Tectonic and climatic controls on knickpoint retreat rates and landscape response times

Abstract

[1] The rate at which transient knickpoints propagate through a landscape fundamentally controls the rate of geomorphic response to tectonic and climatic perturbation. Here we present knickpoint retreat rates upstream of active faults for 19 bedrock catchments in Turkey and 11 bedrock catchments in Italy where we have very good constraints on both the magnitude and timing of the tectonic perturbation and where climate histories are well documented. We show that the knickpoints have average retreat rates of between 0.2 and 2 mm/yr for catchments with drainage areas between 6 and 65 km 2 and we test whether differences in rock mass strength and catchment size are sufficient to explain this range in retreat rates. Our analysis suggests that even accounting for these two variables, knickpoint propagation velocities differ markedly, and we show that channels crossing faults with higher throw rates have knickpoints that are retreating faster. The dependence of knickpoint retreat velocity on throw rate is at least as important as catchment drainage area. These results indicate, counterintuitively, that landscapes forced by large amplitude tectonic perturbations will have shorter response times than those perturbed by smaller amplitude changes. The link between the knickpoint propagation velocity and throw rate is largely (but not completely) explained by channel narrowing in areas of high uplift rate. Channel steepening upstream of the active faults may explain all of the residual dependency of knickpoint retreat rate on fault throw rate, but only if the slope exponent, n, in the standard stream power model is greater than 1.3. However, we cannot rule out a role for sediment supply in driving enhanced knickpoint retreat rate in addition to the well-documented channel narrowing effect. Finally, we find that mean knickpoints retreat rates in Turkey are only half of those in Italy, for catchments of equivalent size, crossing faults with similar throw rates. This difference in fluvial response time is accounted for by long-term differences in the ratio of precipitation to infiltration in the two areas over the last 1 My.

Numerical modelling of landscape and sediment flux response to precipitation rate change

“…10). Our results are also consistent with the field-based findings of Whittaker and Boulton (2012), who showed that landscape response times for rivers close to the detachment-limited end-member were shorter for terrain uplifted by faster-slipping active faults. Figure 13.…”

“…Taking the drainage length to be directly proportional to the catchment area, l d ∝ a, and given that catchment length is proportional to drainage area raised to the Hack exponent, h, we can re-write Eq. (14) as Therefore, in the case that h = 0.5 and m = 0.5, as in the numerical model here, the response time becomes independent of system length (see Whittaker and Boulton, 2012). If h < m then response times would decrease with increasing drainage basin size, and if h > m then response times would increase with drainage basin size.…”

“…Third, the sediment transport model has a greater magnitude of peak sediment flux and is particularly sensitive to catchment size. Finally, we note that response time in both models is a function of uplift rate for n > 1, which means that in such cases, perhaps counter-intuitively, the more perturbed the system the faster it responds (see Whittaker and Boulton, 2012).…”

“…These apparently long response timescales to tectonic perturbations have been supported by field observations of landscapes upstream of active faults (e.g. Whittaker et al, 2007;Cowie et al, 2008;Whittaker and Boulton, 2012), although the precise appropriateness of any time-integrated erosion law to specific field sites is not always easy to establish. Sediment flux response times for the advective stream power law have been previously characterized by Whipple (2001) and Baldwin et al (2003), and for the transport model they have been studied by Armitage et al (2011) and Armitage et al (2013), but not systematically or using 2-D models.…”

Patterns of Quaternary uplift of the Corinth rift southern border (N Peloponnese, Greece) revealed by fluvial landscape morphometry

“…However, possibly because the control of K on steepness is damped by sorting and attrition effects on bedload grain size with distance, the lower segments of the I<rios and Trikalitikos do not show the low steepness expected from their lithological conditions. This not only confirms the minor role of bedrock resistance to erosion in the long-term process of river incision (Roberts and White, 2010;Whittaker and Boulton, 2012;Beckers et al, 2014), but also shows that, except in the far west of the study area, the current episode of uplift is more intense than the previous one.…”