What is the topographic signature of lateral advection of bedrock?
Although the tectonic geomorphology community has advanced its understanding of landscape development above vertically uplifting bedrock, the geomorphology that develops where a significant component of the bedrock’s velocity is horizontal is poorly understood. Yet such a velocity field is exactly what exists in many active orogens. I have conducted numerical experiments using the Channel-Hillslope Integrated Landscape Development (CHILD) model to investigate the steady-state fluvial geomorphology of individual ridges formed over fault-bend folds in foreland fold-and-thrust belts [Miller et al., 2007, JGR]. The lateral component of the bedrock velocity field has a significant role in determining the steepness and concavity of rivers and therefore the morphology of mountain ranges (e.g., cross-range topographic profile asymmetry, such as seen in the figures below). Topography can also be advected, in certain cases across the drainage divide [Miller and Slingerland, 2006, Geology]. Model results predict well the observed geomorphology of the Siwalik Hills in Nepal that are formed above the active Main Frontal and Main Dun Thrusts. These results demonstrate that added information on long-term bedrock kinematics (e.g., vergence direction, slip rate, fault geometry) can be gleaned from geomorphic data. Future models will explore the role of sediment in erosion of settings with lateral advection, which is expected to be important.
Work in my dissertation also showed how the fluvial geomorphology of fault-bend folds is more sensitive to precipitation rate than are model mountain ranges undergoing vertical uplift alone. To test these ideas, I am investigating the regional precipitation gradient, using calibrated Tropical Rainfall Measuring Mission (TRMM) satellite data, and geomorphology of the Siwalik Hills in Nepal and India with collaborators Eric Kirby (Penn State) and Bodo Bookhagen (Stanford). Preliminary results show that east-to-west changes in height, asymmetry, and drainage basin shape correspond to a decrease in precipitation. With a better understanding of the response of geomorphology to climate and to tectonics, we will be better able to assess whether variations in geomorphology among mountain ranges or segments of mountain ranges reflect different tectonic or climatic regimes. Future work will also investigate the role of sediment flux and hillslope erosion on fault-bend fold topography.
Simple cartoon showing the combined effect of lateral advection, topographic slope, and topographic aspect on erosion rates above a simple fault block experiencing uniform rock uplift rates. Note that erosion rates are generally faster on the distal side of the ridge than the proximal side, and that erosion rates vary in the downstream direction due to changes in channel gradient.
Stream profiles in the Siwalik Hills, Nepal, which are (here) fault-bend folds overlying the south-verging Main Frontal Thrust. Slope-area data show differences in steepness and concavity indices as a function of location on the ridge, as predicted by the landscape evolution model.
Perspective view comparing Siwalik Hills (Google Earth image) and output from the landscape evolution model (fault-dip = 30 degrees, nondimensional erosion number = 5).
Simple cartoon showing how we envision valleys on the proximal side of a fault-bend fold are advected, resulting in beheading and the formation of wind-gaps. The size of these wind-gaps are related to fault dip and nondimensional erosion number, and determine whether distal drainage basins align with proximal basins. Simply, distal basins preferentially self-organize where less material (phi) is shoved across the drainage divide.