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Didier Bourles - One of the best experts on this subject based on the ideXlab platform.
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controls on holocene Denudation rates in mountainous environments under mediterranean climate
Earth Surface Processes and Landforms, 2017Co-Authors: Nicolas Freslon, Julien Moreau, Stéphane Molliex, Gwenael Jouet, Christine Authemayou, Didier Bourles, Marina RabineauAbstract:The Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional Denudation rates in the framework of a source-to-sink approach. Indeed, fluvial sedimentation in East-Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz-rich crystalline rocks allowing cosmogenic nuclide Be-10 measurements. In this paper, Holocene Denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced Be-10 concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated Denudation rates range from 15 to 95 mm ka(-1). When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho-climatic parameters including sun exposure and the direction of the main wet wind, so-called Libecciu'. This distribution, as well as the basement rock strength seems to play a significant role in the Denudation distribution. We thus suggest that the landscape reached a geomorphic steady-state due to the specific Mediterranean climate and that Holocene Denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport-limited...
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controls on holocene Denudation rates in mountainous environments under mediterranean climate
Earth Surface Processes and Landforms, 2017Co-Authors: Nicolas Freslon, Julien Moreau, Stéphane Molliex, Gwenael Jouet, Christine Authemayou, Didier Bourles, Marina RabineauAbstract:The Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional Denudation rates in the framework of a source-to-sink approach. Indeed, fluvial sedimentation in East-Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz-rich crystalline rocks allowing cosmogenic nuclide Be-10 measurements. In this paper, Holocene Denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced Be-10 concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated Denudation rates range from 15 to 95 mm ka(-1). When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho-climatic parameters including sun exposure and the direction of the main wet wind, so-called Libecciu'. This distribution, as well as the basement rock strength seems to play a significant role in the Denudation distribution. We thus suggest that the landscape reached a geomorphic steady-state due to the specific Mediterranean climate and that Holocene Denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport-limited...
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evolution and degradation of flat top mesas in the hyper arid negev israel revealed from 10be cosmogenic nuclides
Earth Surface Processes and Landforms, 2014Co-Authors: Ronen Boroda, Didier Bourles, Ari Matmon, Rivka Amit, Itai Haviv, Maurice Arnold, Georges Aumaitre, Karim Keddadouche, Yehuda Eyal, Yehouda EnzelAbstract:Mesas are ubiquitous landforms in arid and semiarid regions and are often characterized by horizontal stratified erodible rocks capped by more resistant strata. The accepted conceptual model for mesa evolution and degradation considers reduction in the width of the mesa flat-top plateau due to cliff retreat but ignores possible Denudation of the mesa flat-top and the rates and mechanism of erosion. In this study we examine mesas in the northeastern hyperarid Negev Desert where they appear in various sizes and morphologies and represent different stages of mesa evolution. The variety of mesas within a single climatic zone allows examination of the process of mesa evolution through time. Two of the four sites examined are characterized by a relatively wide (200–230 m) flat-top and a thick caprock whereas the other two are characterized by a much narrower remnant flat-top (several meters) and thinner caprock. We use the concentration of the cosmogenic nuclide 10Be for: (a) determining the chronology of the various geomorphic features associated with the mesa; and (b) understanding geomorphic processes forming the mesa. The 10Be data, combined with field observations, suggest a correlation between the width of flat-top mesa and the Denudation and cliff retreat rates. Our results demonstrate that: (a) cliff retreat rates decrease with decreasing width of the flat-top mesa; (b) vertical Denudation rates increase with decreasing width of the flat-top mesa below a critical value (~60 m, for the Negev Desert); (c) the reduction in the width of the flat-top mesa is driven mainly by cliff retreat accompanied by extremely slow vertical Denudation rate which can persist for a very long time (>106 Ma); and (d) when the width of the mesa decreases below a certain threshold, its rate of Denudation increases dramatically and mesa degradation is completed in a short time. Copyright © 2014 John Wiley & Sons, Ltd.
