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Articles |
School of Earth, Ocean and Planetary Sciences, Cardiff University, Wales
Present address: School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Williamson Building, Oxford Road, Manchester M13 9PL, UK (e-mail: Neil.Mitchell{at}manchester.ac.uk)
Far less is known of the processes involved in erosion of submarine channels compared with channels eroded subaerially by water runoff, but geometrical properties derived for canyons of the USA Atlantic continental slope reveal some intriguing similarities. Slope-confined canyons are concave-upwards, displaying decreasing channel gradient with increasing contributing area, as observed in many bedrock-eroding rivers. Tributaries join principal channels at the same elevation (without intervening waterfalls), in effect obeying Playfair's law, as do many river networks. Gradient and contributing area data for channels at confluences also reveal a tendency for tributaries to have steeper gradients than their associated principal channels, reflecting their smaller drainage areas. The concavities of bedrock-eroding rivers are often explained by a balance between river discharge, which increases with increasing rainfall catchment area, and gradient, which declines to offset the erosive effect of the discharge. It is unclear, however, if such a balance can be invoked for submarine canyons because erosion is probably caused when sedimentary flows are active only in individual canyon branches, originating from isolated slope failures. Instead, the frequency of sedimentary flows experienced by canyon floors may increase downstream simply because the area of unstable canyon walls available to source sedimentary flows increases, and this effect becomes compensated by declining gradient. Knickpoints created by faults in tectonically active slopes provide a further way to infer the form of erosion by sedimentary flows. Such knickpoints typically lie upstream of the faults that probably generate them, implying that detachment-limited erosion is enhanced where sedimentary flows become more vigorous on steep gradients, leading to knickpoint migration.
