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1. Tectonic advection of contacts enhances landscape transience

   https://doi.org/10.1002/esp.5559  

   Mitchell, Nate ; Forte, Adam M. ( February 2023 , Earth Surface Processes and Landforms)

   Contrasts in bedrock erodibility have been shown to drive landscape transience, but it is unclear whether horizontal tectonic displacements would enhance such effects. Furthermore, one might expect these factors to coexist, as tectonic convergence helps to create rock strength contrasts in settings like the Himalayas. How do landscapes respond when contacts separating units are raised vertically and shifted horizontally by tectonics? To evaluate such questions, we use landscape evolution models to simulate the exposure of a weak unit in a landscape equilibrated to a strong unit. We explore different simulations varying factors like weak unit erodibility, diffusivity, contact dip, and topographic advection rate. In these simulations, we assess the migration of the main drainage divide as well as changes in channel steepness and topographic relief within the strong unit. Our model results show that the horizontal movement of a contact does enhance drainage divide migration and increases in channel steepness, especially when the contact migrates along rivers with low drainage areas. Across all simulations, however, increases in topographic relief are minimal and temporary. Unexpected behaviors emerge in our simulations in which the mass balance of topography is influenced by horizontal tectonic displacements. For example, the exposure of the weak unit causes a gradual decline in the steepness of the strong unit. We interpret such behaviors to be artifacts related to the fixed boundaries of our domain and likely unrepresentative of natural landscapes. Instead, we focus on simulations where advection does not influence the mass balance of topography. These models show that the horizontal movement of contacts can enhance landscape transience, but this transience is marked by features one can use as diagnostic characteristics. Detecting such characteristics in natural landscapes featuring tectonic convergence would be difficult, however, due to the natural coincidence of factors such as faulting, folding, and landslides.

    

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2. Resistant rock layers amplify cosmogenically‐determined erosion rates

   https://doi.org/10.1002/esp.4730  

   Darling, Andrew ; Whipple, Kelin ; Bierman, Paul ; Clarke, Brian ; Heimsath, Arjun ( January 2020 , Earth Surface Processes and Landforms)

   Prior numerical modeling work has suggested that incision into sub‐horizontal layered stratigraphy with variable erodibility induces non‐uniform erosion rates even if base‐level fall is steady and sustained. Erosion rates of cliff bands formed in the stronger rocks in a stratigraphic sequence can greatly exceed the rate of base‐level fall. Where quartz in downstream sediment is sourced primarily from the stronger, cliff‐forming units, erosion rates estimated from concentrations of cosmogenic beryllium‐10 (¹⁰Be) in detrital sediment will reflect the locally high erosion rates in retreating cliff bands. We derive theoretical relationships for threshold hillslopes and channels described by the stream‐power incision model as a quantitative guide to the potential magnitude of this amplification of¹⁰Be‐derived erosion rates above the rate of base‐level fall. Our analyses predict that the degree of erosion rate amplification is a function of bedding dip and either the ratio of rock erodibility in alternating strong and weak layers in the channel network, or the ratio of cliff to intervening‐slope gradient on threshold hillslopes. We test our predictions in the cliff‐and‐bench landscape of the Grand Staircase in southern Utah, USA. We show that detrital cosmogenic erosion rates in this landscape are significantly higher (median 300 m/Ma) than the base‐level fall rate (~75 m/Ma) determined from the incision rate of a trunk stream into a ~0.6 Ma basalt flow emplaced along a 16 km reach of the channel. We infer a 3–6‐fold range in rock strength from near‐surface P‐wave velocity measurements. The approximately four‐fold difference between the median¹⁰Be‐derived erosion rate and the long‐term rate of base‐level fall is consistent with our model and the observation that the stronger, cliff‐forming lithologies in this landscape are the primary source of quartz in detrital sediments. © 2020 John Wiley & Sons, Ltd.

    

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3. Impact of fault damage on eastern Tibet topography

   https://doi.org/10.1130/G48179.1  

   Kirkpatrick, Heather M. ; Moon, Seulgi ; Yin, An ; Harrison, T. Mark ( August 2020 , Geology)

   null (Ed.)