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Denudation and retreat of the serra do mar escarpment in southern brazil derived from in situ produced 10be concentration in river sediment
Earth Surface Processes and Landforms, 2014Co-Authors: Andre Augusto Rodrigues Salgado, Didier Bourles, Breno Ribeiro Marent, Luis Felipe Soares Cherem, Leonardo Jose Cordeiro Santos, Regis Braucher, Helen Nebias BarretoAbstract:The Serra do Mar escarpment, located along the southeastern coast of Brazil, is a high-elevation passive margin escarpment. This escarpment evolved from the Denudation of granites, migmatites and gneisses. The granites outcrop in the form of a ridge along the escarpment crest, due to its differential erosion (‘sugarloaf’ hills) from the surrounding lithologies. Several studies suggest that the passive margin escarpments are actively retreating toward the interior of the continent. However, no prior study has calculated the long-term Denudation rates of Serra do Mar to test this hypothesis. In this study, we measured the in situ-produced 10Be concentration in fluvial sediments to quantify the catchment-wide long-term Denudation rates of the Serra do Mar escarpment in southern Brazil. We sampled the fluvial sediments from ten watersheds that drain both sides of the escarpment. The average long-term Denudation rate of the oceanic side is between 2.1- and 2.6-fold higher than the rate of the continental side: 26.04 ± 1.88 mm ka-1 (integrating over between 15.8 ka-1 and 46.6 ka-1) and 11.10 ± 0.37 mm ka-1 (integrating over between 52.9 ka-1 and 85.4 ka-1), respectively. These rates indicate that the coastal base level is controlling the escarpment retreat toward the continental high lands, which is consistent with observations made at other high-elevation passive margins around the globe. The results also demonstrate the differential erosion along the Serra do Mar escarpment in southern Brazil during the Quaternary, where drainages over granites had lower average Denudation rates in comparison with those over migmatites and gneisses. Moreover, the results demonstrate that the ocean-facing catchments have been eroded more intensely than those facing the continent. The results also reveal that drainage over the granites decreases the average Denudation rates of the ocean-facing catchments and the ‘sugarloaf’ hills therefore are natural barriers that slowly retreat once they are exhumed. Copyright © 2013 John Wiley & Sons, Ltd.
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determination of both exposure time and Denudation rate from an in situ produced 10be depth profile a mathematical proof of uniqueness model sensitivity and applications to natural cases
Quaternary Geochronology, 2009Co-Authors: Regis Braucher, P Del Castillo, Lionel Siame, A J Hidy, Didier BourlesAbstract:Measurements of radioactive in situ-produced cosmogenic nuclide concentrations in surficial material exposed to cosmic rays allow either determining the long-term Denudation rate assuming that the surface studied has reached steady-state (where production and losses by Denudation and radioactive decay are in equilibrium) (infinite exposure time), or dating the initiation of exposure to cosmic rays, assuming that the Denudation and post-depositional processes are negligible. Criteria for determining whether a surface is eroding or undergoing burial as well as quantitative information on Denudation or burial rates may be obtained from cosmogenic nuclide depth profiles. With the refinement of the physical parameters involved in the production of in situ-produced cosmogenic nuclides, a unique well-constrained depth profile now permits determination of both the exposure time and the Denudation rate affecting a surface. In this paper, we first mathematically demonstrate that the exponential decrease of the in situ-produced 10Be concentrations observed along a depth profile constrains a unique exposure time and Denudation rate when considering both neutrons and muons. In the second part, an improved chi-square inversion model is described and tested in the third part with actual measured profiles.
Stéphane Molliex - One of the best experts on this subject based on the ideXlab platform.
