Active Tectonics Impact on Society (1986) / Chapter Skim
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12. Morphologic Dating and Modeling Degradation of Fault Scarps
12. Morphologic Dating and Modeling Degradation of Fault Scarps
Pages 181-194
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From page 181... ...
The accuracy of the age calculated for a fault scarp is dependent on the accuracy of the calibration and the accuracy of the initial morphology assumed for the scarp prior to degradation (potentially complex for fault scarps)
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From page 182... ...
Scarps bounded by horizontal or gently inclined bases and crests, such as fault scarps produced by normal faulting, of alluvial fans, abandoned fluvial cutbanks (terrace scarps) , and abandoned wave-cut bluffs, on which debris derived from the scarp face accumulates at its base (i.e., debris is not removed by fluvial or wave undercutting)
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From page 183... ...
b, With time, the curvature of the crestal convexity and basal concavity decrease on transport-limited hillslopes. The degraded excess midsection slope angle, ¢, is defined as the angle by which the degraded midsection exceeds 8.
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From page 184... ...
in his study of the degradation of badland slopes. Schumm observed that creep-dominated slopes, those with a continuous cover of loosened debris, FIGURE 12.3 The scarps formed during the 1959 West Yellowstone earthquake iindicated by the arrow)
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From page 185... ...
. The model, sometimes termed the diffusion model of hillslope degradation, is based on the assumption that the volumetric rate at which debris moves downslope at a particular point on a hillslope profile is proportional to the gradient of the profile at that point.
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From page 186... ...
. If the amount of expansion and contraction of the debris mantle is uniform throughout its thickness, the upward and downward displacement of debris decreases linearly with depth, and, therefore, the downslope movement of debris resulting from a single expansion and contraction episode will also decrease linearly with depth (Figure 12.6b)
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From page 187... ...
(1984) fit the pattern of degradation observed on Lake Bonneville scarps and a fault scarps near Drum Mountain, Utah, abandoned sea cliffs along the Califor
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From page 188... ...
At the close of a further equal interval of time, a further layer of rock, again of the same thickness and of equal thickness at every part of the slope, passes over into the reduced form. Removal of a uniform thickness of loosened debris from the scarp face results in parallel retreat with no rounding of the crestal convexity or basal concavity.
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From page 189... ...
USING THE HILLSLOPE DEGRADATION MODELS TO DATE SIMPLE FAULT SCARES Both the transport-limited and the weathering-limited hillslope models offer a means for morphologic dating of some prehistoric fault scarps. Before this can be done, the initial morphology of a scarp must be accurately estimated and the models must be calibrated.
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From page 190... ...
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.
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From page 191... ...
Wallace (1980) found that the free face of most of the fault scarps produced by the 1959 earthquake in West Yellowstone are now nearly buried.
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From page 192... ...
tan 2 tax) FIGURE 1214 The relationship among initial excess midsection slope angle,; degraded excess midsection slope angles JIB; scarp offsets H.; c; and elapsed time since the start of the second stage of scarp degradation, t This relationship forms the basis of the morphologic dating technique used here for dating transport-limited scarps.
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From page 193... ...
Morphologic dating should not be applied to fault scarps that cannot be assumed to have had a simple initial morphology comprising an equally inclined crest and base separated by a straight midsection. Despite these limitations, morphologic dating, when used with considerable care, should prove itself to be a valuable tool for determining the deformational history of areas of active tectonism.
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From page 194... ...
. Morphologic dating of fluvial terraces scarps and fault scarps near West Yellowstone, Montana, Geol.
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