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1.
Transient evolution and adjustment to changing tectonic and climatic boundary conditions is an essential attribute of landscapes, and characterizing transient behavior is a key to understanding their dynamics and history. Developing new approaches to detect such transience has been explored by various methods, in particular to identify landscape response to Late Cenozoic and Quaternary climatic changes. Such studies have often focused on regions of high relief and/or active tectonic activity where interferences between tectonic and climatic signals might complicate the interpretation of the observations. We investigated the case of the hillslopes of the Serra do Cipó quartzitic range in SE Brazil in order to detect and quantify transience in a tectonically quiescent landscape over 100-ka timescales. We determined hilltop curvature from a high-resolution digital surface model derived from Pléiades imagery and measured cosmogenic nuclide (10Be and 26Al) concentrations at these hilltop sites. We compare both observations with predictions of hillslope diffusion theory, observing a distinctive signature of an acceleration of denudation. We performed a joint inversion of topographic and isotopic data to retrieve an evolution of the hillslope sediment transport coefficient through time. The timing of the increase in denudation cannot be unequivocally associated with a single climatic event but is consistent with important, climatically modulated fluctuations in precipitation and erosion in this area during the Middle and Late Pleistocene.  相似文献   

2.
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.  相似文献   

3.
A range of evidence from the Lachlan valley in the southeast Australian highlands is consistent with Neogene isostatic rebound in response to denudational unloading. This evidence is found along the inland edge of the highlands in the transition zone between the highlands proper and the Lachlan's inland alluviated valley and the intracratonic Murray Basin. The amounts and rates of uplift indicated by offsets of suballuvium bedrock profiles and the long profiles of Tertiary valley-filling basalts are consistent with modelling of denudational rebound using known rates of highland denudation and basinal sedimentation, and reasonable crustal properties. The modelling shows that weak to moderately strong strong lithosphere (effective elastic thickness, Te = 1-25 km) and strong lithosphere (Te = 100 km) are all consistent with the observed amounts of rebound. Strong lithosphere must be broken, however, to be consistent with the field data. Even in the Australian setting, which is characterized by very low rates of denudation, isostatic rebound in response to denudational unloading must be a significant factor in maintaining highland elevation and must be incorporated in models of long-term landscape evolution. It would be expected that denudational isostatic rebound would be an even more significant component of long-term landscape evolution in areas of higher denudation rates.  相似文献   

4.
Differences in chemical weathering extent and character are expected to exist across topographic escarpments due to spatial gradients of climatic and/or tectonic forcing. The passive margin escarpment of south‐eastern Australia has a debated but generally accepted model of propagation in which it retreated (within 40 Ma) to near its current position following rifting between Australia and New Zealand 85–100 Ma before present. We focus on this escarpment to quantify chemical weathering rates and processes and how they may provide insight into scarp evolution and retreat. We compare chemical weathering extents and rates above and below the escarpment using a mass balance approach coupling major and trace element analyses with previous measurements of denudation rates using cosmogenic nuclides (10Be and 26Al). We find a slight gradient in saprolite chemical weathering rate as a percentage of total weathering rate across the escarpment. The lowlands area, encompassing the region extending from the base of the escarpment to the coast, experiences a greater extent of chemical weathering than the highland region above the escarpment. Percents of denudation attributable to saprolite weathering average 57 ± 6% and 47 ± 7% at low and high sites respectively. Furthermore, the chemical index of alteration (CIA), a ratio of immobile to mobile oxides in granitic material that increases with weathering extent, have corresponding average values of 73·7 ± 3·9 and 65·5 ± 3·4, indicating lower extents of weathering above the escarpment. Finally, we quantify variations in the rates and extent of chemical weathering at the hillslope scale across the escarpment to suggest new insight into how climate differences and hillslope topography help drive landscape evolution, potentially overprinting longer term tectonic forcing. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

