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1.
A robust finite-element technique is presented for computation of both the internal demagnetization effects and magnetic terrain effects in bodies with arbitrary shape and arbitrary susceptibility distribution. This method facilitates a flexible analysis of the palaeomagnetic deflection problem. Tests on geologically realistic settings of highly magnetic rocks demonstrate that deflections of several degrees may occur even for relatively simple two-dimensional models. Similarly, the magnetic intensity may well be biased by 5-15% by demagnetization effects. The present paper focuses on deflections and intensity variations inside the magnetized body, where we find a systematic shallowing of inclination for bodies with a horizontal elongation. Because the bodies sampled at a typical palaeomagnetic site will have a dominant direction of elongation, the magnetic deflection effect will tend to impose a systematic bias which doesn’t average out. An inversion-based procedure for elimination of the deflection effect is presented. It requires that the magnetic body is quite homogeneous and that its surface geometry is known, as may be the case for historical lava flows. Tests demonstrate that in order to recover both ambient palaeofield direction and the effective susceptibility at blocking temperature it is necessary to sample near strong topographic elements in the magnetic body. Since the surface geometry rarely is known it is proposed as an alternative to inversion that an effective susceptibility is assessed and a horizontal slab correction is applied for samples taken far from topographical features. When shape geometry is unknown and no correction applied, palaeomagnetic conclusions must take into account the possible bias from internal demagnetization and magnetic terrain effects.  相似文献   

2.
The conventional corrections for bedding dip in palaeomagnetic studies involve either untilting about strike or about some inclined axis—the choice is usually governed by the perceived fold hinge orientation. While it has been recognised that untilting bedding about strike can be erroneous if the beds lie within plunging fold structures, there are several types of fold which have plunging hinges, but whose limbs have rotated about horizontal axes. Examples are interference structures and forced folds; restoration about inclined axes may be incorrect in these cases. The angular errors imposed upon palaeomagnetic lineation data via the wrong choice of rotation axis during unfolding are calculated here and presented for lineations in any orientation which could be associated with an upright, symmetrical fold. This extends to palaeomagnetic data previous analyses which were relevant to bedding-parallel lineations. This numerical analysis highlights the influence of various parameters which describe fold geometry and relative lineation orientation upon the angular error imparted to lineation data by the wrong unfolding method. The effect of each parameter is described, and the interaction of the parameters in producing the final error is discussed. Structural and palaeomagnetic data are cited from two field examples of fold structures which illustrate the alternative kinematic histories. Both are from thin-skinned thrust belts, but the data show that one is a true plunging fold, formed by rotation about its inclined hinge, whereas the other is an interference structure produced by rotation of the limbs about non-parallel horizontal axes. Since the angle between the palaeomagnetic lineations and the inclined fold hinge is equal on both limbs in the former type of structure, but varies from limb to limb in the latter, a simple test can be defined which uses palaeomagnetic lineation data to identify rotation axes and hence fold type. This test can use pre- or syn-folding lineations and could be useful in areas of non-coaxial folding.  相似文献   

3.
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   

4.
Summary The paper contains an outline of the palaeomagnetic investigations in Hungary including some of their geophysical and geological applications. More than 500 samples of volcanic rocks were studied by usual palaeomagnetic techniques. The rocks investigated belong to three different geological periods — the Quaternary — upper Tertiary, the middle Miocene and the lower Cretaceous. The palaeomagnetic data obtained for the individual localities are summarized in the Appendix.Paper presented at the IAGA Symposium Madrid, September 1969.  相似文献   

5.
古地磁多磁成分的分离技术   总被引：1，自引：4，他引：1  
在古地磁的研究中，如何从复杂多样的退磁数据中获取能够代表岩石形成时期的古地磁场方向的原生剩磁组分，是关系到古地磁数据是否可靠、古地磁结果是否令人信服的根本环节之一，因此，如何充分利用实验室退磁数据所提供的各种信息去获取同源磁化的一组样品的原生剩磁方向是一个很有研究价值的问题。在总结前人的各种定性或定量的多磁成分分离技术的基础上，详细讨论了重磁化大圆的极和线性坳陷区内退磁平面的极的最佳拟合大圆的最小二乘拟合法，提出了综合使用主成分分析法、线性谱分析法、重磁化圆法以及退磁平面及重磁化大圆的二次拟合法，对来自同源磁化的一组样品进行系统分析，以获取一组样品的特征剩磁方向的方法，并应用于实际研究工作中。  相似文献   

6.
Summary The paper is devoted to a critical evaluation of the hypothesis of the prevalent character of the palaeomagnetic field in the course of the whole Phanerozoic. The study ties in with the synthetic processing of all so far published palaeomagnetic data for Eurasia and Africa respecting some of the models ensuing from the concept of plate tectonics. Initial material for testing the dipolar hypothesis was obtained by statistical processing of the palaeomagnetic pole positions and by analyzing the values of the so-called palaeorotation, the palaeoinclination difference and of palaeodivergence. It was shown that, by defining tectonically stable blocks (plates, palaeoplates) within the scope of the Eurasian continent and within the scope of the African continent as a whole, data were obtained bringing evidence of the predominating dipolar character of the mean geomagnetic field for the period from the Quaternary to the Cambrian, with the exception of the Silurian which has not been studied sufficiently. The deviations of the palaeomagnetic directions from the direction of the dipolar field within the scope of the stable blocks have the character of a Gaussian distribution and the standard deviations only amount to a few degrees.  相似文献   

