印支运动对济阳坳陷构造形态形成演化的影响

三叠纪中期至侏罗纪早期的印支运动动力来源于华北板块与扬子板块的聚敛,聚敛作用形成巨大的NE-SW向挤压力首先导致济阳坳陷北西向压性构造的形成,产生了一系列NW向褶皱隆起带及其伴生的宽缓向斜,其次在对济阳盆地产生多处NW向挤压逆冲构造,伴之以褶皱构造。印支运动还造成本区地势起伏不平,剥蚀程度差异还导致相对隆起区与相对低洼区相间排列,上古生界保留较完整。     

The age,extent and geomorphological significance of the Sassafras basalt,south‐eastern New South Wales

Basalt at Sassafras was erupted in the Middle Eocene. The K‐Ar ages average 45.3 ± 4.9 Ma on whole rock and 48.4 ± 1.9 Ma on plagioclase. The basalt is not limited to a plateau capping, but extends 150 m down into adjacent valleys. Comparison with nearby Eocene basalts shows that there was in excess of 250 m of local relief in the central Shoalhaven valley by the Early Tertiary. The basalts were extruded at high elevation, and denudation of the coastal margin of the upland was already well advanced. Post‐basaltic denudation has been very slow, and the Early Tertiary landscape is well preserved.     

Dimension and entropy in the soil-covered landscape

Both the Hausdorff dimension and the K-entropy supply a measure of the irregularity of the landspace surface. The relationship between the two measures is investigated over a variety of terrains in Britain and a method of calculating the entropy is checked against an independent estimate of the dimension with reasonable agreement. The calculation of the K-entropy requires that the landscape surface be represented by an homogenous ergodic random field. This condition is satisfied by the tendency of soil-covered terrains to progressively approximate to a form well represented by a Gaussian field. Gaussian random fields can either be very smooth, possessing derivatives of all orders at every point or they are highly irregular and non-differentiable everywhere. Within the regular conceptualization the Rice-Kac theory is used to predict the numbers of crossing points and the extent of excursion sets. These predictions are tested against an example terrain from the High Weald of East Sussex with very good agreement, apart from predictions of local maxima. A worked example of the calculation of the K-entropy is given as an appendix. The potential role of information theory in geomorphology extends beyond the use made of entropy in this investigation. In particular ergodic theory has important practical and theoretical implications.     

Rates of cave and landform development in the Yorkshire Dales from speleothem age data

Uranium-series ages have been obtained for 87 speleothems collected from nine major cave systems in the Craven district of northern England. Large systems such as Ease Gill Caverns, the West Kingsdale caves, and Gaping Gill-Ingleborough Cave, which contain relict high-level tunnels, are found to be older than 350,000 years (the limit of the ²³⁰Th/²³⁴U dating method). There is little evidence to indicate a significant enlargement of these passages since this time. Estimates of the age of Victoria Cave from ²³⁴U/²³⁸U isotopic ratios suggest that the cave has been fully relict for more than 500,000 years. Ages of in situ speleothems immediately adjacent to local base level cave streams show that mean maximum downcutting rates in limestone channels are about 2 to 5 cm 1000 yr. These rates are significantly lower than those obtained from direct measurements on limestone bedrock in stream channels but are comparable to areal denudation rates based on solute budgets. Using the present elevation of caves with respect to adjacent valley floor levels, mean maximum valley entrenchment rates are found to range between 5 cm/ky and < 20cm/ky, which corresponds to 6 m to < 24 m of lowering per glacial/interglacial cycle. These rates suggest that upper beds of the limestone were incised to form the Yorkshire Dales between 1 and 2 million years ago. The results indicate that the erosional effects of individual glaciations are not as severe as previously proposed in the literature.     

