QUANTITATIVE MAPPING OF CHANNEL MORPHOLOGY AS AN INDICATOR OF FAULT SYSTEM DEFORMATION

Extracting tectonic information from topography has great potential for advancing our understanding of seismic hazards and deformation processes. River channel morphology can be a useful indicator of the underlying fault geometry associated with active folding because channels respond to changes in base level. Previous studies have estimated rock uplift rate by measuring channel gradients and assuming that channel widths scale with upstream drainage area (a discharge proxy). This assumption is not always valid because changes in uplift rate or substrate erodiblity can also cause channel width to change.

In this study, we explicitly account for channel width variations using new quantitative methods to estimate river incision potential and its relationship to subsurface fault geometry in active landscapes. We apply this method to the Chandigarh and Mohand anticlines, two active fault-bend folds associated with the Himalayan Frontal Thrust (HFT) in northwestern India. We use digital topography and high resolution (5 m) satellite images to measure channel widths and gradients over ~100 channels draining both flanks. We then normalize channel widths and slopes to upstream drainage area yielding two tectonically sensitive morphometrics: normalized width index (k_wn) and normalized steepness index (k_sn).

Our observations show that both k_wn and k_sn vary systematically with changes in fault dip at depth inferred from balanced cross sections. For example, at locations where channels cross a kink in the HFT ramp at depth, k_wn decreases by ~15% and k_sn increases by ~18%. The steeper dipping fault segment translates to higher relative rock uplift rate thereby causing the channels to narrow and steepen. Where channels cross into an erosionally resistant bedrock lithology, k_wn decreases by ~33% and k_sn increases by ~50%. By interpolating these zones of reduced k_wn and high k_sn, we map the change in fault dip at depth and the lithologic contact of the erosionally resistant layer along strike. In future work, we plan to quantify variations in bedrock erodibility to account for both the influence of relative rock uplift rate and substrate strength on channel form with implications for the HFT and more generally, extracting tectonic information from emerging topography in these settings.