5.
We present new data about the morphological and stratigraphic evolution and the rates of fluvial denudation of the Tavoliere di Puglia plain, a low‐relief landscape representing the northernmost sector of the Pliocene‐Pleistocene foredeep of the southern Apennines. The study area is located between the easternmost part of the southern Apennine chain and the Gargano promontory and it is characterized by several orders of terraced fluvial deposits, disconformably overlying lower Pleistocene marine clay and organized in a staircase geometry, which recorded the emersion and the long‐term incision history of this sector since mid‐Pleistocene times. We used the spatial and altimetric distribution of several orders of middle to late Pleistocene fluvial terraces in order to perform paleotopographic reconstruction and GIS‐aided eroded volumes estimates. Then, we estimated denudation rates on the basis of the terraces chronostratigraphy, supported by published OSL and AAR dating. Middle to upper Pleistocene denudation rates estimated by means of such an approach are slightly lower than 0.1 mm yr‐1, in good agreement with short‐term data from direct and indirect evaluation of suspended sediment yield. The analysis of longitudinal river profiles using the stream power erosion model provided additional information on the incision rates of the studied area. Middle to late Quaternary uplift rates (about 0.15 mm yr‐1), calculated on the basis of the elevation above sea level of marine deposits outcropping in the easternmost sector of the study area, are quite similar to the erosion rates average value, thus suggesting a steady‐state fluvial incision. The approach adopted in this work has demonstrated that erosion rates traditionally obtained by quantitative geomorphic analysis and ksn estimations can be successfully integrated to quantify rates of tectonic or geomorphological processes of a landscape approaching steady‐state equilibrium. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
Cosmogenic 10Be concentrations in exposed bedrock surfaces and alluvial sediment in the northern Flinders Ranges reveal surprisingly high erosion rates for a supposedly ancient and stable landscape. Bedrock erosion rates increase with decreasing elevation in the Yudnamutana Catchment, from summit surfaces (13·96 ± 1·29 and 14·38 ± 1·40 m Myr?1), to hillslopes (17·61 ± 2·21 to 29·24 ± 4·38 m Myr?1), to valley bottoms (53·19 ± 7·26 to 227·95 ± 21·39 m Myr?1), indicating late Quaternary increases to topographic relief. Minimum cliff retreat rates (9·30 ± 3·60 to 24·54 ± 8·53 m Myr?1) indicate that even the most resistant parts of cliff faces have undergone significant late Quaternary erosion. However, erosion rates from visibly weathered and varnished tors protruding from steep bedrock hillslopes (4·17 ± 0·42 to 14·00 ± 1·97 m Myr?1) indicate that bedrock may locally weather at rates equivalent to, or even slower than, summit surfaces. 10Be concentrations in contemporary alluvial sediment indicate catchment‐averaged erosion at a rate dominated by more rapid erosion (22·79 ± 2·78 m Myr?1), consistent with an average rate from individual hillslope point measurements. Late Cenozoic relief production in the Yudnamutana Catchment resulted from (1) tectonic uplift at rates of 30–160 m Myr?1 due to range‐front reverse faulting, which maintained steep river gradients and uplifted summit surfaces, and (2) climate change, which episodically increased both in situ bedrock weathering rates and frequency–magnitude distributions of large magnitude floods, leading to increased incision rates. These results provide quantitative evidence that the Australian landscape is, in places, considerably more dynamic than commonly perceived. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

7.
Short‐term (contemporary) and long‐term denudation rates were determined for the Blue Mountains Plateau in the western Sydney Basin, Australia, to explore the role of extreme events (wildfires and catastrophic floods) in landscape denudation along a passive plate margin. Contemporary denudation rates were reconstructed using 40 years of river sediment load data from the Nattai catchment in the south‐west of the basin, combined with an analysis of hillslope erosion following recent wildfires. Long‐term denudation rates (10 kyr–10 Myr) were determined from terrestrial cosmogenic nuclides, apatite fission track thermochronology and post‐basalt flow valley incision. Contemporary denudation rates average several times lower than the long‐term average (5·5 ± 4 mm kyr?1 versus 21·5 ± 7 mm kyr?1). Erosion of sediment following wildfires accounts for only a small proportion (5%) of the contemporary rate. Most post‐fire sediment is stored on the lower slopes and valley floor, with the amount transported to the river network dependent on rainfall–run‐off conditions within the first few years following the fire. Historical catastrophic floods account for a much larger proportion (35%) of the contemporary erosion rate, and highlight the importance of these events in reworking stored material. Evidence for palaeofloods much larger than those experienced over the past 200 years suggests even greater sediment export potential. Mass movement on hillslopes along valleys incised into softer lithology appears to be a dominant erosion process that supplies substantial volumes of material to the valley floor. It is possible that a combination of infrequent mass movement events and high fluvial discharge could account for a significant proportion of the discrepancy between the contemporary and long‐term denudation rates. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