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Late-Pleistocene catchment-wide Denudation patterns across the European Alps
Earth-Science Reviews, 2020Co-Authors: Romain Delunel, Stéphane Molliex, Kevin Norton, Fritz Schlunegger, Pierre Valla, Jean Dixon, Christoph Glotzbach, Kristina Hippe, Florian Kober, Bernhard SalcherAbstract:We compile detrital 10Be concentrations of Alpine rivers, representing the Denudation rates pattern for 375 catchments across the entire European Alps. Using a homogeneized framework, we employ state-of-the-art techniques for inverting in-situ 10Be concentrations into Denudation rates. From our compilation, we find that (i) while lithologic properties and precipitation/runoff do influence erosion mechanisms and rates at the scale of individual catchments and in some specific Alpine regions, such controls do not directly stand for the entire Alps, (ii) as also previously suggested, catchment-wide Denudation rates across the entire European Alps closely follow first-order Alpine topographic metrics at the scale of individual catchments or selected Alpine sub-regions. However, in addition to previous local-scale studies conducted in the European Alps, our large-scale compilation highlights a functional relationship between catchment-wide Denudation and mean catchment slope angle. Catchment-wide Denudation positively correlates with mean catchment slope up to a threshold angle (25–30°). Above this threshold, any correlation between catchment-wide Denudation and slope as well as other catchment metrics breaks apart. We can reconcile these systematic patterns by proposing a regional erosion model based on diffusive-transport laws for catchments located below the slope threshold angle. In oversteepened catchments situated above-threshold slopes, erosion is stochastic in nature, as glacial carving likely caused a partial decoupling between hillslope and fluvial domains with complex topographic relationships and sediment connectivity patterns. Finally, we identify a first-order positive relationship between modern geodetic rock uplift and catchment-wide Denudation for the European Alps. The observed spatial pattern is highly variable and possibly reflects the surface response to deep geodynamic mechanisms prevailing in the different Alpine regions. We conclude that today's topography and geomorphic features of the entire Alps are the result of a millenial-scale geomorphic response to past glacial processes and active rock uplift, highlighting a link between external and internal drivers for mountain erosion.
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controls on holocene Denudation rates in mountainous environments under mediterranean climate
Earth Surface Processes and Landforms, 2017Co-Authors: Nicolas Freslon, Julien Moreau, Stéphane Molliex, Gwenael Jouet, Christine Authemayou, Didier Bourles, Marina RabineauAbstract:The Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional Denudation rates in the framework of a source-to-sink approach. Indeed, fluvial sedimentation in East-Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz-rich crystalline rocks allowing cosmogenic nuclide Be-10 measurements. In this paper, Holocene Denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced Be-10 concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated Denudation rates range from 15 to 95 mm ka(-1). When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho-climatic parameters including sun exposure and the direction of the main wet wind, so-called Libecciu'. This distribution, as well as the basement rock strength seems to play a significant role in the Denudation distribution. We thus suggest that the landscape reached a geomorphic steady-state due to the specific Mediterranean climate and that Holocene Denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport-limited...
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controls on holocene Denudation rates in mountainous environments under mediterranean climate
Earth Surface Processes and Landforms, 2017Co-Authors: Nicolas Freslon, Julien Moreau, Stéphane Molliex, Gwenael Jouet, Christine Authemayou, Didier Bourles, Marina RabineauAbstract:The Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional Denudation rates in the framework of a source-to-sink approach. Indeed, fluvial sedimentation in East-Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz-rich crystalline rocks allowing cosmogenic nuclide Be-10 measurements. In this paper, Holocene Denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced Be-10 concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated Denudation rates range from 15 to 95 mm ka(-1). When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho-climatic parameters including sun exposure and the direction of the main wet wind, so-called Libecciu'. This distribution, as well as the basement rock strength seems to play a significant role in the Denudation distribution. We thus suggest that the landscape reached a geomorphic steady-state due to the specific Mediterranean climate and that Holocene Denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport-limited...
Friedhelm Von Blanckenburg - One of the best experts on this subject based on the ideXlab platform.
