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1.
Samples of oil shales of Autunian age from the Krkono e (Riesengebirge) Piedmont basin, northern Bohemia, which represent rocks with fossil micro-organic matter in the initial stage of carbonification, were subjected to palaeomagnetic investigations. The palaeomagnetic data obtained from these rocks are fully in accordance with those derived previously on rocks of the red-bed facies in the same area. Preliminary results obtained on Lower Carboniferous coal-bearing shales collected from the Upper Silesian basin, Ostrava-Karviná region, indicate also that these rocks, characterized by a higher degree of carbonification, are suitable for palaeomagnetic investigations. These results with those of laboratory carbonification indicate that such rocks as bituminous rocks, black shales, oil shales, coal-bearing shales or generally rocks containing micro-organic matter at different stages of natural carbonification are worthy of being studied palaeomagnetically.  相似文献   

2.
The palaeomagnetic and rock magnetic characteristics of some Cenozoic rocks from the Cairo–Fayum area have been investigated. A total number of 259 oriented core samples were collected at 32 sites located in rocks of Eocene (13 sites), Oligocene (11 sites) and Pliocene (9 sites) ages. Most of these rocks carry a weak but stable remanent magnetisation that is principally carried by hematite. Goethite and magnetite are also found in some samples as subordinate constituents. Careful thermal demagnetisation successfully enabled the isolation of the characteristic remanent magnetisation. Normal and reversed polarities that passed a reversal test have been recorded in the three age groups. This magnetisation is probably of primary origin and reflects the ages of the rocks. The resultant palaeomagnetic poles are considered reliable and represent a good contribution to the African palaeomagnetic database and should help in further refining of the Cenozoic APWP for Africa.  相似文献   

3.
The Svecofennian mobile zone occupies the bulk of the Fennoscandian shield and represents terrain subjected to profound tectonic activity and granite intrusion at ～1800 My. This study covers the palaeomagnetism of basic rocks within this belt in Sweden between 65.5 and 67.5°N (gabbros and diorites of Kallax, Niemisel, Sangis, Stora Lulevatten and the Gällivare, Jokkmokk and Voullerim regions) magnetised during uplift and cooling of this belt at ～1750-1550 My. AF and thermal demagnetisation define a consistent sequence of high to low blocking-temperature components identifying a migration of the geomagnetic field during part of this interval. Together with the Rådmansö gabbro-diorite of central-east Sweden (palaeomagnetic pole 201°E, 36°N) these components yield a comparable sequence of palaeopoles to those derived from uplift magnetisations of the contemporaneous Svecokarelian terrain of Finland. The post-tectonic, Uppsala metabasite suite possesses a magnetite-held (“A”) remanence comparable to Svecofennian uplift magnetisations from elsewhere; within the aureole of the Almunge alkaline complex this has been largely displaced by a low blocking temperature (“B”) remanence, possibly related to a late stage in the Svecofennian uplift cycle. The Hälleforsnäs giant dyke possesses a magnetite-held remanence attributed to initial cooling at 1518 My (palaeomagnetic pole 167°E, 27°N) and at least two high blocking-temperature components. One of these is correlated with the ～1000-800 My Sveconorwegian mobile activity of southwest Sweden; this latter component is represented as the univectorial remanence in dolerite dykes of this age, and sporadically as a secondary component in the adjacent Svecofennian terrain.The results are compiled with other palaeomagnetic poles from the Fennoscandian shield to derive a generalised apparent polar wandering path for the interval ～1750-1550 My. They define segments of a large loop which agrees closely with uplift magnetisations from the contemporaneous Hudsonian mobile terrain of the Laurentian shield on a single reconstruction derived from Upper Proterozoic (1450-1200 My) palaeomagnetic data. The two shields thus appear to have formed an integral continental unit during the interval 1750-1200 My. A geological reconstruction of the Siberian and Laurentian shields is also tested and found to yield general agreement with the palaeomagnetic evidence. The major geological implications of the collective reconstruction are an alignment of major tectonic trends and a gradual restriction of anorthosite-Rapakivi magmatism between the termination of the ～1800 and ～1100 My mobile episodes.  相似文献   

