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Active Tectonics: Studies in Geophysics
FIGURE 7.1 Graphs showing hypothetical uplift history by plotting cumulative uplift versus the length of record. Mountain fronts record long periods of time, whereas fault scarps and historical earthquake data record progressively shorter time periods. The circle-in-square on the mountain fronts graph outlines the time period shown on the fault scarp graph. Likewise, the circle on the fault scarps graph indicates the time period covered by the historical earthquakes graph.
dividing the total (cumulative) uplift by the period of record. The variation of uplift rate estimates reflects short-term tectonic fluctuations that may be superimposed on long-term trends. Thus, long-term averages of past uplift rates serve to constrain forecasts of uplift. How mountain fronts and escarpments evolve and several methods for their use in tectonic geomorphology are discussed below.
The most popular early attempt to relate landform morphology with tectonics was William Morris Davis’ cycle of erosion. Davis (1899) envisioned a closed geomorphic system where, following a pulse of rapid uplift, landforms evolved through a sequence of characteristic landforms. Each landform assemblage in the Davisian sequence of “youth,” “maturity,” and “old age” stages differed morphologically from the others. Davis’ cycle of erosion was highly intuitive and logical but suffered under the blow of equifinality. In other words it was possible to produce the landform assemblages in each stage by variables other than sequential denudation following rapid uplift. For example, rock types and geologic structure differ from area to area, and the effectiveness of erosional geomorphic processes also varies; yet these differences were not accounted for in the cycle of erosion. Despite the shortcomings of the Davisian scheme, the basic concept of landform adjustment following tectonism remains intact. The controversy surrounding the Davisian cycle of erosion may have provided the impetus for quantitative field investigations, the key to unraveling landform history.
Recent studies (Bull and McFadden, 1977; Wallace, 1977, 1978; Bucknam and Anderson, 1979) have examined the interaction of tectonics and landforms using an empirical approach. In these studies dating or determining rates of change of a landform or landform assemblage is an ultimate goal on which tectonic interpretations may be based. Data that allow the rates of landform change to be estimated can be used to determine how landforms may respond to tectonic perturbations. Hillslopes can result from tectonism such as faulting, and, therefore, hillslope evolution and slope processes are fundamental to understanding how erosion and tectonics interact to produce any given landform.
Slopes are basic landform elements that are naturally combined to form landform assemblages. Hillslopes are that portion of the sloping landscape within a drainage basin that contribute sediment (by several transport processes) and runoff to streams. Slope systems denote landform assemblages that operate or evolve as an integrated package of surficial processes and are commonly delineated by their topographic expression.
Equilibrium in slope systems can refer either to slope