Decoding the record of uplift and erosion in landscape topography
About the Event:
The recognition that mass redistribution by erosion governs the geodynamics of active collisional mountain belts is arguably one of the more transformative hypotheses in the geosciences over the past few decades. The community has made significant progress in understanding how the dependence of erosional and transport processes on topographic gradient ultimately dictates the adjustment of landscape relief to uplift and erosion rates. However, disentangling the signals of uplift 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, I briefly review the current understanding of how channel longitudinal profiles adjust to both spatial and temporal variations in rock uplift rate and show how this understanding can be used to place constraints on surface deformation in both an active and an ancient orogen.
Along the eastern margin of the Tibetan Plateau, adjacent to the Sichuan Basin, a long-standing and vigorous debate persists over whether mountain building occurred largely along upper-crustal faults or was the consequence of distributed thickening in the lower crust. In contrast to the intensively studied Longmen Shan, the topographic margin of the Tibetan Plateau north of the Sichuan Basin follows the north-south Min Shan and cuts orthogonally across the structural grain of the Mesozoic West Qinling orogen. The lack of a direct association of topography with upper crustal faults affords an opportunity to evaluate the patterns of differential rock uplift from geomorphology. Employing an empirical calibration of river profile steepness (channel gradient normalized for drainage basin area) and erosion rate from cosmogenic 10Be concentrations in modern sediment, I show how spatial patterns in rock uplift rate are localized along active faults in the Longmen Shan, but are distributed across a broad north-south axis along the Min Shan. A preliminary chronology of terrace formation and abandonment confirms that steep channels in this region have been sustained by differential rock uplift along the flank of the Min Shan. The wavelength of the region of highest incision rates appears to require either 1) a deeply buried tip of a blind fault or 2) flow and thickening of deep Tibetan crust against the foreland of the West Qinling.
The persistence of topography in the tectonically inactive Appalachian orogen, along the eastern seaboard of North America, remains enigmatic. Is topography simply relict from Paleozoic orogeny, or was it rejuvenated in Cenozoic time? Drawing on new analyses of river profiles in the Susquehanna River basin, I show how spatial patterns in channel steepness record a basin-wide response to a temporal change in rock uplift rate. Utilizing a set of empirical calibrations for the scaling of channel steepness to basin-wide erosion rate across a wide range of lithologic substrate, I estimate the timescales associated with the transient response of channel networks to changes in mantle flow or buoyancy over the past 15 Ma. These results are consistent with models of changes in dynamic topography driven by mantle flow and buoyancy and support long-held contentions that the modern topography of the Appalachians reflects Miocene – present uplift in eastern North America.
About the Speaker:
Eric Kirby studies the interplay of erosion and tectonics to understand the growth and evolution of active mountain ranges around the world. His research into the adjustment of fluvial systems to uplift rate has led to new insights in how rivers encode tectonic signals in collisional mountain belts in Asia, the Basin and Range extensional province in western North America, and forearc regions in Japan and Central America. He has conducted research along active fault systems that contributed to new insights regarding the termination of major strike-slip faults in Tibet and low-angle normal faults in western North America. He has worked extensively on the growth and expansion of high topography in northeastern Tibet, and developed new constraints on the rheology of Tibetan crust. He and his students have also been involved in understanding the processes by which soil transport and erosion on hillslopes adjusts to microclimate variations in the eastern United States.
Eric Kirby is a Professor in the College of Earth, Ocean and Atmospheric Sciences at Oregon State University. He holds the R.S. Yeats Chair of Earthquake Geology and Active Tectonics and is currently the Associate Dean of Academic Programs for the college. Prior to moving to OSU in 2013, Prof. Kirby taught on the faculty of Penn State University as Assistant and Associate Professor. He has been a research fellow of the Alexander von Humboldt foundation and is a Fellow of the Geological Society of America. He has served as Science Editor for Lithosphere, and as Associate Editor for Tectonics, GSA Bulletin, and Geology. Eric received his Ph.D. in Geology from MIT in 2001, holds a M.S. in Geology from the University of New Mexico, and a B.A. from Hamilton College.