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1.
Relationship between median destructive field and remanent coercive forces for dispersed natural magnetite, titanomagnetite and hematite 总被引：4，自引：0，他引：4
Peter Dankers 《Geophysical Journal International》1981,64(2):447-461
Summary. Remanent acquisition curves, remanent hysteresis curves and alternating field demagnetization curves were determined for a number of artificial rock specimens containing well-defined grain-size fractions between 5 and 250 μm of natural magnetite, titanomagnetite and hematite. From these curves, the remanent acquisition coercive force H 'cr , the remanent coercive force H cr and the median destructive field of IRM H ½I were determined. Theoretically these parameters should be the same for an assembly of non-interacting, homogeneously distributed, randomly oriented single-domain grains. For a given hematite specimen H 'cr , H cr and H ½I have about the same value in spite of the strong grain-size dependence of these parameters. For each specimen of magnetite and titanomagnetite the value of H 'cr is larger than H cr which again is larger than H ½I . However, the ratios H 'cr / H cr and H ½I / H cr appear to have a (different) constant value. An interesting relationship which appears to hold for dispersed magnetite, titanomagnetite or hematite grains between 5 and 250 μm, independently of grain-size, quantity and packing density of the magnetic material, is: 相似文献
2.
Özden Özdemir 《Geophysical Journal International》2000,141(2):351-356
The temperature dependence of coercive force H c was studied on well-characterized and stoichiometric millimetre-sized single crystals of magnetite at a series of 16 temperatures from 300 to 10 K using a SQUID magnetometer. H c decreases gradually with cooling to the isotropic temperature, T i = 130 K, where the first magnetocrystalline anisotropy constant K 1 becomes zero. H c exhibits a sharp increase at the Verwey transition, T v = 120 K, where the structure changes from cubic to monoclinic. In crossing the Verwey transition, H c increases by more than two orders of magnitude, from 20 μT to 2.4 mT, and the shape of the hysteresis loops becomes wasp-waisted.
Observed coercivity between 300 K and 170 K varies with temperature as λs / M s , where λ s is the magnetostriction constant and M s is the saturation magnetization, indicating that the coercivity in MD magnetite is controlled mainly by internal stress associated with dislocations or other crystal defects. It seems likely that the stable single-domain-like magnetic memory observed in large MD magnetite crystals is due to magnetoelastically pinned domain walls. The discontinuous change in H c at the Verwey transition is controlled by abrupt changes in magnetocrystalline and magnetostriction constants due to crystal deformation from cubic to monoclinic structure. 相似文献
Observed coercivity between 300 K and 170 K varies with temperature as λ
3.
Detailed geological observations and palaeomagnetic analyses were carried out in the Largentière Stephano–Autunian basin and on the Stephanian deposits of the Alès coalfield, both located at the southeastern margin of the French Massif Central. Because of unfavourable rock types, the Alès Stephanian deposits did not yield any results. The palaeomagnetic pole (164.9°E, 45.4°N, K = 89, A 95 = 4.1°) deduced from a study of the Autunian sediments of the Largentière Basin agrees very well with the reference pole for stable Europe. The Lodève–Largentière area, that is the southeastern border of the Massif Central, has been stable since Early Permian time with respect to stable Europe, whereas the western part (the Saint-Affrique Rodez Basin and, probably, the Brive Basin) has been rotated counterclockwise. 相似文献
4.
V.M. Luboshits 《Geophysical Journal International》1987,91(2):349-354
Summary. It is shown that at any orientation of a dipole of direct current at the surface of a uniform anisotropic halfspace there exists a unique critical value ηcr of the "vertical" electric anisotropy η. For η > ηcr not all field lines are closed, and three straight field lines pass through the centre of the dipole. Also there exists a unique value ηeq > ηcr , where equatorial η is equal to zero. If η > ηeq then equatorial ρapp is negative since the electric vector is turned more than through 90° at the equator of dipole, and it is shown that ηcr = 1.79, ηeq = 1.93 if the axis of dipole is directed symmetrically with respect to the axes of anisotropy. Routine observations of variations of ρapp with the aim of earthquake prediction should be carried out taking into account the possibility of great fluctuations on η far from the epicentre of an earthquake. 相似文献
5.
