http://quebec.hwr.arizona.edu/classes/hwr696t/gosse01-cosmogenic-nuclide-review.pdf

Terrestrial in situ cosmogenic nuclides: theory and application

■ Abstract Bedrock rivers set much of the relief structure of active orogens and dictate rates and patterns of denudation. Quantitative understanding of the role of climate-driven denudation in the evolution of unglaciated orogens depends first and foremost on knowledge of fluvial erosion processes and the factors that control incision rate. The results of intense research in the past decade are reviewed here, with the aim of highlighting remaining unknowns and suggesting fruitful avenues for further research. This review considers in turn (a) the occurrence and morphology of bedrock channels and their relation to tectonic setting; ( b) the physical processes of fluvial incision into rock; and (c) models of river incision, their implications, and the field and laboratory data needed to test, refine, and extend them.

/pdf/bedrock-rivers-and-the-geomorphology-of-active-orogens-1m7ptkwy5z.pdf

Bedrock rivers and the geomorphology of active orogens

The topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock-channel incision. Quantitative field assessments of process-based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate. We evaluate and calibrate the shear stress (or similar unit stream-power) bedrock-incision model by studying stream profiles in a tectonically active mountain range. Previous work on emergent marine terraces in the Mendocino triple junction region of northern California provides spatial and temporal control on rock-uplift rates. Digital elevation models and field data are used to quantify differences in landscape morphology associated with along-strike northwest to southeast changes in tectonic and climatic conditions. Analysis of longitudinal profiles supports the hypothesis that the study-area channels are in equilibrium with current uplift and climatic conditions, consistent with theoretical calculations of system response time based on the shear-stress model. Within uncertainty, the profile concavity (𝛉) of the trunk streams is constant throughout the study area (𝛉 ≈ 0.43), as predicted by the model. Channel steepness correlates with uplift rate. These data help constrain the two key unknown model parameters, the coefficient of erosion ( K ) and the exponent associated with channel gradient ( n ). This analysis shows that K cannot be treated as a constant throughout the study area, despite generally homogeneous substrate properties. For a reasonable range of slope-exponent values ( n ), best-fit values of K are positively correlated with uplift rate. This correlation has important implications for landscape-evolution models and likely reflects dynamic adjustment of K to tectonic changes, due to variations in orographic precipitation, and perhaps channel width, sediment load, and frequency of debris flows. The apparent variation in K makes a unique value of n impossible to constrain with present data.

Landscape response to tectonic forcing: Digital elevation model analysis of stream profiles in the Mendocino triple junction region, northern California

[1] Developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate, erosion, and tectonics Current models of long-term river network incision differ significantly in their treatment of the role of sediment flux We analyze the implications of various sediment-flux-dependent incision models for large-scale topography, in an attempt (1) to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and (2) to explain the apparent ubiquity of mixed bedrock-alluvial channels in active orogens Although certain forms of the various models can be discarded as inconsistent with morphological data, we find that the relative intrinsic concavity indices of detachment- and transport-limited systems (defined herein) largely dictate whether the various models can be tied to distinctive steady state morphologies Preliminary data suggest that no such diagnostic differences may exist, and other methods must be developed to test models Accordingly, we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment- versus transport-limited channels as a function of rock uplift rate that may allow discrimination among various models Further, we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate In addition to general differences between detachment- and transport-limited systems our analysis identifies an interesting hysteresis in landscape evolution: “hybrid” channels at the threshold between detachment- and transport-limited conditions are expected to act as detachment-limited systems in response to an increase in rock uplift rate (or base level fall) and as transport-limited systems in response to a decrease in rock uplift rate, especially during postorogenic topographic decline The analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

https://www.public.asu.edu/~kwhipple/papers/Whipple_Tucker_2002_JGR.pdf

Implications of sediment‐flux‐dependent river incision models for landscape evolution