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sediment production and delivery in the amazon river basin quantified by in situ produced cosmogenic nuclides and recent river loads
Geological Society of America Bulletin, 2011Co-Authors: Hella Wittmann, Friedhelm Von Blanckenburg, Laurence Maurice, Jeanloup Guyot, Naziano Filizola, P W KubikAbstract:We use cosmogenic nuclide-derived Denudation rates from in situ–produced 10 Be in river sediment to determine sediment production rates for the central Amazon River and its major tributaries. Recent developments have shown that this method allows calculating Denudation rates in large depositional basins despite intermediate sediment storage, with the result that fluxes of the sediment-producing hinterland can now be linked to those discharged at the basins’ outlet. In rivers of the central Amazonian plain, sediment of finer grain sizes (125–500 μm) yields a weighted cosmogenic nuclide-derived Denudation rate of 0.24 ± 0.02 mm/yr that is comparable to the integrated rate of all main Andean-draining rivers (0.37 ± 0.06 mm/yr), which are the Beni, Napo, Mamore, Ucayali, and Maranon rivers. Coarser-grained sediment (>500 μm) of central Amazonian rivers is indicative of a source from the tectonically stable cratonic headwaters of the Guyana and Brazilian shields, for which the Denudation rate is 0.01–0.02 mm/yr. Respective sediment loads can be calculated by converting these cosmogenic nuclide-derived rates using their sediment-producing areas. For the Amazon River at Obidos, a sediment production rate of ∼610 Mt/yr results; non-Andean source areas contribute only ∼45 Mt/yr. A comparison with published modern sediment fluxes shows similarities within a factor of ∼2 with an average gauging-derived sediment load of ∼1000 Mt/yr at Obidos, for example. We attribute this similar trend in cosmogenic versus modern sediment loads first to the absence of long-term deposition within the basin and second to the buffering capability of the large Amazon floodplain. The buffering capability dampens short-term, high-amplitude fluctuations (climatic variability in source areas and anthropogenic soil erosion) by the time the Denudation rate signal of the hinterland is transmitted to the outlet of the basin.
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relation between rock uplift and Denudation from cosmogenic nuclides in river sediment in the central alps of switzerland
Journal of Geophysical Research, 2007Co-Authors: Hella Wittmann, Friedhelm Von Blanckenburg, Tina Kruesmann, Kevin P Norton, Peter W KubikAbstract:[1] A north-south traverse through the Swiss Central Alps reveals that Denudation rates correlate with recent rock uplift rates in both magnitude and spatial distribution. This result emerges from a study of in situ–produced cosmogenic 10Be in riverborne quartz in Central Alpine catchments. As a prerequisite, we took care to investigate the potential influence of shielding from cosmic rays due to snow, glaciers, and topographic obstructions; to calculate a possible memory from Last Glacial Maximum (LGM) glaciation; and to identify a watershed size that is appropriate for systematic sampling. Mean Denudation rates are 0.27 ± 0.14 mm/a for the Alpine foreland and 0.9 ± 0.3 mm/a for the crystalline Central Alps. The measured cosmogenic nuclide-derived Denudation rates are in good agreement with post-LGM lake infill rates and are about twice as high as Denudation rates from apatite fission track ages that record Denudation from 9 to 5 Ma. In general, Denudation rates are high in areas of high topography and high crustal thickness. The similarity in the spatial distribution and magnitude of Denudation rates and those of rock uplift rates can be interpreted in several ways: (1) Postglacial rebound or climate change has introduced a transient change in which both uplift and Denudation follow each other with a short lag time; (2) the amplitude of glacial to interglacial changes in both is small and is contained in the scatter of the data; (3) both are driven by ongoing convergence where their similarity might hint at some form of long-term quasi steady state; or (4) enhanced continuous Quaternary erosion and isostatic compensation of the mass removed accounts for the distribution of present-day rock uplift.
