Rock Uplift Rates Research Articles

Cover effect in bedrock abrasion: A new derivation and its implications for the modeling of bedrock channel morphology

The sediment load of a bedrock river plays an important role in the fluvial incision process by providing tools for abrasion (the tools effect) and by covering and thereby protecting the bed (the cover effect). We derive a new formulation for the cover effect, in which the fraction of exposed bed area falls exponentially with increasing sediment flux or decreasing transport capacity, and explore its consequences for the model of bedrock abrasion by saltating bed load. Erosion rates predicted by the model are higher than those predicted by earlier models. In a closed system, the maximum erosion rate is predicted to occur when sediment supply is equal to transport capacity for a flat bed. By optimizing the channel geometry to minimize the potential energy of the stream and using representative values for both discharge and grain size, we derive equations for the geometry of a bedrock river and explore how predictions for width, slope, and bed cover vary as functions of drainage area, rock uplift rate, and rock strength. The equations predict a dependence of channel width on drainage area similar to the relations using a simple shear stress incision law. The slope‐area relationship is predicted to be concave up in a log‐log regime, with a curvature dependent on uplift rate. However, this curvature does not deviate sufficiently from a straight line to allow discrimination between models using empirical data. Dependence of channel width and slope on rock uplift rate can be separated into two domains: for low uplift rates, channel geometry is largely insensitive to uplift rate due to a threshold effect. At high uplift rates, there is a power law dependence. Bed cover is predicted to increase progressively downstream and to increase with increasing uplift rate. In our model, the width‐to‐depth ratio is a function of both tectonic and climatic forcing. This indicates that the scaling between channel width and bed slope is neither a unique indicator of tectonic forcing at steady state nor a signature of transience or steady state. We conclude that sediment effects need to be taken into account when modeling bedrock channel morphology.

Transpressional tectonics and stream terraces of the Gobi‐Altay, Mongolia

We studied the patterns, rates and evolution of fluvial terraces and fault system during the building process of an intracontinental transpressional mountain in the Gobi‐Altay (Mongolia). By analyzing incisions and offsets of fluvial terraces and alluvial fans, we show that the massif has grown by outward migration of thrust faults through time. On the northern flank, the present bounding thrust fault began its activity ∼600 ka ago, while a more internal sub‐parallel fault was still active until ∼200‐100 ka. Vertical offset of an alluvial fan abandoned ∼100 ka ago allows an estimate of 0.1 mm/yr Upper Pleistocene ‐ Holocene uplift rate. The morphology of the catchment‐piedmont system strongly suggests a periodical formation of the alluvial surfaces, controlled by the climatic pulses, at the beginning of the wet interglacial periods. The abandonment of the alluvial terraces lags by several thousand years the abandonment of the alluvial fans, showing a diachronous incision propagating upstream. The incision rate deduced from the different elevations of straths exceeds of one order of magnitude the rock uplift rate. This excess is mostly due to ongoing drainage network growth at the core of the massif, and incision due to alluvial apron entrenchment near the outlet. This implies that fluvial response is mainly controlled by drainage growth, interaction with piedmont and cyclic climatic variations, rather than by rock uplift.

Characterization of topographic steady state in Taiwan

The arc-continent collision in Taiwan has resulted in surface uplift, high relief and high rates of erosion. Because the collision is older in the north and has propagated southward, Taiwan offers the opportunity to study the evolution of topography as it evolves towards a large-scale steady state. In this paper, we analyze a 40-m digital elevation model of Taiwan to determine how topographic steady state is expressed in the landscape; particular attention is paid to the eastern Central Range, where major drainage basins are approximately aligned in the direction of the collision propagation. Analyses of orogen size, regional topographic slope, drainage basin hypsometry, and local topographic slope reveal a geomorphological transition 100–125 km north of the southern tip of the island. To the south of this transition, topographic characteristics change with distance along the island; to the north, these characteristics are relatively constant, which we interpret as representing large-scale topographic steady state. Assuming that the collision has propagated southward at a rate of 55 mm/yr, the time to topographic steady state is 1.8–2.3 Myr after emergence above sea level. Within the steady state region, we evaluate regional variability in the topography and find that the most prominent deviations from a steady form are associated with large-scale structures, though our analyses of local topographic slope and river profile form suggest that spatial variations in rock strength and rock uplift rate also contribute to regional topographic variability.

