Search

[1] Incised fluvial systems are typically interpreted as recording geologically recent changes in either climate or tectonics. However, few diagnostic tools exist to evaluate whether particular incised landscapes primarily reflect climatic or tectonic perturbation. Here we summarize the results of a simple fluvial sediment transport model that allows us to contrast patterns of fluvial incision driven by changes in hydrology and sediment flux (“climate”) with those driven by changes in rock uplift patterns relative to sea level (“tectonics”). Our modeling suggests that there may be diagnostic differences between the spatial and temporal patterns of incision caused by these different processes. In particular, incision driven by climate change is most commonly accompanied by downstream migrating waves of incision and decreases in channel gradient, while under most circumstances incision driven by tectonics will be accompanied by upstream migrating incision and increases in channel gradient. We apply our modeling to a case study of the North American High Plains, where regionally elevated surfaces east of the Rockies have been incised up to 500 m by the fluvial systems draining the core of the range. Although this incision has been interpreted as reflecting a tectonic rejuvenation of the High Plains, our analysis suggests that climate change is at least as plausible an explanation to explain the incision of this landscape. Diagnostic differences between the climate and tectonic end-member scenarios might be obtained via detailed study of the spatial and temporal patterns of terrace abandonment, providing a potentially fruitful target for field investigation.

[1] The morphology of a relict landscape provides important insight into erosion rates and processes prior to base level fall. Fluvial knickpoints are commonly thought to form a leak-proof moving boundary between a rejuvenated landscape below and a relict landscape above. We argue that fluvial rejuvenation may leak farther upstream, depending on the rate and style of knickpoint migration. The outer margin of a relict landscape should therefore be used with caution in tectonic geomorphology studies, as channel steepening upstream of knickpoints could reduce the relict area. We explore the response of the Roan Plateau to knickpoint retreat triggered by late Cenozoic upper Colorado River incision. Multiple knickpoints (100-m waterfalls) separate a low-relief, upper landscape from incised canyons below. Two digital elevation model data sets (10-m U.S. Geological Survey and 1-m Airborne Laser Swath Mapping) indicate steeper channels above waterfalls relative to concave channels farther upstream. The steepened reaches are several kilometers long, correspond to doubling of slope, and exhibit channel narrowing and an increase in hillslope angle. We compare two mechanisms for generating steepened reaches. The first uses a recent model for erosion amplification due to flow acceleration at the waterfall lip. The second acknowledges that waterfall lips may be limited to the outcrop of a resistant formation. Subtle structural warping of the stratigraphy can lead to lowering of the waterfall lip as it retreats, thus lowering base level for upstream channels. Results of numerical modeling experiments suggest the latter mechanism is more consistent with our observations of long, mildly steepened reaches.

[1] We use cosmogenic radionuclide (CRN) exposure ages from polished, striated bedrock to constrain numerical simulations of deglaciation in the Middle Boulder Creek Valley, Colorado Front Range, and the Animas River Valley, San Juan Mountains, Colorado. In both valleys, the cosmogenic ages suggest initiation of deglaciation ∼20 ka and ongoing retreat until 12–13 ka. While the first-order trend in CRN concentrations in each valley suggests a monotonic glacial retreat, we evaluate other retreat scenarios with different implications for post-Last Glacial Maximum regional climate. We use a 2-D numerical glacier simulation with a CRN layer to investigate how CRN-based deglaciation records are affected by retreat histories that are punctuated by periods of glacier readvance. The CRN layer simulates both production during periods of exposure and reduction by glacial erosion during readvances. We simulate glacial occupation of the valleys as they respond to equilibrium line altitude (ELA) histories characterized by stepwise change, gradual rise, or a rise punctuated by short periods of lowering. Each scenario generates a distinct spatial pattern of concentrations in the CRN layer. These results and the spatial pattern of measured concentrations in bedrock constrain the range of ELA histories that reproduce the CRN pattern in each valley. In the Animas River Valley, the exposure ages are well explained by a linear ELA rise from full glacial to deglacial conditions. Ages in Middle Boulder Creek Valley are best explained by a deglaciation history including a stillstand or partial readvance between 16 and 14 ka, followed by rapid retreat.

