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1.
Spectral analyses of long magnetic anomaly profiles of Cretaceous age in the Phoenix and Hawaiian lineations within the Pacific Basin show two to five non-harmonic wavelength peaks, with probabilities >90 per cent, within the range of several tens to hundreds of kilometres for each profile. For each profile with four peak wavelengths, their ratios correlate strongly ( r > 0.98) with the ratios for the Earth's orbital eccentricity periodicities for this time, and hence the profile peaks necessarily correlate with each other. These observations suggest that the magnetic anomaly signal is influenced by the Earth's changing orbital parameters, probably through their influence on the internal geomagnetic field. This implies that assumptions usually made to convert the magnetic signal to the magnetization of the oceanic crust may be suspect. The observations, if confirmed, indicate that orbital periodicities can assist in improving the age calibration of the global polarity timescale and that the predicted orbital behaviour of the Earth is confirmed for the last 150 Ma.  相似文献   

2.
The Antarctic magnetic anomaly map compiled marine and airborne surveys collected south of 60°S through 1999 and used Magsat data to help fill in the regional gaps between the surveys. Ørsted and CHAMP satellite magnetic observations with greatly improved measurement accuracies and temporal and spatial coverage of the Antarctic, have now supplanted the Magsat data. We combined the new satellite observations with the near-surface survey data for an improved magnetic anomaly map of the Antarctic lithosphere. Specifically, we separated the crustal from the core and external field components in the satellite data using crustal thickness variations estimated from the terrain and the satellite-derived free-air gravity observations. Regional gaps in the near-surface surveys were then filled with predictions from crustal magnetization models that jointly satisfied the near-surface and satellite crustal anomalies. Comparisons in some of the regional gaps that also considered newly acquired aeromagnetic data demonstrated the enhanced anomaly estimation capabilities of the predictions over those from conventional minimum curvature and spherical harmonic geomagnetic field models. We also noted that the growing number of regional and world magnetic survey compilations involve coverage gaps where these procedures can contribute effective near-surface crustal anomaly estimates.  相似文献   

3.
Summary . An elongated zone of positive magnetic anomalies extends from Northern Ireland to the Shetland Islands. From the total magnetization of the body causing the anomaly and published palaeomagnetic directions corresponding to a range of different possible ages for the remanent magnetization of this body, associated ranges of susceptibility are calculated. These ranges favour a Siluro–Devonian age as most plausible for the remanent magnetization. Such an age is compatible with the known geology of the area.  相似文献   

4.
《Polar Science》2014,8(4):342-356
We developed small computer-controlled unmanned aerial vehicles (UAVs, Ant-Plane) using parts and technology designed for model airplanes. These UAVs have a maximum flight range of 300–500 km. We planned aeromagnetic and aerial photographic surveys using the UAVs around Bransfield Basin, Antarctica, beginning from King George Island. However, we were unable to complete these flights due to unsuitable weather conditions and flight restrictions. Successful flights were subsequently conducted from Livingston Island to Deception Island in December 2011. This flight covered 302.4 km in 3:07:08, providing aeromagnetic and aerial photographic data from an altitude of 780 m over an area of 9 × 18 km around the northern region of Deception Island. The resulting magnetic anomaly map of Deception Island displayed higher resolution than the marine anomaly maps published already. The flight to South Bay in Livingston Island successfully captured aerial photographs that could be used for assessment of glacial and sea-ice conditions. It is unclear whether the cost-effectiveness of the airborne survey by UAV is superior to that of manned flight. Nonetheless, Ant-Plane 6-3 proved to be highly cost-effective for the Deception Island flight, considering the long downtime of the airplane in the Antarctic storm zone.  相似文献   