Response of glacial landscapes to spatial variations in rock uplift rate

The response of glaciated landscapes to rapid rock uplift, driven by tectonic convergence, is an important, often neglected, aspect of proposed interactions between plate tectonic processes and climate change. Rivers typically respond to more rapid rock uplift in part through increasing channel gradients. In contrast, the “glacial buzzsaw” hypothesis suggests that glaciers can erode as quickly as the fastest rock uplift rates (6–10 mm/yr) without any increase in mean elevations. However, it has not been established how this is achieved. We examined moving window maps, swath and longitudinal profiles, hillslope relief, and hypsometry for glacierized and formerly glacierized basins in areas of spatially variable rock uplift rate in the Southern Alps, New Zealand, and around Nanga Parbat, Pakistan, to determine whether glaciers have a specific response to rapid rock uplift. The response of these glaciated landscapes to rapid rock uplift (6–10 mm/yr) comprises (1) modest steepening of the longitudinal profiles in smaller glaciated basins, (2) maintenance of shallow downvalley slopes in larger glaciated basins (>∼30 km2, Southern Alps; >∼100 km2, Nanga Parbat), (3) development of tall headwalls, and (4) steepening of the basin as a whole, dominated by hillslope lengthening. Around Nanga Parbat, headwalls several kilometers high constitute >50% of the basin relief. At rapid rock uplift rates, although glaciers can incise the valley floor swiftly, they cannot prevent headwalls from reaching exceptional heights. The associated increase in mean distance between cirque heads (i.e., a decrease in drainage density) causes regional mean elevation to rise with increasing rock uplift rate. However, this is much less than the changes in elevation expected in unglaciated ranges.

Insights into subduction‐related uplift along the Hikurangi Margin, New Zealand, using numerical modeling

We investigate the causes of regional uplift along the Hikurangi Margin (North Island, New Zealand). Late Quaternary rock uplift rates and postglacial fluvial incision rates delineate broad changes in rock uplift pattern along the margin. Comparison with the Neogene rock uplift rates, the erosion surface on basement, and the margin morphology suggests that these uplift patterns are long‐lived and relate to deep‐seated subduction processes. We use two‐dimensional numerical (finite element) models to place constraints on margin geometry and uplift rates resulting from subduction processes inferred to drive regional uplift. The models indicate that subduction of relatively buoyant and thick Hikurangi Plateau crust is the most likely mechanism for low rates (<1 mm yr−1) of uplift along the entire margin. Sediment underplating is probably the major cause of higher rates of uplift of the central and northern axial ranges (2–4 mm yr−1). Seamount subduction could account for short wavelength zones of higher uplift rate on the order of 1 mm yr−1 and could contribute to tectonic erosion, which may be driving uplift in the northern margin. This study shows the value of using numerical models to link regional patterns of rock uplift and surface morphology to deep‐seated subduction processes.

Predictions of steady state and transient landscape morphology using sediment‐flux‐dependent river incision models