   Tectonic deformation can influence spatiotemporal patterns of erosion by changing both base level and the mechanical state of bedrock. Although base-level change and the resulting erosion are well understood, the impact of tectonic damage on bedrock erodibility has rarely been quantified. Eastern Tibet, a tectonically active region with diverse lithologies and multiple active fault zones, provides a suitable field site to understand how tectonic deformation controls erosion and topography. In this study, we quantified erosion coefficients using the relationship between millennial erosion rates and the corresponding channel steepness. Our work shows a twofold increase in erosion coefficients between basins within 15 km of major faults compared to those beyond 15 km, suggesting that tectonic deformation through seismic shaking and rock damage significantly affects eastern Tibet erosion and topography. This work demonstrates a field-based, quantitative relationship between rock erodibility and fault damage, which has important implications for improving landscape evolution models. 

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4. Spatially Variable Increase in Rock Uplift in the Northern U.S. Cordillera Recorded in the Distribution of River Knickpoints and Incision Depths

   https://doi.org/10.1029/2018JF004880  

   Mitchell, Nate A. ; Yanites, Brian J. ( May 2019 , Journal of Geophysical Research: Earth Surface)

   Landscape evolution is driven by factors like tectonics and climate, and unraveling such factors can reveal the history recorded in landscape morphology. The northern U.S. Cordillera features many potential drivers, such as the Yellowstone plume, the extrusion of a large igneous province, and the drainage of large lakes. Among this complex geologic history, the drivers of transient incision in the Clearwater and Salmon watersheds of central Idaho are not well understood. To constrain the pattern of regional incision, we analyze the morphologies of 80 individual tributaries underlain by single lithologies. From north to south across our study area, knickpoint elevations increase from about 800 to 2,200 m, and incision depths increase from about 300 to 1,200 m. We use both numerical and analytical models to demonstrate that such a gradient could represent spatial variations in rock uplift. These findings suggest that transience is driven by a spatially variable increase in rock uplift that has disrupted a low‐relief paleolandscape, and the high steepness values of main drainages suggest that high rock‐uplift rates are still maintained to the present. Changes in rock uplift may be related to interactions between the Yellowstone plume and the lithosphere, although base level fall from the drainage of the Lake Idaho down the proto‐Snake River may be superimposed over these patterns in rock uplift. We show that careful, quantitative analyses of river profiles in geologically complex regions can differentiate between the influences of rock uplift and far‐field base level changes.

    

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5. Building a bimodal landscape: bedrock lithology and bed thickness controls on the morphology of Last Chance Canyon, New Mexico, USA

   https://doi.org/10.5194/esurf-11-995-2023  

   Anderson, Sam ; Gasparini, Nicole ; Johnson, Joel ( October 2023 , Earth Surface Dynamics)

   We explore how rock properties and channel morphology vary with rock type in Last Chance Canyon, Guadalupe Mountains, New Mexico, USA. The rocks here are composed of horizontally to near-horizontally interbedded carbonate and sandstone. This study focuses on first- and second-order channel sections, where the streams have a lower channel steepness index (ksn) upstream and transition to higher ksn values downstream. We hypothesize that differences in bed thickness and rock strength influence ksn values, both locally by influencing bulk bedrock strength and also nonlocally through the production of coarse sediment. We collected discontinuity intensity data (the length of bedding planes and fractures per unit area), Schmidt hammer rebound measurements, and measured the largest boulder at every 12.2 m elevation contour to test this hypothesis. Bedrock and boulder mineralogy were determined using a lab-based carbonate dissolution method. High-resolution orthomosaics and digital surface models (DSMs) were generated from drone and ground-based photogrammetry. The orthomosaics were used to map channel sections with exposed bedrock. The United States Geological Survey (USGS) 10 m digital elevation models (DEMs) were used to measure channel slope and hillslope relief. We find that discontinuity intensity is negatively correlated with Schmidt hammer rebound values in sandstone bedrock. Channel steepness tends to be higher where reaches are primarily incising through more thickly bedded carbonate bedrock and lower where more thinly bedded sandstone is exposed. Bedrock properties also influence channel morphology indirectly, through coarse sediment input from adjacent hillslopes. Thickly bedded rock layers on hillslopes erode to contribute larger colluvial sediment to adjacent channels, and these reaches have higher ksn values. Larger and more competent carbonate sediment armors both the carbonate and the more erodible sandstone and reduces steepness contrasts across rock types. We interpret that in the relatively steep, high-level ksn downstream channel sections, the slope is primarily controlled by the coarse alluvial cover. We further posit that the upstream low-level ksn reaches have a base level that is fixed by the steep downstream reaches, resulting in a stable configuration, where channel slopes have adjusted to lithologic differences and/or sediment armor. 

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