7.
Sediments spanning the last 9000 y from two sites in lake Vatnsdalsvatn (Lat. 66°N; Long. 23°E) in northwest Iceland hold repeatable palaeomagnetic direction records. The Vatnsdalsvatn sediments have mean palaeomagnetic inclinations of 76° close to that expected for a geocentric axial dipole field, and direction fluctuations of around 20° from the mean. The palaeomagnetic directions are stable under alternating field partial demagnetization experiments. A time scale for the Vatnsdalsvatn sediments has been estimated from 14C dating. The pattern of palaeomagnetic secular change shows few similarities with British records 2000 km distant and a central North American record 5000 km distant.  相似文献   

8.
Two hundred and nineteen palaeomagnetic results are combined in calculating Gondwanan apparent polar wander paths, their confidence intervals, and the residual mean square errors about the paths, for six reconstructions.A quantitative assessment of the six reconstructions of Gondwanaland is made on the basis of the consistency of palaeomagnetic pole positions using a new statistical technique. Smith and Hallam's [1] Gondwanan reconstruction is the most effective in accounting for the distribution of the palaeomagnetic data and is used in calculating our preferred apparent polar wander path.  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.
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   

11.
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   

12.
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   

13.
Of 16 sites collected in the Taru grits (Permian) and Maji ya Chumvi beds (Permo-Triassic) of East Africa only 6 sites from the Maji ya Chumvi sediments gave meaningful palaeomagnetic results. After thermal cleaning the 6 sites (32 samples) give an Early Triassic pole at 67°N, 269°E with A95 = 17° in excellent agreement with other African Mesozoic poles. There are now 26 Mesozoic palaeomagnetic poles for Africa from widely diverse localities ranging in present latitude from 35°N to 30°S. The poles subdivide into Triassic (17 poles) and Cretaceous (9 poles) groups whose means are not significantly different. The palaeomagnetic pole for Africa thus remained in much the same position for 170 m.y. from Early Triassic to Late Cretaceous. The data form an especially good set for estimating the palaeoradius using Ward's method. Values of 1.08 ± 0.15 and 1.03 ± 0.19 times the present radius are deduced for the Triassic and Cretaceous respectively with a mean value of 1.08 ± 0.13 for all the Mesozoic data combined. The analysis demonstrates that hypotheses of earth expansion are very unattractive.  相似文献   

14.
Increases in the production rate of cosmogenic radionuclides associated with geomagnetic excursions have been used as global tie-points for correlation between records of past climate from marine and terrestrial archives. We have investigated the relative timing of variations in 10Be production rate and the corresponding palaeomagnetic signal during one of the largest Pleistocene excursions, the Iceland Basin (IB) event (ca. 190 kyr), as recorded in two marine sediment cores (ODP Sites 1063 and 983) with high sedimentation rates. Variations in 10Be production rate during the excursion were estimated by use of 230Thxs normalized 10Be deposition rates and authigenic 10Be/9Be. Resulting 10Be production rates are compared with high-resolution records of geomagnetic field behaviour acquired from the same discrete samples. We find no evidence for a significant lock-in depth of the palaeomagnetic signal in these high sedimentation-rate cores. Apparent lock-in depths in other cores may sometimes be the result of lower sample resolution. Our results also indicate that the period of increased 10Be production during the IB excursion lasted longer and, most likely, started earlier than the corresponding palaeomagnetic anomaly, in accordance with previous observations that polarity transitions occur after periods of reduced geomagnetic field intensity prior to the transition. The lack of evidence in this study for a significant palaeomagnetic lock-in depth suggests that there is no systematic offset between the 10Be signal and palaeomagnetic anomalies associated with excursions and reversals, with significance for the global correlation of climate records from different archives.  相似文献   

15.
Palaeomagnetic remanence may be reset by orogeny in a manner analogous to isotopic systems. The heavy mineral seams and associated psammite in the deformed and metamorphosed Lower Morar Division of the Moine Assemblage of northwest Scotland have a palaeomagnetic signature that is characteristic over a wide area. The various components of the magnetization are resolved by detailed thermal demagnetization. These components are post-tectonic, as they are not affected by any structural fabric. They may be classified according to blocking temperature and, when statistically distinguishable, show a systematic change of direction from high to lower temperature possibly associated with apparent polar wander. Comparison to the European palaeomagnetic record suggests a late Silurian to early Devonian age of magnetization for the localities south of the Strath Conon Fault. The distribution of remanance directions and blocking temperatures is not consistent with the pattern of potassium-argon ages in this part of the British Caledonides. If the magnetization is purely of thermal origin, the palaeomagnetic record indicates higher temperatures at later time than does the potassium-argon record. This remagnetization event is related to the closing stages of the Caledonian orogeny.  相似文献   