Recent experimental and theoretical studies support the notion that bed load in mountain rivers can both enhance incision rates through wear and inhibit incision rates by covering the bed. These effects may play an important role in landscape evolution and, in particular, the response of river channels to tectonic or climatic perturbation. We use the channel‐hillslope integrated landscape development (CHILD) numerical model with two different bedrock incision models that include the dual role of the sediment flux to explore the transient behavior of fluvial landscapes. Both models predict that steady state channel slopes increase in landscapes with higher rock uplift rates. However, the incision models predict different transient responses to an increase in uplift rate, and the behavior of each incision model depends on both the magnitude of change in uplift rate and the local drainage area. In some cases, the transient channel behavior is indistinguishable from that predicted for transport‐limited alluvial rivers. In other cases, knickpoints form in some or all of the drainage network, as predicted by the detachment‐limited stream power model. In all cases the response in the lower parts of the network is highly dependent on the response in the upper parts of the network as well as the hillslopes. As the upper parts of the network send more sediment downstream, channel incision rates may rise or fall, and slopes in the lower parts of the channel may, in fact, decrease at times during the transient adjustment to an increase in rock uplift rate. In some cases, channel incision in the upper parts of the network ceases during the transient while the hillslopes adjust to the new uplift rate; drainage density may also change as a function of uplift rate. Our results suggest that if the sediment flux strongly controls bedrock incision rates, then (1) the transient fluvial response will take longer than predicted by the detachment‐limited stream power model, (2) changes in channel slope may be much more complex than predicted by the detachment‐limited stream power model, and (3) changes in the fluvial system will be closely tied to sediment delivery from the hillslopes. Importantly, our results outline quantitative differences in system behavior produced by competing models and provide a framework for identifying locations in natural systems where differences in channel morphology can be used to discern between competing fluvial erosion models.

Coupling glacial erosion and tectonics at active orogens: A numerical modeling study

Climate change indirectly alters the distribution of tectonic uplift at active orogens by modifying the action of surface processes, which in turn alters mountain topography. The impact of alpine glaciation on tectonic activity is explored here. The predictions of previous analytical, critical wedge models are compared with the output of a numerical model that explicitly couples rock uplift produced by convergence with surface erosion. Glacial and fluvial and erosive processes are calculated over a two‐dimensional grid, which uses an ice evolution model to calculate the rate of glacial erosion due to ice sliding. Tectonic uplift is determined by a two‐dimensional finite element convergence model. The simulations performed support the predictions of previous one‐dimensional analytical modeling of fluvially dominated orogens: Increasing precipitation decreases orogen width by the one quarter power. The simulations show that glacially dominated landscapes have similar dependencies. The total glacial erosion yield is determined to be proportional to the glacial coverage, sublinearly dependent on the precipitation rate, and linearly dependent on the glacial erosion constant that relates erosion with sliding speed. Climate also determines the distribution of tectonic uplift in the model. If the equilibrium line altitude (ELA) is high and glacial processes are limited to peak regions, the center of the mountain range experiences the highest rate of rock uplift. Extensive glaciation increases rock uplift rates on the flanks of the range. The style of glacial erosion is also important: A simple glacial buzzsaw does not create the same tectonic response as models that physically simulate glacial erosion. The simulations support the idea that a climate cooling of the magnitude recorded in the late Cenozoic has the potential to more than double the rate of rock uplift in appropriate orogens. The implications of the model are discussed for observations from the Olympic Mountains of Washington State, the Southern Alps of New Zealand, the Alaskan coastal range, and the southern Andes.

Long‐term landscape evolution: linking tectonics and surface processes

Long‐term landscape evolution: linking tectonics and surface processes

Transient channel incision along Bolinas Ridge, California: Evidence for differential rock uplift adjacent to the San Andreas fault

The rates and spatial distribution of active deformation provide critical constraints on the geodynamics of deforming lithosphere, yet such data are often difficult to acquire in eroding landscapes where poor preservation of geomorphic or stratigraphic markers hinders strain reconstruction. Recent advances in understanding of the relationship between bedrock channel profile form and erosion rate have led to their use as an index of rock uplift rate in steady state landscapes. Here we extend this analysis to landscapes experiencing a transient increase in erosion rate using an example from the Marin County region of northern California. We characterize channel and hillslope gradients in a series of small watersheds along a monolithologic portion of the Franciscan terrane in Marin County. Channel steepness indices vary strongly from north to south along the ridge and correspond with the progressive development of relief on threshold hillslopes along valley walls. These patterns argue that recent channel incision has engendered a transient adjustment of hillslope gradient, as incision outpaces soil production rates. These differences in landscape form and inferred incision rate are explained by differential rock uplift within the region east of the San Andreas fault. Relationships between channel gradient and incision rate suggest a threefold to fivefold difference in incision rate across the region and place a minimum bound on differential rock uplift rates. Our study highlights how landscape analysis can place bounds on the distribution of Earth deformation in both space and time and thus lends insight into the processes driving that deformation.

