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891.
G S Kimbell R W Gatliff† J D Ritchie† A S D Walker J P Williamson 《Basin Research》2004,16(2):259-278
A series of three‐dimensional models has been constructed for the structure of the crust and upper mantle over a large region spanning the NE Atlantic passive margin. These incorporate isostatic and flexural principles, together with gravity modelling and integration with seismic interpretations. An initial isostatic model was based on known bathymetric/topographic variations, an estimate of the thickness and density of the sedimentary cover, and upper mantle densities based on thermal modelling. The thickness of the crystalline crust in this model was adjusted to equalise the load at a compensation depth lying below the zone of lateral mantle density variations. Flexural backstripping was used to derive alternative models which tested the effect of varying the strength of the lithosphere during sediment loading. The models were analysed by comparing calculated and observed gravity fields and by calibrating the predicted geometries against independent (primarily seismic) evidence. Further models were generated in which the thickness of the sedimentary layer and the crystalline crust were modified in order to improve the fit to observed gravity anomalies. The potential effects of igneous underplating and variable upper mantle depletion were explored by a series of sensitivity trials. The results provide a new regional lithospheric framework for the margin and a means of setting more detailed, local investigations in their regional context. The flexural modelling suggests lateral variations in the strength of the lithosphere, with much of the margin being relatively weak but areas such as the Porcupine Basin and parts of the Rockall Basin having greater strength. Observed differences between the model Moho and seismic Moho along the continental margin can be interpreted in terms of underplating. A Moho discrepancy to the northwest of Scotland is ascribed to uplift caused by a region of upper mantle with anomalously low density, which may be associated with depletion or with a temperature anomaly. 相似文献
892.
In order to understand the significance of detrital grain ages in sedimentary basins, a new approach is presented. Five characteristic paths, identified by the change in age of detrital grain populations combined with the change in lagtime over time, can be related to different geodynamic settings in the source regions. When lagtime and grain age increase over time, a change in source must be invoked – this is usually a direct response to a geological event. A constant cooling age, a vertical path, associated implicitly with increasing lagtime, implies erosion of materials that had passed through the closure temperature rapidly – exhuming sufficient rock to supply detritus over the time of the path. Constant lagtimes, regardless of the lagtime itself, are indicative of thermochronological stability in the source region. This can involve fast or slow cooling. Finally, decreasing lagtimes support the notion of increasing cooling rates in the source regions over time. A test study is presented from sediments of the northern Ecuadorian Sub‐Andean Zone where geological events had previously been identified using alternative methods. The addition of heavy‐mineral studies increased the precision in the interpretation. At 90 Ma, rapidly decreasing lagtimes point to a phase of tectonic activity. From about 85 Ma until about 60 Ma the lagtimes were approximately zero. This represents a phase of rapid exhumation of the source regions correlating with the previously identified Pallatanga event. An associated increase of metamorphic minerals occurs over this time span, pointing to increased erosion from deeper horizons. At about 70 Ma, the oldest source region, the shield to the east, was switched off. This timing correlates with a change from marine to continental conditions in the basin, a change in palaeocurrent directions from the east to the west, as well as an associated influx of material from the growing Cordillera Real. At about 55 Ma, a change in source is identified by a change in slope of the lagtime curve together with a change in heavy minerals. From 50 to 35 Ma a renewed period of tectonism in the source region is correlated with the docking of the Macuchi terrane which clearly had an effect of increased erosion in the Cordillera Real bringing in higher grade metamorphic minerals. From about 32 Ma onwards the lagtime has been somewhat constant at about 30 Myr. This does not imply, however, a steady‐state environment as it is well known from other geological evidence that there have been other events within this time frame. One must be cautious about over‐interpreting the lagtime as a method to determine steady state in any region. It is a matter of scale. 相似文献
893.
The changing nature of, and approaches to, UK coastal management at the start of the twenty-first century 总被引:2,自引:0,他引:2
Peter W French 《The Geographical journal》2004,170(2):116-125
As we enter the twenty-first century, effective coastal management will become more of a central issue for coastal users and governors alike. The new century sees the review of many of the UK's existing management plans, and the subsequent publication of revised versions. Some of these new editions will have to show changes from their predecessors because of natural processes of erosion and accretion, yet other changes will be because of human interference. In this, we mean the continued ignoring of previous management plan recommendations regarding new development and coastal usage. Coastal management needs to adapt and develop, yet along with this, increased education and awareness is also necessary to allow users to understand and appreciate what is being recommended in management plans. 相似文献
894.
Rainfall thresholds for landsliding in the Himalayas of Nepal 总被引:5,自引:0,他引:5
Emmanuel J. Gabet Douglas W. Burbank Jaakko K. Putkonen Beth A. Pratt-Sitaula Tank Ojha 《Geomorphology》2004,63(3-4):131-143
Landsliding of the hillslope regolith is an important source of sediment to the fluvial network in the unglaciated portions of the Himalayas of Nepal. These landslides can produce abrupt increases of up to three orders of magnitude in the fluvial sediment load in less than a day. An analysis of 3 years of daily sediment load and daily rainfall data defines a relationship between monsoonal rainfall and the triggering of landslides in the Annapurna region of Nepal. Two distinct rainfall thresholds, a seasonal accumulation and a daily total, must be overcome before landslides are initiated. To explore the geomorphological controls on these thresholds, we develop a slope stability model, driven by daily rainfall data, which accounts for changes in regolith moisture. The pattern of rainfall thresholds predicted by the model is similar to the field data, including the decrease in the daily rainfall threshold as the seasonal rainfall accumulation increases. Results from the model suggest that, for a given hillslope, regolith thickness determines the seasonal rainfall necessary for failure, whereas slope angle controls the daily rainfall required for failure. 相似文献
895.
