THE GEOMORPHIC SIGNATURE OF MANTLE FLOW: A ‘GREAT’ CHALLENGE FOR TECTONICS (Invited Presentation)

Differential vertical motion of the Earth’s surface in response to differences in mantle buoyancy and flow generates long-wavelength, low-amplitude topographic gradients which drive erosion and deposition over timescales of millions of years. Disentangling the signals of dynamic topography in erosional landscapes from the confounding effects of variations in rock mass strength, consequent differences in grain size, and the variability of discharge through time remains a first-order problem. Here, we explore the question of whether changes in mantle buoyancy or flow through time could have driven differential rock uplift in two examples, the Rocky Mountains of Colorado and the Appalachians along the eastern seaboard of North America. In each site, we show how integration of stream profile analysis with measures of erosion/incision rate yield insight into the evolution of topography through time. Along the western slope of the Colorado Rockies, north-south variations in channel steepness co-vary with the position of a low-velocity seismic anomaly, but do not appear to depend on variations in substrate. New estimates of the onset of incision along the main stem of the Colorado River suggest that rapid incision occurred between 8 – 6 Ma, prior to integration of the lower watershed. In the Appalachians, we develop a set of empirical calibrations for the scaling of channel steepness to basin-wide erosion rate. In conjunction with an analysis of channel topography, we develop a model for the timescales associated with the transient response of channel networks to changes in mantle flow or buoyancy over the past 15 Ma. Drawing on these examples, we conclude that signals of mantle flow are encoded in the topography of erosional landscapes, but that climate, rock strength and network reorganization strongly filter landscape response.