4.
The proposition that movements between multiple continental fragments driven by plate tectonics have taken place during the earlier eons of geological time can be simply tested. A random distribution will be derived when the collective palaeomagnetic poles are rotated into any quasi-rigid reconstruction. This elementary test surmounts the most serious limitation of Precambrian palaeomagnetic data, namely that the ages of magnetisation are mostly poorly known. In the event, the late Archaean and Proterozoic poles (2850–590 Ma) produce a non-random distribution with a single peak which is only compatible with a quasi-rigid continental configuration occupying a preferential position on the globe. This solution is not favoured because uncertainties in definition of the poles contribute to their scatter and bias the solution towards the random one. Hence confirmation of the quasi-rigid premise is derived in a debased form, but is not suppressed, by the deficiencies of the database.This palaeomagnetic constraint explains the distinctive isotopic, geochemical and lithofacies signatures of the Proterozoic eon and is independently supported by the trend and long temporal continuity of crustal lineaments. It does not preclude the opening of ocean basins between the shields, but it does predict that development of these basins was limited by the influence of controlling mantle systems. This constrained the continental crust into a form which was essentially rigid on the gross scale resolvable by palaeomagnetism.  相似文献   

5.
Summary In the Middle Miocene claystones containing fossil micro-organic matter and overlying the brown-coal seams in the Jii quarry of the Sokolov Brown-Coal Basin, a strongly magnetic layer was found and subjected to detailed palaeomagnetic investigations. The principal carrier of magnetism in this bed (called Kocián's bed) is the ferrimagnetic mineralization of greigite or greigitesmythite showing pronounced metastable properties. This mineralization must be treated with caution during laboratory tests aimed at deriving palaeomagnetic directions. In Kocián's bed, about two metres thick, two zones of palaeomagnetic field transition were identified. The high degree of demagnetization achieved by using thermal stepwise procedures and a MAVACS apparatus enabled the origin of the self-reversal of remanence to be defined after heating to 360°C. The process of chemo-remanent magnetization of the authigenic greigite-(smythite) mineralization fossilizing the palaeomagnetic field was relatively fast; the transition of the palaeomagnetic field is recorded in layers not exceeding a thickness of 2 × 10–2 m.Presented at 2nd conference on New Trends in Geomagnetism, Castle of Bechyn, Czechoslovakia, September 24–29, 1990.  相似文献   

6.
Summary The dolerite dyke of post-lower Gondwana age reported earlier by the authors [5]2) has been taken for detailed magnetic study. Seven magnetic traverses have been taken across the dyke using Schmidt vertical force variometer. Negative anomalies varying from 1400 to 2200 gammas have been observed. Magnetic susceptibility of samples from the dyke has been determined on both cores and crushed material, using the Model MS-3 susceptibility bridge. The average susceptibility of 20 samples has been found to be 1930x10–6 C.G.S. units. Direction and intensity of remanence on 10 oriented samples have been determined by an Astatic magnetometer. The average intensity of remanence has been found to be 3.13x10–3 C.G.S. units and the average direction has a declination of 323° and inclination-68° (up dip). The Koenigsberger ratio varies from 1.6 to 5.6. The high negative magnetic anomalies have been explained in terms of remance-that more than 80% of the anomaly is due to remance and that negative anomalies are due to negative inclination of remanent direction of the rocks. The magnetic direction for the dyke gave the position for Dhanbad as 51° south latitude in Jurassic period. This is in conformity with the deductions made by other workers from the palaeomagnetic studies of the Deccan and Rajmahal traps. The virtual geomagnetic pole in Jurassic period as deduced from the palaeomagnetism of the dyke has a position of latitude 8 1/2°N and longitude 71°W, in the southern Carribean. On palaeomagnetic evidence this dyke has been correlated with Rajmahal traps of eastern Bihar.  相似文献   

7.
A new analysis of palaeomagnetic data for igneous rocks from Deception Island, 25 de Mayo Island (King George Island) and Cape Spring, are given.K-Ar age determinations indicate that most of the igneous samples from 25 de Mayo Island included in the palaeomagnetic study are of Late Mesozoic/Early Tertiary age. The significance of these palaeomagnetic-radiometric data on the hypothesis of oroclinal bending of the Antarctic Peninsula and on the apparent polar movement of Antarctica is discussed.The positions of palaeomagnetic poles for the Andean igneous complex indicate that there has not been any apparent post-Late Cretaceous/Early Tertiary oroclinal bending in the Antarctic Peninsula from 74°S to 62°S.A comparison of the positions of palaeomagnetic poles for Antarctica and Australia suggests that the direction of apparent polar movement relative to Antarctica reversed after the Miocene.  相似文献   

