Catchment-wide Denudation Rates Research Articles

Cosmogenic (un-)steadiness revealed by paired-nuclide catchment-wide denudation rates in the formerly half-glaciated Vosges Mountains (NE France)

Although catchment-wide denudation rates inferred from in situ cosmogenic nuclide concentrations measured in stream sediments has represented a ground-breaking progress in geomorphology over the last three decades, most of these studies rely on 10Be concentrations only. It seems that this current and routine one-nuclide approach to infer catchment-wide denudation rates has somehow overshadowed two key assumptions that are cosmogenic steady-state and short sediment transit time at the catchment scale. Although a paired-nuclide approach allow testing these assumptions, it is rarely performed on stream sediments and this can become highly problematic in slow-eroding, formerly glaciated contexts. In this study, we thus measure both 10Be and 26Al in stream sediments pertaining to twenty-one rivers draining an entire low mountain range: the Vosges Massif (NE France). The latter exhibits a sharp gradient between its southern and northern part in terms of lithology, morphometry and climate. Moreover, if its northern part remained void of glacial cover during Quaternary cold stages, its southern part was significantly and repeatedly glaciated. We aim to assess the factors that control the denudation of the Vosges Mountains and to quantitatively explore the impact of both repeated glacial cover and storage of glacially derived sediments on 26Al/10Be ratios, hence cosmogenic (un-)steadiness in modern river samples. Our results first show that elevation, slope, channel steepness and precipitation are primarily organised along a N-S increasing trend. 10Be- and 26Al-derived catchment-wide denudation rates accordingly range from 34 ± 1 to 66 ± 2, and 41 ± 3 to 73 ± 7 mm/ka, respectively, in thirteen investigated catchments that are in cosmogenic equilibrium. Lithological contrasts may control the pattern of denudation with a higher erodibility of the sandstone-dominated catchment to the north compared to the crystalline-dominated catchments to the south.Our results also show that catchments in strong cosmogenic disequilibrium (26Al/10Be ratios from 1.4 to 5.2) spatially cluster in the SW part of the Vosges Mountains that was the most intensively glaciated during Quaternary cold stages. If this precludes any conclusion about controlling factors at the whole massif scale, this study is the first to quantify the impact of past glaciations on cosmogenic (un-)steadiness measured in stream sediments. A statistically significant relationship between the degree of depletion of the 26Al/10Be ratios and the spatial pattern of glaciation is found: the larger the former glacial cover in each catchment, the lower the 26Al/10Be ratio. Equally important is the significant correlation reported between the degree of depletion of the 26Al/10Be ratios and the proportion of glacial and fluvio-glacial deposits within each catchment. These two relationships underline the link between cosmogenic unsteadiness in the stream cosmogenic signal and long-lasting and repetitive ice shielding, and complex sediment routing systems in glacial environments, respectively. We thus argue to systematically measure 26Al in complement to 10Be and to test the steady-state assumption when it comes to infer catchment-wide denudation rates from modern stream sediments, especially in slow eroding, formerly glaciated landscapes.

Late Quaternary–Recent intraplate topographic uplift of the St. Francois Mountains (Ozark Plateau), Missouri (USA), driven by differential density and erosion

AbstractThe origin of relatively high topography in intraplate settings is elusive. The St. Francois Mountains, a tectonically inactive region with high relief, a Mesoproterozoic granite and rhyolite core, and Paleozoic sedimentary flanks, provide insights into the evolution of intraplate landscapes. We determined 14 in situ10Be exposure ages, 23 10Be catchment-wide denudation rates (CWDRs), and geomorphic indices using a geographic information system, and we modeled surface uplift using an isostatic-flexure model. CWDRs varied from ~5 to 10 ± 1.7 mm/k.y. and averaged 1.5 times greater in streams draining granite and rhyolite than in those draining flanking sedimentary rocks. In situ results from bedrock strath terraces gave Quaternary incision ages and incision rates that were tenfold higher than CWDRs. Major knickpoints (locally called shut-ins), all at 200 ± 70 m elevation, are located at contacts between the igneous and sedimentary rocks. Our exposure and denudation results inform isostatic-flexural modeling, which together support the conclusion that late Quaternary–Recent surface uplift driven by differential density and erosion focused in the igneous core is creating significant topography and relief in this intraplate setting.