Development and response of a coupled catchment fan system under changing tectonic and climatic forcing

Sediment fans are a potentially useful and underexploited recorder of Earth's climatic and tectonic history, but historical observations have led to conflicting views on the importance of tectonic, climatic, and lithologic variables in controlling fan morphology and deposition. A one‐dimensional model of a sediment fan and its associated catchment is used to explore the sensitivity of such simple sediment routing systems to perturbations in fault slip and precipitation rates. A transport‐limited catchment is coupled to a fan whose surface slope is set by the balance between catchment sediment efflux and the available tectonically generated basin accommodation. Rock uplift rate is spatially variable across the model space. Increasing the fault slip rate, or decreasing the precipitation rate, leads to an increase in fan slope, temporary back‐stepping of the fan toe, and a pronounced angular unconformity. Conversely, a decrease in slip rate, or an increase in precipitation rate, results in a decrease in fan slope, and progradation and eventual stabilization of the fan toe. Once perturbed, the system evolves toward a new equilibrium state with time constants of ∼0.5 to 2 Myr; these response times are insensitive to slip rate but are strongly dependent on precipitation rate. Variations in fan slope are well described by a dimensionless parameter that expresses equilibrium slope as a function of slip rate, precipitation rate, system size, and catchment lithology. This parameter holds promise as a predictive tool in inverting the morphology of natural fans for environmental variables.

Subsurface magnetostratigraphy of Pleistocene sediments from the Po Plain (Italy): Constraints on rates of sedimentation and rock uplift

We used facies analysis to reconstruct the Pleistocene sedimentary evolution of seven cores from the central-northern Po Plain, Italy. The cores record an overall regressive sequence consisting of shallow-water marine and fl uvial-deltaic deposits overlain by fully continental sediments. We used magnetostratigraphy to date marine and fl uvial-deltaic sediments to the early Pleistocene and continental sediments to the middle–late Pleistocene. Sediment accumulation rates were ~30–40 cm/k.y. in the early Pleistocene, whereas relevant unconformities and/or an overall reduction in sediment accumulation rates characterized the middle–late Pleistocene. A simple Airy compensation model was applied to restore actual sediments elevations to elevations at times of deposition expressed in meters above current sea level. The correlation of isostatically corrected sedimentary facies to a sea-level curve obtained from classic oxygen-isotope studies shows that an event of rock uplift on the order of ~70–120 m occurred in the middle–late Pleistocene. Literature studies of vegetational cyclicity, used in conjunction with the sea-level curve, allowed us to link sedimentary facies to climate variability. We propose that the onset of fully and persistently continental sedimentation occurred in response to the waxing of the fi rst major Pleistocene glaciation in the Alps, currently correlated to marine isotope stage 22 at ca. 0.87 Ma, and that the episode of uplift occurred (at least in part) as a consequence of erosion and crustal rebound of the Alpine chain triggered by Pleistocene glacialinterglacial cycles.

Self‐formed bedrock channels

An effective physics‐based rule for the evolution of bedrock channel cross sections is required for quantitative modeling of the roles of climate, tectonics, and sediment supply in setting bedrock longitudinal profiles and landscape form. Here we propose a modeling strategy in which the spatial pattern of erosion rates in a channel cross section is calculated, allowing exploration of the origin of the channel cross‐sectional profile, and of the dependence of channel width on flow discharge and channel slope. Our approach reproduces many of the scaling relationships observed in natural systems, including power‐law width‐discharge (W∼Q0.4) and width‐slope (W∼S−0.2) relationships. Models of channel cross‐sections linked in series and subject to varying rock uplift (baselevel lowering) rates produce concave‐up longitudinal profiles with power‐law slope‐uplift (S∼B1.31) and width‐uplift (W∼B−0.24) relationships. Our modeling strategy can easily be adapted to handle i) better representations of erosional processes, ii) better approximations of the flow structure, and iii) the role of non‐uniform sediment mantling of the bed.

