A copy of Guangbo jiemu bao [Broadcast Program Report] was being passed from hand to hand among a group of young people eager to be the first to read the article introducing the program "What Is Revolutionary Love?" It said: "… Young friends, you are certainly very concerned about this problem'. So, we would like you to meet the young women workers Meng Xiaoyu and Meng Yamei and the older cadre Miss Feng. They are the three leading characters in the short story ‘The Place of Love.’ Through the description of the love lives of these three, the story induces us to think deeply about two questions that merit further examination.

This is How the Discussion Started

This work constructs a fracture mechanics framework for conceptualizing mechanical rock breakdown and consequent regolith production and erosion on the surface of Earth and other terrestrial bodies. Here our analysis of fracture mechanics literature explicitly establishes for the first time that all mechanical weathering in most rock types likely progresses by climate-dependent subcritical cracking under virtually all Earth surface and near-surface environmental conditions. We substantiate and quantify this finding through development of physically based subcritical cracking and rock erosion models founded in well-vetted fracture mechanics and mechanical weathering, theory, and observation. The models show that subcritical cracking can culminate in significant rock fracture and erosion under commonly experienced environmental stress magnitudes that are significantly lower than rock critical strength. Our calculations also indicate that climate strongly influences subcritical cracking—and thus rock weathering rates—irrespective of the source of the stress (e.g., freezing, thermal cycling, and unloading). The climate dependence of subcritical cracking rates is due to the chemophysical processes acting to break bonds at crack tips experiencing these low stresses. We find that for any stress or combination of stresses lower than a rock's critical strength, linear increases in humidity lead to exponential acceleration of subcritical cracking and associated rock erosion. Our modeling also shows that these rates are sensitive to numerous other environment, rock, and mineral properties that are currently not well characterized. We propose that confining pressure from overlying soil or rock may serve to suppress subcritical cracking in near-surface environments. These results are applicable to all weathering processes.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017RG000557

Mechanical weathering and rock erosion by climate-dependent subcritical cracking

. Topography is a reflection of the tectonic and geodynamic processes that act to uplift the Earth's surface and the erosional processes that work to return it to base level. Numerous studies have shown that topography is a sensitive recorder of tectonic signals. A quasi-physical understanding of the relationship between river incision and rock uplift has made the analysis of fluvial topography a popular technique for deciphering relative, and some argue absolute, histories of rock uplift. Here we present results from a study of the fluvial topography from south-central Crete, demonstrating that river longitudinal profiles indeed record the relative history of uplift, but several other processes make it difficult to recover quantitative uplift histories. Prior research demonstrates that the south-central coastline of Crete is bound by a large ( ∼  100 km long) E–W striking composite normal fault system. Marine terraces reveal that it is uplifting between 0.1 and 1.0 mm yr−1. These studies suggest that two normal fault systems, the offshore Ptolemy and onshore South-Central Crete faults, linked together in the recent geologic past (ca. 0.4–1 My BP). Fault mechanics predict that when adjacent faults link into a single fault the uplift rate in footwalls of the linkage zone will increase rapidly. We use this natural experiment to assess the response of river profiles to a temporal jump in uplift rate and to assess the applicability of the stream power incision model to this setting. Using river profile analysis we show that rivers in south-central Crete record the relative uplift history of fault growth and linkage as theory predicts that they should. Calibration of the commonly used stream power incision model shows that the slope exponent, n, is  ∼  0.5, contrary to most studies that find n  ≥  1. Analysis of fluvial knickpoints shows that migration distances are not proportional to upstream contributing drainage area, as predicted by the stream power incision model. Maps of the transformed stream distance variable, χ, indicate that drainage basin instability, drainage divide migration, and river capture events complicate river profile analysis in south-central Crete. Waterfalls are observed in southern Crete and appear to operate under less efficient and different incision mechanics than assumed by the stream power incision model. Drainage area exchange and waterfall formation are argued to obscure linkages between empirically derived metrics and quasi-physical descriptions of river incision, making it difficult to quantitatively interpret rock uplift histories from river profiles in this setting. Karst hydrology, break down of assumed drainage area discharge scaling, and chemical weathering might also contribute to the failure of the stream power incision model to adequately predict the behavior of the fluvial system in south-central Crete.

