Synclinal-horst basins: examples from the southern Rio Grande rift and southern transition zone of southwestern New Mexico, USA     

Greg H. Mack  William R. Seager  Mike R. Leeder† 《Basin Research》2003,15(3):365-377

In areas of broadly distributed extensional strain, the back‐tilted edges of a wider than normal horst block may create a synclinal‐horst basin. Three Neogene synclinal‐horst basins are described from the southern Rio Grande rift and southern Transition Zone of southwestern New Mexico, USA. The late Miocene–Quaternary Uvas Valley basin developed between two fault blocks that dip 6–8° toward one another. Containing a maximum of 200 m of sediment, the Uvas Valley basin has a nearly symmetrical distribution of sediment thickness and appears to have been hydrologically closed throughout its history. The Miocene Gila Wilderness synclinal‐horst basin is bordered on three sides by gently tilted (10°, 15°, 20°) fault blocks. Despite evidence of an axial drainage that may have exited the northern edge of the basin, 200–300 m of sediment accumulated in the basin, probably as a result of high sediment yields from the large, high‐relief catchments. The Jornada del Muerto synclinal‐horst basin is positioned between the east‐tilted Caballo and west‐tilted San Andres fault blocks. Despite uplift and probable tilting of the adjacent fault blocks in the latest Oligocene and Miocene time, sediment was transported off the horst and deposited in an adjacent basin to the south. Sediment only began to accumulate in the Jornada del Muerto basin in Pliocene and Quaternary time, when an east‐dipping normal fault along the axis of the syncline created a small half graben. Overall, synclinal‐horst basins are rare, because horsts wide enough to develop broad synclines are uncommon in extensional terrains. Synclinal‐horst basins may be most common along the margins of extensional terrains, where thicker, colder crust results in wider fault spacing.  相似文献   

Weather as a Chronic Hazard for Road Transportation in Canadian Cities   总被引：2，自引：1，他引：2  

Jean Andrey  Brian Mills  Mike Leahy  Jeff Suggett 《Natural Hazards》2003,28(2-3):319-343

Inclement weather creates a chronic hazard for Canadian travellers. Past studies indicate that road collision rates increase during precipitation, although the magnitude of theincrease varies from study to study, partly as a result of variations in weather and driving conditions,but also because of differences in methods. The goal of the current study is to improve ourunderstanding of the links between weather and travel risk in mid-sized Canadian cities by using astandardized method for analyzing data from six cities with different climates: Halifax-Dartmouth,Ottawa, Québec, Hamilton, Waterloo Region, and Regina. The study has four interrelated objectives: (1) Toconduct a sensitivity analysis to determine the extent to which risk estimates vary depending onthe criteria used to define precipitation events and `normal' conditions; (2) To compare therelative risk of collision and injury during precipitation relative to `normal' conditions; (3) Todetermine the extent to which weather-related risks vary fordifferent Canadian cities; and (4) To explore any differences in collision characteristics between events and controls, especially as theyvary from city to city. Results are based on a matched-pair analysis, using six-hour time blocks over afour-year period, 1995 to 1998. Results indicate only modest sensitivity to the criteria used to define precipitation events and `normal' conditions. On average, precipitation is associated with a 75 percentincrease in traffic collisions and a 45 percent increase in related injuries, as compared to `normal'seasonal conditions, but risk levels vary depending on the characteristics of the weather event.Both sensitivity to specific weather conditions and weather-related accident profiles vary from city tocity in ways that are not easily explained.  相似文献   

Fluxes of Sr into the headwaters of the Ganges   总被引：1，自引：0，他引：1  

Mike J Bickle  Judith Bunbury  Nigel B.W Harris  Talat Ahmad 《Geochimica et cosmochimica acta》2003,67(14):2567-2584

Himalayan weathering is recognized as an important agent in modifying sea water chemistry, but there are significant uncertainties in our understanding of Himalayan riverine fluxes. This paper examines causes of the variability, including that of the seasons, by analysis of downstream variations in Sr, ⁸⁷Sr, and major ions in the mainstream, in relation to the composition of tributary streams from subcatchments with differing geologic substrates.Water samples were collected over four periods spanning the premonsoon, monsoon, and postmonsoon seasons. Uncertainties in the relative fluxes have been estimated, using Monte Carlo techniques, from the short-term variability of mainstream chemistry and the scatter of tributary compositions. The results show marked seasonal variations in the relative inputs related to high monsoon rainfall in the High and Lesser Himalaya, contrasting with the major contribution from glacial melt waters from the Tibetan Sedimentary Series (TSS) at times of low rainfall. Much of the spread in previously published estimates of the sources of Sr in Himalayan rivers may result from these seasonal variations in Sr fluxes.The annual fluxes of Sr into the headwaters of the Ganges are derived from the three main tectonic units in the proportions 35 ± 1% from the TSS, 27 ± 3% from the High Himalayan Crystalline Series (HHCS), and 38 ± 8% from the Lesser Himalaya. The particularly elevated ⁸⁷Sr/⁸⁶Sr ratios characteristic of the HHCS and the Lesser Himalaya enhance their influence on seawater Sr-isotope composition. The TSS contributes 13 ± 1%, the HHCS 30 ± 3%, and the Lesser Himalaya 57 ± 11% of the ⁸⁷Sr flux in excess of the seawater ⁸⁷Sr/⁸⁶Sr ratio of 0.709.  相似文献   

