Gliding and overthrust nappe tectonics in the Barberton Greenstone Belt     

Maarten J. de Wit 《Journal of Structural Geology》1982,4(2):117-136

Regional recumbent folds, inverted stratigraphy, nappes and olistostromes are described from the southern part of the 3.3–3.5 Ga Barberton Greenstone Belt. Overthrusting of thin rigid silicified slabs with minimum dimensions of 25 km² and up to 500 m in thickness, occurred over minimum distances of 86 km. More ductile and coherent units were overfolded up to at least 2 km during their emplacement. The glide planes on which these nappes travelled were zones of high fluid pressures related to hydrothermal fluid circulation patterns, driven by heat sources from igneous intrusions. The upwelling areas of the geothermal convection cells were sites of mud-pools and hydrothermal vents which may mark the trailing edges (pull aparts) of the overthrust units. Progressive silica and carbonate precipitation due to decreasing temperatures, within the zones of fluid migration distant from the areas of high heat flow, probably acted as built-in braking systems below the travelling slabs. Active sedimentation and metasomatism during this tectonism indicates a protracted history for the evolution of the greenstone belt. The recognition of nappe and overthrust tectonics in the Barberton Belt, processes which may have been commonplace in Archaean terrains, necessitates a re-evaluation of the stratigraphy of this belt.  相似文献   

The bottom friction of the sea-bed off Melkbosstrand,South Africa: A comparison of a quadratic with a linear friction model     

Dr. Maarten P. Van Ieperen 《Ocean Dynamics》1975,28(2):72-88

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Immobilization of Ni by Al-modified montmorillonite: A novel uptake mechanism   总被引：1，自引：0，他引：1  

Maarten Nachtegaal  André M. Scheidegger  Rainer Dähn  Gerhard Furrer 《Geochimica et cosmochimica acta》2005,69(17):4211-4225

The sorption capacity of montmorillonite clay minerals for small cations, such as Ni²⁺, can be greatly enhanced by modifying the clay mineral with Al(III). In this study, the mechanisms of Ni uptake by Al-modified montmorillonite were studied using extended x-ray absorption fine structure (EXAFS) spectroscopy of powders and polarized EXAFS spectroscopy of self-supporting clay films to delineate the binding structure of Ni formed as a function of the reaction conditions. Analysis of powder EXAFS spectra of wet pastes, collected from Ni-treated Al-modified montmorillonites reacted at pH 5-8, 25°C or 80°C (to accelerate the reaction process), and reaction times ranging from 1 month to 9 yrs, showed that Ni was surrounded on average by 6 O atoms at a distance of 2.05 Å and 6 Al atoms at 3.01 Å, suggesting the incorporation of Ni into a gibbsite-like structure. Only at pH 8, Ni-containing precipitates were congruently formed. Polarized EXAFS spectroscopy of self-supporting Ni-reacted Al-modified montmorillonite clay films showed a pronounced angular dependency of the spectra of the Ni-doped gibbsite, indicating that the orientation of this Ni-doped gibbsite coincided with the layering of the montmorillonite. Data analysis suggested that Ni is included slightly above and below the vacant octahedral sites of the postulated interstitial gibbsite monolayer. This newly identified mechanism of metal uptake by Al-modified montmorillonite provides a large metal sorption capacity and, because the metal is included in a monolayer gibbsite or gibbsite “islands” formed in the interstitial space of the clay mineral, it potentially leads to a permanent sequestration of the metal from the environment.  相似文献   

Phase shift at caustics along rays in anisotropic media     

P. M. Bakker 《Geophysical Journal International》1998,134(2):515-518

In isotropic ray tracing, the ray approximation to the wavefield undergoes a phase shift when the ray crosses a caustic. The cumulative number of such phase shifts along a ray is usually called the KMAH index. The sign of these phase shifts is prescribed by the sign of the angular frequency in combination with the sign convention used for the Fourier transformation. In isotropic media the KMAH index always increases by one or by two, depending on the type of caustic crossed. For (quasi-)shear waves in anisotropic media the KMAH index may decrease. This is the case if the associated slowness sheet is locally concave in one or two of its principal directions of curvature.  相似文献   

Changing trends in tropospheric methane and carbon monoxide: A sensitivity analysis of the OH-radical     