8.
Landscape evolution is modulated by the regional tectonic uplift,climate change,and river dynamics.However,how to distinguish these mechanisms through the research of surface exhumation and fluvial incision remains controversial.In this study,cosmogenic~(10)Be,~(26)Al,and~(21)Ne concentrations in quartz from cave deposits,modern river sediments,and bedrocks were measured to constrain the applicability of cosmogenic~(21)Ne and discuss Quaternary landscape evolution history in the Guizhou Plateau,southeast China.Using the~(26)Al-~(10)Be and~(21)Ne-~(10)Be pairs to distinguish the cosmogenic~(21)Ne concentration from the excess~(21)Ne,we found that the nucleogenic~(21)Ne produced by the U and Th decay in quartz is significant in the samples although there is the possibility of inherited cosmogenic~(21)Ne.Combining with previous studies,we suggest that the precise approach for applying the cosmogenic~(21)Ne could be reached by (1) estimating the contribution from nucleogenic~(21)Ne,(2) avoiding samples with complex burial histories to exclude inherited cosmogenic~(21)Ne,and (3) combining the~(10)Be-~(26)Al-~(21)Ne nuclides method for the Quaternary samples.In addition,both pre-burial basin denudation rates and burial ages derived from the~(26)Al-~(10)Be pair were used to determine the different timescale surface denudation rate and fluvial incision rate in relation to previous work.The consistency of the different timescales pre-burial basin denudation rate,~(36)Cl surface denudation rate,and modern basin denudation rate indicates that the landscape-scale surface denudation has been likely stabilized since the Quaternary in the Guizhou Plateau area.The slightly higher river incision rates than the local surface denudation rate show that the river dynamics may not have reached a steady-state due to the regional tectonic uplift in the Guizhou Plateau.  相似文献   

9.
The ～900 km long Darling Scarp in Western Australia is one of the most prominent linear topographic features on Earth. Despite the presence of over‐steepened reaches in all westerly flowing streams crossing the scarp, and significant seismic activity within 100 km of the scarp, there is no historical seismicity and no reported evidence for Quaternary tectonic displacements on the underlying Darling Fault. Consequently, it is unclear whether the scarp is a rapidly evolving landform responding to recent tectonic and/or climatic forcing or a more slowly evolving landform. In order to quantify late Quaternary rates of erosion and scarp relief processes, we obtained measurements of the cosmic‐ray produced nuclide beryllium‐10 (10Be) from outcropping bedrock surfaces along the scarp summit and face, in valley floors, and at stream knickpoints. Erosion rates of bedrock outcrops along the scarp summit surface range from 0·5 to 4·0 m Myr?1. These are in the same range as erosion rates of 2·1 to 3·6 m Myr?1 on the scarp face and similar to river incision rates of 2·6 to 11·0 m Myr?1 from valley floor bedrock straths, indicating that the Darling Scarp is a slowly eroding ‘steady state’ landform, without any significant contemporary relief production over the last several 100 kyr and possibly several million years. Knickpoint retreat rates determined from 10Be concentrations at the bases of two knickpoints on small streams incised into the scarp are 36 and 46 m Myr?1. If these erosion rates were sustained over longer timescales, then associated knickpoints may have initiated in the mid‐Tertiary to early Neogene, consistent with early‐mid Tertiary marginal uplift. Ongoing maintenance of stream disequilibrium longitudinal profiles is consistent with slow, regional base level lowering associated with recently proposed continental‐scale tilting, as opposed to differential uplift along discrete faults. Cosmogenic 10Be analysis provides a useful tool for interpreting the palaeoseismic history of intraplate near‐fault landforms over 105 to 106 years. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

