MANTLE BUOYANCY-DRIVEN DIFFERENTIAL INCISION OF THE GRAND CANYON

Despite decades of study, the processes controlling Grand Canyon’s incision continue to be debated. The Colorado River’s profile is the product of tectonics, which created relief, and erosion which has modulated topography; however, rates and timing of uplift and erosion are often difficult to determine. In Grand Canyon, this is a result of a lack of full understanding about the processes controlling intracratonic uplift and deformation through time and an inability to quantify the factors controlling erosion. We present new incision rates through time at four key locations in Grand Canyon, based on U/Th dating of travertine in stacked strath terraces and cosmogenic burial dating, with the goal of better understanding the processes modulating incision. Each locality shows temporally steady incision, but rates increase from 101 ± 3 m/Ma in western Grand Canyon (over the last 4 Ma), to 97 +77 -30 m/Ma in central Grand Canyon (over the last 1.5 Ma), to 160 ± 8 m/Ma in eastern Grand Canyon (over the last 600 ka). Steady incision at each site precludes migration of transient knickzones through eastern Grand Canyon in the last 0.6 Ma and western Grand Canyon in the last 4 Ma. We interpret the west-to-east increase in temporally steady bedrock incision to be due to differential mantle-driven uplift. Other potential controls on incision rates such as climate oscillations, bedrock strength, sediment load effects, base-level fall or isostatic response to differential denudation are unable to explain the spatial and temporal patterns of incision. The highest incision rates (eastern Grand Canyon and in the upper Virgin River) overlie one of the largest mantle-velocity gradients in the western United States and lower incision rates farther west overlie uniformly low velocity mantle velocities similar to that of the Basin and Range province. We interpret the higher rates to reflect mantle-driven uplift associated the sharp gradient in mantle velocity rather than low velocity mantle itself. This zone may mark the locus of ongoing mantle tectonism (convective heating, melt transfer, small-scale convection, and lithospheric removal) and resulting increased mantle buoyancy that is tracking the Miocene to present propagation of basaltic volcanism from the margins of the western Colorado Plateau toward its center.