A low‐dimensional model of bedrock weathering and lateral flow coevolution in hillslopes: 2. Controls on weathering and permeability profiles,drainage hydraulics,and solute export pathways

The advance of a chemical weathering front into the bedrock of a hillslope is often limited by the rate weathering products that can be carried away, maintaining chemical disequilibrium. If the weathering front is within the saturated zone, groundwater flow downslope may affect the rate of transport and weathering—however, weathering also modifies the rock permeability and the subsurface potential gradient that drives lateral groundwater flow. This feedback may help explain why there tends to be neither “runaway weathering” to great depth nor exposed bedrock covering much of the earth and may provide a mechanism for weathering front advance to keep pace with incision of adjacent streams into bedrock. This is the second of a two‐part paper exploring the coevolution of bedrock weathering and lateral flow in hillslopes using a simple low‐dimensional model based on hydraulic groundwater theory. Here, we show how a simplified kinetic model of 1‐D rock weathering can be extended to consider lateral flow in a 2‐D hillslope. Exact and approximate analytical solutions for the location and thickness of weathering within the hillslope are obtained for a number of cases. A location for the weathering front can be found such that lateral flow is able to export weathering products at the rate required to keep pace with stream incision at steady state. Three pathways of solute export are identified: “diffusing up,” where solutes diffuse up and away from the weathering front into the laterally flowing aquifer; “draining down,” where solutes are advected primarily downward into the unweathered bedrock; and “draining along,” where solutes travel laterally within the weathering zone. For each pathway, a different subsurface topography and overall relief of unweathered bedrock within the hillslope is needed to remove solutes at steady state. The relief each pathway requires depends on the rate of stream incision raised to a different power, such that at a given incision rate, one pathway requires minimal relief and, therefore, likely determines the steady‐state hillslope profile.     

Episodic bedrock erosion by gully‐head migration,Colorado High Plains,USA

This study explores the frequency of bedrock exposure in a soil‐mantled low‐relief (i.e. non‐mountainous) landscape. In the High Plains of eastern Colorado, gully headcuts are among the few erosional features that will incise through the soil mantle to expose bedrock. We measured the last time of bedrock exposure using optically stimulated luminescence dating of alluvial sediment overlying bedrock in gully headcuts. Our dating suggests that headcuts in adjacent gullies expose bedrock asynchronously, and therefore, the headcuts are unlikely to have been triggered by a base‐level drop in the trunk stream. This finding supports the hypothesis that headcuts can develop locally in gullies as a result of focused scour in locations where hydraulic stress during a flash flood is sufficiently high, and/or ground cover is sufficiently weak, to generate a scour hole that undermines vegetation. Alluvium dating also reveals that gullies have been a persistent part of this landscape since the early Holocene. Copyright © 2016 John Wiley & Sons, Ltd.     

Implications of the saltation–abrasion bedrock incision model for steady‐state river longitudinal profile relief and concavity

The saltation–abrasion model predicts rates of river incision into bedrock as an explicit function of sediment supply, grain size, boundary shear stress and rock strength. Here we use this experimentally calibrated model to explore the controls on river longitudinal profile concavity and relief for the simple but illustrative case of steady‐state topography. Over a wide range of rock uplift rates we find a characteristic downstream trend, in which upstream reaches are close to the threshold of sediment motion with large extents of bedrock exposure in the channel bed, while downstream reaches have higher excess shear stresses and lesser extents of bedrock exposure. Profile concavity is most sensitive to spatial gradients in runoff and the rate of downstream sediment fining. Concavity is also sensitive to the supply rate of coarse sediment, which varies with rock uplift rate and with the fraction of the total sediment load in the bedload size class. Variations in rock strength have little influence on profile concavity. Profile relief is most sensitive to grain size and amount of runoff. Rock uplift rate and rock strength influence relief most strongly for high rates of rock uplift. Analysis of potential covariation of grain size with rock uplift rate and rock strength suggests that the influence of these variables on profile form could occur in large part through their influence on grain size. Similarly, covariation between grain size and the fraction of sediment load in the bedload size class provides another indirect avenue for rock uplift and strength to influence profile form. Copyright © 2008 John Wiley & Sons, Ltd.     

The map of vertical crustal movements in Czechoslovakia and its interpretation

In addition to the measurements carried out in the framework of the 1973–1978 international relevellings, further levelling lines have been remeasured on the territory of Czechoslovakia till now. Considering all new relevellings on the region under study, an independent adjustment of annual velocities of vertical movements in whole network was performed. The values of vertical movements were determined relatively to the fundamental benchmark Želešice in the central part of Czechoslovakia, monumented in the bedrock of the southeastern border of the Bohemian Massif. The results are presented in a map of vertical movements and preliminarily interpreted in connection to the geological features of the territory under study.     

