Geomorphic signatures of the transient fluvial response to tilting

Abstract

Abstract. Nonuniform rock uplift in the form of tilting has been documented in convergent margins, postorogenic landscapes, and extensional provinces. Despite the prevalence of tilting, the transient fluvial response to tilting has not been quantified such that tectonic histories involving tilt can be extracted from river network forms. We used numerical landscape evolution models to characterize the transient erosional response of a river network initially at equilibrium to rapid tilting. We focus on the case of punctuated rigid-block tilting, though we explore longer-duration tilting events and nonuniform uplift that deviates from perfect rigid-block tilting such as that observed when bending an elastic plate or with more pronounced internal deformation of a fault-bounded block. Using a model river network composed of linked 1-D river longitudinal profile evolution models, we show that the transient response to a punctuated rigid-block tilting event creates a suite of characteristic forms or geomorphic signatures in mainstem and tributary profiles that collectively are distinct from those generated by other perturbations, such as a step change in the uniform rock uplift rate or a major truncation of the headwater drainage area, that push a river network away from equilibrium. These signatures include (1) a knickpoint in the mainstem that separates a downstream profile with uniform steepness (i.e., channel gradient normalized for drainage area) from an upstream profile with nonuniform steepness, with the mainstem above the knickpoint more out of equilibrium than the tributaries following forward tilting toward the outlet, versus the mainstem less out of equilibrium than the tributaries following back tilting toward the headwaters; (2) a pattern of mainstem incision below paleo-topography markers that increases linearly up to the mainstem knickpoint or vice versa following back tilting; and (3) tributary knickzones with nonuniform steepness that mirrors that of the mainstem upstream of the slope-break knickpoint. Immediately after a punctuated tilting event, knickpoints form at the mainstem outlet and each mainstem–tributary junction. Time since the cessation of rapid tilting is recorded by the mainstem knickpoint location relative to base level and by the upstream end of tributary knickzones relative to the mainstem–tributary junction. Tilt magnitude is recorded in the spatial gradient of mainstem incision depth and, in the forward tilting case, also by the spatial gradient in tributary knickzone drop height. Heterogeneous lithology can modulate the transient response to tilting and, post tilt, knickpoints can form anywhere in a stream network where more erodible rock occurs upstream of less erodible rock. With a full 2-D model, we show that stream segments flowing in the tilt direction have elevated channel gradient early in the transient response. Tilting is also reflected in network topologic changes via stream capture oriented in the direction of tilt. As an example of how these geomorphic signatures can be used in concert with each other to estimate the timing and magnitude of a tilting event, we show a sample of rivers from two field sites: the Sierra Nevada, California, USA, and the Sierra San Pedro Mártir, Baja California, Mexico, two ranges thought to have been tilted westward toward river outlets in the late Cenozoic.

Response of transient rock uplift and base level knickpoints to erosional efficiency contrasts in bedrock streams

“…Recently, SPIM-based modeling studies have examined how landscapes and river profiles respond to detachment-limited scenarios that do not conform to the "standard model". For example, Beeson & McCoy (2020) modeled tectonic tilting scenarios, while Cook et al (2009), Imaizumi et al (2015), Forte et al (2016), Perne et al (2017), Yanites et al (2017), and Darling et al (2020) each investigated the role of rock contacts on erosion rates and landscape evolution. As summarized by Scheingross et al (2020), these types of experiments are important because a variety of forcings can perturb bedrock streams from erosional steady state and obfuscate signals of climate and tectonics.…”

The Direction of Erosion

“…at constant erosion rate (see, e.g., Lague, 2014) and for low-to-moderate slope angles for which tan β ≈ sin β. To keep complexity to a minimum, in the remainder of the paper we fix the exponent ratio (aka "concavity index") at a constant µ/η = 1 2 such that the resulting slope-area scaling is close to that typically observed (e.g., Beeson and McCoy, 2020;Flint, 1974;Lague, 2014;Royden and Perron, 2013).…”

Comment on esurf-2021-59

“…This model generates a channel profile with the asymptotic scaling slope ∼ area −µ/η (77) at constant erosion rate (see, e.g., Lague, 2014) and for low-to-moderate slope angles for which tan β ≈ sin β. To keep complexity to a minimum, in the remainder of the paper we fix the exponent ratio (aka "concavity index") at a constant µ/η = 1 2 such that the resulting slope-area scaling is close to that typically observed (e.g., Beeson and McCoy, 2020;Flint, 1974;Lague, 2014;Royden and Perron, 2013).…”