the Cenozoic. Accumulation of Cretaceous-Cenozoic sediments accounts for only 300 m of uplift of the Colorado Plateau and 400 m of the western Great Plains, leaving 1.3 km and 0.9 km, respectively, unexplained. To determine the physical causes of this enigmatic epeirogeny, we derived three-dimensional (3-D) lithospheric density models from seismic velocity, gravity, topography, and heatflow data. Lower-crustal density decreases systematically westward across the Great Plains, accounting nearly perfectly for the remaining 900 m of uplift of the western Great Plains and the modern east-west topographic gradient. Lower-crustal dedensification beneath the Colorado Plateau accounts for a similar 900 m of uplift. Lower-crustal xenoliths in both regions show progressive hydration-induced retrogression of garnet-bearing assemblages with increasing modern elevation, and Th-Pb dating of the Colorado Plateau retrogression gives end-Cretaceous dates (xenoliths from the Great Plains have not yet been dated). We hypothesize that lower-crustal density variations-and much of the surface relief-in North America's Proterozoic interior terranes reflect varying degrees of metasomatic retrogression, such as by fluids exsolved from the Farallon slab. The remaining 400 m of Colorado Plateau uplift is most plausibly due to elevated mantle temperature. We present thermal models that suggest that 25-70 km of Cenozoic lithospheric thinning can explain the modern elevation and density structure.

Formation and subsequent modification processes of lower crust play important roles in evolution and rheological models for continental lithosphere. In the last two decades, numerous xenolith studies have documented metasomatism of continental mantle in the Rocky Mountain region of North America. However, much less attention has been paid to whether and to what extent these processes may have affected the crust. We address the nature and timing of hydrous alteration of extant deep Proterozoic crust in the Colorado Plateau through a petrological and geochronological study of a xenolith from the Red Mesa diatreme in the 30-20 Ma Navajo volcanic field. Fluid-related alteration is widespread in sample RM-21, with the main features recorded by breakdown effects in feldspar, garnet, and allanite, and the production of secondary assemblages characterized by lower Ca plagioclase (or albite) + muscovite + biotite + calcite + monazite ± zoisite at estimated conditions of 0.8-0.7 GPa (or ~27 km depth) and 480 °C. Th-Pb dating by ion probe reveals a period of monazite crystallization at 70-65 Ma, interpreted to reflect late alteration of the high-temperature metamorphic assemblage with hydrous fluid introduced by the shallowly subducting Farallon slab. Similar to previous suggestions involving mantle hydration, the growth of low-density hydrous phases at the expense of high-density, anhydrous minerals, which are abundant in unaltered Proterozoic crust, if sufficiently widespread, could have contributed to elevated topography in the Colorado Plateau and regionally across the Rocky Mountains and High Plains.