Richard L. Reynolds Joseph G. Rosenbaum Josh Rapp Michael W. Kerwin J. Platt Bradbury Steven Colman David Adam 《Journal of Paleolimnology》2004,31(2):217-233
Petrological and textural properties of lacustrine sediments from Upper Klamath Lake, Oregon, reflect changing input volumes of glacial flour and thus reveal a detailed glacial history for the southern Cascade Range between about 37 and 15 ka. Magnetic properties vary as a result of mixing different amounts of the highly magnetic, glacially generated detritus with less magnetic, more weathered detritus derived from unglaciated parts of the large catchment. Evidence that the magnetic properties record glacial flour input is based mainly on the strong correlation between bulk sediment particle size and parameters that measure the magnetite content and magnetic mineral freshness. High magnetization corresponds to relatively fine particle size and lower magnetization to coarser particle size. This relation is not found in the Buck Lake core in a nearby, unglaciated catchment. Angular silt-sized volcanic rock fragments containing unaltered magnetite dominate the magnetic fraction in the late Pleistocene sediments but are absent in younger, low magnetization sediments. The finer grained, highly magnetic sediments contain high proportions of planktic diatoms indicative of cold, oligotrophic limnic conditions. Sediment with lower magnetite content contains populations of diatoms indicative of warmer, eutrophic limnic conditions. During the latter part of oxygen isotope stage 3 (about 37–25 ka), the magnetic properties record millennial-scale variations in glacial-flour content. The input of glacial flour was uniformly high during the Last Glacial Maximum, between about 21 and 16 ka. At about 16 ka, magnetite input, both absolute and relative to hematite, decreased abruptly, reflecting a rapid decline in glacially derived detritus. The decrease in magnetite transport into the lake preceded declines in pollen from both grass and sagebrush. A more gradual decrease in heavy mineral content over this interval records sediment starvation with the growth of marshes at the margins of the lake and dilution of detrital material by biogenic silica and other organic matter. 相似文献
896.
Studies were conducted in the central Mojave Desert to quantify how creosote bushes (Larrea tridentata) respond to physical damage during large-scale military training exercises. Creosote bush possesses a resilient growth form that recovers from repeated physical damage via resprouts arising from meristems in stem bark below severed or crushed canopy units. At high levels of disturbance by heavy vehicles, nearly all individuals showed measurable breakage, but without additional damage each plant can regain a full canopy within 5 years under arid field conditions. Resprouts exhibited more vigorous growth and doubled the biomass accumulation stimulated by high rainfall of 1998, an El Niño year, vs. a normal year. New shoots of resprouted individuals were markedly different in morphological traits than canopy old growth and had slightly higher predawn shoot water potentials. The natural ability of this evergreen species to recover from cutting and crushing bodes well for re-establishment of creosote bush desert scrub communities following episodes of severe damage by vehicles. 相似文献
897.
Spatial Dimensions of Urban Commuting: A Review of Major Issues and Their Implications for Future Geographic Research* 总被引:3,自引:0,他引:3
Mark W. Horner 《The Professional geographer》2004,56(2):160-173
Modern urban transportation systems continuously challenge, and are challenged by, the changing nature of 21st‐century travel demand. Today, congestion is the norm in cities of the United States, and researchers and practitioners are seeking solutions to these problems. Urban commuting is identified as contributing to the suboptimal performance of transportation systems. This paper offers a review and critique of recent research on urban commuting, emphasizing geographical dimensions of this topic. Three broad areas of research related to urban commuting are discussed. These include (1) urban sustainability, (2) land use, and (3) geographic information systems (GIS). Major themes are examined in an effort to elicit thought on future geographic research. At the conclusion of the paper, summary remarks are provided and avenues for research are outlined. 相似文献
898.
899.
P C W Davies 《Astronomy& Geophysics》2005,46(1):1.26-1.29
Why does time apparently fly one way, when the laws of physics are actually time-symmetrical? Paul Davies proposes a quantum solution, in the Whitrow Lecture 2004, given on 8 October 2004. 相似文献
900.
The reflection of plane waves in a poroelastic half-space saturated with inviscid fluid 总被引:8,自引:2,他引:8
Chi-Hsin Lin Vincent W. Lee Mihailo D. Trifunac 《Soil Dynamics and Earthquake Engineering》2005,25(3):205-223
This paper discusses surface displacements, surface strain, rocking, and energy partitioning during reflection-of-plane waves in a fluid-saturated poroelastic half-space. The medium is modeled by Biot's theory, and is assumed to be saturated with inviscid fluid. A linear porosity-modulus relation based on experimental data on sandstones is used to determine the material parameters for Biot's model. Numerical results in terms of angle of incident waves and Poisson's ratio are illustrated for various porosities and degrees of solid frame stiffness. The results show that the amount of solid frame stiffness controls the response of a fluid-saturated porous system. A poroelastic medium with essentially dry-frame stiffness behaves like an elastic medium, and the influence of pore fluid increases as dry-frame stiffness is reduced. The effects of a second P-wave become noticeable in poroelastic media with low dry-frame stiffness. 相似文献