5.
Remanence directions, measured at 2  cm intervals along a composite 88  m bore-core, enable mean palaeomagnetic poles to be defined at 13.6°S, 25.2°W and 13.6°N, 154.8°E. The directions of remanence vary very smoothly away from each palaeomagnetic pole, extending more than 90° from them. This raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles. For practical purposes, intermediate polarity is defined as directions whose virtual poles lie more than 25° from the mean pole, enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three-quarters of the core. The stratigraphic thickness between these subchrons shows a very high linear correlation ( r >0.99) with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age. The analysed sequence contains wavelength spectra which, when transformed to the temporal realm, match periodicities determined for three marine magnetic anomaly profiles of similar age. These also match planetary orbital periodicities for the Cretaceous. These observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits. Such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically.  相似文献   

6.
Summary. Fifty-six orientated samples were collected from 13 sites on five dolerite dykes (between lat.14°23°N, long.77°43'E and lat.14°08'N, long. 77°49'E), which adjoin the south-western margin of the Cuddapah basin in Anantapur district, Andhra Pradesh. After af demagnetization, two dykes (five sites) striking ENE possess similar magnetic directions, (1) D = 57°, 1=-69° (K = 52, α95= 7°) and (2) D = 71°, I = -72° ( K = 260, α95= 5°). Again dykes (3) (three sites), and (4) (two sites) have similar strike (NE) and magnetic directions, D = 64°. I=-7 α( K = 142, α95 = 8°) and D = 53°, I =-8° (K = 142, α95= 6°) and dyke (5) (two sites) striking NW shows D = 320°, I = -34° ( K = 68, α95= 13°). Remanent directions estimated from total field magnetic anomaly data agree well with these results. Synthesis of these data with 10 other published palaeomagnetic studies of Precambrian dolerite dykes on the Indian peninsula, suggest that these three systems of dykes adjoining the Cuddapah basin had been emplaced prior to the basin formation perhaps representing the initial thermal event responsible for the basin development and also that there have been at least three separate periods of dyke emplacement on this shield. The radiometric data, however, are very sparse and these periods cannot be dated with confidence.  相似文献   

7.
Summary. If one can measure the anomalous horizontal magnetic field associated with a (locally bounded) two-dimensional conductivity anomaly, the transfer function which results from correlating the vertical with the anomalous horizontal magnetic field directly indicates the depth to an equivalent line-current. A. numerical model can be used to illustrate this. If three-dimensional effects (including current channelling) produce the current anomaly, interpretation in terms of conductive structure would be less clear. It has been claimed (Babour & Mosnier etc.) that such three-dimensional effects are experimentally observed in the highly coherent transfer functions determined from differential geomagnetic sounding experiments. These effects are, specifically, the 'linear polarization' of the anomalous fields, and the invariance of the phase of the measured anomalous field across the anomaly. It is suggested in this present paper that both these effects can be explained in terms of simple local induction models.
If the embedded two-dimensional anomaly is sufficiently close to the Earth's surface, the transfer function (between the vertical and the total horizontal field) contains more information than is usually interpreted. With this in mind, the magnetovariational data collected by Rooney & Hutton in the Kenyan Rift is re-examined.  相似文献   

8.
The low-temperature magnetic properties of magnetite are reviewed, and implications for rock magnetism considered. The behaviour of fundamental properties of magnetite at low temperatures near the Verwey transition ( T v ) are documented, and attention is given to various Verwey transition theories. The low-temperature behaviour of the magnetic energies that control domain structure is reviewed in detail. For the first time in rock magnetic literature, the low-temperature anomaly in spontaneous magnetization ( M s ) is documented and the differences between the saturation magnetization and M s near the Verwey transition are discussed. It is argued that the low-temperature behaviour of the magnetocrystalline anisotropy, and in particular the anomaly at T v , is most likely to affect multidomain remanence during low-temperature cycling. For multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through T v is likely to reduce the stability of closure domains.  相似文献   

