New constraints on bedrock erodibility and landscape response times upstream of an active fault

Abstract

Considerable progress has been made in modelling the response of rivers to tectonic perturbation in order to decode the tectonic signals embedded in river long profiles and planform geometry. Whilst studies have showed the importance of rock type on the morphology of rivers responding to tectonics on a local scale, these effects are often not captured in landscape evolution models. In fact, current models of fluvial response to tectonic perturbation such as active faulting require carefully collected data sets to fully constrain or calibrate key parameters, including the effect of bedrock lithology on substrate erodibility and timescales for tectonic signal propagation in bedrock river systems. Here we constrain the role of bedrock in controlling fluvial incision for a 240 km 2 catchment draining into the Gulf of Corinth, which has excellent tectonic constraints and a variety of bedrock lithologies. An active normal fault at the downstream end of the catchment (the East Eliki Fault) is known to have initiated at 0.7 Ma, with average Quaternary uplift and incision rates of 1.00-1.25 mm/yr. The initiation of the East Eliki Fault is recorded in the river as a prominent knickzone 7-16 km upstream of the fault. Detailed field data collected within this catchment at 500 m intervals along the main channel length at this tectonically wellconstrained field site show an order of magnitude increase in river channel slopes (y/x = 0.02 to 0.18) and stream powers (1 kWm -2 to 27 kWm -2 ) at the lithological boundary between weak and resistant bedrock. The weak conglomerates and strong limestones have Schmidt hammer compressive strengths of ca. 30 and 50 respectively, based on in-situ rock strength measurements. Consequent erodibility values of 1.8 ± 0.3 × 10 -14 and 5 -6 ± 2 × 10 -15 ms 2 kg -1 show a factor of three to four decrease in erodibility for a two-fold increase in rock strength. A simple simulation of tectonic signal propagation through the river catchment based on these erodibility values indicates that the observed plan view position of the knickpoint is consistent with our calculated knickpoint positions based on erodibility values derived from incision rate and stream power data. However, the tectonic signal associated with faulting would have propagated completely through the catchment within 1 Ma if it were entirely composed of weak rock for similar climatic and tectonic conditions, indicating that lithology has a major control on landscape response times.Our results help constrain the effect of lithology on river channel geometry and allow erosional parameters to be derived that are crucial for effective modelling of tectonic rates from topography.

A New Data‐Driven Bayesian Inversion of Fluvial Topography Clarifies the Tectonic History of the Corinth Rift and Reveals a Channel Steepness Threshold

“…∼6.2 × 10 −6 m 1−2 m yr −1 . These results are consistent with the previously reported range of erodibility estimates known for the area for comparable rock types (Table 3; Pechlivanidou et al, 2019;Zondervan et al, 2020).…”

“…This assertion is supported by river profiles and hillslope gradient maps that do not show distinct stepped morphologies (beyond those artifacts in the DEM) that are commonly observed in horizontal or subhorizontal strata with strong differences in erodibility (Figures 2e and S1). Furthermore, several other geomorphic studies assume that comparable units within Corinth maintain spatially and temporally uniform erodibility and generate results that mimic geological observations (Pechlivanidou et al, 2019;Watkins et al, 2018;Zondervan et al, 2020). As noted above, there are small amounts of valley aggradation upstream in the river network associated with back tilting due to flexural uplift (Fernández-Blanco et al, 2020), but such deposits are spatially limited.…”

“…These authors use hydraulic geometry, calculations of bank-full discharge, and estimates of the contemporary rock uplift velocity to determine erodibility for several rock units incised by the river. The incision model calibrated by Zondervan et al (2020) is a variant of the stream power model that explicitly accounts for channel geometry and discharge, which in our equations are represented by a scaling relationship between both variables and drainage area. Converting their erodibility to stream power units assuming that m = 0.5 and n = 1 yields a value of K of ∼6 × 10 −6 m 1-2 m yr −1 for rock types comparable to those in the Eastern Sector.…”

Geometry of Flexural Uplift by Continental Rifting in Corinth, Greece

“…In the west rift, westward of the reversed drainages, the largest catchments have river morphologies that are similar albeit less marked, with steep gorges and narrow valleys in a broad upwarped zone that separates a low‐gradient, low‐relief landscape upstream from a steeper, high‐relief landscape downstream (e.g., Seger & Alexander, ; Zondervan et al, ). As in the rift center, west rift sequences of offlapping stranded deltas become younger coastward (Ford et al, , ; Seger & Alexander, ).…”

River patterns reveal two stages of landscape evolution at an oblique convergent margin, Marlborough Fault System, New Zealand

“…Recent studies focusing on lithology and fractures in bedrock channels have also revealed the importance of material strength in setting incision rates and response times of channels to base level forcings (e.g., Stock and Montgomery, 1999;Whipple and Tucker, 2002;Cook et al, 2009;Whittaker and Boulton, 2012;Forte et al, 2016;Yanites et al, 2017;DiBiase et al, 2018;Zondervan et al, 2020) as well as in setting patterns in the position and orientations of rivers (e.g., Koons et al, 2012;Roy et al, 2015;2016a, b, c;Upton et al, 2018;Scott and Wohl, 2019). Detailed field and numerical modeling studies of rivers in faulted landscapes in particular demonstrate the sensitivity of fluvial incision to gradients in erodibility between weak fault zones and the surrounding stronger bedrock (Roy et al, 2016c).…”