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Active Tectonics: Studies in Geophysics
index, is a valuable reconnaissance tool when evaluating effects of active vertical tectonics. The Smf index is particularly attractive because it can be quickly and easily measured from aerial photographs, satellite or other high-altitude imagery, or topographic maps.
Ratio of Valley-Floor Width to Valley Height
The ratio of the width of valley floor to valley height Vf may be expressed by
where Vfw is the width of valley floor, Eld and Erd are the respective elevations of the left and right valley divides, and Esc is the elevation of the valley floor (Bull and McFadden, 1977). In determining Vf, the data are measured at a given distance up from the mountain front. The index reflects differences between broad-floored canyons with relatively high values of Vf and V-shaped canyons with relatively low values. Comparison of Vf values measured from valleys emerging from different mountain fronts or different parts of the same front provides an indication of whether the streams are actively downcutting (forming V-shaped valleys with low Vf) in response to active tectonics or are being eroded laterally (forming broad valleys with high Vf) in response to relative stability of the front.
The Vf index was tested by Bull and McFadden (1977) for mountain fronts north and south of the Garlock Fault. They found that values of the index varied from 0.05 to 4.7, with the lower values being derived from valleys north of the fault where mountain fronts are tectonically active. The index was also tested by Rockwell and Keller (in press) for mountain fronts near Ventura, California, where Vf ratios show similar trends to that established by Bull and McFadden—being lower for relatively active fronts than for fronts with lesser rates of uplift.
TECTONIC GEOMORPHOLOGY AND LANDFORM ASSEMBLAGE
A genetic classification of landforms is possible because different geomorphic processes tend to produce a characteristic assemblage of landforms. For example, the discussion of geomorphic indices suggested that active vertical tectonics tends to produce straight mountain fronts with V-shaped canyons and streams with relatively steep gradients for a particular rock type. On a more local scale, as for example along a specific mountain front or fault zone, active tectonics often modifies or produces characteristic landform assemblages. For example, alluvial fans have a variable morphology somewhat dependent on tectonic processes, and active strike-slip faulting produces a specific set of tectonic landforms. A tacit assumption in the evaluation of landform assemblages produced by active tectonics is that the more pristine or fresh appearing the landforms are, the younger the tectonics is assumed to be. Discussion of alluvial-fan morphology and tectonic activity as well as the assemblage of landforms associated with strike-slip faulting will illustrate the above concepts.
An alluvial fan is the end point of an erosional-depositional system in which sediment eroded from a mountain source is transported to the mountain front. There it is deposited as a cone or fan-shaped body of fluvial and/or debris-flow deposits (Bull, 1977b). The stream is the connecting link between the erosional and depositional parts of the system (Bull, 1977b) and therefore has a significant influence on the morphology of the alluvial fan. Radial profiles for most fans are composed of several segments, which together are gently concave. Breaks in slope mark boundaries between segments, and younger segments may be identified from older ones based on relative soil profile development, weathering of alluvial clasts, dissection of the surface by small streams, and development of desert varnish [see Bull (1964, 1977b) for a more detailed discussion of segmented alluvial fans].
Alluvial-fan morphology is an indicator of active tectonics because the fan form reflects varying rates of tectonic processes such as uplift of the source mountain along a range-bounding fault or tilting of the fan surface. When the rate of uplift of the mountain front is high relative to rate of stream-channel downcutting in the mountain and to fan deposition, then fanhead deposition tends to occur, and the youngest fan segment is near the apex of the fan. If the rate of uplift of the mountain front is less than or equal to the rate of downcutting of the stream in the mountain, then fanhead trenching occurs and deposition is shifted downfan. Younger fan segments will then be found well away from the mountain front (Figure 8.3 shows these two conditions). Change in sediment or water yield may also cause fanhead trenching, but this tends to be temporary if mountain-front uplift persists (Bull, 1964).
The above model of segmented alluvial fans relative to active tectonics has been successfully tested for alluvial fans in Death Valley, California (Hooke, 1972). Hooke found that eastward tilting and normal faulting produced segmented alluvial fans. On the east side of