10.
Escarpments are prominent morphological features along high-elevation passive margins. Recent studies integrating geomorphology, thermochronology, and cosmogenic nuclide-based denudation rate estimates suggest a rapid phase of denudation immediately after the earliest stages of seafloor spreading, and subsequent slow denudation rates since. To constrain the geomorphic evolution of passive margins, we have examined the development of the Sri Lankan escarpment. Cosmogenic nuclide data on river sediment along a north–south transect across the southern escarpment reveal that the landscape is eroding ten times more rapidly in the escarpment zone (26 to 71 mm kyr− 1) than in the high-elevation plateau above it and in the lowland plain beneath it (2.6 to 6.2 mm kyr− 1). Unlike these low denudation rate areas, the escarpment denudation is strongly and linearly hill slope-dependent. This shows that denudation and retreat are tightly interlinked within the escarpment, which suggests that the escarpment is evolving by rift-parallel retreat, rather than by escarpment downwearing. Supporting evidence is provided by the morphology of rivers draining the escarpment zone. These have steep bedrock channels which show sharp and prominent knickpoints along their longitudinal profiles. It appears that fluvial processes are driving escarpment retreat, as rivers migrate headwards were they incise into the high-elevation plateau. However, the average catchment-wide denudation rates of the escarpment zone are low compared to the denudation rates that are estimated for constant escarpment retreat since rifting. In common with other escarpments worldwide, causes for this slow down can be tectonic change related to flexural bending of the lithosphere, climate change that would vary the degree of precipitation focused into the escarpment, or the decrease in the contributing catchment area, which would reduce the stream power available for fluvial erosion.  相似文献   

11.
A sharp increase in deposited sediment volume since Pliocene times has been observed worldwide and in particular around the European Alps. This phenomenon has been linked to a rise in denudation rates controlled by an increase of either climatic or tectonic forcing. Observation of in-situ cooling histories for orogens is critical to assess the reality of the inferred increase in denudation rates, and to determine whether this phenomenon is widespread or localized at active tectonic structures. We exploit the unique density of fission-track ages in the Western European Alps to reconstruct cooling isoage surfaces and to estimate exhumation rates on the orogen scale between 13.5 and 2.5 Ma. Our novel technique is based on the association of isoage contours with age–elevation relationships. It uses map-view interpolation, enabling a spatio-temporal analysis of exhumation rates over the entire Western Alps. The resulting exhumation histories reconstructed for eight areas of the Western Alps display strong similarities in timing and rates with orogen-wide average denudation rates inferred from sediment volumes. This consistency validates the use of both techniques for the study of an orogen characterized by strong relief and high recent exhumation rates. We conclude that exhumation rates in the Western Alps have increased more than twofold since Late Miocene times. This increase may have been locally modulated by the distinct response of different tectonic units.  相似文献   

12.
Concentrations of in‐situ‐produced cosmogenic nuclides 10Be and 26Al in quartz were measured by accelerator mass spectrometry for bedrock basalts and sandstones located in northwest Tibet. The effective exposure ages range between 23 and 134 ka (10Be) and erosion rates between 4·0 and 24 mm ka?1. The erosion rates are significantly higher than those in similarly arid Antarctica and Australia, ranging between 0·1 and 1 mm ka?1, suggesting that precipitation is not the major control of erosion of landforms. Comparison of erosion rates in arid regions with contrasting tectonic activities suggests that tectonic activity plays a more important role in controlling long‐term erosion rates. The obtained erosion rates are, however, significantly lower than the denudation rate of 3000–6000 mm ka?1 beginning at c. 5‐3 Ma in the nearby Godwin Austen (K2) determined by apatite fission‐track thermochronology. It appears that the difference in erosion rates within different time intervals is indicative of increased tectonic activity at c. 5–3 Ma in northwest Tibet. We explain the low erosion rates determined in this study as reflecting reduced tectonic activity in the last million years. A model of localized thinning of the mantle beneath northwest Tibet may account for the sudden increased tectonic activity at c. 5–3 Ma and the later decrease. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