Frost-weathering control on the rate of late Quaternary landscape evolution, western flank of the Taebaek Mountain Range, Korea: a case of passive margin landscape evolution

ABSTRACT Passive continental margins can show anomalously high topography and exhibit a discrete steep escarpment, divide, and gentle slope from the exterior to the interior of the margin. Compared with active (i.e. convergent and strike-slip) tectonic regions, the processes and rates of change of high-altitude landscapes driven by tectonics and/or climate in tectonically inactive (passive) continental margins are poorly understood. We used 10Be catchment-wide denudation rates of fluvial sands (n = 29) collected in 17 catchments and 12 sub-catchments, as well as topographic analysis, to quantify the rate of landscape change along the western flank of the Taebaek Mountain Range (TMR). The denudation rates range from ∼20 to ∼70 mm/ka. These rates show no significant difference between upstream and downstream areas, implying that denudation is not (or is only negligibly) affected by deep-seated mass wasting processes and human impact. 10Be denudation rates in the northern TMR are 1.6 times higher than in the south. In addition, the relationship between denudation rates and geomorphic parameter values also differs from north to south. These observed spatial differences in the rate of denudation and geomorphic response can be explained by intense frost weathering rather than lithological control. Our quantitative analysis of denudation rates and topography suggests that southwest-directed migration of the range’s main divide occurs and that the range's western flank (low relief) is likely to be in a geomorphic state of quasi-equilibrium whereas the eastern flank (steep) still remains transient.

Spatio-temporal variability and controlling factors for postglacial denudation rates in the Dora Baltea catchment (western Italian Alps)

Abstract. Disentangling the influence of lithology from the respective roles of climate, topography and tectonic forcing on catchment denudation is often challenging in mountainous landscapes due to the diversity of geomorphic processes in action and of spatial and temporal scales involved. The Dora Baltea catchment (western Italian Alps) is an ideal setting for such investigation, since its large drainage system, extending from the Mont Blanc Massif to the Po Plain, cuts across different major lithotectonic units of the western Alps, whereas this region has experienced relatively homogeneous climatic conditions and glacial history throughout the Quaternary. We acquired new 10Be-derived catchment-wide denudation rates from 18 river-sand samples collected both along the main Dora Baltea river and at the outlet of its main tributaries. The inferred denudation rates vary between 0.2 and 0.9 mm yr−1, consistent with previously published values across the European Alps. Spatial variability in denudation rates was statistically compared with topographic, environmental and geological metrics. 10Be-derived denudation rates do not correlate with modern precipitation and rock geodetic uplift. We find, rather, that catchment topography, in turn conditioned by bedrock structures and erodibility (lithotectonic origin) and glacial overprint, is the main driver of 10Be-derived denudation patterns. We calculated the highest denudation rate for the Mont Blanc Massif, whose granitoid rocks and long-term tectonic uplift support high elevations, steep slopes and high relief and thus favour intense glacial and periglacial processes and recurring rockfall events. Finally, our results, in agreement with modern sediment budgets, demonstrate that the high sediment input from the Mont Blanc catchment dominates the Dora Baltea sediment flux, explaining the constant low 10Be concentrations measured along the Dora Baltea course even downstream the multiple junctions with tributary catchments.

Geomorphic constraints on the development of a blind-thrust induced landform, south-central Mongolia: Insights into foreberg growth

