River incision and anticlines

In many areas of active deformation, landforms that suggest that uplift and river down-cutting keep pace are very common.  Stream courses cross zones of uplift, rather than being diverted or ponded up to form lakes.  Traditionally, geomorphologists have described such drainages as “antecedent”, i.e. rivers that were present before uplift began.  They can be seen on all scales up to examples such as the Indus and Brahmaputra rivers that carve their way across the actively rising Himalaya.  The most common are anticlines through which streams flow in canyons perpendicular to the fold axes.  A curious and common feature is that the canyons are not haphazard, but often cut the fold where its amplitude is greatest and its axis plunges away from the site of incision.  The stupendous rates at which crustal rocks are eroded and transported away in the courses of the Indus and Brahmaputra, and in lesser drainages on the flanks of major extensional orogens, such as the Red Sea, clearly removes load from the crust.  Consequently there is an isostatic component to the uplift involved in the two cases at a grand scale.  Peter Molnar and Phillip England suggested an erosional role in large-scale uplift over a decade ago.  Intervening ridges rise higher than they would if erosion was slower or non-existent.  In major rift systems, the highest peaks are often within the escarpments rather than at the lip of uplift, sometimes more than 500 m higher.   Bearing this well-known process in mind, Guy Simpson of ETH Zurich, has sought evidence that it functions on much smaller scales (Simpson, G. 2004.  Role of river incision in enhancing deformation.  Geology, v. 32, p. 341-344).  That comes from the surprising symmetry of doubly plunging anticlines that are cut by rivers at their highest point.  His modelling suggests that the phenomenon can occur when the crust deforms plastically, allowing isostatic response to erosion on even minor scales during compression.  When deformation is by brittle means, any uplift of rigid crust is flexural and has long wavelengths, so that rivers bear no relation to local structures


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