J.N. Prasad S.K. Patil M. Venkateshwarlu P.D. Saraf S.C. Tripathi & D.R.K. Rao 《Geophysical Journal International》1998,133(3):519-528
A palaeomagnetic study of 115 samples (328 specimens) from 22 sites of the Mid- to Upper Cretaceous Bagh Group underlying the Deccan Traps in the Man valley (22° 20'N, 75° 5'E) of the Narmada Basin is reported. A characteristic magnetization of dominantly reverse polarity has been isolated from the entire rock succession, whose depositional age is constrained within the Cretaceous Normal Superchron. Only a few samples in the uppermost strata have yielded either normal or mixed polarity directions. The overall mean of reverse magnetization is D m =144°, I m =47° ( α 95 =2.8°, k =152, N =18 sites) with the corresponding S-pole position 28.7°S, 111.2°E ( A 95 =3.1°) and a palaeolatitude of 28°S±3°. The characteristic remanence is carried dominantly by magnetite. Similar magnetizations of reverse polarity are also exhibited by Deccan basalt samples and a mafic dyke in the study area. This pole position falls near the Late Cretaceous segment of the Indian APWP and is concordant with poles reported from the Deccan basalt flows and dated DSDP cores (75–65 Ma) of the Indian Ocean. It is therefore concluded that the Bagh Group in the eastern part of the Narmada Basin has been pervasively remagnetized by the igneous activity of Deccan basalt effusion. This overprinted palaeomagnetic signature in the Bagh Group indicates a counter-clockwise rotation by 13°±3° and a latitudinal drift northwards by 3°±3° of the Indian subcontinent during Deccan volcanism. 相似文献
6.
Palaeoclimatic record from a loess–soil profile in northeastern Bulgaria—I. Rock magnetic properties
A detailed record of mineral magnetic properties of a loess–palaeosol profile comprising seven loess horizons, six interbedded palaeosols and recent soil at the top in NE Bulgaria is analysed. A strong contrast between the soil and loess susceptibilities as well as other concentration-dependent hysteresis parameters is present, similar to the well-documented magnetic characteristics of the Chinese loess ( Hus & Han 1992 ; Maher & Thompson 1992 ; Heller & Evans 1995 ; Hunt et al. 1995 ). The magnetic enhancement of the palaeosol units is caused by very fine-grained pedogenic magnetite with superparamagnetic behaviour. Thermomagnetic analyses on bulk material suggest magnetite and maghemite as the main ferrimagnetic carriers in both soil and loess horizons. Their relative proportions are shown to reflect different palaeoclimatic conditions. Chernozem soils, which include recent soil S0 and first and second palaeosols S1 and S2 developed under steppe vegetation, show a high degree of low-temperature oxidation of the pedogenic magnetite to maghemite. This material is characterized by coercive force H c showing even higher values than those of the parent loess material. The older palaeosols (S4 to S6 ) were formed during more humid climatic conditions and therefore probably developed as forest types. Rock magnetic data suggest the existence here of only partly oxidized magnetite grains. The behaviour of the thermomagnetic curves, characterized by a kink at 200 °C, may be due to either a release of internal stress (built up as a result of partial low-temperature oxidation) or interactions between two phases. 相似文献
7.
A continental sequence of red beds and interbedded basaltic layers crops out in the Sierra Chica of Córdoba Province, Argentina (31.5°S, 64.4°W). This succession was deposited in a half-graben basin during the Early Cretaceous. We have carried out a palaeomagnetic survey on outcrops of this basin (147 sites in seven localities). From an analysis of IRM acquisition curves and detailed demagnetization behaviour, three different magnetic components are identified in the volcanic rocks: components A, B and X are carried by single- or pseudo-single-domain (titano) magnetite, haematite and multidomain magnetite, respectively. Component A is interpreted as a primary component of magnetization because it passes conglomerate, contact, tilt and reversal tests. The carrier of the primary magnetization, fine-grained (titano)magnetite, is present in basalts with a high degree of deuteric oxidation. This kind of oxidation is interpreted to have occurred during cooling. Components B and X are discarded because they are interpreted as recent magnetizations. In the sedimentary rocks, haematite and magnetite are identified as the carriers of remanence. Both minerals carry the same component, which passes a reversal test. The calculated palaeomagnetic pole, based on 55 sites, is Lat. 86.0°S, Long. 75.9°E ( A 95 =3.3, K =35). This palaeomagnetic pole supersedes four with anomalous positions reported in previous papers. 相似文献
8.
George Purcaru 《Geophysical Journal International》1975,42(1):61-79
Summary. In this paper some aspects of the deviations of frequency distributions of earthquakes from a linear trend are considered. For recurrence curves with inflexions, the formula
is proposed as a magnitude-frequency relation, with critical magnitude Mcr . Comparison with actual data shows better agreement than other formulae. The slope of the frequency graph is considered in the most general case to be a function of M and some parameters p 1 , p 2 , …. A classification of the types of magnitude-frequency relation is given and all known types of relations N : M and N: K ( K = log E , E = seismic energy) are summarized. 相似文献
is proposed as a magnitude-frequency relation, with critical magnitude M
9.