Forebergs are landforms characterized by low, elongated ridges or hills rising above the surrounding alluvial fans or floodplains, and are typically formed by folding associated with thrust faulting. Forebergs in the Gobi-Altay range, south-central Mongolia, have developed in the forelands of mountains transpressionally uplifted in restraining bends along the E–W-trending sinistral strike-slip Bogd fault. Along the easternmost part of the Bogd fault, six forebergs have formed in the foreland of the Artz Bogd restraining-bend. We surveyed one of these forebergs to ascertain the fault geometry associated with its formation and understand how it has developed over time. This foreberg is ~6 km long and ~1 km wide and rises about 40 m above the low-lying floodplain. We also constrained its evolution using cosmogenic 10Be denudation rates measured from individual catchments along the foreberg, and morphometric analyses using a high-resolution DEM generated from images captured by an unmanned aerial vehicle. We suggest that the foreberg evolved via fault-propagation folding, developing an asymmetric across-foreberg topographic profile. This asymmetry results in differences in 10Be catchment-wide denudation rates and geomorphic metrics between its northern and southern slopes, causing migration of the drainage divide of the growing foreberg to adjust to the tectonic regime. By analyzing the along-foreberg variation in denudation rates and geomorphic metrics, we infer that the foreberg formed via lateral growth and linking of four discrete segments. Our study illuminates the formation of a unique landform and sheds light on how a structure enlarges both vertically and laterally during the early stage of thrust-related fold developments.

Unsteady migration of Taebaek Mountain drainage divide, Cenozoic extensional basin margin, Korean Peninsula

The Taebaek Mountain Range (TBR) initially formed via extension of a back-arc basin in the East Sea during the early-Miocene (ca. ~22 Ma) and exemplifies a typical escarpment on a passive continental margin. The TBR acts as a major watershed and divide across which topography changes from gentle western side slopes to steep eastern slopes. Compared to the geologic history of the flanking extensional basin, which is well known from analysis of its sedimentary fill, the post-extensional geomorphic history of the escarpment and basin margin has been minimally studied because of a lack of terrestrial archives. We determined the rate and cause of divide migration of the TBR using a suite of relatively new geomorphic and geochronologic tools. We used geomorphic analyses of relief, slope, river long profiles, swath profiles, and χ parameters to study this transient topography. Catchment-wide denudation rates (CWDR) using 10Be facilitated comparisons of denudation rates of cross-divide paired basins. Denudation rates east of the divide from drainages with steep slopes average nearly two times counterparts with gentle slopes west of the divide. Gilbert metrics used to assess divide mobility strongly suggest that catchments in the eastern TBR may have captured counterpart catchments in the west. Both CWDR and Gilbert metric results reveal that retreat of the main TBR divide likely accelerated in the late Quaternary following tectonic inversion, surface uplift, and tilting or flexure in the eastern TBR. Geomorphic disequilibrium across the current divide of the TBR continues until the present.

Asymmetric Hillslope Retreat Revealed from Talus Flatirons on Rock Peak, San Tan Mountains, Arizona, United States: Assessing Caprock Lithology Control on Landscape Evolution

Talus flatirons (TFs) are morphostratigraphic markers of prior talus deposition that are now disconnected from the active hillslope. Three generations of TFs (TF1, TF2, TF3) exist flanking a Sonoran Desert inselberg, Rock Peak, in a welded tuff caprocks-controlled landscape bounded by pediments. TFs at Rock Peak enable estimation of slope retreat rates through the application of cosmogenic 10Be, optically stimulated luminescence dating, and catchment-wide denudation rates (CWDR). We estimate disconnection of TF1 on Rock Peak at 88.9 ± 7.8 ka (northern slope) and 29.1 ± 2.5 ka (southern slope). Rates of hillslope retreat measure between 311.6 mm·ka−1 (northern slope) and 728.5 mm·ka−1 (southern slope). Asymmetry in retreat rates is consistent with CWDR, with southern slopes denuding ∼1.5 times faster. The asymmetry is interpreted as the result of the southward structural dip of strata present (>10°). Denudation rates on the summit of Rock Peak (54.3 ± 19.4 mm·ka−1 welded tuff; 111.2 ± 15.3 mm·ka−1 sandstone conglomerate) support interpretation that removal of welded tuff caprock accelerates denudation of this landscape and amplifies the impact of the structural dip. Given this, we interpret that Rock Peak will evolve into a rounded residual hill as pediments flanking the inselberg lengthen through time, similar to landforms observed in the surrounding landscape where the welded tuff and underlying sedimentary caprocks are no longer present. Using the range of slope retreat rates from Rock Peak, we provide a first estimate for the length of time necessary for pediments to form via hillslope retreat in the Sonoran Desert. Key Words: caprock, landscape evolution, pediment association, talus flatiron, 10Be exposure dating.

