Browsing by Author "Cazes, G"
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- Item26Al/10Be ratios reveal the source of river sediments in the Kimberley, NW Australia(Wiley, 2020-02-07) Cazes, G; Fink, D; Codilean, AT; Fülöp, RH; Fujioka, T; Wilcken, KMWe use cosmogenic 10Be and 26Al in both bedrock and fluvial sediments to investigate controls on erosion rates and sediment supply to river basins at the regional scale in the Kimberley, NW Australia. The area is characterised by lithologically controlled morphologies such as cuestas, isolated mesas and extensive plateaus made of slightly dipping, extensively jointed sandstones. All sampled bedrock surfaces at plateau tops, ridgelines, and in the broader floodplain of major rivers over the region show similar slow lowering rates between 0.17 and 4.88 m.Myr-1, with a mean value of 1.0 ± 0.6 m.Myr-1 (n=15), whilst two bedrock samples collected directly within river-beds record rates that are one to two orders of magnitude higher (14.4 ± 1.5 and 20.9 ± 2.5 m.Myr-1, respectively). Bedrock 26Al/10Be ratios are all compatible with simple, continuous sub-aerial exposure histories. Modern river sediment yield lower 10Be and 26Al concentrations, apparent 10Be basin-wide denudation rates ranging between 1.8 and 7.7 m.Myr-1, with a median value of 2.6 m.Myr-1, more than double the magnitude of bedrock erosion rates. 26Al/10Be ratios of the sediment samples are lower than those obtained for bedrock samples. We propose that these depleted 26Al/10Be ratios can largely be explained by the supply of sediment to river basins from the slab fragmentation and chemical weathering of channel gorge walls and plateau escarpments that result in diluting the cosmogenic nuclide concentration in river sediments measured at the basin outlets. The results of a mass-balance model suggest that ~60–90% of river sediment in the Kimberley results from the breakdown and chemical weathering of retreating vertical sandstone rock-walls in contrast to sediment generated by bedrock weathering and erosion on the plateau tops. This study emphasises the value of analysing two or more isotopes in basin-scale studies using cosmogenic nuclides, especially in slowly eroding post-orogenic settings. © 2019 John Wiley & Sons, Ltd.
- ItemBurial ages, ghost-rocks and karst network structure. Insights from the Vis canyon (Southern France)(Groupe français de géomorphologie, 2020-10-21) Malcles, O; Vernant, P; Chéry, J; Ritz, JF; Cazes, G; Fink, DNew burial ages by measuring 26 Al / 10 Be isotopic ratios in three cavities near the Vis canyon (south of France, south-eastern limit of Grands-Causses) challenge the classic karstogenesis model. used to explain the structuring of networks. Indeed, the subhorizontal levels of underground galleries are generally used as markers of the position of the regional base level ( ie , river). If the relationship between burial age and height relative to the regional base level expected using the classical model per descensumfor the Scorpions caves (1.2 ± 0.4 Ma, +105 m relative to the base level -rnb-) and Bergougnous (1.2 ± 0.3 Ma, +117 m rnb) is in agreement with the quantification of the average regional incision rate (83 ± 35 m / Ma), it does not explain the age of 0.94 ± 0.07 Ma obtained for the infills of the horizontal levels of the Rocas located much higher in altitude (+317 m rnb). The first two cavities open directly into the gorges, while the third opens onto the plateau less than 4 km from the gorges. To our knowledge, this is the first time that quantitative data support the proposition that the model of regional karstogenesis controlled by the position of the base level is not universal. These dates make it possible to suggest that ghosting plays a primordial role in controlling the location of the sub-horizontal levels. This process internal to the karst relegates the role of the position of the regional base level to that of a simple external triggering of the hollowing out of the alteration products generated by the phantomization. © French group of geomorphology
- ItemChallenging intraplate orogens: from geomorphology to lithospheric dynamic. The French Massif Central case study(Copernicus GmbH, 2020-05-04) Malcles, O; Vernant, P; Ritz, JF; Fink, D; Cazes, G; Fujioka, T; Braucher, R; Camps, PIn the 60’s, the formulation of the plate tectonic theory changed our understanding of the Earth dynamics. Aiming at explaining the earth first order kinematics, this primary theory of plate tectonic assumed rigid plates, a necessity to efficiently transfer stress from one boundary to another. If successful to explain, at first order, the plate-boundary evolutions, this theory fails when compared to the unpredicted but identified deformation located inside the plate-domains: the intraplate orogens. Indeed, the intraplate regions are thought to be slowly, if at all, deforming. Therefore, it is expected that intraplate regions do not show important finite deformation, that is to say, no mountains. Some intraplate regions, however, have important relief: the Snowy Mountains (Australia), the Ural Mountains (Russia) or the Massif Central (France) for examples. Traditionally, such regions are interpreted as old structures that are slowly eroded, interpretations that are most of the time weakly constrained. Our study is aiming at providing stronger constraints and then a better understanding of such challenging area that are the intraplate orogen domains. Because direct measurements of deformations (e.g. GNSS: Global Navigation Satellite System or InSAR: Interferometric Synthetic Aperture Radar) are most of the time below the precision level, it is necessary to derive this information from the landscape evolution. To do so, terrestrial cosmogenic nuclide (TCN) technics are a key method, allowing to constraint the temporal landscape evolution. Classically, two TCN-based approaches are used to quantify the landscape evolution rate: burial ages and watershed-wide denudation rates, based on measurement in quartz sediment of 10Be and 26Al concentrations, two radioactive cosmogenic isotopes. Using the Massif Central (France) as study area, we show that this region is currently deforming. From new geochronological constraints and a geomorphometric study, we propose that the region undergoes an active uplift encompassing the last c.a. 4 Ma. It can be explained by the combination of at least two phenomena: the first one is the uplift triggering event, that has yet to be clearly identified, and the second one: the erosional isostatic adjustment enhancing the first one and possibly continuing after the end of the first one.
