Long-term Denudation Rates Research Articles

Long-term denudation rates of actively uplifting hillcrests in the Boso Peninsula, Japan, estimated from depth profiling of in situ-produced cosmogenic 10Be and 26Al

Depth profiling of in situ-produced cosmogenic 10Be and 26Al was applied to estimate the rate of long-term denudation on actively uplifting hillcrests made of weakly consolidated sandstone in the Boso Peninsula, Japan. The nuclide concentration in the topmost meter of the hillcrest has reached an equilibrium value owing to surface denudation at 90 mm/kyr. This rate is much smaller than the regional uplift rate, and is consistent with the current isolated residual topography of the hillcrest. By considering the epoch of emergence of the sandstone hillcrests, we estimate the summit denudation rate of the adjacent mudstone hill formation to be 720 ± 110 mm/kyr.

Erosional and climatic effects on long-term chemical weathering rates in granitic landscapes spanning diverse climate regimes

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and total denudation in granitic landscapes in diverse climatic regimes. Our 42 study sites encompass widely varying climatic and erosional regimes, with mean annual temperatures ranging from 2 to 25 °C, average precipitation ranging from 22 to 420 cm·year −1, and denudation rates ranging from 23 to 755 t·km −2·year −1. Long-term chemical weathering rates range from 0 to 173 t·km −2 year −1, in several cases exceeding the highest granitic weathering rates on record from previous work. Chemical weathering rates are highest at the sites with rapid denudation rates, consistent with strong coupling between rates of chemical weathering and mineral supply from breakdown of rock. A simple empirical relationship based on temperature, precipitation and long-term denudation rates explains 89–95% of the variation in long-term weathering rates across our network of sites. Our analysis shows that, for a given precipitation and temperature, chemical weathering rates increase proportionally with fresh-material supply rates. We refer to this as “supply-limited” weathering, in which fresh material is chemically depleted to roughly the same degree, regardless of its rate of supply from breakdown of rock. The temperature sensitivity of chemical weathering rates is two to four times smaller than what one would expect from laboratory measurements of activation energies for feldspar weathering and previous inter-comparisons of catchment mass-balance data from the field. Our results suggest that climate change feedbacks between temperature and silicate weathering rates may be weaker than previously thought, at least in actively eroding, unglaciated terrain similar to our study sites. To the extent that chemical weathering rates are supply-limited in mountainous landscapes, factors that regulate rates of mineral supply from erosion, such as tectonic uplift, may lead to significant fluctuations in global climate over the long term.

Erosion of the submarine flanks of the Canary Islands

Surveying with multibeam echo sounders around old (≫1 Ma) volcanic ocean islands reveals that their submarine flanks contain a strong downslope‐oriented ridge‐and‐valley corrugation, which modifies the original volcanic morphology of lava terraces and cones. By analogy with canyons in other settings, this corrugation was probably caused by channel incision by erosive sedimentary mass flows such as turbidity currents and debris flows. We adapt a method first used in subaerial geomorphology to isolate the erosion depth (exhumation) and apply it to the eroded flanks of the 6–8 Ma Anaga massif of Tenerife. The channels formed around this massif divert around local topographic highs. These highs, which are probably original volcanic cones, are therefore preferentially preserved during erosion, so that their elevations can be used to construct an artificial reference surface. Terrain depth was calculated by subtracting this reference surface from measured bathymetry. Comparison of the terrain depth of the old, eroded submarine flank of Anaga with that of the young, mostly unaltered submarine flank of El Hierro allows us to infer the mean depth of Anaga's submarine erosion, which is ∼100 m. Volcanic terrains can be dated by radiometric methods, so they also provide a way of quantifying long‐term denudation rates. We infer that submarine denudation of Anaga has occurred at comparable rates to that of subaerial lowlands and much slower than denudation of highlands, illustrated locally by the more extensive erosion of the subaerial Anaga edifice.