Detecting landscape transience with in situ cosmogenic 14C and 10Be

Abstract Constraining past variations in rates of erosion remains a key challenge in geomorphology, as estimates of erosion rate on different timescales must be bridged. The Late Pleistocene to Holocene represents a key timescale for studying landscape transience, as climatic change, increasing anthropogenic activity, and/or tectonic activity changed the rate of surface processes in many landscapes. Few chronometers are suitable for studying surface processes and landscape transience on these timescales, but, although not yet widely used, in situ 14C holds much promise due to its appropriate half-life of 5.7 kyr. Furthermore, by pairing in situ 14C with a stable or longer-lived nuclide, such as 10Be, 21Ne or 26Al, it is possible to compare surface process rates on distinctly different timescales. In this paper, we explore how these paired chronometers can be used to study landscape transience in non-glacial landscapes on Late Pleistocene to Holocene timescales. Indeed, we find that paired measurements of in situ 14C and a long-lived cosmogenic nuclide (CN) in samples from eroding landscapes enable the detection of changes in erosion rate during the Late Pleistocene to Holocene, if the increase or decrease in erosion rate was larger than a factor of two, and the landscape is eroding at rates that are typical for fluvial landscapes (5–500 mm/kyr). Similarly, detecting changes in catchment-wide denudation rates using paired CN measurements in stream-sediment samples is also possible in catchments with suitable conditions, such as short sediment transport times, minimal erosion from deep-seated mass movement, etc. (von Blanckenburg, 2006).

Spatial distribution of cosmogenic 10Be derived denudation rates between the Western Tian Shan and Northern Pamir, Tajikistan

The northward motion of the Pamir mountains relative to Eurasia results in uplift and shortening of the Pamir and Tian Shan orogens. The Vakhsh River catchment (Tajikistan) is situated along a thrust system in the collision zone between the Northern Pamir and the Western Tian Shan. In this study, spatial variations in catchment-wide denudation rates are reported from the Vakhsh River catchment with in situ-produced cosmogenic 10Be and 26Al of 22 samples from active river channels. Samples distributed along 140 km of the main Vakhsh River yield denudation rates between 1.28 ± 0.16 to 1.94 ± 0.26 mm/yr. In detail, samples from rivers draining the Northern Pamir indicate denudation rates of ~1.7 mm/yr, and a river draining the Alai Valley indicates denudation rates of 1.14 ± 0.14 mm/yr. In contrast, rivers draining the Western Tian Shan show increasing denudation rates from 0.18 ± 0.02 mm/yr in the west to 2.70 ± 0.36 mm/yr in the east, coincident with a small increase in the fluvial normalized steepness index (ksn). Measured 26Al/10Be ratios range from 5.2 to 7.6, indicating a low influence of sediment storage and reworking of partially shielded material. In addition, locations resampled one year later confirm previously calculated denudation rates along the main trunk. Analysis of in situ-produced cosmogenic 10Be from depth profiles collected from a sequence of terraces preserved along the main trunk of the Vakhsh River indicate terrace incision starting at ~3 kyr as well as paleo-denudation rates ranging between 1.75 ± 0.54 to 2.01 ± 0.39 mm/yr (2 s.d. errors). All these results suggest consistent high denudation rates in the Northern Pamir block, and a spatial (along strike) gradient in Western Tian Shan denudation rates. The most likely explanation for the spatial gradient in Western Tian Shan and Vakhsh River denudation rates is a regional gradient in tectonic rock uplift.