[1] In many tectonically active regions on Earth, landslides dominate sediment delivery to channels. While cosmogenic radionuclides (CRN) represent a valuable tool to document basin-averaged erosion rates, a better understanding of CRN dynamics in these systems is required before erosion rates can be reliably inferred from CRN concentrations. The stochastic nature of landslides results in variability of CRN concentrations in both alluvial and hillslope samples that is both difficult to document and poorly understood. To guide sampling strategies, we developed a 2-D plan view model to simulate the influence of landslides on CRN dynamics. We employ a sediment mixing model, in which concentrations of CRNs from material eroded by both landslides and steady processes are combined to evaluate how best to infer erosion rates from CRN concentrations. This model is further used to test a new analytical solution for the distribution of CRN concentrations on hillslopes in a landslide-prone system. Results suggest that for landscapes dominated by deep (>5 m) landslides, only large drainage basins (≥100 km2) provide consistently reliable estimates of erosion rates from CRN concentrations. For landscapes dominated by shallow landslides (0.7–2 m), smaller basins can produce reliable estimates. Results further suggest that the distribution of concentrations from hillslope samples could be used to deduce the relative contribution of material derived from landslide and steady processes. These findings underscore the need to consider carefully the sources of natural variability in sampling and analyzing CRNs; if properly done, the variability can reveal useful information about geomorphic processes in landslide-prone areas.

The Microscopic Imager (MI) on the Mars Exploration Rover Opportunity has returned images of Mars with higher resolution than any previous camera system, allowing detailed petrographic and sedimentological studies of the rocks and soils at the Meridiani Planum landing site. Designed to simulate a geologist's hand lens, the MI is mounted on Opportunity's instrument arm and can resolve objects 0.1 mm across or larger. This paper provides an overview of MI operations, data calibration, and analysis of MI data returned during the first 900 sols (Mars days) of the Opportunity landed mission. Analyses of Opportunity MI data have helped to resolve major questions about the origin of observed textures and features. These studies support eolian sediment transport, rather than impact surge processes, as the dominant depositional mechanism for Burns formation strata. MI stereo observations of a rock outcrop near the rim of Erebus Crater support the previous interpretation of similar sedimentary structures in Eagle Crater as being formed by surficial flow of liquid water. Well-sorted spherules dominate ripple surfaces on the Meridiani plains, and the size of spherules between ripples decreases by about 1 mm from north to south along Opportunity's traverse between Endurance and Erebus craters.

[1] The cross-sectional shape of a natural river channel controls the capacity of the system to carry water off a landscape, to convey sediment derived from hillslopes, and to erode its bed and banks. Numerical models that describe the response of a landscape to changes in climate or tectonics therefore require formulations that can accommodate evolution of channel cross-sectional geometry. However, fully two-dimensional (2-D) flow models are too computationally expensive to implement in large-scale landscape evolution models, while available simple empirical relationships between width and discharge do not adequately capture the dynamics of channel adjustment. We have developed a simplified 2-D numerical model of channel evolution in a cohesive, detachment-limited substrate subject to steady, unidirectional flow. Erosion is assumed to be proportional to boundary shear stress, which is calculated using an approximation of the flow field in which log-velocity profiles are assumed to apply along vectors that are perpendicular to the local channel bed. Model predictions of the velocity structure, peak boundary shear stress, and equilibrium channel shape compare well with predictions of a more sophisticated but more computationally demanding ray-isovel model. For example, the mean velocities computed by the two models are consistent to within ∼3%, and the predicted peak shear stress is consistent to within ∼7%. Furthermore, the shear stress distributions predicted by our model compare favorably with available laboratory measurements for prescribed channel shapes. A modification to our simplified code in which the flow includes a high-velocity core allows the model to be extended to estimate shear stress distributions in channels with large width-to-depth ratios. Our model is efficient enough to incorporate into large-scale landscape evolution codes and can be used to examine how channels adjust both cross-sectional shape and slope in response to tectonic and climatic forcing.

[1] We argue that by fracturing rock, not by raising it relative to base level, tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes. Tectonic deformation riddles the upper crust with fractures, which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes. Bends in active faults require straining of adjacent rock masses. Aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust. At least some aftershocks must nucleate or lengthen cracks, which contribute to the comminution of these rock masses. Scaling rules suggest that dimensions of ruptures for very small (M < −2) earthquakes can be meters or less. The Gutenberg-Richter recurrence relationship implies that such earthquakes are common, as high-magnification seismographs in low-noise environments confirm. Moreover, large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another; some faults must intersect. Thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller. Dismembered rock arrives at the Earth's surface already prepared to be transported away. As a corollary, both deeply exhumed lower crust and posttectonic igneous rock, never deformed under brittle conditions and not deformed recently, should be less susceptible to detachment and subsequent transport than fractured rock.