Radiometric ages of laterites and constraints on long-term denudation rates in West Africa

Research Article| February 01, 2003 Radiometric ages of laterites and constraints on long-term denudation rates in West Africa Yanni Gunnell Yanni Gunnell 1Laboratoire de Géographie Physique, CNRS-UMR 8591, 1 Place A. Briand, 92120 Meudon, France Search for other works by this author on: GSW Google Scholar Author and Article Information Yanni Gunnell 1Laboratoire de Géographie Physique, CNRS-UMR 8591, 1 Place A. Briand, 92120 Meudon, France Publisher: Geological Society of America Received: 14 Jun 2002 Revision Received: 05 Oct 2002 Accepted: 12 Oct 2002 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2003) 31 (2): 131–134. https://doi.org/10.1130/0091-7613(2003)031<0131:RAOLAC>2.0.CO;2 Article history Received: 14 Jun 2002 Revision Received: 05 Oct 2002 Accepted: 12 Oct 2002 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yanni Gunnell; Radiometric ages of laterites and constraints on long-term denudation rates in West Africa. Geology 2003;; 31 (2): 131–134. doi: https://doi.org/10.1130/0091-7613(2003)031<0131:RAOLAC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In stable cratonic regions, most tropical weathering mantles evolve over long time scales and record long-term environmental change. They may therefore also reflect tectonism and its denudation-related signals detected through apatite fission-track thermochronology, cosmogenic radionuclide dating, and the age bracketing of laterites by 40Ar/39Ar dating of potassium- and manganese-rich oxides. Based on an existing Cenozoic pedimentation model for the West African craton, this study uses the three combined radiometric methods to define rates of Cenozoic denudation. Denudation rates of <2 m/m.y. on the lateritic plateaus, in comparison to rates of 7–13 m/m.y. adjacent to them, fit ages of 45–50 Ma for late stages of bauxite development and 24–25 Ma for one phase of lateritization. Together, they support the theory implicit in the landscape model that depths of denudation in cratonic interiors are equal to, or not significantly greater than, existing elevation differences between lateritic landforms. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The long-term denudation rate in the Southern Apennines Chain (Italy): a GIS-aided estimation of the rock volumes eroded since middle Pleistocene time

The long-term denudation rate has been evaluated within an actively deforming area (the Southern Apennines Chain), using both geomorphological and GIS-aided analysis of a 2738 km 2 representative sample area. The rock volume contained between a reconstructed palaeotopography and the present topography of the study area has been calculated. Palaeotopography has been reconstructed on the basis of the relics of an irregular surface, whose tentative age is 0.8–0.6 Ma, and also includes remnants of a more ancient, more suspended and badly preserved palaeosurface. To represent the modern topography, a certified DTM of Italy, with grid 230×230 m 2, was adopted. The calculated rock volume eroded since the beginning of the dissection of the palaeosurface is 478 km 3, which implies an average erosional downwearing of 175 m. However, downwearing is not uniformly distributed over the area, as it is concentrated along valleys. In particular, the same major valley trunks that already existed at the beginning of the Middle Pleistocene, show downcutting amounting to 400 m and more, while the amount of erosion on the divide areas is of the order of a few tens of metres. The resulting mean denudation rate falls between 0.22 and 0.30 mm/a.

Late Neogene passive margin denudation history—cosmogenic isotope measurements from the central Namib desert

