EVOLUTION OF EXPERIMENTAL LANDFORM WITH RAINFALL EROSION AND UPLIFT

Development of experimental landforms with rainfall-erosion and uplift of various rates on a square mound of a mixture of fine sand and kaolinite suggests the existence of two thresholds in uplift rate. The experiments start from a flat surface, and certain low relief dominates while the uplift rate is below the lower threshold (“characteristic relief phase”). Erosion is exclusively fluvial under the detachment-limited condition in this phase. Relief increases with the periodic “erosion with knickpoints” promoted by fault displacement, and decreases with the normal “erosion of declining slopes.” The characteristic relief, which is mostly determined by the material erodibility and rainfall intensity, would be maintained by the reciprocal effect of these two types of erosion. The erosion rate with this characteristic relief can be higher (or lower) than the uplift rate, and the average height of the uplifted area may decrease (or increase). When the uplift rate exceeds the lower threshold, uplift starts to prevail in the upstream most areas where fluvial erosion works less. Hills grow until slope failures occur and reduce their heights. Sediment supply from slopes increases, and the condition of fluvial erosion becomes transport-limited. Slope failures inside the uplifted area do not change the average height of this area unless deposited sediments are carried away by fluvial processes. When the uplift rate becomes higher, hills grow more and sediment supply from slopes increases, but the resultant increase in stream gradients helps streams to carry more sediments. Uplift and erosion would become balanced and the average height tends to stay constant. A similar landscape can be seen for a long time, while the configuration of landform changes in detail. This state may be defined as a steady-state, but continuous changes in landform configuration and gradual thinning of hills through the process of repeated slope failures give a different impression. The name “quasi-steady state phase” seems to be appropriate. When the uplift rate becomes even higher and crosses the upper threshold, uplift overwhelms erosion. Hills grow into high mountains in this “mountain building phase.” The mountains will stop rising when they become high enough for the material produced by slope failures to move directly out of the area of deposition.