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the control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment
Earth and Planetary Science Letters, 2005Co-Authors: Friedhelm Von BlanckenburgAbstract:The study of a sample of river sediment enables the determination of spatially averaged Denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape. These measurements are done with in-situ produced cosmogenic nuclides (e.g. 10Be, 26Al), mostly in quartz from alluvial sediment. Cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the Earth's surface. They are produced within a characteristic depth scale of about 1 m, which means that the measured concentrations record an integrated Denudation history while material passed through this depth interval. Depending on the Denudation rate the resulting integration time scales are 103 to 105 years, and one obtains a robust long-term estimate of natural Denudation that is relatively insensitive to short-term changes. The last 10 years have seen significant research activity using these methods, and an array of fascinating tectono-geomorphologic and geochemical insights are emerging. Amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change. A compilation of world-wide Denudation rates in non-glaciated areas, that however, does not yet include some of the world's most active mountain belts, has resulted in the following findings, some of which have been unexpected: (1) No obvious relationship between precipitation or mean annual temperature and total Denudation is apparent. (2) Topographic relief alone does not result in high rates of Denudation. (3) Denudation rates are high in areas of landscape rejuvenation; that is triggered and controlled by tectonic activity (faulting, escarpment formation and retreat, rifting, surface uplift). (4) Rates of weathering (using a combination of cosmogenic nuclides and zirconium-normalised cation loss balances) co-vary primarily with physical erosion rates and much less with temperature or precipitation. (5) In some areas of high land use short-term rates (from river load gauging) exceed those from cosmogenic nuclides by several orders of magnitude, which serves to highlight the severity of geomorphic change caused by human action. In the future, the control mechanisms over Denudation will be determined on all spatial scales, ranging from the single soil section to entire river basins. The same analysis can be done back through time on well-dated terraces, lake records, and marine sediment cores, which is possible with 10Be for the past 1–2 My. The rates obtained will be used to develop a quantitative understanding of tectonic, geomorphologic, and geochemical landscape processes, which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic, climate, and anthropogenic forcing.
Marina Rabineau - One of the best experts on this subject based on the ideXlab platform.
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controls on holocene Denudation rates in mountainous environments under mediterranean climate
Earth Surface Processes and Landforms, 2017Co-Authors: Nicolas Freslon, Julien Moreau, Stéphane Molliex, Gwenael Jouet, Christine Authemayou, Didier Bourles, Marina RabineauAbstract:The Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional Denudation rates in the framework of a source-to-sink approach. Indeed, fluvial sedimentation in East-Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz-rich crystalline rocks allowing cosmogenic nuclide Be-10 measurements. In this paper, Holocene Denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced Be-10 concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated Denudation rates range from 15 to 95 mm ka(-1). When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho-climatic parameters including sun exposure and the direction of the main wet wind, so-called Libecciu'. This distribution, as well as the basement rock strength seems to play a significant role in the Denudation distribution. We thus suggest that the landscape reached a geomorphic steady-state due to the specific Mediterranean climate and that Holocene Denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport-limited...
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controls on holocene Denudation rates in mountainous environments under mediterranean climate
Earth Surface Processes and Landforms, 2017Co-Authors: Nicolas Freslon, Julien Moreau, Stéphane Molliex, Gwenael Jouet, Christine Authemayou, Didier Bourles, Marina RabineauAbstract:The Mediterranean domain is characterized by a specific climate resulting from the close interplay between atmospheric and marine processes and strongly differentiated regional topographies. Corsica Island, a mountainous area located in the western part of the Mediterranean Sea is particularly suitable to quantify regional Denudation rates in the framework of a source-to-sink approach. Indeed, fluvial sedimentation in East-Corsica margin is almost exclusively limited to its alluvial plain and offshore domain and its basement is mainly constituted of quartz-rich crystalline rocks allowing cosmogenic nuclide Be-10 measurements. In this paper, Holocene Denudation rates of catchments from the eastern part of the island of Corsica are quantified relying on in situ produced Be-10 concentrations in stream sediments and interpreted in an approach including quantitative geomorphology, rock strength measurement (with a Schmidt Hammer) and vegetation cover distribution. Calculated Denudation rates range from 15 to 95 mm ka(-1). When compared with rates from similar geomorphic domains experiencing a different climate setting, such as the foreland of the northern European Alps, they appear quite low and temporally stable. At the first order, they better correlate with rock strength and vegetation cover than with morphometric indexes. Spatial distribution of the vegetation is controlled by morpho-climatic parameters including sun exposure and the direction of the main wet wind, so-called Libecciu'. This distribution, as well as the basement rock strength seems to play a significant role in the Denudation distribution. We thus suggest that the landscape reached a geomorphic steady-state due to the specific Mediterranean climate and that Holocene Denudation rates are mainly sustained by weathering processes, through the amount of regolith formation, rather than being transport-limited...
