Interpretation of aeromagnetic data across the central crystalline shield area of Nigeria     

D. E. Ajakaiye  D. H. Hall  T. W. Millar 《Geophysical Journal International》1985,83(2):503-517

Summary. A residual map of the total magnetic field (above 25 000 nT base) is presented for a portion of the central crystalline shield area of Nigeria and overlapping small portions of the Chad basin and the Benue rift (8°30'−12° 00'lat, and 7°−10°30' long). The map (based on a dataset digitized from recently released aeromagnetic sheets of Nigeria) leads to four results. (1) A magnetic boundary, evident on the map, separates the Younger Granite complexes into two groups. The groups are petrologically different, and the boundary may be a fault line with uplift to the south. (2) South of the boundary the map is dominated by a system of sub-parallel anomalies striking NE–SW, possibly representing major tectonic trends, and a set of fractures through which the Younger Granite complexes were intruded. The trend of the system parallels the Benue rift and lineaments in the oceanic crust off West Africa. (3) Negative magnetic anomalies lie over most of the known ring complexes, and over some suspected buried ring complexes and other intrusions. (4) 2½-and 3-D modelling shows that the larger complexes extend to 12 km depth, and the smaller ones to 6 km. They have nearly vertical sides, and magnetization contrasts range from 0.3 to 0.5 A m⁻¹.  相似文献   

To move or not to move: an applied study of veterinary clinic location in Wellington,New Zealand     

Colin Adrian  Christopher Kissling 《Applied geography (Sevenoaks, England)》1985,5(2):117-130

This paper demonstrates the utility, in an applied context, of a standard Teitz and Bart heuristic for solving location-allocation problems. The example discussed is the location of veterinary clinics in Wellington, New Zealand. A distinct ‘service-poor’ area is identified in southern Wellington suggesting that the existing system of clinics, especially the largest multi-veterinary practice, is vulnerable to intrusion by competitors. A variety of strategies, involving new clinics and clinic relocation are analysed to ascertain alternative courses of action for both existing clinic operators and competitors. The appropriateness of this type of analysis as an aid to decision-making is reflected in the locational response of one practice to protect its share of the market for veterinary services.  相似文献   

No major thermal event on the mid-Cretaceous Côte d'Ivoire–Ghana Transform Margin     

T. Wagner  & T. Pletsch 《地学学报》2001,13(3):165-171

Transient mid-Cretaceous thermal uplift induced by lateral heating from passing oceanic lithosphere is often invoked as a mechanism for the formation of the Côte d'Ivoire–Ghana basement ridge in the Equatorial Atlantic. This heating event should have affected mid-Cretaceous sedimentary rocks along the ridge. However, organic maturity and clay mineral data on the thermal evolution of these rocks suggest that burial temperatures did not exceed 80 °C and that palaeo–geothermal gradients are not anomalous. Optical petrography and the stratigraphic pattern of temperature-sensitive parameters indicate that higher palaeotemperature estimates are related to admixtures of preheated, detrital organic and inorganic matter. Erosion brought the sediments to their present shallow burial depths. Lack of evidence for significant thermal alteration implies that either thermal exchange between oceanic and continental lithosphere along the Côte d'Ivoire–Ghana Transform Margin was negligible, or that lateral heating by oceanic lithosphere was not strong enough to affect the sedimentary cover of the basement ridge.  相似文献   