Han Van Dop  Maarten Krol 《Journal of Atmospheric Chemistry》1996,25(3):271-288

A sensitivity analysis is performed in order to study recently observed changes in atmospheric methane and carbon monoxide trends. For the analysis we have adapted a one-dimensional transport/chemistry model in order to comply with changes in vertical transport, stratosphere-troposphere flux of ozone, the water vapour cycle and the short-wave radiative transfer. In addition we have formulated an improved relationship which expresses the steady state OH concentration in terms of longer lived compounds which has a fair agreement with the one-dimensional model results. An analysis of the observed changes and trends in methane and carbon monoxide shows that both emissions and changes in global OH concentrations can be main causes for the observed changes. Average methane emissions have slowed down, particularly in the NH, in the last five years, though perhaps not very significantly. Carbon monoxide emissions are decreasing faster in the last couple of years than in the period 1983–1990. The study suggests that climate fluctuations (tropospheric water vapour, temperature and convective activity) and the stratospheric ozone depletion (tropospheric UV radiation) have a significant influence on tropospheric composition and thus on trends in methane and carbon monoxide concentrations.The IMAU is partner in the Netherlands Centre for Climate Research (CCR).  相似文献   

Crustal thickness, discontinuity depth, and upper mantle structure beneath southern Africa: constraints from body wave conversions     

Jacek Stankiewicz  S  bastien Chevrot  Rob D. van der Hilst  Maarten J. de Wit 《Physics of the Earth and Planetary Interiors》2002,130(3-4):235-251

The technique of receiver function analysis is applied to the study of crustal and upper mantle structures beneath the Kaapvaal craton in southern Africa and its surroundings. Seismic data were recorded by the seismic array of 82 sites deployed from April 1997 to April 1999 across southern Africa, as well as a dense array of 32 sites near Kimberley, in operation from December 1998 to June 1999. Arrival times for phases converted at the Moho are used to determine crustal thickness. The Moho depth in the south–western section of the craton was found to vary between 37 and 40 km, except for one station that recorded a depth of 43 km (SA23). Farther north along the western block of the craton (into Botswana) the depth increases up to 43 km. The depth increases even further in the north–eastern section of the craton, where results vary from 40 to 52 km. Just north of the Kaapvaal craton, in the neighbouring Zimbabwe craton, the crustal thickness drops significantly. The results obtained there varied from 36 to 40 km. For the Kimberley area, using the dense array, the Moho depth was found to be 37.3 km. Arrivals of the Ps and Ppps phases were used to determine the Poisson’s ratio in the region. This was found to be 0.26±0.01. Arrivals of phases from the 410 and 660 km mantle discontinuities are used to interpret the relative positions of these discontinuities, as well as for comparison of mantle temperatures and seismic velocities in the region with global averages. In the Kimberley area the 410 and 660 km discontinuities were found at their expected depth, implying that mantle temperatures in the region are close to the global average. The seismic velocities above the ‘410’ were found up to 5% faster than the averages from the global iasp91 model, which is fast even by Precambrian standards. In other sections of the Kaapvaal craton, the velocities are also faster than global averages, but not as fast as beneath Kimberley. In these sections, the ‘410’ is also slightly elevated, while the ‘660’ is depressed, which implies a slightly lower mantle temperature relative to the global average. Beneath the Kaapvaal craton we find evidence suggesting the presence of a zone with a reduced wavespeed gradient at an upper bound of approximately 300 km, which may mark the lower chemical boundary of the craton.  相似文献   

Saharan Dust Transport and High-Latitude Glacial Climatic Variability: The Alboran Sea Record   总被引：1，自引：0，他引：1  

Ana Moreno  Isabel Cacho  Miquel Canals  Maarten A. Prins  María-Fernanda Snchez-Goi  Joan O. Grimalt  Gert Jan Weltje 《Quaternary Research》2002,58(3):318

Millennial to submillennial marine oscillations that are linked with the North Atlantic's Heinrich events and Dansgaard–Oeschger cycles have been reported recently from the Alboran Sea, revealing a close ocean-atmosphere coupling in the Mediterranean region. We present a high-resolution record of lithogenic fraction variability along IMAGES Core MD 95-2043 from the Alboran Sea that we use to infer fluctuations of fluvial and eolian inputs to the core site during periods of rapid climate change, between 28,000 and 48,000 cal yr B.P. Comparison with geochemical and pollen records from the same core enables end-member compositions to be determined and to document fluctuations of fluvial and eolian inputs on millennial and faster timescales. Our data document increases in northward Saharan dust transports during periods of strengthened atmospheric circulation in high northern latitudes. From this we derive two atmospheric scenarios which are linked with the intensity of meridional atmospheric pressure gradients in the North Atlantic region.  相似文献   