In this paper, we review ideas on the geomorphological history of the southwest African passive margin, focusing on the central Namib sector and presenting new evidence on the late Neogene landscape evolution of this region. The hyperarid central Namib Desert occupies the 100–150-km-wide pediment at the foot of the Great Escarpment and forms part of the southwest African passive margin, which formed after breakup in the South Atlantic at around 118 Ma. Previous apatite fission track (AFT) and cosmogenic isotope studies and numerical models of coupled tectonic-surface processes in the same area suggest that long-term denudation rates of this passive margin (after a period of significant post-rift denudation) have been very low, ∼5 m/my. Aridity of the Namib Desert is generally assumed to have started with the onset of Benguela upwelling in the SE Atlantic at 10–15 Ma and to have prevailed ever since. It has been implied that during this period, the landscape has undergone only marginal change. Here, we present new evidence from in situ cosmogenic 21Ne dating of pediment surfaces, river terraces and river cut bedrock benches. Correlation of terrace sediment bodies defining ancient river courses is supported by a sediment provenance study using heavy mineral analyses. Our results generally confirm previous work, adding new insights in the late Neogene landscape evolution of the central Namib. They indicate that denudation rates since at least 5 Ma have been even lower (of the order of 0.5–1 m/my) than those calculated by previous studies of inselbergs. In the central Namib, inselbergs appear to be located in or near areas of Plio/Pleistocene incision and badland erosion. These inselbergs therefore are not the best sites to obtain representative passive margin denudation rates. We obtained a 21Ne age of 5.18±0.18 Ma from a resistant quartz vein projecting above pediment unaffected by recent incision, the oldest exposure age from the Namib so far. The pediment surface stretching from the base of the escarpment to the ocean, on average, probably has undergone less than 10 m of surface lowering since the mid Miocene. Our results also indicate that a late Neogene episode of accelerated denudation in the central Namib Desert and incision of deep canyons started around 2.81±0.11 Ma ago. We interpret this as a signal of increasingly wet conditions in this region, in response to Plio/Pleistocene global cooling. As aggradation of the river systems is followed by degradation, their drainage patterns change from braided to meandering. Evidence of, yet undated, earlier incision episodes suggests that the denudation history of this passive margin is one of slow long-term denudation—as other workers concluded on the basis of AFT and cosmogenic nuclide analyses—interrupted by pulses of rapid denudation.

Large-scale geomorphology and fission-track thermochronology in topographic and exhumation reconstructions of the Southern Rocky Mountains

Long-term landscape evolution is the integrative sum of constructive rock-uplift processes and destructive erosional processes. For the Southern Rocky Mountains, it has been proposed that Phanerozoic rock uplift and erosion have been influenced by crustal structure that was inherited from the time of assembly of the continent in the Proterozoic. This paper compares large-scale geomorphology and long-term rock exhumation histories in three distinct crustal terranes to assess possible differences in Laramide and post-Laramide deformation of the Southern Rocky Mountains. We analyze modern topographic data and fission-track thermochronology for several portions of the Southern Rockies with distinctly different post-Laramide geologic histories and estimate the amount of Laramide and post-Laramide rock uplift. The areas of investigation include: (1) the Colorado Front Range, an area of regional elevated heat flow in the Yavapai province; (2) the New Mexico Taos Range, a region of localized high heat flow through-out the late Tertiary and Quaternary along the Rio Grande rift in the Mazatzal province; and (3) the New Mexico Sierra Nacimiento, a region of localized high heat flow in the Quaternary associated with the Jemez Mountains, also in the Mazatzal province. Both the Taos Range and Sierra Nacimiento lie in proximity to the Jemez lineament, which is thought to be a significant Proterozoic structural feature that has influenced Quaternary volcanism along its trend. We utilize and test a dimensionless topographic index called the ZR ratio, which is the ratio between mean elevation and mean relief, as a quantitative measure of the differences in orogen-scale topography. Digital topography (DEM) and GIS software (ARC/INFO) allow for the rapid determination of this ratio at various length scales. Our results show that the Taos Range has the lowest ZR ratio (most rugged topography), and the Sierra Nacimiento has the highest ratio (least rugged topography). Fission-track thermochronology results show that a partial annealing zone (PAZ) is preserved in the Front Range and the northern portion of Sierra Nacimiento (the region incidentally not on the high-heat flow Jemez lineament); both the Taos Range and the southern portion of Sierra Nacimiento have had the PAZ removed by erosion. Reconstructed amounts and timing of rock denudation based on the fission-track data are consistent with the ZR ratios in that the most rugged topography exhibits the greatest and most recent denudation, whereas the least rugged topography experienced far less denudation, most of which occurred immediately after the Laramide orogeny. Fission-track thermochronology and geomorphic results support an overall model of significant crustal thickening during the Laramide orogeny, followed by relatively low and differential rates of rock uplift, erosional exhumation, and isostatic rebound. Specific regions of greater post-Laramide rock exhumation and rugged topography are highly correlated with regions of known localized high heat flow and late Cenozoic volcanism, a finding consistent with the hypothesis that at least locally, the high mean elevation of the Southern Rockies is supported by a buoyant mantle. Both the style of Laramide deformation and subsequent magmatic input from a low-velocity, buoyant mantle may have been influenced by older crustal structure. All three factors, Proterozoic crustal structure, Laramide deformation, and post-Laramide up-lift, appear to play an important role in the ability of streams to integrate through the Rocky Mountain foreland. As rates of erosion are strongly tied to local relief of well-drained landscapes, stream integration at the large scale may explain the correlation between long-term denudation rates and modern topographic ruggedness ( ZR ratio), and be the limiting factor in the processes driving rock uplift in the post-Laramide Southern Rockies.