Transient response of a circular cavity in a poroelastic medium     

T. Senjuntichai  R. K. N. D. Rajapakse 《国际地质力学数值与分析法杂志》1993,17(6):357-383

This paper considers the transient response of a pressurized long cylindrical cavity in an infinite poroelastic medium. To obtain transient solutions, Biot's equations for poroelastodynamics are specialized for this problem. A set of exact general solutions for radial displacement, stresses, pore pressure and discharge are derived in the Laplace transform space by using analytical techniques. Solutions are presented for three different types of prescribed transient radial pressures acting on the surface of a permeable as well as an impermeable cavity surface. Time domain solutions are obtained by inverting Laplace domain solutions using a reliable numerical scheme. A detailed parametric study is presented to illustrate the influence of poroelastic material parameters and hydraulic boundary conditions on the response of the medium. Comparisons are also presented with the corresponding ideal elastic solutions to portray the poroelastic effects. It is noted that the maximum radial displacement and hoop stress at the cavity surface are substantially higher than the classical static solutions and differ considerably from the transient elastic solutions. Time histories and radial variations of displacement, hoop stress, pore pressure and fluid discharge corresponding to a cavity in two representative poroelastic materials are also presented.  相似文献   

A new method of palaeofield magnitude correction for thermally altered samples and its application to Lower Carboniferous lavas     

T. C. Rolph  J. Shaw 《Geophysical Journal International》1985,80(3):773-781

Summary. Some lavas of Early Carboniferous age from Scotland and Derbyshire have been analysed for palaeofield magnitude by the application of a new correction to data obtained by the Shaw method. Its application has yielded field magnitude values from data which had previously been rejected. A modern pottery specimen and recent lavas from Sicily and Westman Island have also been analysed.
An average virtual dipole moment (VDM, Smith) of 2.5 ± 0.4 × 10²² A m² was determined for the Early Carboniferous. This is only 30 per cent of the present-day VDM.  相似文献   

Auroral zone effects on hydrogen geocorona structure and variability     

T.E. Moore  A.P. Biddle  J.H. Waite  T.L. Killeen 《Planetary and Space Science》1985,33(5):499-505

The instantaneous structure of planetary exospheres is determined by the time history of energy dissipation, chemical, and transport processes operative during a prior time interval set by intrinsic atmospheric time scales. The complex combination of diurnal and magnetospheric activity modulations imposed on the Earth's upper atmosphere no doubt produce an equally complex response, especially in hydrogen, which escapes continuously at exospheric temperatures. Vidal-Madjar and Thomas (1978) have discussed some of the persistent large scale structure which is evident in satellite ultraviolet observations of hydrogen, noting in particular a depletion at high latitudes which is further discussed by Thomas and Vidal-Madjar (1978). The latter authors discussed various causes of the H density depletion, including local neutral temperature enhancements and enhanced escape rates due to polar wind H⁺ plasma flow or high latitude ion heating followed by charge exchange. We have reexamined the enhancement of neutral escape by plasma effects including the recently observed phenomenon of low altitude transverse ion acceleration. We find that, while significant fluxes of neutral H should be produced by this phenomenon in the auroral zone, this process is probably insufficient to account for the observed polar depletion. Instead, the recent exospheric temperature measurements from the Dynamics Explorer-2 spacecraft suggest that neutral heating in and near the high latitude cusp may be the major contributor to depleted atomic hydrogen densities at high latitudes.  相似文献   

Voids in the Jagiellonian Field     

T. Grabińska 《Astrophysics and Space Science》1985,115(2):369-376

This paper presents arguments relating to the phenomenon of scantily populated regions with intergalactic obscuration.  相似文献   

Simulation of double cold cores of the 35°N section in the Yellow Sea with a wave-tide-circulation coupled model   总被引：1，自引：0，他引：1  

Xia Changshui  Qiao Fangli  Zhang Mengning  Yang Yongzeng  Yuan Yeli 《中国海洋湖沼学报》2004,22(3):292-298