Spatially and temporally variable catchment-wide denudation rates – clues from the Alps

Spatially and temporally variable catchment-wide denudation rates – clues from the Alps

Spatially and temporally variable catchment-wide denudation rates – clues from the Alps

Abstract. THESIS ABSTRACT

Spatial and temporal variations in denudation rates derived from cosmogenic nuclides in four European fluvial terrace sequences

The denudation of landscapes is affected by temporal and spatial variations in tectonics, climate, and vegetation. However, deciphering the contributions of these different processes has proven challenging. In this study, cosmogenic nuclide-derived modern and paleo catchment-wide denudation rates in four European rivers are investigated. We present 12 new and 4 recalculated cosmogenic nuclide-derived denudation rates from modern river sediments and 14 paleo-denudation rates from terraces deposited over the last 2 Ma. The catchments studied are located in regions with minimal Quaternary tectonic activity and span different climates over 12o latitude. Results indicate that modern denudation rates range between 16 ±11 and 51 ±7 mm/ka with no clear latitudinal variation. Modern denudation rates are compared with catchment geomorphic indices including slope, fluvial steepness index, and relief. The denudation rates correlate better to catchment topographic indices (R2 ≈ 0.4) rather than climate. Paleo-denudation rates range from 8 ±7 to 56 ±7 mm/ka and are associated with a possible increase in the average paleo-denudation rates over the past 2 Ma. Taken together, the results indicate that quantification of catchment-wide denudation rates over long (Quaternary) time scales because of climate change is difficult. Future work to study climate influence on denudation rates should focus on the successes of previous work that document transient denudation rates over shorter and more recent time scales, i.e., from the Last Glacial Maximum to present.

Cosmogenic 10 Be denudation rates and geomorphometric analysis in the Ambato range (28°–29°S), Sierras Pampeanas, Argentina

Abstract Quantifying denudation rates along orogenic systems is crucial to understand how tectonics and climate interact with each other to create topography. Long-term mean denudation rates estimated from geologic data in the Ambato range (Sierras Pampeanas) is 0.34 mm/yr. When compared with short-term denudation rates from other studies northward and southward along the Argentine South Central Andes foreland, our results are similar to those found in regions with higher mean annual precipitation. Here we utilize concentrations of 10Be cosmogenic radionuclide (CRN) in river sediments from catchments along the eastern slope of the Ambato range to understand the relationship among short-term (102–104 yrs) catchment-wide denudation rates and long-term denudation rates estimated from geologic data. CRN denudation rates range between ∼0.038 and ∼0.12 mm/yr. The relationship between geomorphometric parameters (local relief, slope, hypsometric integral, and catchment mean channel steepness index, Ksn) and CRN denudation rates shows linear behavior. The best correlation concerned Ksn indicating a strong connection in the Ambato range between tectonics and denudation. When analyzed together with denudation rates from other studies northward and southward along the South Central Andes foreland, our results are similar to those found in more arid regions and suggest that bedrock strength variations should be contemplated in a regional analysis as well as climate.

Evaluating process domains in small arid granitic watersheds: Case study of Pima Wash, South Mountains, Sonoran Desert, USA

