GEOCHEMICAL AND GEOCHRONOLOGIC EVIDENCE FOR UPWELLING MANTLE ASTHENOSPHERE UNDER THE COLORADO PLATEAU: MORE SUPPORT FOR NEOGENE UPLIFT

Recent models hypothesizing ~600 m of Neogene and ongoing uplift of the Colorado Plateau, due to upper-mantle convection, can be tested by examining the geochemistry and geochronology of young basalts as a record of lithosphere-asthenosphere interactions. Basaltic lavas are important probes because they are extracted from discrete depth ranges, can be dated directly, and have isotopic signatures that can identify their mantle source reservoirs. Thus by combining geochemical data from basalts and geophysical images of the mantle we can resolve whether low velocity mantle regions are depleted mantle asthenosphere or partially melted enriched lithosphere and track the lithosphere-asthenosphere boundary through time.

Geochronologic data from the southern margins of the Colorado Plateau (CP) show an inboard radial migration of Neogene basaltic magmatism, at rates of up to 8 km/my. Nd and Sr isotopic data show that as basaltic volcanism migrates inboard it also becomes increasingly more asthenospheric. For example, in western Grand Canyon ε_Nd values increase at a rate of 0.3 ε units/my. Strongly asthenospheric alkali basalt (ε_Nd > 4) appears on the western (W) plateau margin at about 5 Ma, on the southeasterm (SE) margin at 7 Ma, and is lacking from the plateau’s other margins. Tomographic data suggest that low velocity mantle underlies almost all recent (<1 Ma) basaltic volcanism in a ring around much of the CP at a depth of 80 km. The combined isotopic and tomographic data indicate that the low velocity mantle is asthenosphere in the W and SE, but modified lithosphere around the remaining margins. Temporal and spatial patterns suggest a process by which upwelling asthenosphere is progressively infiltrating and replacing lithospheric mantle especially where Proterozoic boundaries exist. This model explains the dramatic velocity contrast seen well inboard of the physiographic edge of the plateau, the inboard sweep of Neogene magmatism, the isotopic evidence that much (but not all) of the low velocity mantle is asthenospheric, and supports geodynamic models that ongoing uplift of the edges of the CP is driven by upwelling asthenospheric mantle.