J. Gagnepain-Beyneix 《Geophysical Journal International》1987,89(2):681-704
Summary. Spectral attenuation of coda waves has been studied in the range 2–40 Hz from local events recorded in the western Pyrenean range from 1980 to 1982. Q c was obtained using a single scattering model of S -waves for different segments of the coda. An increase of Q c with lapse time was found and attributed to a rapid increase of Q β with depth.
Three groups of events were selected from distinct focal areas. Two data sets are mainly composed of aftershocks of moderate earthquakes of magnitude 5.1 and 4.8, respectively. No moderate earthquake occurred in the third area in the few years preceding or following the selected events. Use of stations close to epicentres allowed sampling of the coda at very short lapse times and then study of small, distinct scattering volumes. Noticeable differences were found between the three studied areas and attributed to spatial rather than temporal variations.
The Qc frequency dependence was studied according to Q c = qf α . α is found to range from 0.7 to 1.1 and q from 30 to 140. These values are in agreement with those found in other tectonic areas. It is shown that scattering is the dominant attenuation process below 10Hz. 相似文献
Three groups of events were selected from distinct focal areas. Two data sets are mainly composed of aftershocks of moderate earthquakes of magnitude 5.1 and 4.8, respectively. No moderate earthquake occurred in the third area in the few years preceding or following the selected events. Use of stations close to epicentres allowed sampling of the coda at very short lapse times and then study of small, distinct scattering volumes. Noticeable differences were found between the three studied areas and attributed to spatial rather than temporal variations.
The Q
10.
Non-linear altimetric geoid inversion for lithospheric elastic thickness and crustal density 总被引：1，自引：0，他引：1
The inversion of high-resolution geoid anomaly maps derived from satellite altimetry should allow one to retrieve the lithospheric elastic thickness, T e , and crustal density, c . Indeed, the bending of a lithospheric plate under the load of a seamount depends on both parameters, and the associated geoid anomaly is correspondingly dependent on the two parameters. The difference between the observed and modelled geoid signatures is estimated by a cost function, J , of the two variables, T e and c . We show that this cost function forms a valley structure along which many local minima appear, the global minimum of J corresponding to the true values of the lithospheric parameters. Classical gradient methods fail to find this global minimum because they converge to the first local minimum of J encountered, so that the final parameter estimate strongly depends on the starting pair of values ( T e , c ). We here implement a non-linear optimization algorithm to recover these two parameters from altimetry data. We demonstrate from the inversion of synthetic data that this approach ensures robust estimates of T e and c by activating two search phases alternately: a gradient phase to find a local minimum of J , and a tunnelling phase through high values of the cost function. The accuracy of the solution can be improved by a search in an iteratively restricted parameter subspace. Applying our non-linear inversion to the Great Meteor Seamount geoid data, we further show that the inverse problem is intrinsically ill-posed. As a consequence, minute geoid (or gravity) data errors can induce large changes in any recovery of lithospheric elastic thickness and crustal density. 相似文献
11.
Michel Westphal Jean-Pierre Pozzi Yao Xiu Zhou Li Sheng Xing Xian Yao Chen 《Geophysical Journal International》1983,73(2):507-521
Summary. A palaeomagnetic study of Middle to late Cretaceous redbeds from Linzhou basin (Lhasa block), north of the Yarlung Zangbo suture zone, gives a stable palaeomagnetic direction of magnetization with a positive fold-test: six sites, 57 samples, D = 333°, I = 38°, k = 78, α95 = 8°, pole 64°N, 348°E. We discuss the problem of a possible remagnetization but consider that this direction of magnetization gives a good approximation for the palaeolatitude of the Lhasa block during Middle to late Cretaceous time. Results from more recent Tibetan formations are also presented: late Cretaceous to Palaeocene sediments and volcanics give a lower palaeolatitude of 10° N and but more recent andesites have emplaced about 30°N, close to the present-day latitude. An interpretation is proposed whereby the Lhasa block, which was a part of Asia in the early Cretaceous, has undergone first a southward motion accompanied by an anticlockwise rotation and then, after the Palaeocene, a northward motion under the constraint of the colliding India. 相似文献
12.
Review of the low-temperature magnetic properties of magnetite from a rock magnetic perspective 总被引：4，自引：0，他引：4
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
13.