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River profile response to normal fault growth and linkage: an example from the Hellenic forearc of south-central Crete, Greece

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Position paper: Open web-distributed integrated geographic modelling and simulation to enable broader participation and applications

The importance of high-magnitude, short-lived flood events in controlling the evolution of bedrock landscapes is not well understood. During such events, erosion processes can shift from one regime to another upon the passing of thresholds, resulting in abrupt landscape changes that can have an important long lasting legacy on landscape morphology.Here we use topographic analysis and cosmogenic 3He surface exposure dating of fluvially sculpted surfaces to determine the timing of extreme flood events within the Jokulsargljufur canyon (North-East Iceland) and to constrain the mechanisms of bedrock erosion during these events. Once a threshold flow depth has been exceeded, the dominant erosion mechanism becomes the toppling and transportation of basalt lava columns and erosion occurs through the upstream migration of knickpoints. Surface exposure ages allow identification of three catastrophic erosive flood events about 9, 5 and 2 ka ago when multiple active knickpoints retreated large distances (> 2 km). Despite sustained high discharge of sediment-rich glacial meltwater (ranging from 100 to 500 m3 s-1), there is no evidence for a transition to an abrasion-dominated erosion regime since the last erosive flood event: the vertical knickpoints have not diffused over time and there is no evidence of incision into the canyon floor.We hypothesise that the erosive signature of the extreme events is maintained in this landscape due to the nature of the bedrock, the large scale of the river, large knickpoints and associated plunge pools, and the lack of transported coarse sediment (greater than gravel size). We explore these controls with an experimental study to define the possible influence of the following parameters on the dynamics of knickpoint migration and morphology in a controlled environment: discharge, flow regime, knickpoint height and initial topographic slope.

Erosion during Extreme Flood Events Dominates Holocene Canyon Evolution in North-East Iceland

The most common detachment-limited river incision models ignore the effects of sediment on fluvial erosion, yet steep reaches of mountain rivers often host clusters of large (>1 m) blocks. We argue that this distribution of blocks is a manifestation of an autogenic negative feedback in which fast vertical river incision steepens adjacent hillslopes, which deliver blocks to the channel. Blocks inhibit incision by shielding the bed and enhancing form drag. We explore this feedback with a 1-D channel-reach model in which block delivery by hillslopes depends on the river incision rate. Results indicate that incision-dependent block delivery can explain the block distribution in Boulder Creek, Colorado. The proposed negative feedback may significantly slow knickpoint retreat, channel adjustment, and landscape response compared to rates predicted by current theory. The influence of hillslope-derived blocks may complicate efforts to extract base level histories from river profiles.

Hillslope-derived blocks retard river incision

. Models of landscape evolution by river erosion are often either transport-limited (sediment is always available but may or may not be transportable) or detachment-limited (sediment must be detached from the bed but is then always transportable). While several models incorporate elements of, or transition between, transport-limited and detachment-limited behavior, most require that either sediment or bedrock, but not both, are eroded at any given time. Modeling landscape evolution over large spatial and temporal scales requires a model that can (1) transition freely between transport-limited and detachment-limited behavior, (2) simultaneously treat sediment transport and bedrock erosion, and (3) run in 2-D over large grids and be coupled with other surface process models. We present SPACE (stream power with alluvium conservation and entrainment) 1.0, a new model for simultaneous evolution of an alluvium layer and a bedrock bed based on conservation of sediment mass both on the bed and in the water column. The model treats sediment transport and bedrock erosion simultaneously, embracing the reality that many rivers (even those commonly defined as bedrock rivers) flow over a partially alluviated bed. SPACE improves on previous models of bedrock–alluvial rivers by explicitly calculating sediment erosion and deposition rather than relying on a flux-divergence (Exner) approach. The SPACE model is a component of the Landlab modeling toolkit, a Python-language library used to create models of Earth surface processes. Landlab allows efficient coupling between the SPACE model and components simulating basin hydrology, hillslope evolution, weathering, lithospheric flexure, and other surface processes. Here, we first derive the governing equations of the SPACE model from existing sediment transport and bedrock erosion formulations and explore the behavior of local analytical solutions for sediment flux and alluvium thickness. We derive steady-state analytical solutions for channel slope, alluvium thickness, and sediment flux, and show that SPACE matches predicted behavior in detachment-limited, transport-limited, and mixed conditions. We provide an example of landscape evolution modeling in which SPACE is coupled with hillslope diffusion, and demonstrate that SPACE provides an effective framework for simultaneously modeling 2-D sediment transport and bedrock erosion.

https://gmd.copernicus.org/articles/10/4577/2017/gmd-10-4577-2017.pdf

The SPACE 1.0 model: a Landlab component for 2-D calculation of sediment transport, bedrock erosion, and landscape evolution

