Geomorphology of laterally advected fault blocks in convergent orogens

Although the kinematics of bedrock in tectonically active mountain ranges commonly includes large horizontal components, few studies have addressed what the effect of this motion might be on geomorphology. Using a 2-D landscape evolution model, I investigate the role of lateral bedrock motion in steady-state topography developed on fault-bend folds and on extruding plateau margins. The first study, of fault-bend fold topography, examines the role that lateral bedrock motion has on the longitudinal profile shapes of detachment-limited streams. The primary effect is that streams on the fold that flow in the direction of lateral advection have greater steepness and concavity indices than streams that flow the opposite direction. This results from the fact that steady-state erosion rate is a function of bedrock velocity as well as topographic slope and aspect. Erosion rates are greater on a given surface sloping in the direction of advection than on a similar surface facing the opposite direction. Similarly, steeper slopes facing in the direction of advection must erode at faster rates than gentler slopes. This effect is enhanced at faster bedrock velocities, at intermediate fault dips, for thinner hanging walls, and for lower erosion coefficients. Because stream profiles set the relief structure of most mountain ranges, the cross-sectional asymmetry of their range-scale topography results from this mechanism. Hillslope diffusion does not strongly influence this asymmetry at the common length-scales of mountain range half-widths and for reasonable diffusivities. Model results compare favorably to active fault-bend fold topography in the Siwalik Hills, Nepal. The second study shows how topography, such as transversely oriented valleys and ridges, can be advected across the crest of a fault-bend fold. This advection results in a cross-correlation between elevations on the two sides of the mountain range, promoted by the same conditions listed above. The third study demonstrates how lateral advection of an extruding plateau margin, where bedrock motion is limited to a block bounded by an upper normal fault and a lower thrust fault, creates convexo-concave detachment-limited stream longitudinal profiles that are shown to be consistent with observations of trans-Himalayan streams as they cross the Higher Himalaya.