New Mexico Geological Society Annual Spring Meeting — Abstracts

Post-Laramide rock uplift in western United States has been the subject of a long-standing debate. Central to the debate are the following questions: has the West experienced rock uplift, where have rocks come up and what is nature of the uplift mechanism? Workers speculate that the rugged, high topography of the Rocky Mountains reflects late Cenozoic rock uplift, generated perhaps in part by active mantle processes. Recent teleseismic data suggest that plumes oflow velocity, dynamic mantle currently buoy an almost uniformly over-thickened crust. Apatite fission-track cooling histories of various mountain chains seem to corroborate this story. Paleobotanical data, however, indicate that the Laramide Rocky Mountains stood at equal or greater elevations than the modern Rockies. These high elevations have been attributed to up basin hydrological shifts such as those that accompany global climate change and glacial-interglacial cycles. While the presence of deeply entrenched river systems of the Colorado Plateau, Great Plains and Basin and Range can be used to support either scenario; we believe that geomorphic evidence found in a major river system on the high plateaus of the western Great Plains argues for late Cenozoic mantle-driven, epeirogenic rock uplift.

The Jemez Lineament is one of many distinct zones of high heat flow and prolific late Cenozoic volcanism identified as 'fingers' of low velocity mantle that extend beneath parts ofthe western United States. Regional-scale warping of the Tertiary Ogallala formation implies Neogene-Quaternary deformation across the Lineament. The Canadian River canyon, northeastern New Mexico, straddles this crust-penetrating structure and cuts through the warped Ogallala cap rock. A broad convexity in the longitudinal profile of the Canadian where it traverses the Jemez Lineament provides further evidence for rock uplift. No such broad convexities exist in the profiles of other major river systems (e.g. Arkansas River, South Platte River, and the Pecos River) which drain the eastern flank of the southern Rockies. Fluvial stratigraphy of the Canadian River canyon is temporally constrained by ⁴⁰Ar/³⁹Ar dating of a basalt flow within the canyon and by radiocarbon dating of organic material found in the terrace deposits. Numerical age dates yield uniform incision rates over the past 1.5 Ma. These uniform rates coupled with nearly parallel terrace profiles suggest that incision is controlled by relatively uniform, mantle-driven rock uplift along the Jemez Lineament and not by hydrologic shifts linked to climate change. These combined data provide strong evidence for late Cenozoic epeirogenic rock uplift associated with localized mantle anomalies. Rock uplift in the western United States, therefore, may be localized to regions where the crust is underlain by buoyant mantle, but not necessarily associated with areas of high elevation and high relief.