The simplicity and apparent mechanistic basis of the stream power river incision law have led to its wide use in empirical and theoretical studies. Here we identify constraints on its calibration and application, and present a mechanistic theory for the effects of sediment supply on incision rates which spotlights additional limitations on the applicability of the stream power law. On channels steeper than about 20%, incision is probably dominated by episodic debris flows, and on sufficiently gentle slopes, sediment may bury the bedrock and prevent erosion. These two limits bound the application of the stream power law and strongly constrain the possible combination of parameters in the law. In order to avoid infinite slopes at the drainage divide in numerical models of river profiles using the stream power law it is commonly assumed that the first grid cell is unchanneled. We show, however, that the size of the grid may strongly influence the calculated equilibrium relief. Analysis of slope-drainage area relationships for a river network in a Northern California watershed using digital elevation data and review of data previously reported by Hack reveal that non-equilibrium profiles may produce well defined slope-area relationships (as expected in equilibrium channels), but large differences between tributaries may point to disequilibrium conditions. To explore the role of variations in sediment supply and transport capacity in bedrock incision we introduce a mechanistic model for abrasion of bedrock by saltating bedload. The model predicts that incision rates reach a maximum at intermediate levels of sediment supply and transport capacity. Incision rates decline away from the maximum with either decreasing supply (due to a shortage of tools) or increasing supply (due to gradual bed alluviation), and with either decreasing transport capacity (due to less energetic particle movement) or increasing transport capacity (due less frequent particle impacts per unit bed area). We use this model to predict longitudinal profiles under varying boundary conditions and sediment supply rates and find that even in actively downcutting rivers, the river slope needed to maintain incision may be only slightly greater than the slope required to transport the imposed load. Hence, the channel slope-drainage area relationships of rivers actively cutting through bedrock may predominately reflect the grain size and supply rate of sediment and only secondarily the influence of bedrock resistance to erosion.

River Longitudinal Profiles and Bedrock Incision Models: Stream Power and the Influence of Sediment Supply

Quantifying passive margin denudation and landscape development using a combined fission-track thermochronology and cosmogenic isotope analysis approach

Denudation at passive continental margins occurs over time as erosional escarpments propagate inland, cutting through regions of elevated topography flanking ocean basins. Understanding the actual processes and time variability in propagation rates associated with the advance of escarpments across passive margins remains a largely unsolved problem for tectonic geomorphology. Here we report results from new analyses of detailed river longitudinal profiles and surface exposure age measurements using cosmogenic radionuclides from the New England Tableland portion of the southeast Australian passive margin. In that area, many plateau-draining tributaries of the Macleay River cascade into narrow gorges across large-scale river knickpoints that represent the tips of the leading edge of the inland-advancing escarpment. Previous river profile analyses showed most knickpoints to be the same distance, about 200 km, upstream from the mouth of the river, despite order-of-magnitude variations in the areas drained at the gorge heads. The implication is that all knickpoints have migrated upstream at a speed of about 2 km/Myr averaged over the ∼100 Myr history of the margin. The new profile analyses confirm that, in general, distance upstream from the Macleay River mouth of the knickpoints is not related to area drained at the gorge head. We conclude that if sufficient fluvial transport power is available, the rate of upstream knickpoint migration is governed by slope failure mechanisms and the frequency of resulting mass wasting events on the steep rock slopes in the gorge head vicinity. In terms of the stream power model for channel incision, Z t A m' S n (where A is drainage area and S is channel gradient), the river profile analyses imply that the drainage area exponent m' = 0 (i.e., no dependence on drainage area so long as a threshold is met), and the slope exponent n > 1. Average erosion rates calculated from Be and Al radionuclide concentrations in samples collected across the knickpoint on Baker's Creek indicate that the plateau surface and the channel upstream of the gorge head - knickpoint are eroding slowly, ∼5 m/Myr, whereas the channel at and downstream of the knickpoint is eroding much more rapidly, >100 m/Myr. The pattern of increasing erosion rates across the knickpoint in the downstream direction is consistent with the paradigm of inland escarpment retreat across passive continental margins.

