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Active Tectonics: Studies in Geophysics
evidence that must be supported with other morphologic evidence of aggradation, degradation, or survey data. In many areas the evidence will be circumstantial. Nevertheless, anomalous reaches that are not related to artificial controls or to tributary influences may reasonably be assumed to be the result of active tectonics.
Examples from the Near East, Pakistan, and the lower Mississippi Valley illustrate the impact of active tectonics on the effective utilization of rivers and canals.
Where man has built long-lasting structures, active-tectonic effects are recorded by displacement of these features. For example, in the southeastern corner of Iran, the Shaurn anticline forms a range of low hills. Folding began in late Pliocene, and it still continues (Ambraseys, 1978). In the first or second century A.D. two canals were cut across the anticline in order to lead water from a canal system on the northeast flank to the more extensive and fertile plains on the southwest. These channels afford a unique opportunity for measuring the uplift of the anticlines since the canals were built. One canal still carries water, but, where it crosses the anticline it has cut down about 3.5 m below its original bed. The other canal has been abandoned. An accurate survey along its alignment shows that, along the anticlinal axis, the bed of the canal has risen at an average rate of approximately 1 m per century (Lees, 1955).
In the Tigrus and Euphrates Valley, where there is active tectonics (Lees, 1955; Adams, 1965; Mirjayar, 1966), canals have also been abandoned. They show reversed gradients and incision.
There are numerous active faults in the Indus Valley (Kazmi, 1979). The most spectacular effect of active faulting is due to the Rann of Cutch Fault zone in the lower Indus Valley. In 1819 a severe earthquake resulted in the 6-m uplift of a 16-km-wide and 81-km-long tract of alluvial land. This feature was locally known as Alah Bund (Oldham, 1926), and it blocked an eastern branch of the Indus River. The channel at that time was dry, but flow was re-established during a flood in 1828.
Lyell (1857, p. 462) stated that “for several years after the convulsion of 1819, the course of the Indus was very unsettled, and at length, in 1826, the river threw a vast body of water into its eastern arm, forcing its way in a more direct course to the sea, burst through all the artificial barriers that had been thrown across the channel, and at length cut right through the Alah Bund.” For discussion of recent history of the Indus see Holmes (1968), and for an interesting hypothesis concerning the decline of an Indus civilization see Dales (1966), who suggested that one ancient Indus valley city (Mohenjodaro) was flooded as a result of major tectonic activity forming a dam in the Indus Valley. This is possible as a result of valley-floor warping, but not as the result of a major natural alluvial dam (Lambrick, 1967). Finally, the westward shift of the Indus River during the last few thousand years suggests major avulsive changes owing to westward tilting of the Indus River valley (Wilhelmy, 1969).
Lake County Uplift The great 1811–1812 earthquakes near New Madrid, Missouri, have created considerable concern about the possibility of a recurrence. Therefore, extensive studies have been carried out in this area, and the literature relating to the geophysics and geology of the Mississippi Embayment between Memphis and Cairo is abundant (McKeown and Pakiser, 1982).
The area of deformation near New Madrid is referred to as the Lake County Uplift (Figure 5.6). The surface of the uplift is as much as 10 m above the general level of the Mississippi River Valley. The deformed area has a maximum length of about 50 km and a maximum width of about 23 km. Its relief is uneven, and the surface is dominated by two elongated bulges.
The Lake County Uplift consists of part of four different geomorphic surfaces, the modern Mississippi meander belt and three separate Mississippi River braided-stream terraces. Lateral migration of the Mississippi River during and following the most active periods of deformation has eroded a considerable amount of the uplifted surface. During great floods of the past, the as yet uneroded portions of the Lake County Uplift existed as islands on the Mississippi River alluvial plain.
Russ (1982) cited the following evidence of active deformation of the Lake County Uplift: (1) profiles of the Lake County Uplift reveal that the structure is significantly higher than the natural occurring landforms of the modern meander belt (Figure 5.7A); (2) the longitudinal profiles of abandoned river channels and natural levees have been significantly warped, some to the extent that the original river flow direction has been reversed; (3) the modern floodplain is also warped (Figure 5.7B); and (4) the Reelfoot scarp vertically offsets abandoned Mississippi River channels, which once flowed across the area.