9.
Abstract Reconstructions of the relative positions of the Indian, African, and Antarctic plates and their uncertainties are given for the times of selected magnetic anomalies that could be identified on adjacent pairs of these plates. Among the most certain reconstructions are those for the Antarctic and African plates, which can be determined directly from recently published magnetic anomalies from both sides of the Southwest Indian Ridge. As Patriat and his colleagues reported, there was an important change in direction and a decrease in rate of separation between Africa and Antarctica between the times of anomalies 33 and 20. India moved rapidly away from both Africa and Antarctica in the Late Cretaceous and early Tertiary periods, but slowed markedly near the time of anomaly 20 (≅ 45 Myr). The positions of the Indian plate with respect to the others are poorly constrained between the times of anomaly 5 (≅ 10 Myr) and anomaly 23 (≅ 54 Myr), but using the reconstructions of the African and Antarctic plates, the uncertainties can be reduced. Despite the relatively large uncertainties, the positions of anomalies 5, 6, and 13 on the Antarctic and Indian plates apparently cannot be described by the same parameters that describe the history of separation of Australia and Antarctica. Therefore, Stein and Okal's contention that Australia and India lie on separate plates appears to be valid not only for the present, but for the last 35 Myr.  相似文献   

10.
Summary. Processes involved in the generation of arc maginatism, which are associated with the evolution of subduction zones, may in certain cases be able to record the variations of the Earth's magnetic field with time. A mechanism is suggested which may generate lineated magnetic anomaly patterns, similar to those observed over oceanic areas, with bands of alternating normal and reverse polarity parallel to the subduction zone. It is suggested that magmatic arc rocks acquire remanent magnetizations creating a zone of normal or reverse polarity in the magmatic arc zone, and if the geometric arrangement of plate subduction changes, i.e. the region of primary magma generation is displaced normal to the trench by changes of either the Wadati—Benioff zone dip angle or the trench position, then active arc magmatism is displaced accordingly, creating another magnetic anomaly zone. An important factor is the rate of displacement of arc magnatism with time; estimates for the south-western North America magmatic arc gives a value of about 1.2 cm yr−l for the interval 100–55 Myr ago, which is comparable to sea-floor spreading rates. The elongated patterns of positive and negative magnetic anomalies, trenchward of the Japan trench and the Kurile—Kamchatka trench may have been produced by this process. Elongated intense magnetic anomalies are also observed in other magmatic arc assemblages, such as in western North America. Processes which may obscure these lineated anomaly patterns include, e.g. tectomc rotations within the magmatic arc zone, changes in thermal conditions and igneous activity, remagnetization events and decay of remanent magnetism. The anomalies may be related to, and better preserved by, intrusive rocks or buried rocks.  相似文献   

11.
The tectonic history of the Western Mediterranean region during the Oligocene and Early Miocene is illustrated through a series of plate reconstructions, from chron C13n to chron C6n. The reconstructions are based on a new interpretation of published magnetic anomaly data and two-ships seismic data, and their integration with known geological constraints, in order to determine style and timing of the backarc extension processes in the Liguro-Provençal, Valencia and Algerian basins. In particular, a reinterpretation of the regional magnetic anomaly field allowed the calculation of the instantaneous Euler poles associated with the motion of 11 microplates relative to Eurasia and Iberia. Furthermore, a quantitative analysis of the acoustic basement morphology and the balancing of deep crustal profiles were used to estimate the closure and pre-rift rotation angles associated with the Euler poles of opening of the Ligurian, Provençal, Valencia and Algerian basins. This rigorous reconstruction of the geometry of the pre-rift continental margins of Iberia and Eurasia could furnish important insights into the study of Mediterranean tectonics for older times.  相似文献   

12.
Summary. A simplified model of the solar quiet-time ionospheric current system is used to calculate the induced currents in a model earth. The conductivity is assumed to be constant below a depth of about 400 km and zero above that depth. The current induced in the north—south conductivity anomaly under the Rocky Mountains is then estimated from the time-varying potential difference between points at 30 and 45° latitude at the surface of the conducting sphere. The purpose of these calculations is to investigate whether variations in the latitude of the northern hemisphere current system vortex will substantially alter the relationship between the observed magnetic field components at the Earth's surface and the local magnetic field gradient caused by the conductivity anomaly. We find that a 10° shift in the latitude of the ionospheric current focus causes a change of 6 per cent or less in the transfer function from the field components to the gradient in the total field. Thus such latitude shifts cannot explain much of the magnetic field gradient variation at periods near 24 hr that has been observed near Boulder, Colorado.  相似文献   