Experimental channel response to tectonic uplift

We have performed a series of experiments to investigate the relationship between channel geometry and tectonic forcing in steady state landscapes at various uplift rates. The experimental setup consists of uniformly uplifted silica paste eroded by artificial rainfall. In this setup, erosional landscapes evolve by growth and amalgamation of incisions, which organize into a drainage network whose dynamics are a function of the interaction between vertical channel cutting, hillslope erosion, and sediment transport. High‐precision (0.5 mm pixel size) digital elevation models were constructed using a stereogrammetric camera system. From this data, channel bed slope was found to be independent of discharge and position on the experimental surface and to increase linearly with uplift rate. Geometric parameters of the flow, such as channel width or depth, could not be observed directly during the experiments. Using laminar flow equations for the mean flow velocity, these parameters were back calculated, and relationships to the imposed substrate uplift rate were derived. Channel width, cross‐sectional area, and wetted perimeter decrease with increasing uplift rate to a limit value, hydraulic radius and flow depth increase slightly, and flow velocity increases approximately linearly with increasing uplift rate. These results are qualitatively consistent with recent field surveys and highlight the importance of channel width and slope variations in accommodating channel response to variable rock uplift rates.

Neotectonics of the central Nepalese Himalaya: Constraints from geomorphology, detrital 40Ar/39Ar thermochronology, and thermal modeling

The southern flanks of the central Nepalese Himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate. This transition can be explained by either (1) accretion of footwall material to the hanging wall across a ramp in the décollement separating India from Eurasia, thereby enhancing rock uplift rates above the zone of accretion or (2) out‐of‐sequence surface thrust faulting at the physiographic transition. Here we use geomorphic data, 649 new detrital 40Ar/39Ar cooling ages, and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central Nepalese Himalaya. We first define and delineate the physiographic transition in central Nepal using maps of knickpoints, river steepness indices, local relief, and the distribution of thick alluvial fill deposits. We then report new detrital 40Ar/39Ar data from two trans‐Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition. Thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions. We contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out‐of‐sequence surface thrusting within the Lesser Himalayan metasedimentary sequence, approximately 15–30 km south of the mapped surface trace of the Main Central Thrust system. An important finding of this work is that there are substantial along‐strike variations in physiography and thermal history that reflect along‐strike changes in the degree and location of out‐of‐sequence surface thrusting.

The role of sediment in controlling steady-state bedrock channel slope: Implications of the saltation–abrasion incision model

The saltation–abrasion model is a mechanistic model for river incision into bedrock by saltating bedload, which we have previously derived and used experimental data to constrain all parameter values. Here we develop a method for applying the saltation–abrasion model at a landscape scale, and use the model as a reference for evaluating the behavior of a wide range of alternative incision models, in order to consider the implications of the saltation–abrasion model, as well as other models, for predicting topographic steady-state channel slope. To determine the single-valued discharge that best represents the effects of the full discharge distribution in transporting sediment and wearing bedrock, we assume all runoff can be partitioned between a low-flow and a high-flow discharge, in which all bedload sediment transport occurs during high flow. We then use the gauged discharge record and measurements of channel characteristics at a reference field site and find that the optimum discharge has a moderate magnitude and frequency, due to the constraints of the threshold of grain motion and bed alluviation by high relative sediment supply. Incision models can be classified according to which of the effects of sediment on bedrock incision are accounted for. Using the predictions of the saltation–abrasion model as a reference, we find that the threshold of motion is the most important effect that should be represented explicitly, followed in order of decreasing importance by the cover effect, the tools effect and the threshold of suspension effect. Models that lack the threshold of motion over-predict incision rate for low shear stresses and under-predict the steady-state channel slope for low to moderate rock uplift rates and rock strengths. Models that lack the cover effect over-predict incision rate for high sediment supply rates, and fail to represent the degree of freedom in slope adjustment provided by partial bed coverage. Models that lack the tools effect over-predict incision rate for low sediment supply rates, and do not allow for the possibility that incision rate can decline for increases in shear stress above a peak value. Overall, the saltation–abrasion model predicts that steady-state channel slope is most sensitive to changes in grain size, such that the effect of variations in rock uplift rate and rock strength may affect slope indirectly through their possible, but as yet poorly understood, influence on the size distribution of sediments delivered to channel networks by hillslopes.