Potential Effects on Ground Water of a Hypothetical Surface Coal Mine in Indiana

The possible mine will remove a gently, less than 50 feet per mile, westerly dipping Springfield coal from an area covered by glacial till and some channel sands and gravel. The area is flat, with less than 20 feet of relief in a square mile. The channel sands and gravels, the till and the bedrock are capable of yielding ground water at 5 to 75,3 to 10, and 1 to 10 gallons per minute (gpm), respectively. The ground water in the drift and the shallow bedrock is calcium-bicarbonate type, contrasting with the sodium-bicarbonate type in the deep bedrock. The surface mine will feature selective handling of overburden. The probable hydrologic consequences of the mine will be 1) a short-term, areally limited dewatering, 2) an increase in dissolved solids, 3) a change in ground water chemistry in some areas to a calcium-bicarbonate sulfate water, 4) an increase in ground water storage, and 5) a new integrated surface water system. The proposed ground water monitoring system will include seven monitoring wells in the glacial material and one in the bedrock. The primary effort in ground water monitoring to the west of the mine will be to detect changes in the quality of the ground water, whereas to the east, changes in both quality and quantity will need to be monitored intensively.     

Erosion rates of alpine bedrock summit surfaces deduced from in situ Be and Al

We have measured the concentration of in situ produced cosmogenic ¹⁰Be and ²⁶Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My⁻¹. Individual measurements vary between 2 and 19 m My⁻¹. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My⁻¹). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.

We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.     

Influence of bedrock groundwater on streamflow characteristics in a volcanic catchment

Groundwater movements in volcanic mountains and their effects on streamflow discharge and representative elementary area (REA) have remained largely unclear. We surveyed the discharge and chemical composition of spring and stream water in two catchments: the Hontani river (NR) catchment (6.6 km²) and the Hosotani river (SR) catchment (4.0 km²) at the southern part of Daisen volcano, Japan. Daisen volcano is a young volcano (17 × 10³ years) at an early stage of erosion. Our study indicated that deep groundwater that moved through thick lava and pyroclastic flows and that could not be explained by shallow movements controlled by surface topography contributed dominantly to streamflow at larger catchment areas. At the NR catchment, the deep groundwater contribution clearly increased at a catchment boundary defined by an area of 3.0 km² and an elevation of 800 m. At the SR catchment, the contribution deep groundwater to the stream also increased suddenly at a boundary threshold of 2.0 and 700 m. Beyond these thresholds, the contributions of deep bedrock groundwater remained constant, indicating that the REA is between 2 and 3 km² at the observed area. These results indicate that the hydrological conditions of base flow were controlled mainly by the deep bedrock groundwater that moved through thick lava and pyroclastic flows in the undissected volcanic body of the upper part of the catchment. Our study demonstrates that deep and long groundwater movements via a deep bedrock layer including thick deposits of volcanic materials at the two catchments on Daisen volcano strongly determined streamflow discharge instead of the mixing of small‐scale hydrological conditions. Copyright © 2015 John Wiley & Sons, Ltd.     

青藏高原东缘岩石圈结构对现今地表垂向运动影响的数值分析

参考青藏高原东缘松潘-甘孜地块至四川盆地陡变地形起伏和地壳密度结构的横向差异,本文建立了二维牛顿黏性流体有限元模型,计算分析构造加载、陡变地形和重力效应控制下青藏高原东缘岩石圈变形特征,探讨横向不均匀的地壳密度结构、陡变地形和岩石圈流变性质对区域现今垂向运动的影响.计算结果显示：在构造加载作用下,松潘-甘孜地块至四川盆地地表抬升微弱.区域横向不均匀的地壳密度结构驱使松潘-甘孜地块地壳整体抬升,速率高达2 mm·a^-1,四川盆地整体下沉,速率约1 mm·a^-1,与龙门山两侧现今观测到的地表垂向变形模式相近.龙门山地区陡变地形驱使柔性地壳流动,调整区域地壳局部变形;岩石圈流变结构影响重力驱动作用下的模型变形量值和岩石圈变形耦合程度,松潘-甘孜地块较低的中地壳黏滞系数引起上、下地壳的变形解耦;模型较高的岩石圈地幔黏滞系数使重力驱动作用下区域垂向变形量降低.因此,青藏高原东缘地壳密度结构差异、地形起伏和岩石圈流变性质是现今区域垂向变形的重要动力学控制因素.     