Present, past and potential denudation rates: is there a link? Tentative evidence from fission-track data, river sediment loads and terrain analysis in the South Indian shield

Ahnert [Ahnert, F., 1970. Functional relationships between denudation, relief and uplift in large mid-latitude drainage basins. Am. J. Sci. 268, 243–263] proposed that a denudation rate D could be predicted by the relation D=0.1535 h, where h is local relief, a substitute for mean slope. The present study seeks to explore the value of this simple equation as a normative geomorphological tool. In two successive and complementary stages, its principles are applied to the Western Dharwar Craton of South India. First, the robustness of local relief as a reliable substitute for mean slope is examined. Following the demonstration that the equation can successfully be used in the region of interest pending correction factors for rock resistance, a potential denudation map of the Western Dharwar Craton is established. The pattern of potential denudation values is compared with known measurements of both long-term (∼10 2 Ma) denudation rates inferred from apatite fission track data and modern (∼10 −5 Ma) denudation rates derived from available sediment load measurements in relevant river basins. Unexpectedly close similarities between long-term, short-term and potential denudation rates prompt a discussion on the merits of potential denudation as a possible standard in the appraisal of measured rates and as a tool for establishing permissible denudation rates when assessing the impact of man-induced erosion. A landscape-specific basal metabolic rate of erosion is defined as a new concept. The methodological pitfalls in extrapolating denudation rates from the measurement of sediment loads in rivers are reviewed and suggestions for tighter investigation are made.

Late Cenozoic tectonics and denudation in the Central Kenya Rift: quantification of long-term denudation rates

Long-term average denudation rates were calculated for the last 3.4 million years (Ma) using digital topographic data related to the present surface of the intrarift Bahati-Kinangop Plateau in the Central Kenya Rift. Well established relations between stratigraphy and faulting chronology, and a detailed digital topographic database allow reconstruction of the idealized uneroded plateau surface for an area of 710 km 2. The reconstruction refers to the time when the plateau came into existence (3.4-2.6 Ma) due to widespread deposition of ashflow tuffs and subsequent faulting. The reconstruction of this palaeosurface is based on erosional surface remnants occurring within the present plateau surface. Relict surfaces elements were determined by analyzing the present digital surface representation and geological information within a Geographical Information System (GIS). After generating digital elevation models of the present and palaeosurfaces, long-term denudation rates were calculated using the elevation difference method. About 80% of the study area are characterized by denudation rates between 0 (relict parts) and 20 m per one million years (m/Ma). For deeply eroded plateau regions denudation reaches 80–100 m/Ma. These rates are comparable to denudation rates which are known from other studies in similar settings. The GIS-based approach provides spatially high-resolution denudation rates allowing for further regional analysis of factors controlling spatial variations in sediment supply to the rift basin.