Based on the MASNUM wave-tide-circulation coupled numerical model, the temperature structure along 35°N in the Yellow Sea was simulated and compared with the observations. One of the notable features of the temperature structure along 35°N section is the double cold cores phenomena during spring and summer. The double cold cores refer to the two cold water centers located near 122°E and 125°E from the depth of 30m to bottom. The formation, maintenance and disappearance of the double cold cores are discussed. At least two reasons make the temperature in the center (near 123°E) of the section higher than that near the west and east shores in winter. One reason is that the water there is deeper than the west and east sides so its heat content is higher. The other is invasion of the warm water brought by the Yellow Sea Warm Current (YSWC) during winter. This temperature pattern of the lower layer (from 30m to bottom) is maintained through spring and summer when the upper layer (0 to 30m) is heated and strong thermocline is formed. Large zonal span of the 35°N section (about 600 km) makes the cold cores have more opportunity to survive. The double cold cores phenomena disappears in early autumn when the west cold core vanishes first with the dropping of the thermocline position. Supported by the National Basic Research Program of China (No. G1999043809) and the National Science Foundation of China (No. 49736190).  相似文献   

A dated cave sediment record of Pleistocene transgressions on Berry Head,Southwest England     

Christopher J. Proctor  Peter L. Smart 《第四纪科学杂志》1991,6(3):233-244

Berry Head, a limestone headland in Torbay, southwest England, exhibits a series of subaerial marine erosion platforms and raised beaches spanning an altitude range of 97 m. Solution caves on the headland show preferred horizontal development at elevations that are correlated with the marine erosion platforms, and developed in a marine/freshwater mixing zone whose position was controlled by high sea-level still-stands. Corbridge Cave in Berry Head Quarry lies below the raised beaches in Torbay, and contains evidence of three marine transgressions in the form of fine-grained marine ponding deposits with a marine microfauna. Uranium-series dating of intercalated speleothems indicates that a transgression during oxygen isotope stage 5e reached an elevation of 5.8 m OD, while an earlier transgression (probably during stage 7) reached at least 7.2 m OD. These findings are used to constrain possible interpretations of the aminostratigraphy of raised beaches in Southwest Britain, and a correlation of the Unnamed Stage of Bowen, Sykes, Reeves, Miller, Andrews, Brew and Hare with oxygen isotope stage 5e is proposed.  相似文献   

Dynamical History of the Earth–Moon System     

G. A. Krasinsky 《Celestial Mechanics and Dynamical Astronomy》2002,84(1):27-55

Differential equations describing the tidal evolution of the earth's rotation and of the lunar orbital motion are presented in a simple close form. The equations differ in form for orbits fixed to the terrestrial equator and for orbits with the nodes precessing along the ecliptic due to solar perturbations. Analytical considerations show that if the contemporary lunar orbit were equatorial the evolution would develop from an unstable geosynchronous orbit of the period about 4.42 h (in the past) to a stable geosynchronous orbit of the period about 44.8 days (in the future). It is also demonstrated that at the contemporary epoch the orbital plane of the fictitious equatorial moon would be unstable in the Liapunov's sense, being asymptotically stable at early stages of the evolution. Evolution of the currently near-ecliptical lunar orbit and of the terrestrial rotation is traced backward in time by numerical integration of the evolutional equations. It is confirmed that about 1.8 billion years ago a critical phase of the evolution took place when the equatorial inclination of the moon reached small values and the moon was in a near vicinity of the earth. Before the critical epoch t _cr two types of the evolution are possible, which at present cannot be unambiguously distinguished with the help of the purely dynamical considerations. In the scenario that seems to be the most realistic from the physical point of view, the evolution also has started from a geosynchronous equatorial lunar orbit of the period 4.19 h. At t < t _cr the lunar orbit has been fixed to the precessing terrestrial equator by strong perturbations from the earth's flattening and by tidal effects; at the critical epoch the solar perturbations begin to dominate and transfer the moon to its contemporary near-ecliptical orbit which evolves now to the stable geosynchronous state. Probably this scenario is in favour of the Darwin's hypothesis about originating the moon by its separation from the earth. Too much short time scale of the evolution in this model might be enlarged if the dissipative Q factor had somewhat larger values in the past than in the present epoch. Values of the length of day and the length of month, estimated from paleontological data, are confronted with the results of the developed model.  相似文献