Geomorphic signatures of the transient fluvial response to tilting

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 a punctuated rigid-block tilting event. Using a model river network composed of linked 1-D river longitudinal profile evolution models, we show that the transient response to punctuated tilting creates characteristic forms or geomorphic signatures in mainstem and tributary profiles that are distinct from those generated by other perturbations such as a step change in uniform rock uplift rate or major truncation of 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 towards the outlet, versus the mainstem less out of equilibrium than the tributaries following back tilting towards the headwaters; 2) a pattern of mainstem incision below paleotopography 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 tilting, knickpoints form at the mainstem outlet and each mainstem-tributary junction. Time since tilting onset is recorded by mainstem knickpoint location relative to base level and by the upstream end of tributary knickzones relative to tributary-mainstem junctions. Tilt magnitude is recorded in the spatial gradient of mainstem incision depth and, in the forward tilting case, by 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 during the transient and that the magnitude of tilt can be recovered from the relationship between channel gradient and azimuth, but only shortly after tilting. 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 to estimate timing and magnitude of a tilting event, we show a sample of rivers draining the west slope of the Sierra Nevada, California, USA, a range long thought to have been tilted westward towards river outlets in the late Cenozoic.