8.
Summary Over the shield region of Northern Sweden, the Geological Survey runs a low altitude (30 m) aeromagnetic survey and regional gravity surveys cover almost the same areas. The production of detailed geological maps would be almost impossible without these geophysical measurements. To enhance their interpretation, a study of the appropriate petrophysical properties was started by measurements of density, susceptibility and remanence on all rock specimens collected by field geologists. Up to now more than 30,000 specimens have been measured and almost as many in situ susceptibility determinations have been made. About 10% of the samples are oriented. The information obtained is useful not only for the direct interpretation of geophysical surveys but also reflects the effects of various geological processes acting during and after the formation of the rocks considered.In precambrian rocks, density is obviously closely correlated to mineral composition and thus to chemical composition. This is demonstrated for igneous rocks by the correlation trends between density and SiO2-content and the CM/AF-index.Susceptibility mainly reflects the magnetite content of rocks. As magnetite is an accessory mineral it is seldom considered by geologists. However, a closer study of the magnetic susceptibility of rocks reveals that its extreme complexity reflects the effects of primary and secondary geological processes. The susceptibility spectrum of a certain rock may prove to be a useful classification tool — at least regarding intermediate and basic igneous rocks.The combination of the physical parameters density and susceptibility in 2-dimensional frequency distributions seems to be a promising approach to the understanding of certain petrological processes and makes possible the delineation of local or regional secondary processes. Magmatic differentiation and serpentinization can be demonstrated in this manner.Remanent magnetization plays a secondary role in the majority of precambrian rocks. Still there are places where the natural remanence dominates and shows pronounced directions deviating from today's magnetic field. So far we have only one case where a follow up by demagnetization has been attempted, but intensified paleomagnetic research should most certainly add to our understanding of precambrian geology.Combination of susceptibility and remanance shows some characteristic correlations for highly remanent rocks, reflecting mainly exosolution phenomena among magnetic opaques and grain size distributions. Even these features might be used as diagnostic or classifying tools. They also explain some of the diversity of susceptibility spectra.  相似文献   

9.
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   

10.
The rocks of the early Precambrian proto-north China platform in general have an isotopic age of more than 1800–1900 Ma and thus belong to the Archaean and early Proterozoic.Based on a study of six representative regions, four evolutionary stages have been recognized in terms of volcano-sedimentary and/or sedimentary mega-cycles, two for the Archaean and two for the early Proterozoic. The geochronological division between the first and second mega-cycles is ca. 2800—3000 Ma, that between the second and third ca. 2500–2600 Ma, and that between the third and fourth ca. 2200–2300 Ma.Prior to ca. 2800–3000 Ma ago the area probably already possessed characteritics of a proto-crustal basement on which basaltic, dacitic and neritic clastic rocks had been deposited. This was followed during the second stage (cycle) by the formation of basaltic and intermediate volcanics, silty graywackes and even carbonate rocks. Most of these rocks were metamorphosed to granulite facies and amphibolite facies respectively during Archaean time. They were also influenced by an earlier sodic migmatization and a later potassic one and related magmatic activity and at least two episodes of folding. They probably produced an elevated, rather widespread sialic basement.The third stage was characterized by the accumulation of volcano-sedimentary rocks in local basins, troughs and rift-depressions which differ in actual setting from place to place. These rocks were later affected by varying degrees of tectonism and insignificant magmatism and were metamorphosed to amphibolite to greenschist facies.The last stage was one of deposition, near the earlier basins or troughs, of dominant clastic and dolomitic carbonate rocks with locally developed minor volcanics. This stage reflects comparatively stable tectonic conditions, seemingly transitional into those of the more stable middle to late Proterozoic. Most of the rocks were metamorphosed to greenschist facies before 1800–1900 Ma ago.  相似文献   

11.
The results of palacomagnetic studies made on the Deccan Traps by various workers are reviewed in the light of the recent palaeomagnetic data on these rocks and the general geological information. It is suggested that: (a) the earlier altitude-polarity classification of the Deccan Traps, suggesting that the flows below the general elevation of 2000±200 feet above mean sea level are of reversed magnetic polarity while those above this horizon are normal, is not without exceptions; (b) the geomagnetic field reversed its polarity several times during the eruption of these lavas; (c) the Deccan Trap eruptions probably consisted of several phases of volcanicity over a protracted period; and (d) the phases of Deccan Trap volcanism, the phases of Himalayan upheaval, and the northward drift of the Indian landmass were rather concrescent events.  相似文献   