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2017JF004575

Variable‐Threshold Behavior in Rivers Arising From Hillslope‐Derived Blocks

Quantifying off-fault deformation (OFD) rates on geomorphic timescales (10^2-10^5 yr) along strike-slip faults is critical for resolving discrepancies between geologic and geodetic slip-rate estimates, improving knowledge of seismic hazard, and understanding the influence of tectonic motion on landscapes. Quantifying OFD over these timescales is challenging without displacement markers such as offset terraces or geologic contacts. We present a landscape evolution model coupled with distributed lateral tectonic shear to show how drainage basins sheared by lateral tectonic motion can reveal OFD rates. The model shows that OFD rate can control the orientation of drainage basin topography: the faster the OFD rate, the greater the deflection of drainage basins towards a fault-parallel orientation. We apply the model to the southern San Andreas Fault near the Mecca Hills, where drainages basins change in orientation with proximity to the fault. Comparison of observed and modeled topography suggests that the OFD rate in the Mecca Hills follows an exponential-like spatial pattern with a maximum rate nearest the fault of 3.5 ± 1.5 mm/yr, which decays to approximately zero at ~600 m distance from the fault. This rate is applicable since the initiation of differential rock uplift in the Mecca Hills at approximately 760 ka. Our results suggest that OFD in this 800 m study area may be as high as 10% of total plate motion. This example demonstrates that curved drainage basins may be used to estimate OFD rates along strike slip faults.

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Off-fault deformation rate along the southern San Andreas fault at Mecca Hills, southern California, inferred from landscape modeling of curved drainages