[1] The Roan Plateau in western Colorado constitutes a natural experiment for studying landscape response to a drop in base level. Late Cenozoic incision of the upper Colorado River led to elevational isolation of the Plateau and initiation of a wave of incision into its southern edge. Knickpoints (oversteepened reaches that contain waterfalls 60–110 m in height) mark the upstream extent of this headward propagating wave. That this incision has occurred in a laterally extensive, well-stratified, and essentially flat-lying bedrock and in an area with relatively uniform climate, implies that it should serve as a good test of existing knickpoint propagation models. We predict the locations of knickpoints by using a stream power-based celerity model, in which knickpoint recession rate is a power function of drainage area and is proportional to rock susceptibility to erosion. Models of the Parachute and Roan drainages (17 and 16 knickpoints, respectively) show expected rapid initial knickpoint propagation rates, which decline as drainage area decreases stepwise at tributary junctions. The modeled positions of knickpoints match well with observed features, using a single combination of parameters to model retreat in both drainage basins. We compare our celerity model results with past studies and explore how longitudinal profile analysis may be used to derive independently the exponent on drainage area in the celerity model.

[1] The high relief of the modern Rocky Mountain landscape formed in the late Cenozoic by downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores of adjacent ranges. Using a numerical model of fluvial erosion and the flexural isostatic response to the associated unloading, we first calculate the expected pattern and pace of incision caused by rock uplift related to migration of the Yellowstone hot spot and to growth of the northern portion of the Rio Grande rift. Calculated incision rates are

[1] Glacial occupation of alpine valleys results in a distinct signature in the long-valley profile, including steepening of the profile in the headwaters, flattening at lower elevations, and a step in the profile at the convergence of headwater tributaries. We present analytic results for glacial erosion patterns by making the following assumptions: (1) the initial profile is linear, (2) the width of the valley is uniform, (3) the annual mass balance varies linearly with elevation, (4) the glacier at any time is quasi-steady, (5) the erosion rate is proportional to ice discharge per unit valley width, and (6) glacial erosion rates far exceed fluvial erosion rates. A steady glacier under these conditions would erode a parabolic divot in the longitudinal valley profile, with its maximum depth coinciding with the down-valley position of the equilibrium line altitude (ELA). The calculated flattening of the valley floor down valley of the ELA and the steepening of it up valley captures the essence of the glacial signature. When a reasonable probability distribution of ELAs is allowed, the predicted erosion peaks at 30-40% of the down-valley distance to the glacial limit, and the pattern merges smoothly with the steeper fluvial profile downstream of the glacial limit. Profiles of mass balance that are capped at a maximum value produce shorter glaciers and a slight asymmetry in the expected erosion pattern. Only when we mimic the tributaries of glacial headwaters by specifying a valley width distribution do we obtain the upper step observed in many valley profiles.

[1] Glaciers draining westward from the Sierra Nevada divide, California, during the Last Glacial Maximum (LGM) were � 7 times longer than east-draining glaciers. We address the degree to which this difference may be attributed to the topographic asymmetry of the west-tilted Sierran block and the climate asymmetry resulting from orographic modification of Pacific Ocean storms. We simulate kilometer-scale glaciers within the 50 � 50 km Kings Canyon region of the southern Sierra by employing a two-dimensional numerical model that is driven by simple, spatially variable climates and treats ice transport by deformation, sliding, and avalanching. In numerical experiments, we match simulated termini to LGM moraine positions to constrain the parameters of different climate scenarios. The 38-km-long LGM glacier in Kings Canyon was reproduced by a climate specified by an equilibrium line altitude (ELA) of 3170 m, a mass balance gradient of 0.01 m/yr/m, and a maximum positive balance of 2 m/yr. This climate generates much shorter (average � 6 km long) east-draining glaciers that, however, overshoot the LGM moraines by � 1 km. Roughly 97% of the E-W difference in glacier lengths can therefore be attributed to topographic asymmetry alone. A second experiment suggesting a 120-m-higher ELA of 3290 m east of the divide can explain the shorter east-draining glaciers. An experiment in which orographic precipitation is explicitly simulated and melt is prescribed using a positive degree-day algorithm matches both Kings Canyon and the average east-draining glacier length with an LGM climate that was 5.6� C cooler and � 2 times wetter than the modern Sierra Nevada.

per unit area of drainage basin (an effective precipitation rate P )t oa: a � 1 / P n , where n � 1.6. Using this relationship, we confirm that rivers in arid regions should incise less rapidly as climate becomes yet more arid. (If no water flows, the ‘‘river’’ transports no sediment.) Whether aridification of an initially humid environment leads to increased or decreased incision rates, however, depends on the minimum (threshold) discharge capable not only of transporting bed load but also sufficient to scour alluvium from the riverbed and then erode the bedrock. The curve fit relating P to a implies that for aridification to accelerate incision, floods that recur only once or twice per millennium (or less frequently) must carry out most of the incision. An overestimate of n could permit smaller, more frequent floods to incise, but it appears that in only special circumstances will aridification accelerate steam incision.