This paper provides support for the concept of geomorphic process domains developed by Montgomery (1999) by linking geomorphic processes to ecological variations seen in the Pima arid granitic watershed of the Sonoran Desert, Phoenix, Arizona. Closer joint spacing shows a statistically significant correlation with lower percentages of mineral grain attachment as measured by digital image processing of backscattered electron microscope imagery. Lower mineral grain attachment leads to more frequent spalling of rock surfaces, as measured by varnish microlamination (VML) ages of the last spalling event. In contrast, more distant joint spacing leads to in situ 10Be erosion rates of 3.4–8.5mm/ka and the emergence of low domes and kopje granitic landforms; these low domes also serve as knickpoints along ephemeral washes. Distant jointing thus plays a key role in generating the bare bedrock surfaces that funnel limited precipitation to bedrock margins — enhancing the canopy cover of perennial plants next to the bare bedrock. Joint-influenced geomorphic processes at Pima Wash generate four distinct process domains: (PD1) armored drainage divides; (PD2) slopes with different granite landforms; (PD3) mid- and upper basin channels that mix knickzones, strath floodplains, and sandy alluvial sections; and (PD4) the main ephemeral channel transitioning to the piedmont. Distant jointing promotes bedrock exposure and rock armoring along drainage divides in PD1 that then concentrates runoff and promotes perennial plant growth. More distant joint spacing on slopes in PD2 promotes exposure of granitic bedrock forms that shed overland flow to their margin and promotes flora and fauna growths along the margins of low granitic domes and kopjes. Similarly, wider joint spacing along ephemeral washes in PD3 leads to knickpoints, which in turn act to concentrate moisture immediately downstream. The stream terraces in PD4 influence the ecology through xeric desert pavements on terrace treads and roofs for coyotes (Canis latrans) and gray fox (Urocyon Cinereoargenteus) dens on terrace scarps via stage 3 pedogenic carbonate. These four process domains occur in six other randomly selected granitic watersheds with drainage areas <5km2 in the Mojave and Sonoran Deserts. Results on rates of geomorphic processes in the Pima Wash watershed provide new insight in the desert geomorphology of small granitic watersheds. Catchment-wide denudation rates (CWDRs) recorded by 10Be sampled along the main ephemeral wash vary between 15 and 23mm/ka and do not appear to be influenced by knickpoint or knickzone occurrence; instead slightly lower CWDRs appear to be associated with sediment contributions by subbasins with more abundance of bare bedrock forms. Resampling for CWDR after a 500-year flood event from hurricane moisture at two sites along the main ephemeral channel revealed no detectable changes; this finding confirms the average representativeness of CWDR as a long-term denudation proxy and also means that sediment transport on these arid granitic hillslopes must be incremental and without rapid crest to wash transport. The first reported measurements of incision rates into a small granitic Sonoran Desert watershed, using 10Be and VML, reveal rates on the order of 70–180mm/ka in the lower quarter of Pima Wash for the last 60ka — producing a narrow and deep trench. As this base-level fall propagates upstream, erosion focuses on weaker material with higher joint densities; this facilitates the emergence of domes and kopje landforms with more widely spaced jointing.

Geomorphic disequilibrium in the Eastern Korean Peninsula: Possible evidence for reactivation of a rift-flank margin

Tectonically inactive since the middle Miocene, the Korean Peninsula is generally considered a typical passive continental margin with asymmetric relief with only low levels of seismic activity. Recent reports of uplift rates as high as 150–350mmka−1 from geomorphic markers using numerical dating methods (e.g. radiocarbon, optically stimulated luminescence (OSL), and cosmogenic nuclides), however, cast doubt on the tectonic quiescence of Korea. Thus, we evaluate the geomorphic status of eastern Korea using denudation rates at geomorphic timescales of 103–106years, which are equivalent to the rates previously reported at local points. To infer regional denudation rates, we measured catchment-wide denudation rates (CWDR) in 13 watersheds using in situ cosmogenic 10Be and 14C analyses of riverine sediment samples in eastern Korea. These CWDR data suggest that the mean denudation rate during the past 5–14ka centers around ~74.7±25.4mm·ka−1, which is consistent with the long-term, geologic exhumation rate we derive of ~74±10.1mm·ka−1 since the middle Miocene. However, our CWDR data are ~2–3 times lower than rates of coastal uplift and river incision of 150–350mm·ka−1 derived from data on marine and fluvial terraces, respectively. In addition to isotopic analyses, we quantified topographic indices such as elevation, relief, and slope, and then compared these indices with CWDRs to examine morphological changes that might be related to tectonic disturbances since the beginning of late Pleistocene. The relationship between CWDRs and topographic indices indicates that most of the study area rests in a near steady-state condition. However, in contrast to the smooth concave longitudinal profile expected for a steady-state landscape, topographic analyses reveal the presence of major knickzones (confirmed by field reconnaissance). These knickzones may have been produced by changes in base level caused by tectonic events, lithologic variations or sea-level changes. Our 1-D modeling of knickpoint propagation suggests that tectonic disturbance is the factor most likely to produce knickzones in the study area, raising the possibility that erosional signals caused by late-Pleistocene tectonic disturbance affected channel gradients, but that the knickpoints have not yet propagated through the entire catchment area. Our results suggest that the eastern part of the Korean Peninsula is approaching, but has not completely reached, a geomorphic steady-state, and that transient conditions have been induced by reactivation of the rift-flank margin no later than the late Pleistocene, causing tilting of the central Korean Peninsula to the west at a rate of 7.0×10−10m/m per year over the past ~125ka.