13.
Low-temperature thermochronology, such as that provided by apatite fission-track analysis, provides a valuable means of establishing the timing of major denudational events and associated rates of denudation over geological time-scales of 106–108 Ma. Care must be taken, however, in deriving denudation rates from the crustal cooling histories documented by thermochronologic techniques, especially in rapidly eroding terrains, since, in such cases, apparent denudation rates derived from thermochronologic data will usually overestimate true rates if the advective effect of denudation is not included. This is likely to be resolvable where the rate of denudation exceeds 300 m Ma−1 and when the depth of denudation occurring at these rates exceeds several kilometres prior to the sample cooling below the appropriate closure temperature. Because the time at which a sample cools below a particular closure temperature is relatively insensitive to advection, the initiation of denudation can be accurately established, even given uncertainties in the estimation of depths and rates of denudation. Where thermal events originate from a source within or below the lower crust, the cooling through denudation will dominate the low-temperature history of the shallow crust if denudation occurs coevally with the subsurface heating. © 1997 by John Wiley & Sons, Ltd.  相似文献   

14.
Relief generation in non‐glaciated regions is largely controlled by river incision into bedrock but datable fluvial terraces that allow quantifying incision rates are not always present. Here we suggest a new method to determine river incision rates in regions where low‐relief surfaces are dissected by streams. The approach consists of three steps and requires the 10Be concentrations of a stream sediment sample and a regolith sample from the low‐relief surface. In the first step, the spatial distribution of 10Be surface concentrations in the given catchment is modelled by assuming that denudation rates are controlled by the local hillslope angles. The slope–denudation rate relation for this catchment is then quantified by adjusting the relation between slope angle and denudation rate until the average 10Be concentration in the model is equal to the one measured in the stream sediment sample. In the second step, curved swath profiles are used to measure hillslope angles adjacent to the main river channel. Third, the mean slope angle derived from these swath profiles and the slope–denudation relation are used to quantify the river incision rate (assuming that the incision rate equals the denudation rate on adjacent hillslopes). We apply our approach to two study areas in southern Tibet and central Europe (Black Forest). In both regions, local 10Be denudation rates on flat parts of the incised low‐relief surface are lower than catchment‐wide denudation rates. As the latter integrate across the entire landscape, river incision rates must exceed these spatially averaged denudation rates. Our approach yields river incision rates between ~15 and ~30 m/Ma for the Tibetan study area and incision rates of ~70 to ~100 m/Ma in the Black Forest. Taking the lowering of the low‐relief surfaces into account suggests that relief in the two study areas increases at rates of 10–20 and 40–70 m/Ma, respectively. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

15.
Quiet daily magnetic variations recorded by magnetometer arrays in Australia are analysed to obtain electromagnetic response parameters for two parts of the Australian continent remote from electrical conductivity anomalies. The parameters are based on measurements of vertical-field and horizontal-field spatial gradient, and three different methods are followed in their computation. The response parameters are checked for consistency with a compilation of globally-determined Earth apparent resistivities, and are then interpreted for one-dimensional conductivity structure in the two different parts of the continent. There is evidence that the rise in electrical conductivity from 10?1 S m?1 to 100 S m?1 which occurs at a depth of order 500 km beneath central Australia may occur at a substantially shallower depth of order 230 km beneath southeast Australia.  相似文献   

16.
Landscape evolution and surface morphology in mountainous settings are a function of the relative importance between sediment transport processes acting on hillslopes and in channels, modulated by climate variables. The Niesen nappe in the Swiss Penninic Prealps presents a unique setting in which opposite facing flanks host basins underlain by identical lithologies, but contrasting litho‐tectonic architectures where lithologies either dip parallel to the topographic slope or in the opposite direction (i.e. dip slope and non‐dip slope). The north‐western facing Diemtigen flank represents such a dip slope situation and is characterized by a gentle topography, low hillslope gradients, poorly dissected channels, and it hosts large landslides. In contrast, the south‐eastern facing Frutigen side can be described as non‐dip slope flank with deeply incised bedrock channels, high mean hillslope gradients and high relief topography. Results from morphometric analysis reveal that noticeable differences in morphometric parameters can be related to the contrasts in the relative importance of the internal hillslope‐channel system between both valley flanks. While the contrasting dip‐orientations of the underlying flysch bedrock has promoted hillslope and channelized processes to contrasting extents and particularly the occurrence of large landslides on the dip slope flank, the flank averaged beryllium‐10 (10Be)‐derived denudation rates are very similar and range between 0.20 and 0.26 mm yr?1. In addition, our denudation rates offer no direct relationship to basin's slope, area, steepness or concavity index, but reveal a positive correlation to mean basin elevation that we interpret as having been controlled by climatically driven factors such as frost‐induced processes and orographic precipitation. Our findings illustrate that while the landscape properties in this part of the northern Alpine border can mainly be related to the tectonic architecture of the underlying bedrock, the denudation rates have a strong orographic control through elevation dependent mean annual temperature and precipitation. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