Relations between postglacial fluvial incision rates and uplift rates in the North Island, New Zealand

Fluvial incision rates are increasingly being used as a proxy for rock uplift rates for inland settings. We test the validity of this approach by comparing incision rates and rock uplift rates along the Hikurangi Margin, New Zealand. Postglacial fluvial incision rates were calculated by measuring the difference in altitude between the Last Glacial Maximum (LGM) terrace tread (18 ± 2 calendar ka) and the present river bed for 10 major rivers (catchment areas ∼1400–5900 km2). Incision rates have thus been measured over a half glacial cycle. They also represent average rates over the last 18 kyr, during which time variations are likely due to processes such as knickpoint retreat. Rock uplift rates have been estimated from (1) altitude differences between the treads of pairs of fluvial fill terraces (∼55 and ∼18 calendar ka) assumed to have formed at the same altitude (i.e., rock uplift relative to the geoid), (2) present‐day altitudes of Late Neogene paleoshoreline marker horizons (1.6–5.7 Ma) (i.e., rock uplift relative to sea level), and (3) present‐day altitudes of Neogene mudstones for which maximum depth of burial has been calculated from porosity (i.e., rock uplift relative to sea level). These rock uplift rates overestimate tectonic uplift rates because they do not take into account isostatic compensation due to erosion. Mean incision rates range up to 11.7 mm yr−1, and mean rock uplift rates range up to 4, 1, and 0.9 mm yr−1 for the three methods, respectively. We have made some qualitative and semiquantitative observations about nontectonic controls on postglacial incision rates and find that localized influence only is exerted by rainfall, base level, and bedrock lithology, while negative or no correlations are shown with LGM fluvial fill thickness and stream power. Incision rates and uplift rates display a rough correlation, with incision rates typically 1.5–5 times uplift rates. This relation suggests that tectonic uplift is likely to exert an important control on fluvial incision in the Hikurangi Margin.

The effects of rock uplift and rock resistance on river morphology in a subduction zone forearc, Oregon, USA

AbstractRelationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along‐strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a−1, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co‐vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a−1, to subsidence (<0 mm a−1). These observations are consistent with models of transient river response to a decrease in uplift rate. Conversely, the rivers between 44° and 43°N have similar concavities and flow on the same mapped bedrock unit as the central region, but have bedrock channels and irregular longitudinal profiles, suggesting the river profiles reflect a transient response to an increase in uplift rate. If changes in rock uplift rate explain the differences in river profile form and morphology, it is unlikely that rock uplift and erosion are in steady state in the Oregon coastal mountains. Copyright © 2006 John Wiley & Sons, Ltd.

Orogen response to changes in climatic and tectonic forcing

Despite much progress, many questions remain regarding the potential dynamic coupling between atmospheric and lithospheric processes in the long-term evolution of mountain belts. As a complement to recent efforts to discover the interrelationships among climate, topography, erosion, and rock deformation under conditions of mass-flux steady state, we explore orogen response to changes in climate and tectonic influx. We derive an analytical model that predicts a powerful climatic control on orogen evolution and distinct, potentially diagnostic, responses to climatic and tectonic perturbations. Due to isostatic compensation, the near-surface rock uplift rate during transients is tightly coupled to climate-modulated erosional efficiency. System response is approximately exponential, with a characteristic response timescale that is inversely proportional to the climate- and lithology-modulated erosional efficiency, and is largely insensitive to initial conditions, tectonic influx, and both the sign and magnitude of perturbations.