13.
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14.
The magnetic structure of convection-driven numerical dynamos   总被引：1，自引：1，他引：0  
The generation of a magnetic field in numerical simulations of the geodynamo is an intrinsically 3-D and time-dependent phenomenon. The concept of magnetic field lines and the frozen-flux approximation can provide insight into such systems, but a suitable visualization method is required. This paper presents results obtained using the Dynamical Magnetic Field line Imaging (DMFI) technique, which is a representation of magnetic field lines accounting for their local magnetic energy, together with an algorithm for the time evolution of their anchor points. The DMFI illustrations are consistent with previously published dynamo mechanisms, and allow further investigation of spatially and temporally complex systems. We highlight three types of magnetic structures: (i) magnetic cyclones and (ii) magnetic anticyclones are expelled by, but corotate with axial flow vortices; (iii) magnetic upwellings are amplified by stretching and advection within flow upwellings, and show structural similarity with helical plumes found in rotating hydrodynamic experiments. While magnetic anticyclones are responsible for the regeneration of a stable axial dipole, here we show that excursions and reversals of the dipole axis are caused by the emergence of magnetic upwellings, which amplify and transport a generally multipolar magnetic field from the inner to the outer boundary of the models. Geomagnetic observations suggest the presence of magnetic structures similar to those found in our models; thus, we discuss how our results may pertain to Earth's core dynamo processes. In order to make DMFI a standard tool for numerical dynamo studies, a public software package is available upon request to the authors (supplementary material is available at: http://www.ipgp.jussieu.fr/~aubert/DMFI.html ).  相似文献   

15.
The magnetostratigraphy of five new sections through the Neogene Siwalik Group of Nepal is presented. Rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary DRM or a PDRM. After stepwise thermal demagnetization of specimens from all sections, directions of characteristic remanent magnetization were obtained. The results show a positive reversal test and exhibit inclination errors of about 20. Due to gaps in exposures and extremely variable demagnetization behaviour of the haematite-bearing sediments, the resulting polarity sequences are poorly defined for some parts of the sections. However, double-sampled parts of some sections yield similar results. A correlation with the Surai Khola section (Appel, Rösler & Corvinus 1991) is proposed, based on rock magnetic parameters. AMS results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes. Correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the Surai Khola section.  相似文献   

16.
Magnetic data were collected during the Wilkes (1973) and Seacarib II (1987) cruises to the Cayman trough. A new interpretation of magnetic data is carried out. An isochron pattern is drawn up from our anomaly identifications. An early Eocene age (49 Ma, Ypresian) for Cayman trough opening is proposed instead of the late Oligocene or middle Eocene ages suggested by previous studies. Our plate tectonic reconstruction is simpler and fits the on-land geology (Jamaica and Cuba) and the tectonics. Our reconstruction shows a southward propagation of the spreading centre between magnetic anomalies 8 and 6 (26 and 20 Ma). The trough width increases by 30 km in this period. The southward propagation of the Cayman spreading centre from the Middle Oligocene to the Early Miocene induced the development of the restraining bend of the Swan Islands, the formation of a 1 km high scarp on the eastern trace of the Cayman trough transform fault (Walton fault) and the formation of a pull-apart basin (Hendrix pull-apart). Magnetic anomalies and magnetization maps give information about the deformation and the rocks. The proposed evolutionary model of the Cayman trough from the inception of seafloor spreading to the present configuration is presented in relation to the tectonic escape of the northern boundary of the Caribbean plate from the Maastrichtian to the Present.  相似文献   