17.
The measurement of hillslope erosion can be a difficult, costly and time‐consuming activity. Many techniques are available, ranging from using environmental tracers, to LiDAR. Erosion measurements using erosion pins are assessed and compared with regional scale erosion data, hillslope data obtained using 137Cs and erosion modelling results. The pins produced erosion rates which are within the range determined using 137Cs and model data but above that of regional denudation rates. Our findings demonstrate that inexpensive erosion pins can provide reliable data on hillslope erosion. © 2015 Commonwealth of Australia. Hydrological Processes © 2015 John Wiley & Sons Ltd.  相似文献   

18.
《Journal of Geodynamics》2010,49(3-5):284-291
The island of Taiwan is located on the convergent boundary between the Philippine Sea plate and the Chinese continental margin. It offers very active mountain building and collapsing processes well illustrated by the rugged topography, rapid uplift and denudation, young tectonic landforms, active faulting and numerous earthquakes. In this paper, using simple models, we have estimated vertical movements and associated absolute gravity variations which can be expected along a profile crossing the southern part of the island and probably suffering the highest rates of rising. The two different tectonic styles proposed for the island, thin-skinned and thick-skinned, were taken into account. Horizontal and vertical movements were modeled by an elastic deformation code. Gravity variations due to these deformations are then modeled at a second step. They are dominated by plate and free-air effects, i.e. elevation of the topography, with several μGal yr−1. By comparison, gravity changes generated by mass transfers are weak: maximum 0.1 μGal yr−1 with the thin-skinned tectonic and 0.3 μGal yr−1 with the thick-skinned tectonic. Though elastic rheology has limitations, this modeling offers interesting results on what gravity signal can be expected from the AGTO project (Absolute Gravity in the Taiwanese Orogen), which proposes to study the dynamic of these mountain ranges using absolute gravimetry (AG) and also including relative gravimetry (RG) and GPS measurements.  相似文献   

19.
Tectonics and climate are usually seen as the main controlling factors of denudation rates, which seem to rise with the tectonic activity and to decrease when the climate becomes drier. However, the low denudation rates observed in semi‐arid to arid contexts are generally measured on orogenic plateaus where the respective influence of the flat relief and the dry climate cannot really be unravelled. The Chinese Tian Shan was chosen as a case study. In the northern piedmont of this mountain range, a series of well‐preserved Quaternary alluvial fans offer the opportunity to perform a mass balance study at the scale of several catchment areas and several hundreds of thousands of years. Based on a geometrical reconstruction of these fans, the volumes of sediments exported out of 10 drainage basins during the Middle–Late Pleistocene (from ～300 to ～12 kyr) and the Holocene (from ～12 kyr to present) have been estimated. From these volumes, an average denudation rate of ～135 m/Myr was determined in the Tian Shan Range for the last 300 kyr. In agreement with other mass balances performed in the same area, the typical denudation intensity of the northern Tian Shan is thus of a few hundred meters per million years at most, regardless of the space and time scales considered. From a comparison with denudation rates in other mountain ranges throughout the world, we suggest that a dry climate can dramatically limit the denudation intensity even in active orogenic systems with a high topographic gradient like the Tian Shan. As a result, the time required to reach equilibrium between denudation and rock uplift in these systems could be extremely long (i.e. of more than several million years). Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

20.
How soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium-10 (10Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia. Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr-1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp . The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp . Since then the rates have continuously decreased. We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp , climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation. Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.  相似文献   

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