Inland propagation of erosional escarpments and River profile evolution across the Southeast Australian passive continental margin

We examine the influence of bedrock strength properties, imparted by jointing and fracturing, on the retreat of erosional escarpments across passive continental margins. Two types of escarpment are identified, and focus is placed on the gorge head–knickpoint form of escarpment, where upland streams drain across the escarpment. Along the Macleay River system of eastern Australia, major knickpoints associated with escarpment retreat are currently located about 200 km upstream from the river mouth, despite differences in bedrock lithology and order-of-magnitude differences in drainage areas upstream of the gorge heads, implying an average escarpment retreat rate of 2 km/m.y. since the margin formed 85 m.y. ago. Field observations from Apsley River gorge, a Macleay River tributary, indicate that bedrock jointing profoundly affects the evolution of the gorge headwall and sidewalls. Where dominant jointing dips into hillslopes, failure occurs by block and column toppling. Conversely, where jointing dips out of hillslopes, mass wasting occurs by joint-parallel rock sliding, allowing slopes to remain steep. As material is shed from the gorge sidewalls, the toppling slopes develop lower overall gradients, the gorge becomes asymmetric in cross section, and the channel remains nearer to the steeper sidewall affected by joint-parallel sliding. Coupling between fluvial and hillslope processes is essential to maintain headward propagation of the knickpoint–gorge head and to facilitate mass wasting from the gorge sidewalls. Peak stream discharge across the gorge head must exceed a threshold sufficient to transport debris away from the gorge head to keep the knickpoint rock face free of talus, to incise the stream channel, and to erode the toes of the sidewalls. If these conditions on fluvial processes are met, then hillslope processes, including rockslope failure mechanisms, are the rate controlling processes for knickpoint propagation and therefore determine the rate of escarpment retreat across the passive margin.

Influence of rock strength properties on escarpment retreat across passive continental margins

In situ-produced cosmogenic isotope concentrations in bedrock surfaces provide valuable estimates of site-specific, long-term rates of denudation and provide constraints for numerical landscape-evolution models. Measurements of cosmogenic {sup 10}Be and {sup 26}Al from graphite inselbergs in the arid to hyperarid central Namib Desert, Namibia, indicate a mean rate of summit lowering of 5.07 {+-} 1.1 m/m.y. over the past {ge} 10{sup 5} yr. The persistence of an arid climate in the region suggests that a similar rate may have prevailed for the past {approximately} 10 m.y. and possibly throughout much of the Cenozoic. Some samples have complex exposure histories that can be explained by the mode of inselberg weathering and mass wasting. The denudation rates estimated here are an order of magnitude higher than those reported for inselbergs in a significantly more humid environment in South Australia. This difference may largely be due to active salt weathering in the central Namib as a result of high levels of coastal fog precipitation.

Quantifying denudation rates on inselbergs in the central Namib Desert using in situ–produced cosmogenic 10Be and 26Al

The use of cosmogenic isotopes to determine surface exposure ages has grown rapidly in recent years. The extent to which cosmogenic nuclides can distinguish between mechanistic hypotheses of landscape evolution is an important issue in geomorphology. We present a case study to determine whether surface exposure dating techniques can elucidate the role knickpoint propagation plays in longitudinal profile evolution. Cosmogenically produced 10Be, 26Al, 36Cl, 3He and 21Ne were measured in olivines collected from 5·2 Ma basalt flows on Kauai, Hawaii. Several obstacles had to be overcome prior to the measurement of In situ-produced radionuclides, including removal of meteoric 10Be from the olivine grains. Discrepancies between the radionuclide and noble gas data may suggest limits for exposure dating. Approximate surface exposure ages calculated from the nuclide concentrations indicate that large boulders may remain in the Hawaiian valley below the knickpoint for hundreds of thousands of years. The ages of samples collected above the knickpoint are consistent with estimates of erosion based on the preservation of palaeosurfaces. Although the exposure ages can neither confirm nor reject the nickpoint hypothesis, boulder ages downstream of the knickpoint are consistent with a wave of incision passing upvalley. The long residence time off the coarse material in the valley bottom further suggests that knickpoint propagation beneath a boulder pile is necessary for incision of the bedrock underlying the boulders to occur. © 1997 by John Wiley & Sons, Ltd.

Michele A. Seidl

Papers

Quantifying passive margin denudation and landscape development using a combined fission-track thermochronology and cosmogenic isotope analysis approach

Inland propagation of erosional escarpments and River profile evolution across the Southeast Australian passive continental margin

Influence of rock strength properties on escarpment retreat across passive continental margins

Quantifying denudation rates on inselbergs in the central Namib Desert using in situ–produced cosmogenic 10Be and 26Al

Cosmetic Isotope Analyses Applied to River Longitudinal Profile Evolution: Problems and Interpretations