- ItemConstraining the age of Aboriginal rock art using cosmogenic Be-10 and Al-26 dating of rock shelter collapse in the Kimberley region, Australia(Australian National University, 2019-09-09) Cazes, G; Fink, D; Fülöp, RH; Codilean, ATThe Kimberley region, northwest Australia, possesses an extensive and diverse collection of aboriginal rock art that potentially dates to more than 40,000 years ago. However, dating of such art using conventional techniques remains problematic. Here, we develop a new approach which makes use of the difference in production rates of in-situ 10Be and 26Al between intact rock walls and exposed surfaces of detached slabs from rock art shelters to constrain the age of Aboriginal rock-art. In the prevailing sandstone lithology of the Kimberley region, open cave-like rock shelters with cantilevered overhangs evolve by the collapse of unstable, partially rectangular, blocks weakened typically along joint-lines and fractures. On release, those slabs which extend outside the rock face perimeter will experience a higher production rate of cosmogenic 10Be and 26Al than the adjacent rock which remains intact within the shelter. The dating of these freshly exposed slabs can help reconstruct rock-shelter formation and provide either maximum or minimum ages for the rock art within the shelter. At each site, both the upper-face of the newly exposed fallen slab and the counterpart intact rock surface on the ceiling need to be sampled at their exact matching-point to ensure that the initial pre-release cosmogenic nuclide concentration on slab and ceiling are identical. The calculation of the timing of the event of slab release is strongly dependent on the local production rate, the new shielding of the slab surface and the post-production that continues on the ceiling sample at the matching point. The horizon, ceiling and slab shielding are estimated by modelling the distribution of neutron and muon trajectories in the irregular shaped rock-shelter and slab using 3D photogrammetric reconstruction from drone flights and a MATLAB code (modified from G. Balco, 2014) to estimate attenuation distances and model the production rate at each sample. Five rock-art sites have been dated and results range from 9.8±1.9 ka to 180.8±22.3 ka. While the date obtained for the youngest site can be interpreted as both a maximum and minimum age for the art due to its positioning over different walls of this specific shelter, all the other sites give maximum art ages which are significantly older than presumed human occupation in Australia. However, within the context of regional landscape geomorphology, these relatively young ages give new insights into the contrasting modes of landscape evolution in the Kimberley, and the importance of episodic escarpment retreat overprinted by passive basin-wide denudation which from numerous previous measurements are as low as 1-5 mm/ka (i.e. averaging timescales of ~400 kyr). A large number of similar sites in the region have been mapped and are potential candidates for this new approach which can constrain the controversial relative chronology of the various aboriginal rock art styles.
- ItemDetermining the origin and changing shape of landscape-scale rock formations with three-dimensional modelling: the Borologa rock shelters, Kimberley region, Australia(Wiley, 2021-05-02) Genuite, K; Delannoy, JJ; David, B; Unghango, A; Balanggarra Aboriginal Corporation; Cazes, G; Fulop, R; Fink, D; Codilean, AT; Ouzman, S; Veth, P; Harper, S; Green, H; Finch, D; Urwin, CArchaeologists often wonder how and when rock shelters formed, yet their origins and antiquity are almost never systematically investigated. Here we present a new method to determine how and when individual boulders and rock shelters came to lie in their present landscape settings. We do so through 3D laser (LiDAR) mapping, illustrating the method by example of the Borologa Aboriginal site complex in the Kimberley region of northwestern Australia. Through a combination of geomorphological study and high-resolution 3D modelling, individual blocks of rock are refitted and repositioned t680their originating cliff-line. Preliminary cosmogenic nuclide ages on exposed vertical cliff faces and associated detached boulders above the Borologa archaeological sites signal very slow detachment rates for the mass movements of large blocks down the Drysdale Valley slopes, suggesting relative landscape stability over hundreds of thousands of years (predating the arrival of people). These findings offer hitherto unknown details of the pace of regional landscape evolution and move us toward a better understanding of patterns of human occupation in a context of relatively stable rock outcrops both within the sites and across the region. © 1999-2021 John Wiley & Sons, Inc
- ItemDetermining the Plio-Quaternary uplift of the southern French Massif Central; a new insight for intraplate orogen dynamics(European Geosciences Union, 2020-02-26) Malcles, O; Vernant, P; Chéry, J; Camps, P; Cazes, G; Ritz, JF; Fink, DThe evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m , with the highest peak elevations ranging from 1500 to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age ( 10 Be ∕ 26 Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83+ 17 / - 5 m Ma −1 during the last ca. 4 Myr (Pliocene – Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene – Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate – orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation. © Author (s) 2020