Cooling age record of domal uplift in the core of the Higher Himalayan Crystallines (HHC), southwest Zanskar, India

The cooling and tectonic history of the Higher Himalayan Crystallines (HHC) in southwest Zanskar (along the Kishtwar-Padam traverse) is constrained by K-Ar biotite and fission-track (FT) apatite and zircon ages. A total of nine biotite samples yields ages in the range of 14–24 Ma, indicating the post-metamorphic cooling of these rocks through ∼ 300°C in the Miocene. Overall, the ages become younger away from the Zanskar Shear Zone (ZSZ), which marks the basement-cover detachment fault between the HHC and the Tethyan sedimentary zone, towards the core of the HHC. The same pattern is also observed for the FT apatite ages, which record the cooling of the rocks through ∼ 120°C. The apatite ages range from 11 Ma in the vicinity of the ZSZ to 4 Ma at the granitic core of the HHC. This pattern of discordant cooling ages across the HHC in southwest Zanskar reveals an inversion of isotherms due to fast uplift-denudation (hence cooling) of the HHC core, which is, in turn, related to domal uplift within the HHC. The Chisoti granite gneiss is the exposed domal structure along the studied traverse. Cooling history of two granite gneisses at the core of the HHC is also quantified with the help of the biotite, zircon and apatite ages; the time-temperatures thus obtained indicate a rapid pulse of cooling at ∼ 6 Ma, related to accelerated uplift-denudation of the HHC core at this time. Long-term denudation rates of 0.5–0.7 mm/yr are estimated for the high-grade rocks of the Higher Himalaya in southwest Zanskar over the past 4.0–5.5 m.yr.

Terrestrial sediment yield and the lifetimes of reservoirs, lakes, and larger basins

Water reservoirs, lakes, and larger basins, including their drainage areas, represent sedimentologically closed to semi-closed denudation-accumulation systems. The mean rates of mechanical denudation, DRme, and clastic sedimentation, SRme, are related by the ratio of the drainage/lake area, Ad/Al. If the latter is known, DRme (or the specific sediment yield SY in t per km2/a) can be calculated from SRme, or vice versa. The best data for modern SY mainly come from the sediment fills of artificial reservoirs. Small drainage areas of mountainous regions show SY values up to two orders of magnitude higher than lowlands and approximately one order higher than larger regions of mixed relief. This is also true of arid to semi-arid zones which often provide approximately as much sediment (SY) as humid temperate and even tropical zones of comparable relief. Lithology and climate (river runoff) also may play some role for SY from catchments of limited size. The importance of these factors is exemplified by perialpine lakes and two East African lakes. Sediment yields gained from some large reservoirs compare well with long-term denudation rates derived from geological studies (e.g., the Tarbela dam reservoir along the Indus River). In many other cases, human activities have raised SY by factors of 2–10, locally up to >100. Artificial reservoirs in mountainous regions with SY in the range of 300–2000 t per km2/a tend to become filled within several tens to hundreds of years; some have even shorter lifetimes. Perialpine lakes of the Alps and British Columbia are strongly affected by delta prograding and have lifetimes mostly between 15 and 40 ka. Closed lake systems in deep morphological depressions (Lake Bonneville, Aral Sea, northern Caspian Sea) have a high potential for sediment storage up to the level of spillover and therefore can persist over long time periods. Basins with markedly subsiding basin floors (lakes of the East African rift zone, the southern Caspian Sea, and the Black Sea, both on oceanic crust) can survive for many Ma in the future, despite relatively high terrigenous input.

Models for landform development on passive margins. Some implications for relief development in glaciated areas

Models for pre-glacial relief along passive continental margins must reflect the style and rate of uplift, and the rates of weathering and erosion, during at least the last 100–150 Ma. Passive margins have varied elevation, patterns of rifting, uplift and flexuring. A distinction is recognised between low-lying and steeply rising margins, the latter associated with major crustal upwarps. Continuous flexuring of the crust will maintain disequilibrium in weathering systems and known or possible rates of weathering and erosion appear compatible with weathering penetration during uplift. Under hot-humid conditions a rate of weathering of 50–100 mm ka t1¯ is possible and could produce 15–30 m of saprolite in 1 Ma (with 70% surface loss), while long-term denudation rates seldom exceed 40 mm ka −1. Deep, sandy and corestone profiles in zones of marginal dissection indicate effectiveness of weathering during uplift. Examples from southern Africa, southern Australia, South America and western Africa illustrate 4 morpho-tectonic settings: low elevation with minimum tilt, low domal uplift with coastal tilt, high elevation steeply arched, and high elevation faulted margin with back tilting. These examples are compared with higher latitude marginal landscapes in southeastern USA, Fennoscandia and Scotland. It is concluded that pre-glacial survival implies more than small relict features and includes: exhumed surfaces from beneath sedimentary covers; basins and troughs following zones of deeper weathering, and local planations, now warped and elevated to varying heights, above which inselbergs are locally common. While relict saprolites are found beneath duricrusts in the tropics, and strata-bound in high latitudes, most saprolite is better understood as having a dynamic history throughout the Neogene, including the Quaternary, whether in continuously warm or periodically cold climates.