Climatic forcing of erosion, landscape, and tectonics in the Bhutan Himalayas

A fundamental objective in studies of climate-erosion-tectonics coupling is to document convincing correlation between observable indicators of these processes on the scale of a mountain range. The eastern Himalayas are a unique range to quantify the contribution of tectonics and climate to long-term erosion rates, because uniform and steady tectonics have persisted for several million years, while monsoonal precipitation patterns have varied in space and time. Specifically, the rise of the Shillong plateau, the only orographic barrier in the Himalayan foreland, has reduced the mean annual precipitation downwind in the eastern Bhutan Himalaya at the Miocene-Pliocene transition. Apatite fission-track (AFT) analyses of 45 bedrock samples from an E-W transect along Bhutan indicate faster long-term erosion rates outside of the rain shadow in the west (1.0‐1.8 mm/yr) than inside of it in the east (0.55‐0.85 mm/yr). Furthermore, an AFT vertical profile in the latter segment reveals a deceleration in erosion rates sometime after 5.9 Ma. In this drier segment of Bhutan, there are remnants of a relict landscape formed under a wetter climate that has not yet equilibrated to the present climatic conditions. Uplift and preservation of the paleolandscape are a result of a climate-induced decrease in erosion rates, rather than of an increase in rock uplift rate. This study documents not only a compelling spatial correlation between long-term erosion and precipitation rates, but also a climatically driven erosion-rate change on the scale of the eastern Himalayas, a change that, in turn, likely influences that region’s recent tectonic evolution.

How does alluvial sedimentation at range fronts modify the erosional dynamics of mountain catchments?

AbstractAt the geological time scale, the way in which the erosion of drainage catchments responds to tectonic uplift and climate changes depends on boundary conditions. In particular, sediment accumulation and erosion occurring at the edge of mountain ranges should influence the base level of mountain catchments, as well as sediment and water discharges. In this paper, we use a landform evolution model (LEM) to investigate how the presence of alluvial sedimentation at range fronts affects catchment responses to climatic or tectonic changes. This approach is applied to a 25 km × 50 km domain, in which the central part is uplifted progressively to simulate the growth of a small mountain range. The LEM includes different slope and river processes that can compete with each other. This competition leads to ‘transport‐limited’, ‘detachment‐limited’ or ‘mixed’ transport conditions in mountains at dynamic equilibrium. In addition, two end‐member algorithms (the channellized‐flow and the sheet‐flow regimes) have been included for the alluvial fan‐flow regime. The three transport conditions and the two flow algorithms represent six different models for which the responses to increase of rock uplift rate and/or cyclic variation of the precipitation rate are investigated.Our results indicate that addition of an alluvial apron increases the long‐term mountain denudation. In response to uplift, mountain rivers adapt their profile in two successive stages; first by propagation of an erosion wave and then by slowly increasing their channel gradients. During the second stage, the erosion rate is almost uniform across the catchment area at any one time, which suggests that dynamic equilibrium has been reached, although the balance between erosion and rock uplift rates has not yet been achieved. This second stage is initiated by the uplift of the mountain river outlets because of sedimentation aggradation at the mountain front. The response time depends on the type of water flow imposed on the alluvial fans domains (× by 1.5 for channelized flow regime and by 10 for the sheet flow one).Cyclic variations of precipitation rate generate cyclic incisions in the alluvial apron. These incision pulses create knick‐points in the river profile in the case of ‘detachment‐limited’ and ‘mixed’ river conditions, which could be mistaken for tectonically induced knick‐points. ‘Transport‐limited’ conditions do not create such knick‐points, but nevertheless trigger erosion in catchments. The feedbacks linked to sedimentation and erosion at range front can therefore control catchment incision or aggradation. In addition, random river captures in the range front trigger auto‐cyclic erosion pulses in the catchment, capable of generating incision–aggradation cycles.