Wide-angle seismic velocities in heterogeneous crust   总被引：5，自引：0，他引：5  

John Brittan  Mike Warner 《Geophysical Journal International》1997,129(2):269-280

Seismic velocities measured by wide-angle surveys are commonly used to constrain material composition in the deep crust. Therefore, it is important to understand how these velocities are affected by the presence of multiscale heterogeneities. The effects may be characterised by the scale of the heterogeneity relative to the dominant seismic wavelength (λ); what is clear is that heterogeneities of all scales and strengths bias wide-angle velocities to some degree. Waveform modelling was used to investigate the apparent wide-angle P -wave velocities of different heterogeneous lower crusts. A constant composition (50 per cent felsic and 50 per cent ultramafic) was formed into a variety of 1- and 2-D heterogeneous arrangements and the resulting wide-angle seismic velocity was estimated. Elastic, 1-D models produced the largest velocity shift relative to the true average velocity of the medium (which is the velocity of an isotropic mixture of the two components). Thick (width > λ) horizontal layers, as a result of Fermat's Principle, provided the largest increase in velocity; thin (width ≪λ) vertical layers produced the largest decrease in velocity. Acoustic 2-D algorithms were shown to be inadequate for modelling the kinematics of waves in bodies with multiscale heterogeneities. Elastic, 2-D modelling found velocity shifts (both positive and negative) that were of a smaller magnitude than those produced by 1-D models. The key to the magnitude of the velocity shift appears to be the connectivity of the fast (and/or slow) components. Thus, the models with the highest apparent levels of connectivity between the fast phases, the 1-D layers, produced the highest-magnitude velocity shifts. To understand the relationship between measured seismic velocities and petrology in the deep crust it is clear that high-resolution structural information (which describes such connectivity) must be included in any modelling.  相似文献   

Coastal Problems: Geomorphology, Ecology and Society at the Coast     

Mike Shepherd 《New Zealand geographer》1997,53(1):55-56

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Palaeoecological events during the last 15,000 years: B.E. Berglund, H.J.B. Birks, M. Ralska-Jasiewiczowa and H.E. Wright (eds). John Wiley & Sons Ltd, 1996. Price £105 Hardback. ISBN 0-471-95840-9     

Mike Walker 《Quaternary Science Reviews》1996,15(10):1066-1067

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Inversion of potential field data by genetic algorithms     

Fabio Boschetti  Mike Dentith  Ron List 《Geophysical Prospecting》1997,45(3):461-478

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Finding work in 2001: urban–rural contrasts across England in employment rates and local job availability     

Mike Coombes  Simon Raybould 《Area》2004,36(2):202-222

Early results from the 2001 census of England allow a preliminary analysis of joblessness. People in the prime working ages (25–49) are more likely to be in work than those in their 50s, but the differences are quite subtle: in particular, men without qualifications are not much more likely to be in work when they are younger, despite the suggestions that too many men cease working in their 50s. The evidence supports the view that there is a national shortfall of demand for labour, that the low-skilled are the least able to compete for scarce jobs, and older people are the most likely to be unqualified. Moreover there is a strong spatial clustering of areas where job availability has declined or grown least, undermining the government's claim that there were often available jobs near to concentrations of people without work.  相似文献   

Promoting planetary science     

Mike Hapgood 《Astronomy& Geophysics》2005,46(5):5.30-5.31

Mike Hapgood summarizes the RAS's position on planetary sciences in the UK, a subject that delivers world-class results, but needs focused support in order to continue to thrive.  相似文献   

Methods for the visualization of digital elevation models for landform mapping     

Mike J. Smith  Chris D. Clark 《地球表面变化过程与地形》2005,30(7):885-900

Digital elevation models (DEMs) are increasingly used for landform mapping, particularly with the growing availability of national and global datasets. In this paper we describe a variety of techniques that can visualize a DEM. We then compare five techniques to ascertain which performs the most complete and unbiased visualization. We assess the visualization techniques by comparing landforms mapped from them against a detailed morphological map (derived from mapping of multi‐azimuth relief‐shaded DEMs cross‐checked with stereo aerial photographs). Results show that no single visualization method provides complete and unbiased mapping. The relief‐shaded visualizations are particularly prone to azimuth biasing, although they can highlight subtle landforms. We recommend curvature visualization for initial mapping as this provides a non‐illuminated (and therefore unbiased) image. Initial mapping can then be supplemented with data from relief‐shaded visualizations. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献