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1.
A total of 120 samples from 12 sites were collected from two flanks of a fold. Stepwise thermal demagnetization has successfully revealed characteristic magnetization components from the rocks in each case. A well-defined component determined from red fine-grained sandstone is clustered in the northeasterly direction with shallow upward inclination (D = 29.3°,I= -19.2°,k = 283.7, α95 = 7.3°. tilt-corrected). The pole position (39.5°N, 247.3°E,dp = 4.0°,dm = 7.6°) derived from this component is close to the Permian pole for the Yangtze Block, indicating that the red fine-grained sandstone has been overprinted. The red mudstone reveals two characteristic components Component A with lower unblocking temperature, characterized by northerly declination and moderate to steep inclination corresponds to a pole position overlay with the present North Pole. Component B (D = 129.1°,I=-23.6°,k = 44.6, α95 = 7.8°, tilt-corrected) with higher unblocking temperature, passes fold test, and yields a pole position (39.5°S, 185.l°E,dp = 4.4°,dm = 8.3°) different from the other poles for the Yangtze Block. It is therefore suggested that component B was probably a primary magnetization and the Yangtze Block was situated at low latitudes in the Southern Hemisphere in the Middle Cambrian.  相似文献   

2.
Summary Recently excellent archeomagnetic data sequences have been bublished from several parts of the world. Using these sequences, an attempt is made to trace the secular variation of the virtual geomagnetic dipole field characterized by the three first order spherical harmonic coefficients . The archeomagnetic data (declination, inclination and total intensity) are transformed into the first order coefficients mentioned by a simple mathematical method. The secular variations of these coefficients, however, contain both dipole and non-dipole components. The separation of these is also attempted.Paper presented at the IAGA Symposium, Madrid, September 1969.  相似文献   

3.
Three sections of the Candelaria Hills volcanic sequence, west-central Nevada, appear to have recorded parts of two transitional field records or reversal excursions. Paleomagnetic data and / laser fusion sanidine age estimates for pyroclastic rocks and associated flows show that these rocks recorded the unusual field behavior at about 25.7 Ma and about 23.8 Ma. Fifteen sites yield northeast declination, moderate to shallow negative inclination mean directions and 16 sites yield west to southwest declination, moderate negative inclination directions. Both populations of site mean directions, representing a total of 12 independent eruptive units, are highly discordant to a time-averaged late Tertiary field direction, and neither can be explained by a geologically reasonable magnitude of vertical axis rotation. Virtual paleomagnetic poles (VGPs), estimated from the directional data, lie at low to intermediate latitudes; 29 of the 31 flows at intermediate latitudes (<60°), and 11 at very low latitudes (<30°). Two well-grouped VGP clusters are defined by these data with each cluster roughly corresponding to one of the age groups. Stratigraphically corrected VGPs from most of the 23.8 Ma group roughly cluster at intermediate to low latitudes at about 150°E longitude. The cluster at about 150°E corresponds to VGP clusters that have been interpreted to reflect a long lasting near-dipole configuration during several field reversals. The second stratigraphically corrected cluster lies at intermediate to low latitudes at about 80°E longitude and, notably, is defined by pyroclastic flows of the 25.7 and 23.8 Ma age groups. The VGP data at about 80°E do not fall into any previously identified preferred longitudinal band, however, they are consistent with data from some sedimentary records of reversal excursions in western North America. We recognize that the VGPs returned to a preferred location in both age populations, which we interpret as a preferred directional position, thus reflecting a potentially stable non-dipole component during a complete reversal or a reversal excursion. The observation that the VGPs maintained a preferred location during separate high amplitude events supports the hypothesis that preferred VPG clusters and thus persistent non-dipole field components can factor into the behavior of the geomagnetic field during full reversals or reversal excursions.  相似文献   

4.
Gogorza  C.S.G.  Di Tommaso  I.  Sinito  A.M.  Jackson  B.  Nuñez  H.  Creer  K.  Vilas  J.F. 《Studia Geophysica et Geodaetica》1998,42(1):12-29
The preliminary results of paleomagnetic and radiocarbon dating of late pleistocene-holocene sediments from two lakes of south-western Argentina (41°S, 71.5°W) are presented. The magnetic susceptibility, intensity and direction of the natural remanent magnetisation were measured. The stability of the natural remanent magnetisation was investigated by alternating field demagnetisation. The magnetic parameters allowed the cores within each lake to be correlated. 13 C analysis, total organic content measurements and C 14 dating were carried out. A model of sedimentation is suggested. Using this model and the correlation, curves of variations of magnetic inclination and declination in time are shown.  相似文献   