青藏高原中部温泉盆地西缘的晚新生代正断层作用

在青藏高原中部的温泉盆地西侧发育了1条倾向东的强烈活动的近SN正断层——温泉盆地西缘断裂。它是在印度板块与欧亚板块强烈碰撞的背景下,青藏高原中北部地区自晚新生代以来发生近EW向伸展变形的产物。晚新生代以来,该断裂上的最大垂直错动量不会<21km,错动中生代褶皱地层所暗示的最大垂直位移量为(60±22)km。第四纪期间,该断裂发生了多期活动,形成了山前的多套断层三角面和多级断层陡坎地貌。根据断裂垂直错动晚第四纪期间不同时代的地层和地貌体所形成的断层崖高度估算,其晚第四纪以来的最大活动速率不超过12mm/a,平均活动速率为045mm/a。初步的探槽分析表明,晚更新世末期以来沿该断裂至少发生了3次震级不同的古地震事件。综合该断裂的全新世活动特点推断,它是在未来具有较大可能发生6～7级地震的一条重要控震断裂     

The Baker Creek Research Watershed: Streamflow data highlighting the behaviour of an intermittent Canadian Shield stream through a wet–dry–wet cycle

Baker Creek drains water from subarctic Canadian Shield terrain comprised of a mix of exposed Precambrian bedrock, lakes, open black spruce forest and peat filled depressions. Research in the catchment has focused on hydrological processes at the hillslope and catchment scales. Streamflow is gauged from several diverse sub-catchments ranging in size from 9 to 155 km². The period of record (2003–2019) of streamflow from these sub-catchments extends from 12 to 17 years, and these data are the focus of this note. Such data are unique in this remote region. 2003–2019 was a period that included both historic wet and dry conditions. Observations during such a diversity of conditions are helping to improve understanding of how stream networks that drain this landscape expand and contract in response to short and long hydroclimatic cycles. These data from a distinctly cold and dry region of low relief, thin soils, exposed bedrock and permafrost are a valuable contribution to the global diversity of research catchment data.     

Geology of the Kodiak Shelf,Alaska: environmental considerations for resource development

Geologic features and processes pose several environmental concerns to resource development on the Kodiak Shelf in the Gulf of Alaska. Tectonism causes fault movement, strong seismic ground shaking, and changes in sea-floor elevation. Earthquake epicenters and structural deformation are areally concentrated, which implies areal variation in the severity of tectonic hazards. Exposures of bedrock appear to provide competent foundational material over broad areas of the sea floor, although locally rough topographic expression of inclined bedrock strata might influence the siting of engineering structures. Stable deposits of gravelly unconsolidated sediment, derived from Pleistocene glaciers, are also widespread. Deposits of fine-grained sediment in areas of negative relief might possess less desirable foundational properties than bedrock or coarse unconsolidated sediment, but their thickness is only a few tens of meters in most places. Evidence of sediment slides is rare on the shelf, which implies general slope stability, but large slides, whose activity is related to tectonism, are abundant just seaward of the shelf break. Local occurrences of gas-charged sediment show limited indication of overpressuring and no evidence of instability, but low sediment strength is possible. Fields of large sand waves might be sites of high-energy bed-load transport with the potential for erosion and abrasion problems. As inferred from sediment dispersal patterns, pollutants that become incorporated into bottom sediment could be concentrated and stored long-term in troughs that trend transversely across the shelf.     

A note on the areal distribution of suspended sediment yield in South Africa

Suspended sediment yield is calculated by means of Fournier's (1960) equations for 214 locations in South Africa. The areal distributions agree well with field data. Areas of pronounced relief yield most suspended sediment, especially from the Transvaal Escarpment zone, northeast Lesotho, the Natal Drakensburg and the Cape Mountains. Locally high values occur inland (e.g., Waterberg Plateau north of Nylstroom). Valley rainfall records inadequately define mountain precipitation leading to an underestimate of likely actual values of sediment yield in some areas. Man-induced soil erosion locally increases suspended sediment yields, as do areas of deeply weathered bedrock.     

