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Abstract

Tectonic control of alluvial architecture is commonplace in extensional, transcurrent and compressional tectonic terrains. The primary influence of tectonic activity on floodplain behavior is tilting, which varies areally and temporally and is superimposed on the pre-existing channel gradient. These effects are considered theoretically and illustrated by Pleistocene and Holocene examples from southwest Montana and the Mississippi Valley.

A transverse slope will cause the movement of channel belts toward the area of maximum subsidence either 1) by processes of downslope cutoff and preferential erosion, producing an abnormally wide and asymmetric sandstone body, or 2) in steps, by avulsion. Avulsion may be triggered by tilting or may occur subsequently through preferential flooding of the down-tilted side of the floodplain. Although the channel will move toward the position of maximum subsidence, it will often be offset by fans constructed from the footwall or it may flow into a lake occupying the floodplain low. Differential subsidence strongly affects deposition rates, groundwater behavior, pedogenesis and flood distribution, in addition to channel migration.

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