5.
Oriented specimens of very young lavas have been collected from 3 volcanic sites in Kenya in order to determine the declination and inclination angles at the times the lavas cooled. TheseD andI values have been tentatively fitted to a time scale extending over the past 1000 years based on historical, geological and vegetational evidence. A plot of declination against inclination for this period takes the form of a partial loop, somewhat similar to those obtained at other places.  相似文献   

6.
The time variations in three parameters during the last decades are considered. R(foF2) is the correlation coefficient between the nighttime and daytime values of foF2 for the same day. Stable trends are found for the minimum (R(foF2)(max)) and maximum (R(foF2)(min)) values of R(foF2) during a year. The foF2(night)/foF2(day) ratio demonstrates both, negative and positive trends, and the trend sign depends on the inclination I and declination D of the magnetic field. The correlation coefficient r(h, fo) between foF2 and the 100 hP level in the stratosphere demonstrates a decrease (in the years of maximum and minimum solar activity) from the 1980s to the 1990s. The trends in all three groups of data are considered under the assumption of long-term changes in the circulation in the upper atmosphere.  相似文献   

7.
The ratio of daytime and nighttime values of the foF2 critical frequency is analyzed on the basis of the data of 28 ionospheric stations in the Eastern Hemisphere. It is found that three types of time variations in this ratio are observed after 1980: an increase with time (a positive trend), a decrease with time (a negative trend), and the absence of pronounced changes (a zero trend). The sign of this trend is shown to be governed by the signs of the magnetic declination D and magnetic inclination I at the given ionospheric station. This fact makes it possible to assume that the above trend is caused by long-term variations in the zonal component V ny of the horizontal wind in the thermosphere, the latter component contributing into the vertical drift velocity W. The causes of the systematic changes in the thermospheric circulation regime after 1980 are still unknown; however, it is quite probable that they are related to anthropogenic changes in the atmosphere.  相似文献   

8.
The main objective of this work is to compare directional (declination and inclination) volcanic and archaeomagnetic data for the last four centuries (~1600–1990) with the historical geomagnetic predictions given by the GUFM1 model which spans from 1590 to 1990. The results show statistical agreement between archaeomagnetic data and directions given by the geomagnetic field model. However, when comparing the volcanic data with the model predictions, marked inclination shallowing is observed. This systematically lower inclination has already been observed in local palaeomagnetic studies (Italy, Mexico and Hawaii) for the 20th century, by comparing recent lava flows with the International Reference Geomagnetic Field (IGRF) model. Here, we show how this inclination shallowing is statistically present at worldwide scale for the last 400 years with mean inclination deviation around 3° lower than the historical geomagnetic field model predictions.  相似文献   

9.
Summary In palaeomagnetism it is usually assumed that the distribution of directions of natural remanent magnetization at a site is Fisherian. This assumption is used here for an analysis of dispersion on a sphere when only inclination (and not declination) of NRM directions is known. By this method, in spite of the lack of declination data, the mean inclination and precision of the parent Fisherian population are estimated, together with the probable errors in these estimates. The method is tested against known Fisherian populations and is then used for dealing with data from borecores, where the way-up of cores and dip of the hole are known, but the azimuth of cores is not. Uses of the information derived from this analysis include estimations of age of NRM, support for palaeomagnetic data from surface studies, and comparison of dispersion of the palaeomagnetic field with that of the present geomagnetic field.  相似文献   

10.
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11.
Three closely spaced 6-m piston cores were taken in the central part of Lake Tahoe. Cores were split into two complete replicates for paleomagnetic study and the remaining sections were used for stratigraphic and mineralogical analysis.Stratigraphic correlation of the cores is based on two distinctive horizons (volcanic ash and diatomite) and upon three different sedimentological regimes dominated by (1) poorly bedded silts and muds, (2) well bedded graded units, and (3) finely laminated silts. These correlations served as the standards for the evaluation of the paleomagnetic data. Extrapolation of14C dates obtained in the upper sections of the Lake Tahoe sediments suggests that the lower sections of the cores may reach ages of 25,000–30,000 years B.P.X-ray, optical, Curie point, and hysteresis measurements show that magnetite is the only important magnetic mineral in the sediments and occurs in the size range of 10 μm. Hematite is essentially absent. Based on large changes in the declination and inclination of the natural remanent magnetism (NRM) within single graded layers the paleomagnetic signature is a post-depositional remanent magnetism (PDRM). This PDRM is believed to be caused by magnetic orientation during compaction.Paleomagnetic measurements show three regimes that are correlated with the stratigraphic regimes. NRM declination and inclination data show good correlation between the three cores and agree well with the correlations based on sediment character. NRM intensity variations are due largely to the variations in magnetite content and its occurrence as either single detrital grains or as inclusions within the larger silicates. Thus the variation in paleo intensity was not determined.Comparisons of Lake Tahoe data with that from Mono Lake show fair correlations of declination and inclination. The occurrence of a short-wavelength, high-amplitude event in the lower section of the Lake Tahoe cores may provide confirmation of the Mono Lake geomagnetic excursion.  相似文献   