Quaternary landscape development, alluvial fan chronology and erosion of the Mecca Hills at the southern end of the San Andreas Fault zone

Quantitative geomorphic analysis combined with cosmogenic nuclide 10Be-based geochronology and denudation rates have been used to further the understanding of the Quaternary landscape development of the Mecca Hills, a zone of transpressional uplift along the southern end of the San Andreas Fault, in southern California. The similar timing of convergent uplifts along the San Andreas Fault with the initiation of the sub-parallel San Jacinto Fault suggest a possible link between the two tectonic events. The ages of alluvial fans and the rates of catchment-wide denudation have been integrated to assess the relative influence of climate and tectonic uplift on the development of catchments within the Mecca Hills. Ages for major geomorphic surfaces based on 10Be surface exposure dating of boulders and 10Be depth profiles define the timing of surface stabilization to 2.6 +5.6/–1.3 ka (Qyf1 surface), 67.2 ± 5.3 ka (Qvof2 surface), and 280 ± 24 ka (Qvof1 surface). Comparison of 10Be measurements from active channel deposits (Qac) and fluvial terraces (Qt) illustrate a complex history of erosion, sediment storage, and sediment transport in this environment. Beryllium-10 catchment-wide denudation rates range from 19.9 ± 3.2 to 149 ± 22.5 m/Ma and demonstrate strong correlations with mean catchment slope and with total active fault length normalized by catchment area. The lack of strong correlation with other geomorphic variables suggests that tectonic uplift and rock weakening have the greatest control. The currently measured topography and denudation rates across the Mecca Hills may be most consistent with a model of radial topographic growth in contrast to a model based on the rapid uplift and advection of crust.

Dilution of 10Be in detrital quartz by earthquake-induced landslides: Implications for determining denudation rates and potential to provide insights into landslide sediment dynamics

The concentration of 10Be in detrital quartz (10Beqtz) from river sediments is now widely used to quantify catchment-wide denudation rates but may also be sensitive to inputs from bedrock landslides that deliver sediment with low 10Beqtz. Major landslide-triggering events can provide large amounts of low-concentration material to rivers in mountain catchments, but changes in river sediment 10Beqtz due to such events have not yet been measured directly. Here we examine the impact of widespread landslides triggered by the 2008 Wenchuan earthquake on 10Beqtz in sediment samples from the Min Jiang river basin, in Sichuan, China. Landslide deposit material associated with the Wenchuan earthquake has consistently lower 10Beqtz than in river sediment prior to the earthquake. River sediment 10Beqtz decreased significantly following the earthquake downstream of areas of high coseismic landslide occurrence (i.e., with greater than ∼0.3% of the upstream catchment area affected by landslides), because of input of the 10Be-depleted landslide material, but showed no systematic changes where landslide occurrence was low. Changes in river sediment 10Beqtz concentration were largest in small first-order catchments but were still significant in large river basins with areas of 104–105 km2. Spatial and temporal variability in river sediment 10Beqtz has important implications for inferring representative denudation rates in tectonically active, landslide-dominated environments, even in large basins. Although the dilution of 10Beqtz in river sediment by landslide inputs may complicate interpretation of denudation rates, it also may provide a possible opportunity to track the transport of landslide sediment. The associated uncertainties are large, but in the Wenchuan case, calculations based on 10Be mixing proportions suggest that river sediment fluxes in the 2–3 years following the earthquake increased by a similar order of magnitude in the 0.25–1 mm and the <0.25 mm size fractions, as determined from 10Beqtz mixing calculations and hydrological gauging, respectively. Such information could provide new insight into sediment transfer, with implications for secondary sediment-related hazards and for understanding the removal of mass from mountains.