12.
The palaeomagnetism of Middle Triassic (224 ± 5 m.y.) igneous rocks from the Ischigualasto-Ischichuca Basin (67°40′W, 30°20′S) was investigated through 86 oriented hand samples from 11 sites. At least one reversal of the geomagnetic field has been found in these rocks. Nine sites yield a palaeomagnetic pole at 239°E, 79°S (α95 = 15°, k = 13).The K-Ar age determinations of five igneous units of the Puesto Viejo Formation give a mean age of 232 ± 4 m.y. (Early Triassic). The palaeomagnetism of six igneous units of the Puesto Viejo Formation (68°W, 35°S) was investigated through 60 oriented samples. These units, two reversed relative to the present magnetic field of the Earth and four normal, yield a pole at 236°E, 76°S (α95 = 18°, k = 14).Data from the Puesto Viejo Formation indicate, for the first time on the basis of palaeomagnetic and radiometric data, that the Illawarra Zone, which defines the end of the Kiaman Magnetic Interval, extends at least down to 232 ± 4 m.y. within the Early Triassic. The palaeomagnetic poles for the igneous rocks of the Ischigualasto-Ischichuca Basin and Puesto Viejo Formation form an “age group” with the South American Triassic palaeomagnetic poles (mean pole position: 239°E, 77°S; α95 = 6.6°, k = 190). The Middle and Upper Permian, Triassic and Middle Jurassic palaeomagnetic poles for South America would define a “time group” reflecting a quasi-static interval (mean pole position: 232°E, 81°S; α95 = 4°, k = 131).  相似文献   

13.
Palaeomagnetic results are reported from the continental facies of the Triassic-Jurassic Isalo Group of Madagascar. Stability of the magnetic remanence was tested using the alternating field and progressive thermal demagnetization techniques. Results from 8 sites, 6 located in northwestern Madagascar and 2 from the southwestern region, yield a palaeomagnetic pole at 74.2°S, 97.1°E (N = 8, A95 = 6.3°). Three models previously proposed to describe the drift history of Madagascar relative to Africa are considered and the relevant geological and geophysical information is reviewed. The palaeomagnetic data are only consistent with the pre-drift model which places Madagascar off the east coast of Africa adjacent to Kenya and Tanzania. This is also the continental drift fit favoured on geological grounds.  相似文献   

14.
—A palaeomagnetic investigation has been carried out at 14 sites on Jurassic red nodular limestones from the central and eastern part of the External Zones of the Betic Cordillera (Subbetic and Prebetic Zones). Progressive thermal demagnetisation of samples from the Subbetic Zone reveals the presence of two stable magnetic components of the natural remanent magnetisation: 1) a secondary Neogene syn-folding component and 2) the original Jurassic magnetisation. As similar characteristics have been reported in Jurassic limestones from the western Subbetic Zone, a widespread remagnetisation event took place within <106 years in the entire Subbetic region during Neogene times. In contrast, in the Prebetic region, no evidence for a secondary overprint has been detected. Palaeomagnetic Jurassic declinations indicate variable and locally very large clockwise rotations (35°–140°), but the two sites in the north-westernmost part of the investigated region are not rotated. The use of both components of magnetisation and the incremental fold-test results allowed the timing of block rotations in the Subbetic Zone to be constrained. Rotations in the western Subbetic occurred after the acquisition of the secondary overprint, whereas in the central part of the Subbetic Zone they were completed by the time of the remagnetisation event.  相似文献   

15.
The ability to derive Gauss coefficients, up to and including degree 3, and their variation through a geomagnetic polarity transition is studied using simulated palaeomagnetic data. It is concluded that for a specified distribution of palaeomagnetic sites reasonable estimates of the behaviour of the coefficients can be derived even when uncertainties in the data, and in the compilation of contemporaneous records, are considered. Published palaeomagnetic records of the Matuyama–Brunhes transition are then used as basis for deriving the variation of the Gauss coefficients over a 32 kyear period encompassing the reversal. Individual records are interpolated to uniform time intervals of 0.5 kyear and put on to a common time scale by correlating between sites the variation in the latitude of VGP's through the reversal. Relative palaeointensity data are scaled by the geocentric axial dipole field intensity for 2000 at each site, and the Gauss coefficients derived by a matrix inversion employing singular value decomposition. The derived variation with time of the Gauss coefficients suggests that, over the time span of the data, the dipole and non-dipole fields have approximately equal intensities. Plots of the variation of the surface vertical magnetic field through the reversal suggest that immediately prior to the reversal a large patch of reverse flux appears in the southern hemisphere. This may subsequently have been responsible for the weakening of the vertical field leading into the reversal. A similar patch of reverse flux is observed some 20–15 kyear prior to the actual reversal and may be associated with an observed excursion in VGPs at several sites.  相似文献   