12.
One of the key challenges which are traditionally encountered in studying the paleomagnetism of terrigenous sedimentary strata is the necessity to allow for the effect of shallowing of paleomagnetic inclinations which takes place under the compaction of the sediment at the early stages of diagenesis and most clearly manifests itself in the case of midlatitude sedimentation. Traditionally, estimating the coefficient of inclination flattening (f) implies routine re-deposition experiments and studying their magnetic anisotropy (Kodama, 2012), which is not possible in every standard paleomagnetic laboratory. The Elongation–Inclination (E–I) statistical method for estimating the coefficient of inclination shallowing, which was recently suggested in (Tauxe and Kent, 2004), does not require the investigation of the rock material in a specially equipped laboratory but toughens the requirements on the paleomagnetic data and, primarily, regarding the volume of the data, which significantly restricts the possibilities of the post factum estimation and correction for inclination shallowing. In this work, we present the results of the paleomagnetic reinvestigation of the Puchezh and Zhukov ravine (ravine) reference sections of the Upper Permian and Lower Triassic rocks in the Middle Volga region. The obtained paleomagnetic data allowed us to estimate the coefficient of inclination shallowing f by the E–I method: for both sections, it is f = 0.9. This method was also used by us for the paleomagnetic data that were previously obtained for the Permian–Triassic rocks of the Monastyrskii ravine (Monastirskoje) section (Gialanella et al., 1997), where the inclination shallowing coefficient was estimated at f = 0.6.  相似文献   

13.
The decadal variation in the length of day and in the Earth's magnetic field is analyzed by empirical mode decomposition (EMD). The existence of a periodicity of about 60-years in the Earth's angular velocity has often been inferred and is confirmed here, with a new estimate of approximately 62 years. The existence of a similar periodicity in the geomagnetic field has been controversial. From the time series analyses presented here of the magnetic declination at 10 observatories, it is concluded that a geomagnetic periodicity of about 62?±?3 years does exist. A similar analysis of the magnetic inclination at five observatories gives a period of 58?±?5 years. The declination data from four observatories is found to correlate with the length of day signal, with a correlation coefficient of approximately 0.6.  相似文献   

14.
Paleomagnetic and/or micropaleontological studies have been carried out on approximately 28 sedimentary cores of latest Pleistocene age from the Gulf of Mexico. Sedimentation rates range from 9 cm to 20 cm/1000 yr. A distinct excursion in the earth's magnetic field occurs in the upper parts of 8 of 15 cores for which paleomagnetic studies were conducted and is independently correlated with planktonic foraminiferal zones. An inclination change to zero or negative inclination is often associated with a declination change. The age of the excursion was determined by extrapolation of sedimentation rates from the Z-Y paleontological boundary which is dated at 11,000 B.P. The magnetic excursion occurred between 12,500 and 17,000 yr ago within the upper part of zone Y. This falls within the age range of the Laschamp Event as originally defined.  相似文献   

15.
We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.  相似文献   

16.
Superimposed on a regional pattern of oroclinal bending in the Aegean and west Anatolian regions, the coastal region of western Anatolia, shows a complex and chaotic pattern of coexisting clockwise and counterclockwise rotations. Here, we report new palaeomagnetic data from the eastern Aegean island of Chios, to test whether this fits the regional palaeomagnetic pattern associated with the Aegean orocline, or should be included in the narrow zone of chaotic palaeomagnetic directions. Therefore, a combined palaeomagnetic study of Miocene sediments and volcanic rocks has been carried out. Thermal and AF demagnetization of a 130-m thick Middle Miocene succession from the Michalos claypit allowed a stable component of both polarities to be isolated while rock magnetic experiments showed that the main magnetic carrier is magnetite. When compared with the Eurasian reference, the mean declination of 348 ± 5.1° implies 15° of counterclockwise rotation since Middle Miocene times. The obtained shallow inclination of 38 ± 6.7° was corrected to 61.8 ± 3.9°, by applying the elongation/inclination correction method for inclination shallowing. This result is similar to the expected inclination of 58° for the latitude of Chios. The palaeomagnetic analysis (demagnetization treatment and corresponding rock magnetic measurements) of the volcanic rocks identify a stable, predominantly normal, ChRM with poorly constrained mean declination of about 290 ± 19.8° based on five successfully resolved components. The significantly different palaeomagnetic results obtained from an island as small as Chios (and a very short distance), and the relatively large rotation amounts do not fit the regional palaeomagnetic direction of Lesbos and basins in northwestern Turkey which show little or no significant rotation. We thus prefer to include Chios in the coastal zone of chaotic rotations, which may represent a previously inferred tectonic transfer zone that accommodates lateral differences in extensional strain within the Aegean back-arc.  相似文献   