The geomorphology of Australia's fluvial systems: retrospect, perspect and prospect

This article provides a review of the study and geomorphology of Australia's fluvial systems by offering comment on the development, concerns and future of the subject. Trends in the history of fluvial landform studies in Australia are traced from the observations and comments of the early explorers and visiting scientists through to the emergence and growth of fluvial geomorphology as a study discipline. Subsequent development of the idea of a distinctive geomorphology of Australian fluvial systems that often contrast with Anglo-American observations is outlined and illustrated with particular reference to fluvial studies in south-east Australia. Key features of the Australian setting include low long-term denudation rates, the absence of extensive Quaternary glaciation and the predominance of low gradient fluvial systems over much of the continent. Some of the most important themes in contemporary Australian fluvial research are discussed and include long-term landscape evolution, thresholds and riverine response to secular trends in climate, Quaternary environmental change, arid-environment systems, bedrock channels and applied approaches to study. Consideration is also given to present deficiencies in research and to future priorities. Particular attention is focused on the need firstly to collect additional process data, secondly to shift the bias in research away from south-east Australia, and thirdly to develop links between fluvial process and alluvial stratigraphy/chronology. It is concluded that, given the variety of hydrogeomorphological environments in Australia and the diversity of approaches to study, ongoing research will provide further indications of the unusual nature of many of the continent's fluvial systems.

Constraining variability in south-east Australian long-term denudation rates using a combined geomorphological and thermochronological approach

Constraining variability in south-east Australian long-term denudation rates using a combined geomorphological and thermochronological approach

Sub-aerial denudation of passive margins: regional elevation versus local relief models

Variations in denudation rates, which are an important factor in the modification of the sub-aerial topography of passive margins, have frequently been modelled as being proportionate to regional elevation. To be valid, this assumption requires that mean slope gradient and the related property of local relief show a strong positive relationship to regional elevation since slope angle is a primary determinant of denudation rate. Analysis of the relationships between these variables based on the 1:250,000 scale topographic map coverage of southern Africa demonstrates that regional (modal) elevation explains only 0.5% of the variance in mean sine slope gradient and only 0.14% of the variance in local relief (measured over 15′ × 15′ areas). These results demonstrate that regional elevation cannot be used to predict denudation rates for passive margins, such as those of southern Africa, where a dissected coastal zone is separated from a high interior plateau by a major escarpment. On the basis of a previous regression analysis relating local relief to sediment and solute transport by rivers the data for southern Africa indicate denudation rates of > 80 m Ma −1 over much of the coastal zone and < 20 m Ma −1 across most of the elevated interior. Extrapolation of these results over geological time, however, necessitates either assumptions about the local relief of palaeotopography or independent evidence of long-term denudation rates.

Calculation of the volume of an alluvial fan

Calculation of the volume of a lithostratigraphic unit within an alluvial fan allows a longterm denudation rate to be calculated for the fan's catchment, where sediment is not transported beyond the fan. The calculation of denudation rates in this case depends upon accurate estimates of the volume of sediment in a lithostratigraphic unit of known age. Two methods of volume calculation and their approximations are presented. The first method sums the volume of discrete blocks used to approximate a variety of fan shapes. The second method integrates a quadratic function that approximates both the surface and lower boundary of a lithostratigraphic unit where the edges of the fan are assumed to be vertical. For fans that satisfy these restrictive assumptions, the integration method should be more accurate than the summation method. However, the summation method is more generally applicable and calculations using the two methods, and their approximations, yield remarkably similar results.