A J W Gleadow - One of the best experts on this subject based on the ideXlab platform.
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Denudational and thermal history of the early cretaceous brandberg and okenyenya igneous complexes on namibia s atlantic passive margin
Tectonics, 2005Co-Authors: Matthias Raab, Roderick Brown, Kerry Gallagher, Klaus Weber, A J W GleadowAbstract:[1] The Early Cretaceous Brandberg and Okenyenya igneous complexes provide ideal locations to quantify the postbreakup Denudation history of the continental margin of central Namibia. Apatite fission track analysis has been applied to vertical sample profiles extending over 1–2 km of relief to constrain the Denudation history of this region. An advantage of vertical profiles is that they allow the inference of the paleogeothermal gradient directly from the data. An independent, data-consistent thermal gradient estimate is essential to determine the amount of Denudation as well as to resolve phases of slow and accelerated Denudation. We obtained independent paleogeothermal gradient estimates for the two vertical profiles from 26 ± 32°C to 23 ± 16°C. The uncertainties are large but are considered pessimistic. The best fit values agree well with each other, and the present-day gradients of 22°C. The total amount of Denudation since the Late Cretaceous has been estimated as 5 km in the Brandberg area and 4 km for the Okenyenya area. The data from the two vertical profiles imply a phase of rapid exhumation in the Late Cretaceous between 80 and 60 Ma, at rates between 0.2 and 0.125 km/m.y. These decline rapidly in the Early Tertiary to a maximum Denudation rate between 0.023 and 0.015 km/m.y. The Denudation chronology inferred from the new apatite fission track data is consistent with the observation of a major influx of clastic sediment to offshore basins about 50 m.y. after continental breakup. The study shows that vertical fission track profiles provide key insights into postbreakup tectonic and geomorphic processes in the Central Damara Zone in Namibia, where no Phanerozoic sedimentary record has been preserved.
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Denudational history along a transect across the drakensberg escarpment of southern africa derived from apatite fission track thermochronology
Journal of Geophysical Research, 2002Co-Authors: Roderick Brown, M Summerfield, A J W GleadowAbstract:[1] The Denudational history of a ∼500 km long transect across the Drakensberg Escarpment on the high-elevation passive margin of SE Africa is quantified on the basis of thermal history modeling of apatite fission track data for 15 deep borehole samples, supplemented by an additional 10 outcrop samples. A minimum of 4.5 km of Denudation since formation of the margin ∼130 Myr ago is estimated for the coastal zone, with a marked Early Cretaceous episode of accelerated Denudation broadly coincident with continental breakup. Samples from the Swartberg borehole (SW 1/67) located ∼30 km seaward of the present position of the Drakensberg Escarpment indicate a total depth of Denudation of 3.1 ± 1.2 km since ∼91 Myr, with a phase of accelerated Denudation of 2.1 ± 0.9 km at a mean rate of 95 ± 43 m/Myr between ∼91 and 69 Myr. Samples from the Ladybrand borehole (LA 1/68) west of the Lesotho Highlands indicate 1.7 ± 0.5 km of Denudation since ∼78 Myr, with a phase of accelerated Denudation at 82 ± 43 m/Myr from ∼78 to 64 Myr. Average Denudation rates declined to about 10 m/Myr during much of the Tertiary. Although the apatite fission track data do not provide any direct constraints on the Denudational history of the Lesotho Highlands, interpolation between the two boreholes, constrained by geological evidence and extrapolated in situ-produced cosmogenic 36Cl-derived Denudation rate estimates, suggests a pattern of Denudation compatible with numerical modeling studies of escarpment evolution involving rapid river incision seaward of a preexisting inland drainage divide. These patterns of Denudation are incompatible with constant retreat of the Drakensberg Escarpment from an initial position near the present coast. We suggest that the Drakensberg Escarpment formed by rapid post-breakup river incision seaward of a preexisting drainage divide located just east of the present escarpment location and became pinned at this divide with subsequent retreat rates of only 100–200 m/Myr.