A world model of soil carbon dioxide

Mean growing season soil PCO₂ data were obtained for 19 regions of the world in nine countries. Bivariate and multiple linear regression analysis with soil log(PCO₂) as the dependent variable and TEMP, PRECIP, log(AET), and log(PET) as the four climatic independent variables demonstrated that AET was the best independent predictor of soil PCO₂. An improved soil PCO₂-AET model was developed by assuming (1) that as AET approaches zero, soil PCO₂ approaches the atmospheric value and (2) that there is an upper limit to soil PCO₂ at very high AET. This model has the form log(PCO₂) = ?3·47 + 2·09 (1 ?e^{?0·0172 AET}) where AET is in mm. It explains 67 per cent of the initial variation in the soil PCO₂ data, predicts a soil log(PCO₂) of ? 3·47 at AET = 0, and an upper limit of 3·5 per cent (log(PCO₂) = ? 1·45) for mean growing season soil PCO₂ at AET values of 2000 mm and above. The results of this study suggest that soil PCO₂ levels in tropical areas are, on average, higher than those in temperate, alpine, and arctic regions.     

Contemporary versus long‐term denudation along a passive plate margin: the role of extreme events

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.     

The world''s large lake basins as denudation-accumulation systems and implications for their lifetimes

The mechanical denudation rates of 81 large lake basins (lake area > 500 km²) were determined from long-term river loads and erosion maps. Using the drainage area/lake area ratios the mean sedimentation rates of the lakes were calculated for a porosity of 0.3. The mean sedimentation rates of different lake types vary between 0.1 mm/a (glacial lakes, lowland) and 5.4 mm/a (mostly sag basin lakes). The calculated lifetimes of the lakes are based on the lake volumes and mean sedimentation rates, assuming steady-state conditions and solely clastic material. On average, glacial lakes in highlands and fault-related lakes show the shortest lifetimes (c. 70 ka), glacial lakes in lowlands and rift lakes have the longest lifetimes (c. 1 Ma). Some lakes remain unfilled for very long time spans due to rapid subsidence of their basin floors. The calculated lifetimes are compared with those derived from sediment core studies. Most core studies indicate lower mechanical sedimentation rates than the calculated ones because a major part of the incoming sediment is trapped in deltas. However, a number of lakes (e.g., the Great Lakes of North America) show the opposite tendency which is largely caused by extensive shoreline erosion and resuspension. The lifetimes of large glacial lakes often exceed the duration of interglacials. Hence, their lifetimes are restricted by glaciation and not by sediment infill. Rift lakes persist for long time periods which exceed the calculated lifetimes in some cases. Time-dependent subsidence, basin extension, as well as the impact of climate change are briefly described.     

Geochemical mass balances of major chemical constituents in the watershed of the Changhuajian River in China

Based on the geological background, R-mode factor statistics, and the analysis of the stability diagram for the corresponding system, five weathering reactions controlling the surface-water chemical composition in the watershed of the Changhuajiang River are deduced. In the mass balance model, the precipitation accounts for some solute input, since the rainwater is dilute without pollution. Most of the Ca²⁺ and HCO ₃ ^– ions are from the dissolution of calcite, K⁺, Na⁺, H₄SiO₄ and some of the Mg²⁺ and HCO ₃ ^– come from albite and biotite weathering to kaolinite. The dissolution of dolomite and gypsum controls the mass balances of Mg²⁺ and SO ₄ ^2– . The dissolution of calcite is the dominant chemical weathering reaction in the watershed because of its reactivity and high concentration. In the watershed in 1986, the chemical weathering rate was 0.073 (kg/m² a), and the mechanical denudation rate is 0.093 (kg/m² a). The chemical weathering mass output proportion of carbonate rocks to silicate rocks was about three to one.     

地壳上层水平位移与江西省二叠纪沉积相带的畸变

江西省茅口—龙潭期地层研究的较为详细,可分为3区6小区,反映出当时古地理条件下所形成的不同沉积相带,它们沿走向相当稳定,沿倾向则有规律递变。但目前这种有规律的分布已经错乱,通过地层对比推知在武夷山、武功山及九岭山等地均发生过较大规模的水平位移。正是这种水平位移导致了江西省二叠纪沉积相带的畸变与古地理图失真。目前这些山区的核心部位均无盖层,其原因不仅仅在于风化剥蚀,构造剥蚀可能是更重要的因素。     

Denudation rates of crystalline rock in the Huangling anticline of the Three Gorges of the Yangtze River in China since the Eocene

The processes and rates of the weathering denudation of the bare crystalline rock are important problems in the field of geosciences[1—6]. Essentially, the weathering denudation process is the process of rock destruction and transportation. In different …