Denudation rates and tectonic geomorphology of the Spanish Betic Cordillera

The tectonic control on landscape morphology and long-term denudation is largely documented for settings with high uplift rates. Relatively little is known about the rates of geomorphic response in areas of low tectonic uplift. Here, we evaluate spatial variations in denudation of the Spanish Betic Cordillera based on cosmogenic 10Be-derived denudation rates. Denudation rates are compared to published data on rock uplift and exhumation of the Betic Cordillera to evaluate steady-state topography. The spatial patterns of catchment-wide denudation rates (n=20) are then analysed together with topographic metrics of hillslope and channel morphology. Catchments draining the Betic ranges have relatively low denudation rates (64±54 mmkyr−1), but also show large variation as they range from 14 to 246 mm kyr−1. Catchment-wide denudation is linearly proportional to the mean hillslope gradient and local relief. Despite large spatial variation in denudation, the magnitude and spatial pattern of denudation rates are generally consistent with longer-term local uplift rates derived from elevated marine deposits, fission-track measurements and vertical fault slip rates. This might be indicative of a steady-state topography where rock uplift is balanced by denudation.

Glacial impact on short-wavelength topography and long-lasting effects on the denudation of a deglaciated mountain range

Distinct alpine landforms in most high to mid-latitude mountain chains highlight the importance of glacial erosion in shaping mountain topography. The modifications to the initially, fluvially shaped landscape are associated with a massive and sustainable change in the distribution and magnitude of denudation following deglaciation. In this paper we focus on the glacially induced modifications to the short-wavelength topography of the deglaciated European Central Alps in an attempt to characterize the degree of glacial erosion on mountain topography and to explore the potential impact on millennial scale catchment denudation. We propose that short-wavelength topography is characteristically obliterated by glacial action and a measure of this process is provided by drainage density, which can be obtained by measuring the topographic curvature extracted from a DEM. Drainage density is well correlated with catchment-wide denudation rates from cosmogenic nuclides (10Be), but in two separate domains, identified by the degree of glacial conditioning. At lower elevations, where fluvial erosion processes dominate at present, drainage density tends to increase with denudation rate and mean slope. At higher elevations drainage density tends to decrease with increasing denudation rate but is not sensitive to mean slope. The transition between these domains is approximately coincident with the equilibrium line altitude of the last glacial maximum. Our results indicate that the decreasing drainage density in the higher domain reflects the cumulative impact of glacial erosion. We speculate that the commensurate lengthening of hillslopes increases slope instability and mass flux, thereby resulting in higher denudation rates. Rock mass strength seems to have a further significant effect on these relationships.

Denudation rates of small transient catchments controlled by former glaciation: The Hörnli nunatak in the northeastern Swiss Alpine Foreland

Extensive glaciers repeatedly occupied the northern Alpine Foreland during the Pleistocene and left a strongly glacially overprinted low slope landscape. Only few islands appeared as nunataks standing above the surface of the large piedmont glacier lobes. These nunatak areas kept their original shape, manifested in steep catchments with mean slopes up to 33°. Even though not glaciated, these catchments where significantly affected by base-level changes occurring as a consequence of phases of glacier advances and retreats. Both domains, the glacially eroded and non-eroded, are therefore prone to different mechanisms and time-scales of fluvial and colluvial re-adjustment.In this study we investigate these effects by exploring the spatial distribution and magnitude of denudation in the Hörnli region of the eastern Swiss Alpine Foreland in the present Interglacial. The area represents both domains in a relatively small area with largely uniform tectonic, lithologic and climatic conditions. The differences in Holocene landscape evolution are investigated using topographic analyses and catchment-averaged denudation rates derived from 10Be concentrations in fluvial quartz sand. We find that in formerly non-glaciated, fluvially dominated catchments close hillslope-channel coupling prevails and that these catchments yield high average denudation rates of 350 mm/ka. Glacially overprinted catchments yielded catchment-wide denudation rates an order of magnitude lower. These low denudation rates are hypothesized to be the consequence of both (i) a dominance of slow hillslope processes and (ii) admixture of high concentration, pre-LGM glacial sediment. This suggests that a) a careful field investigation must accompany the denudation rate studies and b) that the concept of area-weighted cosmogenic nuclide denudation rates must be considered in light of the predominant catchment processes.