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Active Tectonics: Studies in Geophysics
forms that tend to persist over time or to conditions where the rate at which regolith or soil develops is balanced by the rate of soil removal. In the latter context, equilibrium slopes are suggested by field observations that show soil (sensu lato) thickness to be relatively constant over a slope. Constant soil thickness requires a balance between production of soil by weathering and its removal by erosion. Departures from this balance are either tipped toward erosion or weathering, and the resulting slopes are termed, respectively, weathering-limited and transport-limited slopes (Young, 1972).
Weathering-limited slopes denote a condition where the rate of soil removal exceeds that of soil production. Bare rock slopes are an example of a weathering-limited slope. Transport-limited slopes refer to a condition where the rate of soil production does not limit the rate of soil removal. A valley-side slope cut by drainages on the erodible materials of “badlands” is an example of a transport-limited slope.
As a slope erodes, its forms may change. Four types of slope evolution that describe some common patterns of slope change are decline, replacement, retreat, and rounding. Each type of slope evolution is the geometric result of the distribution of net erosion along a slope. During the history of some slopes, more than one type of slope evolution can take place. The relation between slope evolution and erosion can be described using a mass-flux diagram (Figure 7.2). Mass flux is a rate of material movement per unit of slope area and is analogous to the concept of erodibility.
A mass-flux diagram illustrates the position on a slope where either erosion or deposition is most efficient. Slope decline is important in areas with stable base-level and mature topography. Though characterized by erosion throughout the length of the slope, decline results
FIGURE 7.2 Mass-flux diagram illustrating the differences in slope models in terms of how erosion or deposition is distributed along the profile. The solid lines mark the initial condition, and the dotted lines indicate profile change after some time. On the lower portion of the figure, white areas of the profile indicate erosion while the dark areas (for rounding and replacement) indicate deposition.
from more effective erosion near the upper portions adjacent to the drainage divides. The situation of parallel retreat occurs when erosion is constant along a slope and therefore vertical lowering of the slope occurs everywhere at the same rate. Parallel retreat is common, but not restricted to, areas where the upper surface is protected from erosion by a resistant cap rock. Slope replacement describes the replacement of the original slope by one controlled by deposition. Material eroded from the escarpment is deposited at the base of the slope and accumulates at a slope less steep than the original escarpment. With time, the depositional slope replaces the original slope.
Slope rounding describes a symmetric erosion-deposition feature of some slopes that decreases the curvature all along a slope. Fault scarps in alluvium show this form of evolution. Other models of slope evolution can be envisioned by relating the amount of material eroded on a slope to some morphologic factor such as slope gradient or slope curvature. Slope rounding and decline are similar except for the deposition that is found in slope-rounding evolution. In addition, as a slope evolves, the dominant pattern of evolution may change. For example, a slope may initially change by slope replacement and later by some other evolutionary type. In the case of fault scarps formed in alluvium, early slope development is characterized by slope replacement and later by slope rounding (see Nash, Chapter 12, this volume).
FAULT-GENERATED MOUNTAIN FRONTS
The Basin and Range physiographic province of the United States is characterized by fault-bounded mountain blocks. These mountains, formed where faults displaced the topographic surface, are termed fault-generated mountains (Figure 7.3). The morphology of mountain fronts is strongly affected by the width of the range. The range width determines the maximum possible drainage basin size that can develop. As virtually all of the area within the mountain block is sloping (i.e., draining), drainage basins fill all available space. For a given mountain range, larger watersheds that tend to have a characteristic shape also have a regular spacing along the mountain front. The spacing depends on basin shape. Regular spacing of basins measured as a ratio between the distances of the basin mouths along the range front to length of the basin was noted by Wallace (1978).
Drainage basin characteristics within the mountain range, primarily shape and size, affect the morphology of the mountain front and how that front may evolve. Large circular watersheds do not effectively fill space at