A paleomagnetic study of the Mull lava succession 总被引：1，自引：0，他引：1
Summary. A paleomagnetic study has been made of a succession of 139 non-overlapping basaltic lavas, representing 91 per cent of the longest remaining succession of flows associated with the Paleogene Mull volcano. All the lavas have experienced considerable hydrothermal alteration, probably at up to several million years after initial magnetization and frequently with alteration to the opaque minerals and the production of new potentially magnetic phases. The question of whether directional remagnetization has taken place while preserving within-unit directional consistency and discreteness of unit mean direction is discussed. Extensive directional remagnetization is excluded as an explanation for the data. If stable directions obtained by alternating field remanence cleaning coincide with original TRM directions then a mean pole position for all temporally independent lava directions from the British Tertiary igneous province is at 71.9° N, 167.2° E, with k:22 and α95 :3.0°. This pole is significantly different from the geographic pole. If the difference in palaeomagnetic and geographic poles is interpreted in terms of absolute plate motion, then 2010 km of northwards motion of the western part of the Eurasian Plate, at 3.7 cm/yr, has taken place over the last 55 Myr. This motion has implications for the geological history of the Arctic and for the complexity of mantle motions. 相似文献
14.
New Silurian and Devonian palaeomagnetic results from the Hexi Corridor terrane, northwest China, and their tectonic implications 总被引：5，自引：0，他引：5
Baochun Huang Yo-ichiro Otofuji Zhenyu Yang & Rixiang Zhu 《Geophysical Journal International》2000,140(1):132-146
A total of 239 orientated drill-core samples from 23 sites were collected for palaeomagnetic study from Silurian and Devonian red beds, marlaceous sandstone, and limestone rocks in the eastern part of the Hexi Corridor, southwest Ningxia, North China. The characteristic high-temperature component resides in both haematite and magnetite. It clusters around a northwesterly and shallow to moderate downward direction and its antipode after tilt correction. The primary origin of this characteristic remanent magnetization (ChRM) is ascertained by positive fold and reversal tests at the 95 per cent confidence level. The corresponding palaeopoles, at 339.0°E, 60.1°N with A 95 = 11.2° (Silurian) and 336.0°E, 56.0°N with A 95 = 9.2° (Devonian), imply that the North China Block (NCB) had a low palaeolatitude of around 15°N in the Northern Hemisphere during the Silurian–Devonian period. Comparison with the Early–Middle Ordovician palaeopole of the NCB suggests that the NCB moved rapidly northwards by 30.8° ± 10.9° to cross the palaeo-equator during the Early–Middle Ordovician to Silurian. In combination with the palaeobiogeographical data from Ningxia, our palaeomagnetic results suggest that the NCB was located close to Australia during the Late Devonian. 相似文献
15.
Summary. We present palaeomagnetic results from the Durgapipal and Rudraprayag formations, which are basic volcanic formations in the Lesser Himalayas of Uttar Pradesh State. NRM measurements and AF demagnetization stability tests were made on specimens cored from oriented block samples collected at representative sites. Mean stable remanent magnetic directions were used for calculating the Virtual Geomagnetic Pole (VGP) positions; where necessary tectonic corrections were applied.
The virtual geomagnetic north poles were found to be located at:
A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature. 相似文献
The virtual geomagnetic north poles were found to be located at:
- (a).
Durgapipal (Permian): λ
p = 10° S, Lp = 42° W; - (b).
Rudraprayag (Silurian-Devonian): λ
p = 30° S, Lp = 12° W.
A new, continuous Phanerozoic apparent polar wandering curve for the Indian subcontinent has been plotted from the available palaeomagnetic data and the VGP positions reported in this paper. As a result, the gap in the Indian palaeomagnetic data from the Lower Carboniferous to the Cambrian has been partially filled. The locations of the pole positions for the two formations on the Phanerozoic polar wandering curve for the Indian subcontinent, have been found to coincide with the stratigraphic ages assigned to them on the basis of rather limited geological and palaeontological evidence.
The Cambrian and Permian poles for the Salt Range in the NW Himalayas and the Permian pole for the Kumaon Himalayas are grouped along with the pole positions of contemporaneous formations of the Peninsular Shield. The palaeomagnetic data thus suggests that the two formations are autochthonous in nature. 相似文献
16.
