ALL SMEARED? GEOMORPHIC RESPONSES TO HORIZONTAL MOVEMENT AT GEOLOGIC TIME SCALES

Landscapes represent the interaction between surface processes, rock properties, and tectonic deformation. In active fold-thrust belts rock uplift is controlled by the geometry of the detachment fault, specifically, the location and angle of fault ramps. These ramps are associated with high topography, significant relief and steep river channels. However, such rock uplift is more complex in that it comprises both vertical and horizontal components of displacement, and thus raises the question whether geomorphic markers of uplift can be laterally displaced. In the hinterland of the Himalaya, where high erosion rates accompany high convergence rates of ~ 20 mm/yr, normalized channel steepness (k_sn) values increase to the north over ~ 25 km with the highest values marking the proposed location of the active ramp. Similarly, in the Andes, where low erosion rates attend low convergence rates of ~ 5-10 mm/yr, k_sn values increase westward over ~ 40 km. These k_sn values that decrease in the direction of movement may indicate decreasing uplift, or may be a function of horizontal displacement. The question arises, how the relationship between convergence rates, rock erodibility and thrust ramp geometry might affect topography, relief, and k_sn values. In this study, we explore the displacement of k_sn values (i) analytically, by quantifying k_sn migration in 1D through a ratio of erodibility over convergence rate, and (ii) numerically by integrating realistic displacement kinematics with the Cascade landscape evolution algorithm in 3D. Our results indicate that a horizontal movement component causes an asymmetry in topography and relief, and a smearing of k_sn in the direction of movement away from the vertical uplift field. The extent of this shift is dependent on the ratio of erodibility over convergence rate for a given structural geometry. As this ratio decreases, such that convergence rates are significantly higher than erosion (~ 5-10 times), the smearing can extend to tens of km from the ramp front. Sets of parameters that are representative for the hinterland of the Himalaya and the Andes have low erodibility to convergence ratios such that k_sn values are smeared over such a distance. Thus, we argue that the interpretation of geomorphic markers at convergent orogens requires consideration of horizontal displacement.