Application of a coil‐type TDR probe for measuring the volumetric water content in weathered granitic bedrock

As a first step toward describing water flow processes in bedrock, a coil‐type time domain reflectometry (TDR) probe capable of measuring volumetric water content, θ, in weathered bedrock at three depths was prepared. Because the coil‐type TDR probe is large in diameter (19 mm), it can be installed even in highly weathered bedrock more easily and appropriately than conventional TDR probes that consists of two or three rods of small diameter (5‐8 mm). The probe calibrations suggest that the values measured by the probe are very sensitive to changes in θ. Using the calibrated probe together with commercially available profile soil moisture sensors, the θ profile was monitored for 1 year. Even rainfall events with relatively small cumulative rainfall of 15 mm increased the bedrock θ, and the increments were comparable to those in the soil. After the end of the rainfall events, the bedrock θ displayed a more rapid drop than the soil, and varied little during the period of no rainfall. The water storage showed similar tendencies. These observations suggest that the bedrock θ is controlled by clearly distinguishable macropores and micropores within the bedrock. It is concluded that the coil‐type TDR probe is very effective in determining θ in weathered bedrock, and that bedrock, conventionally defined by conducting cone penetration tests and treated as impermeable, does conduct and hold substantial amounts of water, and therefore contribute greatly to hydrological processes in headwater catchments. Copyright © 2007 John Wiley & Sons, Ltd.     

海原断裂带中东段地貌差异及其成因探讨

以定量化地形因子为切入点的构造地貌学方法已成为活动构造研究的有效手段,被广泛用于定性或半定量解析地貌对新构造运动的响应及其演化过程。针对海原断裂带中东段现今地貌差异,以SRTM 90m分辨率DEM为基础,利用ArcGIS软件和Matlab程序脚本,提取了海原断裂带中东段高程、坡度、地形起伏、地形侵蚀以及河流陡峭系数等地形因子。从空间分布上看,上述各项地形因子沿断裂走向均呈现 “西高东低”的整体分布特征。西段海拔高、坡度陡、起伏大、侵蚀强、抬升快,中段和东段海拔低、坡度缓、起伏小、侵蚀弱、抬升慢,此外,在断裂带的东南尾端呈略微增加趋势,达到小范围内的峰值。在此基础上,通过对比分析地形因子与年降水量、基岩岩性,初步探讨了构造与降水、岩性等因素对地形地貌的控制作用,认为不同降水条件对地貌后期改造起显著作用,基岩岩性与现今地貌之间并无显著关系,该区域地貌类型主要受构造抬升差异所控制。沿断裂带走向上的现今地貌差异表明,西段处于相对快速的构造隆升和强挤压造山构造背景,中段由于受到黄河下切及河流冲积作用影响,地貌参数记录的抬升特征并不显著,而东段则反映出大型断裂带尾端挤压调整效应。     

Problems of Global Geodynamics

Izvestiya, Physics of the Solid Earth - Global geodynamics is determined by the thermal convection in the mantle which manifests itself on the surface by movements, relief, heat flow, and...     

松软覆盖层对隐伏断层带围陷波的影响研究

利用二维非均匀介质地震波传播的伪谱和有限差分混合方法,通过数值计算,讨论了松软覆盖层对隐伏断层带围陷波特征的影响.在没有覆盖层的情况下,围陷波振幅和围岩上相比明显增加,持续时间变长.覆盖层造成围岩上地面运动振幅增大,围陷波的部分能量传播到覆盖层中,使得围陷波的能量变小.随着覆盖层厚度增加,围陷波的振幅越来越小,和围岩上...     

Bedrock erosion and relief production in the northern Flinders Ranges,Australia

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.     

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon,nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

The quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm-humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km² forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral-derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in-stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.     

Lithological control on the elevation of shore platforms in a microtidal setting

The shore platforms on Shag Point, southern New Zealand, are quasi‐horizontal surfaces and are developed between supratidal and low water spring levels. A range of morphologies occur, with more exposed platforms having a distinct low‐tide cliff, in contrast to low‐tide surfaces where the seaward edge is buried beneath rubble and macro‐algal growth. The platforms range in width from 20 to 80 m and are eroded into Late Cretaceous/Early Tertiary fine marine sandstones and mudstones. Shore platforms have formed in two principal lithological units: a homogeneous unit that is characterized by few discontinuities, and a fractured unit with joints spaced about 0·5 m apart. Rock hardness is low in both units (L‐type Schmidt hammer rebound values of 31 ± 4), and there is little systematic variation in values between the two units in which platforms have developed. Case‐hardened concretions within the sandstone are significantly harder than surrounding rock and cause local relief of metre scale as the spherical diagenetic features are eroded from the bedrock. They do not, however, appear to affect broad‐scale platform geometry. Joints within the bedrock are a primary control on platform elevation. Platforms formed in jointed rock occur at the lower portion of the intertidal zone, in contrast to platforms formed in unjointed bedrock, in which horizontal surfaces occur at or above mean high water spring tide level. Rock structure, therefore, appears to be the primary determinant factor of platform geometry at Shag Point. Copyright © 2006 John Wiley & Sons, Ltd.