dition, the attention of geomorphologists and photogeologists generally has been concentrated on the identification of geologic structures that are assumed to be quiescent (Howard, 1967; Ollier, 1981, p. 180) rather than on the effect of ongoing deformation on alluvial channels. Nevertheless, geomorphologists working in petroleum exploration, e.g., DeBlieux (1951, 1962) and Tator (1958), indicated that fluvial anomalies, such as local development of meanders or a braided pattern, local widening or narrowing of channels, anomalous ponds, marshes or alluvial fills, variations of levee width or discontinuous levees, and any anomalous curve or turn, are possible indicators of active tectonics. In addition, active tectonics can produce nickpoints, convexities or concavities of the longitudinal profile, channel depth variations, and, of course, either aggradation or degradation.

Another problem is that active tectonics takes several forms. Deformation can be along faults (shear, normal, or reverse in a downstream sense) or pairs of faults (horst and graben). These faults should have the same effect as a monocline, dome, or basin. In addition to local structural features, the entire valley may be tilted upstream, downstream, or laterally. The possibilities are great, but, in reality, the primary effect of tectonics will be local steepening or reduction of gradient or cross-valley tilting.

In addition to these primary influences on channel and valley gradient and configuration, there will be secondary effects, as the rivers respond to the changed gradient (aggradation or degradation), and there will be tertiary effects, as decreased or increased sediment loads influence reaches downstream of the deformed reach and as aggradation or degradation in the deformed reach progresses upstream. In addition to tectonic effects, there can be similar influences as a result of differential compaction of sediments (draping) over buried topographic highs.

TECTONIC EFFECTS

Where tectonism has been persistent for long periods of time active deformation will produce a channel response that will be superimposed on the long-term tectonic effects. Major valley deformation or total disruption of the river system can be the result of long-term tectonism.

Valley Change

The most commonly cited evidence for deformation is the warping of alluvial terraces in a valley. If the deformation has persisted the oldest terrace is the most deformed by uplift (convex) or subsidence (concave), and it will show the greatest offset by faulting (Machida, 1960; Zuchiewicz, 1979, 1980). Where there has been uplift or subsidence, terraces are warped upward or downward, and the extent of the displacement can be determined by comparison with the longitudinal profile of the present river if, indeed, the river has adjusted to the past deformation. Machida (1960) assumed that the longitudinal profile of a terrace is described by a negative exponential function in a downstream direction and that deviations from this curve indicate deformation. Valley-floor deformation can also be indicated by depth to bedrock. Alluvium will be thickest over downfaulted or downfolded zones and thinnest over areas of uplift (Kowalski and Radzikowska, 1968).

River Changes

When uplift is too rapid to be accommodated by a river there will be disruption of the drainage pattern (Sparling, 1967; Twidale, 1971, pp. 133–136; Ollier, 1981). For example, Freund et al. (1968) have evidence that movement along the Dead Sea rift in Israel disrupted streams that formerly drained from Jordan and crossed what is now the Dead Sea Valley to the Mediterranean Sea. Portions of the channels of these rivers are now displaced about 43 km as a result of movement along the boundary faults of the Dead Sea rift. The Murray River on the Riverine Plain near Echuca (Victoria, Australia) is an impressive example of channel modification by tectonic activity (Bowler and Harford, 1966). The Cadell Fault block has converted the Murray River from a single channel to an anastomosing system of channels that surround the obstruction. The abandoned segment of the Murray River is preserved on the dipslope of the fault block. A particularly active tectonic area is the eastern side of the East African rift valley near Lake Victoria. Doornkamp and Temple (1966) described the formation of lakes in some valleys, as a result of gradient reduction, and this could be the first stage of drainage disruption.

In many cases, incising channels will encounter resistant strata, which may retard or prevent the maintenance of an antecedent condition. Incised meander patterns can also indicate deformation because the alluvial river pattern is affected by the deformation before it becomes fixed by incision into bedrock (Gardner, 1975).

EVIDENCE FOR ACTIVE TECTONICS

The preceding discussion reviews evidence for past deformation but not necessarily for active tectonics. Modern river and valley-floor morphology and geodetic



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