Palaeoclimatic implications of the magnetic record from loess/palaeosol sequence Viatovo (NE Bulgaria) 总被引：1，自引：0，他引：1
Loess-palaeosol deposits in the lower Danube area represent the southeastern edge of the loess cover in Europe. Detailed rock magnetic investigations of the loess/palaeosol sequence in Viatovo in NE Bulgaria reveal that magnetite and maghemite of very fine superparamagnetic grain size are responsible for the magnetic enhancement of palaeosol units. A detailed palaeoclimatic record is obtained through high-resolution measurements of magnetic susceptibility, frequency dependent magnetic susceptibility and CaCO3 content. Magnetic proxies indicate a more warm and humid climate during the development of the older palaeosol units (S4 –S6 ). 相似文献
17.
Summary. Palaeomagnetic data from 71 hand samples of igneous rocks of Late Ordovician age exposed in western Argentina (31.3°S, 69.4°W, Alcaparrosa Formation) are given. Stable remanent magnetization was isolated in the majority of samples; they yield a palaeomagnetic pole at 56°S 33°E ( N = 8, α95 = 16°). Whole rock K-Ar age determinations yield an age of 416 ± 10 Myr for a pillow lava of the Alcaparrosa Formation.
Palaeomagnetic data for South America, Africa, Australia, Antarctica and India suggest that Gondwana was a unit at least as far back as 1000 Myr. The palaeomagnetic data define a rapid polar migration for Gondwana in Ordovician time which is consistent with the widespread occurrences of Late Ordovician glacial deposits across this supercontinent. 相似文献
Palaeomagnetic data for South America, Africa, Australia, Antarctica and India suggest that Gondwana was a unit at least as far back as 1000 Myr. The palaeomagnetic data define a rapid polar migration for Gondwana in Ordovician time which is consistent with the widespread occurrences of Late Ordovician glacial deposits across this supercontinent. 相似文献
18.
K. Kawasaki D. T. A. Symons R. M. Coveney Jr . 《Geophysical Journal International》2007,171(2):594-602
Palaeomagnetic results are reported from the metalliferous Stark black shale in the Upper Pennsylvanian (Missourian/Kasimovian) Kansas City Group. Palaeomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization in 17 sites of the shale that yields a Late Mississippian to Middle Pennsylvanian palaeopole at 32.2°N 128.5°E (dp = 4.7° and dm = 8.8°). The observed palaeomagnetic age is slightly older than the host rock, indicating that the mineralization of the Stark Shale has, excluding recent alteration, a primary sedimentary or syngenetic origin. The reason for the slightly older age is likely due to trace modern hematite that slightly steepens the remanence inclination. The large oval of 95 per cent confidence is interpreted to be caused by clay–magnetite aggregates that formed during sediment transport and the biasing effect of the gentle palaeocurrent at each site acting on the large aggregates. Therefore, the scattered distribution of the site mean remanence declinations found for the Stark Shale is evidence of a detrital remanent magnetization that is formed by primary sedimentary processes with an enriched metallic content and not remagnetization with mineralization by secondary hydrothermal processes. 相似文献
19.
José E. Mendía 《Geophysical Journal International》1978,54(3):539-546
Summary. Stable components of magnetization have been isolated in 15 lava flows (mean K-Ar age 123 ± 4 Myr) from the alkaline sequence outcropping at El Salto-Almafuerte, Province of Cordoba, Argentina. Magnetic and geologic stratigraphy, as well as K-Ar ages indicate that this sequence was probably extruded in the Lower Cretaceous during the first volcanic cycle of the Sierra de los Cóndores Group (Vulcanitas Cerro Colorado Formation).
The palaeomagnetic pole-position for El Salto-Almafuerte lava flows, computed from the mean of 15 virtual geomagnetic poles and denoted SAK7 , is: 25° E, 72° S ( k = 35, α95 = 6.5°); it is fairly close to other Lower Cretaceous palaeomagnetic poles for South America. The elongated distribution of Cretaceous palaeomagnetic poles suggest recurrent drift for South America in early Cretaceous time.
The palaeomagnetic and radiometric data for the igneous rocks from El Salto-Almafuerte support the magnetic reversal time-scale for the early Cretaceous suggested by oceanic magnetic lineations. 相似文献
The palaeomagnetic pole-position for El Salto-Almafuerte lava flows, computed from the mean of 15 virtual geomagnetic poles and denoted SAK
The palaeomagnetic and radiometric data for the igneous rocks from El Salto-Almafuerte support the magnetic reversal time-scale for the early Cretaceous suggested by oceanic magnetic lineations. 相似文献
20.
Paul W. Layer Margarita Lopez-Martinez Alfred Kröner Derek York & Michael McWilliams 《Geophysical Journal International》1998,135(1):129-145
The pluton has a well-defined palaeomagnetic pole which is dated at 3179±18 (2 σ ) Ma by