17.
Summary. A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three over-lapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies M0 to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies M0 to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly M0 is located in the Early Aptian, close to the Aptian/Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we find in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations.  相似文献   

18.
The use of the integral moments for interpreting magnetic data is based on a very elegant property of potential fields, but in the past it has not been completely exploited due to problems concerning real data. We describe a new 3-D development of previous 2-D results aimed at determining the magnetization direction, extending the calculation to second-order moments to recover the centre of mass of the magnetization distribution. The method is enhanced to reduce the effects of the regional field that often alters the first-order solutions. Moreover, we introduce an iterative correction to properly assess the errors coming from finite-size surveys or interaction with neighbouring anomalies, which are the most important causes of the failing of the method for real data. We test the method on some synthetic examples, and finally, we show the results obtained by analysing the aeromagnetic anomaly of the Monte Vulture volcano in Southern Italy.  相似文献   

19.
Mineral magnetic signatures in a long core from Lake Qarun, Middle Egypt   总被引：1，自引：0，他引：1  
The analysis and interpretation of changes in mineral magnetic signatures from a long (ca. 8.2 m) sedimentary sequence recovered from Lake Qarun, Middle Egypt in 2003 spanning a timescale of approximately the last 2,000 years is reported. A suite of mass specific susceptibility and magnetic remanence measurements were made at irregular intervals downcore on 39 samples. These samples were selected on the basis of trends and abrupt changes in whole-core magnetic susceptibility measured using a Bartington® MS2E sensor and were analysed for low and high temperature loss on ignition and their particle size distribution. Trends in all mineral magnetic concentration parameters are remarkably similar and were initially used to divide the core into three magnetically distinct zones. The upper and lower sections of the core (0–119 cm and 445–822 cm depth) are characterised by low values for all magnetic concentration parameters. Between 153 and 380 cm depth, concentration parameters are considerably higher, although somewhat variable. The S ratio and percentage loss of remanence after 24 h (IRMloss) follow a different trend and are inversely related to each other. A low S ratio (<0.7) is associated with a loss of remanence of >4%. On the basis of these parameters, the core can be divided into four zones, and differences in magnetic mineralogy between these four zones were confirmed by measurement of IRM acquisition curves. The major difference between concentration parameters and ratios or percentage loss of IRM lies in the identification of an additional zone below 619 cm depth where the S ratio is high and IRMloss is low. There is little evidence to suggest that the magnetic signatures are controlled by particle size or by trends in organic matter and/or carbonate content. The signatures appear to be predominantly detrital and show little evidence of post-depositional alteration through dissolution or authigenic addition of bacterial magnetite or greigite. Analysis of Saharan dust deposition rates in Northern Egypt suggests that atmospheric fallout is likely to make only a very minor contribution (<1%) to sedimentation rates in Lake Qarun. The downcore trends therefore appear to reflect major changes in fluvial sediment sources over the ca. 2,000 year time period spanned by this ～8 m core. Preliminary mineral magnetic characterisations of potential local sources suggest that these cannot account for the range of signatures recorded in the Qarun sediments and it is hypothesised that these sediments are derived from Nile river floods.  相似文献   

20.
Summary. A pronounced positive magnetic anomaly of approximately 300 gamma occurs over the eastern edge of the East Shetland Platform at approximately 60°N, 1°E. After the removal of the regional gravity variation and the gravity effect of the known geological structure, it is found that this magnetic high correlates with a negative gravity residual anomaly of approximately 30 mGal. Seismic data indicate that these anomalies occur in an area of relatively shallow basement on the upthrown side of the main Viking Graben margin fault. The presence of a buried granite batholith of approximately 40 × 40km may explain the gravity, magnetic and seismic observations. The observed deviation of the fault defining the edge of the Viking Graben in the proximity of the proposed granite may be explained in terms of the tectonic influence of the buoyant granite block during the taphrogenic development of the graben.  相似文献   

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