16.
华北蓟县中、上元古界古地磁研究   总被引：12，自引：0，他引：12  
本文论述了华北蓟县中、上元古界地层标准剖面古地磁采样及实验室技术,采样地层的磁性特征,多磁成分的分析和测试结果.主要讨论了蓟县中、上元古界地层所代表的古地磁极性、极移路径和古纬度,并与北美大陆及华南(扬子)地块该时期的极移路径进行对比.  相似文献   

17.
Constraining the process by which volcanoes become unstable is difficult. Several models have been proposed to explain the driving forces which cause volcanic edifices to catastrophically collapse. These include models for destabilisation of volcanic flanks by wedging due to dyke intrusion and the weakening of mechanical properties by pressurisation of pore fluids. It is not known which, if any, of the models are relevant to particular sector collapse events. Recent developments in the palaeomagnetic estimation of emplacement temperatures of volcaniclastic rocks have shown that even relatively low emplacement temperatures can be recorded by volcaniclastics with high fidelity. We have carried out a palaeomagnetic study of emplacement temperatures to investigate the role of igneous activity in the initiation of the 9,500 b.p. Murimotu sector collapse of Mt Ruapehu, New Zealand. This debris avalanche deposit has three fades which are stratigraphically superimposed, and the lowermost fades contains three lithological assemblages representing different segments of the edifice which were transported with little internal mixing within the flow. We have determined that some of the dacite-bearing assemblage 1, fades 1 was hot (∼350 °C) during transport and emplacement, whereas none of the other lithological assemblages of fades contained hot material. Our interpretation is that a dacite dome was active on the ancient Ruapehu edifice immediately prior to the Murimotu sector collapse. The partially cooled carapace of the dome and material shed from this part was incorporated into the avalanche deposit, along with cold lavas and volcaniclastics. We have not found evidence for incorporation of material at or close to magmatic temperatures, at least in the sampled locations. Our palaeomagnetic work allows us to develop a comprehensive, new palaeomagnetic classification of volcaniclastics. Published online: 25 January 2003 Editorial responsibility: D. Dingwell  相似文献   

18.
Samples of Upper Devonian sedimentary ironstones from the eastern Hindukush, Chitral (Pakistan), give a characteristic palaeomagnetic direction: declination D = 318°, inclination I = ?6.5°; believed to represent the primary magnetization direction. The samples come from an area which lies north of a major ophiolite zone that recent workers suggest is the southwestern continuation of the Indus Suture. As the present palaeomagnetic results are in fair agreement with palaeomagnetic data from the Siberian platform but not with data from Gondwanaland they can be taken as additional evidence that this suture does indeed constitute the main collision zone between the Gondwanic Indian subcontinent and Asia. The palaeomagnetic data presented here from the Devonian of Chitral suggests additionally: (1) in excess of 100° of counterclockwise rotation of the area, associated most likely with the formation of the regional Hindukush-Pamir-Karakoram syntaxial bend; (2) more than 2000 km of crustal shortening between Chitral and the Siberian platform due to the northward indentation of the Indian Gondwanaland fragment subsequent to collision.  相似文献   

19.
Davie fracture zone and the movement of Madagascar   总被引：1，自引：0，他引：1  
A curvilinear feature extending over 21° of latitude can be recognised in the western Indian Ocean. Its structure and relationship to Madagascar suggest that it is a fracture zone that may have controlled the motion of Madagascar as the island drifted southwards to its present position. The pole of rotation defined by the fracture zone does not allow a good fit between the late Palaeozoic to early Mesozoic palaeomagnetic poles for Africa and Madagascar [3] if the latter was in a position against Kenya at the time. The likely presence of the fracture zone needs to be reconciled with the palaeomagnetic results.  相似文献   

20.
Thermal and alternating-current demagnetization combined with ore microscopy and measurements of the temperature dependence of saturation magnetization have been carried out on some Mesozoic, probably Cretaceous, basaltic lavas from two areas (Seidfjell and Sørlifjell) at Spitsbergen. The experimental studies suggest that the Seidfjell locality has undergone extensive oxidations, which resulted in remagnetization. The estimated palaeomagnetic pole for this area is 77°N 107°E, which suggests a remagnetization, probably some time in the Late Tertiary. On the other hand the experimental data from the Sørlifjell locality suggest that the magnetization is primarily of deuteric origin. The mean palaeomagnetic pole position for this latter formation is at 75°N 235°E, which is significantly different from previously published European Mesozoic data. However, closing the Neo-Arctic basin by rotating Spitsbergen towards the Lomonosov Ridge, makes the suggested Cretaceous pole coincide with poles of similar age from North-America. This suggests that the estimated pole from Sørlifjell is a good approximation for a Late Mesozoic palaeomagnetic pole for Europe and it also confirms that the process of continental separation in the Arctic has taken place in Tertiary time.  相似文献   

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