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Contrasting transient and steady-state rivers crossing active normal faults: new field observations from the Central Apennines, Italy |
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| Authors: | Alexander C Whittaker Patience A Cowie Mikaël Attal Gregory E Tucker† Gerald P Roberts‡ |
| Institution: | Grant Institute of Earth Science, School of GeoSciences, University of Edinburgh, Edinburgh, Scotland, UK; Department of Geological Sciences, Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA; Joint Research School of Earth Sciences, UCL-Birkbeck College, University of London, London, UK |
| Abstract: | We present detailed data on channel morphology, valley width and grain size for three bedrock rivers crossing active normal faults which differ in their rate, history and spatial distribution of uplift. We evaluate the extent to which downstream changes in unit stream power correlate with footwall uplift, and use this information to identify which of the channels are likely to be undergoing a transient response to tectonics, and hence clarify the key geomorphic features associated with this signal. We demonstrate that rivers responding transiently to fault slip-rate increase are characterised by significant long-profile convexities (over-steepened reaches), a loss of hydraulic scaling, channel aspect ratios which are a strong non-linear function of slope, narrow valley widths, elevated coarse-fraction grain-sizes and reduced downstream variability in channel planform geometry. We are also able to quantify the steady-state configurations of channels, that have adjusted to differing spatial uplift fields. The results challenge the application of steady-state paradigms to transient settings and show that assumptions of power-law width scaling are inappropriate for rivers, that have not reached topographic steady state, whatever exponent is used. We also evaluate the likely evolution of bedrock channels responding transiently to fault acceleration and show that the headwaters are vulnerable to beheading if the rate of over-steepened reach migration is low. We estimate that in this setting the response timescale to eliminate long-profile convexity for these channels is ∼1 Myr, and that typical hydraulic scaling is regained within 3 Myr. |
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