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Active Tectonics: Studies in Geophysics
FIGURE 12.10 Model for retreating, loosening-limited scarp. The retreating free face is progressively buried by a basal apron of debris shed from its surface.
ble in many cases. Although dating can be performed on several different types of scarp underlain by different kinds of materials, the following discussion of fault scarp dating is limited primarily to scarps produced by normal faulting of relatively cohesionless alluvium such as those produced by range-front faulting of alluvial fans in the Basin and Range region of the western United States. The initial morphology of such scarps can be quite complex. Frequently the fault surface will splay close to the ground surface to produce an assemblage of smaller scarps rather than producing a single scarp. Repeated faulting often superimposes younger scarps on
FIGURE 12.11 Comparison of the profile of a scarp produced by the 1959 West Yellowstone earthquake with its profile observed 20 years later. The free face of the retreating scarp is nearly buried (compare with Figure 12.10). (Photo courtesy of R.E.Wallace, U.S. Geological Survey.)
FIGURE 12.12 If a basal debris apron is not allowed to accumulate, the free face can retreat great distances. Subsequent movements of the fault may produce a new scarp before the older scarp is eliminated (compare with Figure 12.4).
much older ones. The initial morphology of these complex scarps is difficult or impossible to determine with an accuracy sufficient to permit morphologic dating. Nash (1984) suggested that independent geologic evidence (e.g., from trenching) be used to determine whether a scarp had a complex initial morphology and, if so, to reject it for morphologic dating. The analysis below is for simple scarps. Recently, however, Hanks et al. (1984) have proposed a method for dating more complex scarps produced by repeated fault movements.
Initial Morphology of Fault Scarps
It is assumed here that the initial morphology consists of a single scarp that offsets a straight crest and base inclined at the prefaulting slope of the fan surface (Figure 12.2a). Scarps formed by normal faulting of cohesionless sands and gravels result from active Rankine failure producing failure planes inclined at 45+Φ/2 to the horizontal. Φ, the angle of internal friction, generally varies from 30° to 35° so initial scarp angles range from 60° to 62°. Theoretically, for a purely cohesionless material, a slope cannot exceed Φ (Carson, 1977), therefore, the initial fault scarp should rapidly recline to Φ. A rapid recline to Φ, however, is frequently not observed on normal fault scarps underlain by “cohesionless” sands and gravels, probably because of a weak, ephemeral cohesion among grains resulting from pore water in capillary tension and from minute amounts of cementing at contact points between grains. Instead, a loosening-limited free face is produced that retreats, progressively burying its base with debris. The time necessary for the retreating free face to be completely buried, producing a continuous debris apron that at Φ, varies with