[1] Glacial erosion is known to be driven by sliding, which occurs in brief episodes in the melt season. What is not known is how well this sliding history is reflected in the sediment output from a glacier and to what degree temporary sediment storage in the glacier smoothes the history of sediment production. We document sediment evacuation rates for three seasons at Bench Glacier, a small temperate alpine glacier in the Chugach Range of Alaska. Two lines of evidence suggest that subglacial sediment storage is small. Strong hysteresis in the relationship between sediment and water discharge argues for sediment exhaustion from the subglacial system on seasonal and flood event timescales. Uplift of the glacier surface during periods of enhanced basal motion reflects intimate contact of the glacier sole with up-glacier-inclined stoss slopes of bedrock bumps. Sediment evacuation rates at Bench Glacier should therefore closely track glacial erosion rates over annual timescales. Much of the sediment emerges in a major pulse that closely follows the termination of enhanced sliding each year, suggesting that the conduits and well-connected cavities that allowed removal of englacial water to terminate sliding also promoted efficient transport of sediment. This small glacier efficiently modifies the landscape, lowering its bed by 1–2 mm/yr while sliding only 1–2 m/yr. Our sparse measurements of instantaneous bed load discharge average 41% of concurrent suspended sediment discharge. Assuming that half of the suspended load derives from abrasion of subglacially quarried clasts, roughly two thirds of the sediment evacuated can be attributed to quarrying.

[1] Evolution of a tectonically active coast is driven by geomorphically destructive energy supplied by ocean waves. Wave energy is episodic and concentrated; sea cliffs are battered by the geomorphic wrecking ball every 4–25 s. We measure the response of sea cliffs to wave assault by sensing the ground motion using near-coastal seismometers. Sea cliffs respond to waves in two distinct styles. High-frequency motion (20 Hz) reflects the natural frequency of the sea cliff as it rings in response to direct wave impact. Low-frequency motion in the 0.1–0.05 Hz (10–20 s) band consistently agrees with the dominant nearshore wave period. Integrating microseismic velocities suggests 50 μm and 10 μm displacements in horizontal and vertical directions, respectively. Displacement ellipsoids exhibit simultaneous downward and seaward sea cliff motion with each wave. Video footage corroborates the downward sea cliff flex in response to the imposed water load on the wave cut platform. Gradients in displacement amplitudes documented using multiple seismometers suggest longitudinal and shear strain of the flexing sea cliff on the order of 0.5–4 μ strains during each wave loading cycle. As this sea cliff flexure occurs approximately 3 million times annually, it has the potential to fatigue the rock through cyclical loading. Local sea cliff retreat rates of 10 cm/yr imply that a given parcel of rock is flexed through roughly 109 cycles of increasing amplitude before exposure to direct wave attack at the cliff face.

[1] We report the spatial and temporal pattern of sliding on the 7-km-long Bench Glacier, Alaska. Using five continuously recording GPS antennas following motion of the surface ice, distributed at 1 km spacing along the glacier center line, we documented surface ice motion over 50 days during summer 2002. Surface speeds in two previous winters constrain the motion component associated with ice deformation, allowing isolation of the sliding speed history. We observed two speedup events bracketing 2 weeks of steady slow sliding. The first event was not associated with a meteorological trigger, was more subtle than the second, and propagated up-glacier at a rate of several hundred meters per day. The second event coincided with a warm up-valley wind, which triggered considerable melt of the glacier surface. Sliding speeds in this event reached 0.3 m d−1 and began almost simultaneously at all sites in the ablation area. Both the horizontal and vertical displacement time series can be explained by growth and collapse of cavities in the lee of bumps in the bedrock bed. Cavities grow during rapid sliding and decay by viscous creep. We posit that effective pressure, averaged over some large area of the bed, is inversely proportional to the sliding speed. This effective pressure then controls the collapse rate of cavities, whose dimensions are estimated from a plausible, stepped-bed geometry. This model explains well the horizontal and vertical surface displacement history through the first event and beginning of the second event. The vertical record demands a substantial and abrupt drop in water pressure that departs from the posited sliding-effective pressure relationship. We argue that this pressure drop reflects establishment of efficient subglacial drainage, also manifested in a nearly simultaneous step increase in water discharge in the exit stream. The establishment of an efficient conduit system terminates sliding; its maintenance inhibits further sliding over the remainder of the summer.