- ItemThe French Massif-Central, example of a not so inactive intraplate region(Australian Nuclear Science and Technology Organisation, 2021-11-17) Malcles, O; Phillipe, V; Ritz, JF; Fink, D; Cazes, GPlate tectonic theory postulates that intraplate areas are geodynamically inactive regions, active geologic and seismic deformations being concentrated along more or less narrow areas: the plate boundaries. The tectonic plates are supposed rigid, allowing stress transfer from one boundary to another. Therefore, no deformation is expected within the intraplate regions. True inactivity of intraplate areas is however refuted by evidences of active deformation. Many cases of intraplate earthquakes are known as for instance the lake Muir earthquake in 2018 (Mw = 5.3), the Botswana earthquake in 2017 (Mw= 6.5) or the New-Madrid sequence in 1811-1812 (4 events with Mw > 7). Recent propositions tend to decouple plate tectonics dynamic from intraplate earthquakes (e.g. [1]). In this case, the processes generating the stresses are local (e.g. fluid migration) or transient (e.g. GIA) and therefore long-term intraplate deformations are unlikely to happen. Significant intraplate deformations are however easily recognizable at the earth surface using the topography. Indeed, an important regional relief (Mountains) is the first evidence of earth surface long-term deformation and examples can be found for almost each intraplate region, for example: the Appalachians mountains (Northern America), the Great Dividing Range (Australia) or the Guiana highlands (Southern America). The origins of these topographic features are highly debated and almost every explanation has been given: tectonic stress, past tectonic frontier with old relief, dynamic topography, etc. In many cases, the lack of absolute dating precludes the determination of the landscape evolution rates leading to inaccurate and sometimes physically unsound geomorphologic models. Using the example of the French Massif-Central, we study if the long-term surface processes (erosion and sedimentation) can be responsible for intraplate deformation. Quantification of the surface erosion and incision rates were performed using Terrestrial Cosmogenic Nuclides (TCN), mainly 10Be and 26Al. Using both mean watersheds derived denudation rates (covering the last ~ 15 ka) and long-term incision rates using endokarstic infilling (covering the last ~ 5 Ma) we show, despite local variations due to specific morphology and possible climatic variations, that the region is affected by significant erosion (s.l.) with a mean denudation rate of ~ 60 m/Ma of and an incision rate of ~ 90 m/Ma. Given the current ~ 300 m depth of the valleys, we conclude that this mountainous region is the consequence of a Plio-Quaternay uplift and therefore that intraplate area can be associated with active long-term processes leading to consequent finite deformation. First order numerical model addresses the question of driving processes and show that a combination of thermal isostasy and erosion driven isostatic adjustment can explain both long-term uplift rate and distributed volcanic activity of the area. Such relatively constant long-term uplift is expected to be at the origin of long-term stress concentrations and therefore intraplate earthquakes could be associated with low-frequency seismic cycles modulated by transient or local processes.
- ItemSpatial pattern of denudation in a lithologically controlled sub-tropical flat landscape: insights from the Kimberley region, NW Australia(European Geophysical Union, 2016-01-01) Cazes, G; Fink, D; Codilean, AT; Fülöp, RHThe Kimberley region, northwest Australia, is well known for its expansive and diverse collection of prehistorical aboriginal rock art that potentially dates back to 40,000 years ago. The region is characterized by a tropical, semiarid climate with a monsoonal rainfall distribution and a flat landscape interrupted by massive sandstone mesas and deeply incised bedrock river gorges. In order to constrain the chronology of the rock art it is necessary to quantify the spatial and temporal dimensions of landscape evolution. We report cosmogenic 10Be and 26Al concentrations in modern fluvial sediment collected from 27 catchments with areas spanning several orders of magnitude (13.6 – 13,900 km2 ). All catchments are characterized by a very low topographic gradient (average basin slopes < 3◦ ) and subdued local relief of at most 200m. Assuming negligible sediment storage times and rapid sediment transport driven by the annual monsoonal washout, we calculate 10Be based catchment-wide denudation rates ranging between 1.87 ± 0.23 and 9.48 ± 1.05 m.Myr−1 . These low rates are among the slowest recorded in the world, despite the strong climatic seasonality of the region. Our measured denudation rates exhibit a strong correlation with topographic gradient, which in the overall flat landscapes of the Kimberley, is controlled by the prevailing sandstone bedrock lithology and the presence of numerous escarpments adjacent to the river channels. We present a modelling approach that makes use of the 26Al/10Be ratio in the fluvial sediments as a source tracer (ie escarpment cliffs, river channels, plateau bedrocks), and use this to explore the control and retreat rate of the eroding escarpment cliffs in order to provide information on the spatial distribution of denudation in the landscape. © Author(s) 2016. CC Attribution 3.0 License.