Quantifying landscape response times to active faulting and bedrock erodibility: Field insights from Calabria, Italy
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River topography can provide useful insights into landscape response times in active tectonic regions. Lithology and uplift rate are the two factors widely assumed to control the speed of knickpoint incision, however field constraints quantifying these controls remain sparse. Calabria’s rapidly uplifting landscape since the Late Pleistocene provides an excellent opportunity to explore the influence of lithology and uplift rates in driving knickpoint retreat in rivers crossing active normal faults. First, we use numerical models to estimate knickpoint propagation rates and bedrock erodibility. Then, we use detailed field and structural measurements of in-situ rock strength (Schmidt hammer), fracture density and orientation to quantify rock strength along three rivers with contrasting lithologies crossing the Cittanova, Armo and Santa Eufemia normal faults. Finally, we compare bedrock erodibility estimates with our field measurements of rock strength and published fault slip histories to quantify controls on knickpoint velocities. Modelled bedrock erodibility values in Calabria of ~1.25 x10-5 yr-1 are generally fast when compared with published data for rivers crossing active normal faults. Steepened reaches of the river channels do not spatially correlate with higher compressive rock strength measurements. Instead, we observe that significant scatter in field measurements reflects variable weathering states of the rocks rather than intrinsic compressive bedrock strength. We argue that lithology does not appear to play a significant role in governing the shape or channel narrowing of the river long profiles in Calabria. Instead, our results show that knickzone retreat velocities (5–15 mm yr-1) reflect temporal and spatial patterns of tectonic uplift along mapped active normal faults in the region. These findings suggest uplift rates rather than lithology control landscape response times over timescales of > 105 years in regions of active faulting.