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 t ectonic tilting. As such, recognition of tectonic tilting effects on rivers , and their resultant sediments, can be a useful tool for identifying the o ften cryptic warping associated with incipient and smaller-scale epeirogeni c deformation in both modem and ancient settings. Tectonic warping may resu lt in either longitudinal (parallel to floodplain orientation) or lateral ( normal to floodplain orientation) tilting of alluvial river profiles. Alluv ial rivers may respond to deformation of longitudinal profile by: (1) defle ction around zones of uplift and into zones of subsidence, (2) aggradation in backtilted and degradation in foretilted reaches, (3) compensation of sl ope alteration by shifts in channel pattern, (4) increase in frequency of o verbank 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 (comb ing) where tilt rates are lower. Each of the above effects may have profoun d 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 remini scent of modern fluvial tectonic effects are evidence for past tectonic eff ects where it is closely associated with historically active structures, or where non-tectonic causes cannot be invoked; however, caution must be exer cised when using these effects as criteria for past or current tectonic war ping, as these effects may be caused by non-tectonic factors. These non-tec tonic causes must be eliminated before tectonic interpretations are made. ( C) 1999 Elsevier Science B.V. Ail rights reserved.