Geologists frequently debate the origin of iconic river canyons, as well as the extent to which they record climatic and tectonic signals. Fluvial and hillslope processes work in concert to control canyon evolution; rivers both set the boundary conditions for adjoining hillslopes and respond to delivery of hillslope-derived sediment. But, what happens when canyon walls deliver boulders that are too large for a river to carry? River canyons commonly host large blocks of rock derived from resistant hillslope lithologies. Blocks have recently been shown to control the shapes of hillslopes and channels by inhibiting sediment transport and bedrock erosion. Here, we developed the first process-based model for canyon evolution that incorporates the roles of blocks in both hillslope and channel processes. Our model reveals that two-way negative channel-hillslope feedbacks driven by block delivery to the river result in characteristic plan-view and cross-sectional river canyon forms. Internal negative feedbacks strongly reduce the rate at which erosional signals pass through landscapes, leading to persistent local unsteadiness even under steady tectonic and climatic forcing. Surprisingly, while the presence of blocks in the channel initially slows incision rates, the subsequent removal of blocks from the oversteepened channel substantially increases incision rates. This interplay between channel and hillslope dynamics results in highly variable long-term erosion rates. These autogenic channel-hillslope dynamics can mask external signals, such as changes in rock uplift rate, complicating the interpretation of landscape morphology and erosion histories. INTRODUCTION River canyons, steep-walled valleys often developed in bedrock, evolve through a combination of deepening by river incision and widening by hillslope processes. Considerable effort has been expended on establishing the timing and mechanisms of canyon evolution (e.g., Cook et al., 2009; Schildgen et al., 2009; Flowers and Farley, 2012), with a focus on understanding landscape response to climatic and tectonic forcing. The traditional view of canyon erosion holds that river incision, driven by tectonics, climatic perturbations, or changes in substrate erodibility, lowers the canyon bottom. Adjacent hillslopes then respond to river incision by rockfall, landsliding, and/or diffusive sediment transport (e.g., Mudd and Furbish, 2007; Gallen et al., 2011). Under the assumption that sediment delivered to the channel is mobile, patterns and time scales of river incision control hillslope form and dominate canyon evolution. The majority of prior work on canyon development has embraced this view and drawn conclusions about the timing of canyon evolution under the assumption that hillslopes respond passively to river incision. However, canyon-confined rivers do not operate in isolation from their adjacent hillslopes (Egholm et al., 2013; Attal et al., 2015; Shobe et al., 2016, 2018; Bennett et al., 2016; Golly et al., 2017; DiBiase et al., 2018). Hillslopes make up the majority of canyon plan-view area and are often the primary source of sediment to the rivers. Steep canyon walls with substantial bare bedrock exposure and sufficient fracture density commonly release large pieces of rock (with diameters of several meters) into the channel (Howard and Selby, 1994; Glade et al., 2017; DiBiase et al., 2018; Glade and Anderson, 2018). Large grain delivery to rivers can inhibit incision over large spatial and temporal scales (Shobe et al., 2016, 2018), even damming rivers for short periods of time (Korup et al., 2006; Ouimet et al., 2007; Castleton et al., 2016). We propose that slowing or cessation of river incision must then influence the hillslopes by reducing the rate of hillslope steepening. Block delivery therefore acts as a negative feedback on both river and hillslope erosion. To constrain canyon evolution rates and process dynamics, it is critical to understand the interactions between canyon-confined rivers and their adjacent hillslopes. We developed a numerical model that explicitly treats block dynamics both on hillslopes and in channels. We then examined the influence of these negative channel-hillslope feedbacks on river canyon evolution, with two guiding questions: (1) Are negative channel-hillslope feedbacks necessary and sufficient to explain the cross section and planform shapes of natural canyons? (2) How do these feedbacks affect long-term erosion dynamics in river canyons responding to base-level fall? Block delivery is likely important in any blockproducing landscape. However, as a simplified test case, we focused on river canyons in layered rock (Figs. 1 and 2) in which a resistant cap rock (e.g., sandstone) overlies softer rock (e.g., shale). In this simplified geologic setting, the cap rock acts as a line source of blocks, with block size dictated by cap-rock thickness and joint spacing. The softer, underlying layer produces soil but no blocks. Canyons developed in layered rock often exhibit key morphologic features, such as a characteristic bell-shaped planform during transient response to base-level fall (Figs. 1A and 1B), block-mantled channels, and steep hillslopes (Figs. 1C–1F). Here, we explore how block delivery feedbacks influence canyon form and evolution. CITATION: Glade. R.C., Shobe, C.M., Anderson, R.S., and Tucker, G.E., 2019, Canyon shape and erosion dynamics governed by channel-hillslope feedbacks: Geology, v. 47, p. 650–654, https:// doi .org /10 .1130 /G46219.1 *These authors contributed equally to this work. †E-mails: rachel .glade@ colorado .edu; charles .shobe@ colorado .edu. Manuscript received 13 December 2018 Revised manuscript received 14 April 2019 Manuscript accepted 15 April 2019 https://doi.org/10.1130/G46219.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 2 May 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/7/650/4775270/650.pdf by guest on 06 July 2019 Geological Society of America | GEOLOGY | Volume 47 | Number 7 | www.gsapubs.org 651 CONCEPTUAL MODEL Several processes (e.g., landsliding, debris flows) may influence channel-hillslope coupling. We focused on the delivery of large blocks from hillslopes to channels (Fig. 2), which we hypothesized would control river canyon form. During the early stages of canyon evolution, channel incision causes failure of the cap rock (Ward et al., 2011), which delivers large blocks to the hillslopes and channel. The presence of blocks on the hillslopes inhibits soil erosion, stalling subsequent block release from the cap rock (Glade et al., 2017; Glade and Anderson, 2018). Blocks in the channel, if they are too large to be transported, reduce the river incision rate by armoring the bed and increasing hydraulic roughness (Shobe et al., 2016, 2018). Prolonged inhibition of river incision reduces the rate of hillslope steepening and hence the rate of block delivery to the channel. Thus, as long as hillslopes supply blocks to the channel (Fig. 2), erosion rates both in the channel and on the hillslopes are expected to be highly variable in time. Eventually, the hillslopes retreat far enough from the channel that blocks weather during hillslope transport to a size at which they no longer inhibit river incision (Fig. 2). From then on, the channel lowers at a rate that is unaffected by hillslope-derived blocks. NUMERICAL MODELING METHODS We tested the conceptual model shown in Figure 2 with a series of numerical experiments coupling models for channel (Shobe et al., 2016) and hillslope (Glade et al., 2017) evolution that incorporate the effects of large blocks of rock (see the GSA Data Repository1). The model domain, designed to represent the layered landscapes in Figures 1 and 2, consists of a horizontal resistant layer of rock overlying softer, more-erodible rock (the domain is 2 km wide × 1 km long, with 5 m resolution). A channel of uniform initial slope, forced with a constant base-level fall rate at its downstream end, incises the weaker, underlying rock. The channel permanently occupies the center of the model domain and has a constant width and discharge. The rest of the model domain operates under hillslope process laws (Glade et al., 2017; Glade and Anderson, 2018). We used this model to investigate erosion dynamics and time scales in a river canyon in which blocks released from the resistant cap rock cause interactions that govern both hillslope and channel evoluCanyonlands National Park, Utah Gorges du Tarn, France Great Escarpment, South Africa Columbia River Basalt, Oregon 0 20 30

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Charles M. Shobe

Papers

Hillslope-derived blocks retard river incision

The SPACE 1.0 model: a Landlab component for 2-D calculation of sediment transport, bedrock erosion, and landscape evolution

Variable‐Threshold Behavior in Rivers Arising From Hillslope‐Derived Blocks

Off-fault deformation rate along the southern San Andreas fault at Mecca Hills, southern California, inferred from landscape modeling of curved drainages

Canyon shape and erosion dynamics governed by channel-hillslope feedbacks