16.
Summary The present paper deals with the derivation of the intensity of the geomagnetic field in the Plio-Pleistocene, Central Europe. The intensity was derived from the thermo-remanence of porcellanites and palaeo-slags. These rocks represent a common product of intense thermal alteration of loams due to spontaneous combustion of coal seams. Porcellanites and palaeo-slags show outstanding palaeomagnetic properties, their remanent magnetization is of thermo-remanent origin and they were mineralogically stabilized under natural conditions in the geological past, during the coal combustion. It was found, that the intensity of the geomagnetic field during the Plio-Pleistocene oscillated about the value of the present-day intensity.  相似文献   

17.
Summary In order to obtain basic palaeomagnetic data on Upper Carboniferous, Permian and Triassic sediments collected from the NW Bulgaria, laboratory stability tests were extended from A.C. and thermal treatments to studies of mineral phase changes and to investigations of changes of magnetic anisotropy during laboratory procedures. Laboratory criteria were found which permitted to distinguish samples suitable for palaeomagnetic analyses from those representing rocks totally or almost totally chemically reworked during their history. Palaeomagnetic directions and pole positions derived from Stephanian, Lower Permian and Triassic rocks from the southern margin of the Moesian Platform are compatible with the values obtained for the tectonically stable North-European Platform.  相似文献   

18.
Abundances of major and trace elements were determined for the Tertiary volcanic rocks from SW Hokkaido. The Late Miocene to Pliocene volcanic rocks of this region show geochemical features similar to those of the Quaternary rocks, that is, K/Si, Th/Si and LREE/HREE ratios increasing across the arc, east to west, from the Pacific to the Japan Sea side. In contrast, the Early Miocene volcanic rocks, which are geographically restricted to the Japan Sea coast, are distinct from all later volcanics and show “within-plate” characteristics — in particular, high concentrations of HFS elements. The Quaternary basalts have low Hf/Yb ratios and Hf contents, whereas the Early Miocene basalts are high in Hf/Yb and Hf, similar to Hawaiian alkali basalts. The compositional variation with time may result from the progressive depletion of incompatible HFS elements in the mantle source. Th/Yb ratios increase from Early Miocene to Quaternary, possibly reflecting increase in the LIL element contribution to the mantle source during that time.  相似文献   

19.
A number of lode–gold occurrences are hosted by hydrothermally altered greenstones along the southern boundary of the Palaeoproterozoic Central Lapland Greenstone Belt. The hydrothermally altered and mineralised zones are related to a major thrust and shear zone system that extends much across northern Finland. Spatial correlation between mineralized zones, brittle structural features and chemical alteration was explored and identified from high-resolution aeromagnetic data, in combination with airborne electromagnetic and gamma-ray spectrometric data and coupled with petrophysical and palaeomagnetic studies. The most prominent magnetic, ductile signatures formed during the Svecofennian Orogeny (1900–1800 Ma), resulting in elastic, curved, continuous magnetic patterns. These elastic anomaly patterns were disturbed by tectonic stress from S–SW, resulting in parallel, regularly oriented fracture families and thrust faults normal to the main stress direction. According to aeromagnetic, palaeomagnetic and structural evidence, the thrust zone was active during the latest stage of the orogenic event, but was also reactivated at a later date. Airborne gamma-ray data reveals zones of potassic alteration in the ultramafic rock units in the vicinity of cross-sections of these two fault sets. Chemical and mineralogical changes during alteration and metamorphism strongly affected the mafic and ultramafic host rocks throughout the deformation zone. The strong potassium enrichment and coinciding destruction of magnetic minerals resulted in enhanced potassium concentration and reduction of magnetic anomaly amplitudes. Palaeomagnetic results indicate that the remanent magnetization for the altered ultramafic rocks along the thrust zone is of chemical origin (CRM) and was acquired at 1880–1840 Ma, which is presumed also to be the age of the chemical alteration related to gold mineralization.  相似文献   

20.
Deep in the Earth, the electrical conductivity of geological material is extremely dependent on temperature. The knowledge of temperature is thus essential for any interpretation of magnetotelluric data in projecting lithospheric structural models. The measured values of the terrestrial heat flow, radiogenic heat production and thermal conductivity of rocks allow the extrapolation of surface observations to a greater depth and the calculation of the temperature field within the lithosphere. Various methods of deep temperature calculations are presented and discussed. Characteristic geotherms are proposed for major tectonic provinces of Europe and it is shown that the existing temperatures on the crust-upper mantle boundary may vary in a broad interval of 350–1,000°C. The present work is completed with a survey of the temperature dependence of electrical conductivity for selected crustal and upper mantle rocks within the interval 200–1,000°C. It is shown how the knowledge of the temperature field can be used in the evaluation of the deep electrical conductivity pattern by converting the conductivity-versustemperature data into the conductivity-versus-depth data.  相似文献   

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