17.
The behaviour of the Southern Hemisphere stratosphere has attracted considerable interest, and been compared with the Northern Hemisphere, since the International Geophysical Year (1957–58) when the sudden (explosive or accelerated) springtime warming phenomenon in the Antarctic was first observed. Over the years studies of upper air temperature and wind observations have been made, principally through the spring months when the polar vortex breakdown occurs, utilising both ground-based (rawinsonde, rocket) and more recently, satellite-derived data. Although the radiosonde-derived temperature data are limited both by the number of reporting stations, and the practical difficulty of securing observations much above the 100 hPa level, useful records exist from 1956 or 1957. These have shown that in the 1959 southern spring, the lower stratosphere was relatively colder, and the warming rate through the season was essentially more regular, with little evidence of the marked but short-lived temperature fluctuations usually found. Similar, but not quite such wide-spread conditions occurred again in the 1961 spring. In another study, 30 hPa temperature fields over the Antarctic continent, which could be drawn for the 1967 spring, showed the complexity of the polar vortex breakdown. These features are recalled because extension of the 100 hPa springtime temperature series for the Australian Antarctic station at Casey (66.3°S, 110.5°E) shows that in 1985 and part of 1986, the temperature behaviour there was similar to, but not quite so extreme as that which occurred at Mirny (66.5°S, 93.0°E) in 1959.  相似文献   

18.
Summary The ratios of the amplitudes of bay-type variations of the geomagnetic field along the DSS profile VI to the amplitudes of the same phenomena recorded at permanent geomagnetic observatories at Prhonice and Budkov were studied. The relative variations of the values of the horizontal component, declination and, particularly, the vertical component of the geomagnetic field indicate deep fault zones.  相似文献   

19.
Global electron content (GEC) as a new ionospheric parameter was first proposed by Afraimovich et al. [2006]. GEC is equal to the total number of electrons in the near-Earth space. GEC better than local parameters reflects the global response to a change in solar activity. It has been indicated that, during solar cycle 23, the GEC dynamics followed similar variations in the solar UV irradiance and F 10.7 index, including the 11-year cycle and 27-day variations. The dynamics of the regional electron content (REC) has been considered for three belts: the equatorial belt and two midlatitude belts in the Northern and Southern hemispheres (±30° and 30°–65° geomagnetic latitudes, respectively). In contrast to GEC, the annual REC component is clearly defined for the northern and southern midlatitude belts; the REC amplitude is comparable with the amplitude of the seasonal variations in the Northern Hemisphere and exceeds this amplitude in the Southern Hemisphere by a factor of ～1.7. The dayside to nightside REC ratio, R(t), at the equator is a factor of 1.5 as low as such a GEC ratio, which indicates that the degree of nighttime ionization is higher, especially during the solar activity maximum. The pronounced annual cycle with the maximal R(t) value near 8.0 for the winter Southern Hemisphere and summer Northern Hemisphere is typical of midlatitudes.  相似文献   

20.
Naturally disaggregated specularite-bearing sandstone from the Triassic Moenkopi Formation, artificially deposited in controlled magnetic fields of ～5 × 10?2 mT, acquires a stable remanent magnetization that has systematic errors in inclination and declination. Inclinations about 12° shallower than the applied fields are produced by deposition on a horizontal surface in still water. Deposition from flowing water on a surface inclined 6–10° results in inclination errors of as much as 20°. Water flowing obliquely to the applied field results in declination errors of about 10°, with declinations systematically rotated toward the upstream direction of current flow. These experimental results indicate that specularite-bearing sediment responds to the earth's field in a manner similar to magnetite-bearing sediment, and support observational evidence for a primary magnetization of depositional origin in specularite in red beds of the Moenkopi Formation.  相似文献   

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