EFFECTS OF GEOMECHANICAL HETEROGENEITY ON RIVER DRAINAGE AND LANDSCAPE RESPONSE (Invited Presentation)

The geomechanical properties of rock can affect fluvial processes of detachment and transport in at least two ways. First, the presence of mechanical defects such as fractures and joints facilitates rock disaggregation and particle removal; greater defect frequency leads to smaller blocks that are more easily dislodged by various fluvial processes. Second, rock bodies with more closely spaced defects yield finer grains when disaggregated, and finer grains are more frequently transported by fluid- and gravity-driven processes. Field observations of fault structures in the Southern Alps of New Zealand demonstrate that fluvial processes of erosion and transport are highly sensitive to a significant local increase in erodibility attributed to geomechanical heterogeneity in bedrock, and a comparatively smaller critical discharge required to transport fine grained fault gouge and cataclasites. Results from our coupled geodynamic-landscape evolution model suggest that exposed fault structures erode quickly relative to adjacent intact bedrock if local strength is reduced by a factor of 30 or more, causing a strong structural influence on drainage network pattern evolution and on transient sediment storage deep within active orogens. Geomechanical heterogeneity can therefore have a strong influence on the evolution of drainage network patterns, but can also initiate heterogeneous rates of erosion within a landscape through local controls on knickpoint migration rate. Our sensitivity analysis suggests that the pattern of geomechanical heterogeneity becomes more significant than the magnitude of weakening itself because local reductions in rock strength often exceed three orders of magnitude in New Zealand and elsewhere. Tectonic strain establishes a network of weakened rock that erodes far more quickly than predictions based on the bulk geomechanical properties of the orogen. The orogen-scale response rate to base level perturbations can increase by an order of magnitude or greater when interconnected local heterogeneities in rock strength exist. Further, the link between rock strength and erosion rate has major implications for the development of topographic relief, topographic stress, and strain localization within active orogens.