Geomorphic and sedimentary response of rivers to tectonic deformation: a brief review and critique of a tool for recognizing subtle epeirogenic deformation in modern and ancient settings

Rivers are extremely sensitive to subtle changes in their grade caused by tectonic tilting. As such, recognition of tectonic tilting effects on rivers, and their resultant sediments, can be a useful tool for identifying the often cryptic warping associated with incipient and smaller-scale epeirogenic deformation in both modern and ancient settings. Tectonic warping may result in either longitudinal (parallel to floodplain orientation) or lateral (normal to floodplain orientation) tilting of alluvial river profiles. Alluvial rivers may respond to deformation of longitudinal profile by: (1) deflection around zones of uplift and into zones of subsidence, (2) aggradation in backtilted and degradation in foretilted reaches, (3) compensation of slope alteration by shifts in channel pattern, (4) increase in frequency of overbank flooding for foretilted and decrease for backtilted reaches, and (5) increased bedload grain size in foretilted reaches and decreased bedload grain size in backtilted reaches. Lateral tilting causes down-tilt avulsion of streams where tilt rates are high, and steady down-tilt migration (combing) where tilt rates are lower. Each of the above effects may have profound impacts on lithofacies geometry and distribution that may potentially be preserved in the rock record. Fluvial sedimentary evidence for past tilting is traditionally based on the assumption that depositional features reminiscent of modern fluvial tectonic effects are evidence for past tectonic effects where it is closely associated with historically active structures, or where non-tectonic causes cannot be invoked; however, caution must be exercised when using these effects as criteria for past or current tectonic warping, as these effects may be caused by non-tectonic factors. These non-tectonic causes must be eliminated before tectonic interpretations are made.