Rapid and rhythmic ice sheet fluctuations in western Scandinavia 15-40 Kya–a review     

Lars Olsen  Harald Sveian  Klaas van der  Borg  Bjørn Bergstram  Maarten Broekmans 《Polar research》2002,21(2):235-242

The onshore record of Middle to Late Weichselian sediments and glacial history in Norway indicates a succession of four major ice advances alternating with rapid, considerable ice recessions and interstadial conditions. During all the glacial advances the ice sheet expanded from onshore/inland positions to the shelf areas. The basis for visualizing these variations in glaciation curves constructed along nine transects from inland to shelf, and for interpretation of the palaeoclimatic history, is the regional Quaternary stratigraphy, more than 300 datings, fossil content and some palaeomagnetic data. The methods applied in recent years for AMS radiocarbon dating of glacial sediments with low organic carbon content have given promising results with respect to accuracy and precision, and the results of such datings were an important tool for our reconstructions and for timing of the ice oscillations. The rapid and rhythmic ice fluctuations, as reconstructed in our new model, have been fairly synchronous in most parts of Norway. Ice advances commenced and culminated at 40, 30-28, 24-21 and 18-15 (¹⁴C) Kya. We describe three intervening interstadials from inland sites: Hattfjelldal I, Hattfjelldal II and Trofors. Our stratigraphical record also includes many indications of high, pre-Holocene, relative sea levels, suggesting a considerable glacioisostatic depression of western Scandinavia during the interstadials. In our glaciation model we suggest that, in addition to precipitation, the mountainous fjord and valley topography, glacial isostasy and relative sea level changes were probably more important for the size of the glacial fluctuations than were air temperature changes.  相似文献   

Identification and Explanation of a Change in the Groundwater Regime using Time Series Analysis     

Christophe Obergfell  Mark Bakker  Kees Maas 《Ground water》2019,57(6):886-894

Time series analysis is applied to identify and analyze a transition in the groundwater regime in the aquifer below the sand ridge of Salland in the Netherlands, where groundwater regime refers to the range of head variations throughout the seasons. Standard time series analysis revealed a discrepancy between modeled and observed heads in several piezometers indicating a possible change in the groundwater regime. A new time series modeling approach is developed to simulate the transition from the initial regime to the altered regime. The transition is modeled as a weighted sum of two responses, one representing the initial state of the system, the other representing the altered state. The inferred timing and magnitude of the change provided strong evidence that the transition was the result of significant dredging works that increased the river bed conductance of the main river draining the aquifer. The plausibility of this explanation is corroborated by an analytical model. This case study and the developed approach to identify a change in the groundwater regime are meant to stimulate a more systematic application of time series analysis to detect and understand changes in groundwater systems which may easily go unnoticed in groundwater flow modeling.  相似文献   

Archival photogrammetric analysis of river–floodplain systems using Structure from Motion (SfM) methods          下载免费PDF全文

Maarten Bakker  Stuart N. Lane 《地球表面变化过程与地形》2017,42(8):1274-1286

In this study we evaluate the extent to which accurate topographic data can be obtained by applying Structure from Motion (SfM) photogrammetric methods to archival imagery. While SfM has proven valuable in photogrammetric applications using specially acquired imagery (e.g. from unmanned aerial vehicles), it also has the potential to improve the precision of topographic data and the ease with which can be produced from historical imagery. We evaluate the application of SfM to a relatively extreme case, one of low relative relief: a braided river–floodplain system. We compared the bundle adjustments of SfM and classical photogrammetric methods, applied to eight dates. The SfM approach resulted in data quality similar to the classical approach, although the lens parameter values (e.g. focal length) recovered in the SfM process were not necessarily the same as their calibrated equivalents. Analysis showed that image texture and image overlap/configuration were critical drivers in the tie‐point generation which impacted bundle adjustment quality. Working with archival imagery also illustrated the general need for the thorough understanding and careful application of (commercial) SfM software packages. As with classical methods, the propagation of (random) error in the estimation of lens and exterior orientation parameters using SfM methods may lead to inherent systematic error in the derived point clouds. We have shown that linear errors may be accounted for by point cloud registration based on a reference dataset, which is vital for the further application in quantitative morphological analyses when using archival imagery. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献