共查询到20条相似文献，搜索用时 48 毫秒
1.
George E. Backus 《Geophysical Journal International》1977,51(1):27-45
Let ζΛ and r Λ . be the hypocentral position and time of an extended indigenous seismic source. Backus showed that the force moment tensors of the source, Γ( m +1, n ) (ζΛ , r Λ ), determine and are determined by the motion which the source produces. For small m + n , only the long-period motion is relevant. The glut moment tensor Λ( m,n ) (ζΛ , r Λ .) can be calculated uniquely from γ( m +1, n ) (ζΛ r Λ ) only if m = 0 or m = 1. The tensor G =Λ(2,0) (ζΛ ) gives the spatial variance tensor WΛ of the source, and WΛ . roughly describes the size, shape and orientation of the source region. Therefore the failure of the observed F =Γ(3,0) (ζΛ ) to determine G uniquely is of seismological interest. In the present paper we show that F determines G uniquely if we assume the source to be a simple straight line source (SSLS) or an ideal fault in an isotropic medium with isotropic prestress (IFIMIP). We give tests on F which determine whether it can come from a SSLS, from an IFIMIP or from a simple plane surface source (SPSS). If we assume the source to be a SPSS then knowing F and the fault plane determines G to within an unknown scalar multiple of a certain tensor tangent to the fault plane. Moreover F determines the fault plane uniquely unless F can come from a SSLS. If it can, then F determines this virtual source line uniquely, and F permits the fault plane to be any plane containing the virtual source line. 相似文献
2.
J. P. Cogné 《Geophysical Journal International》1987,91(3):1013-1023
Anisotropic assemblages of multidomain magnetite particles develop an anisotropy of magnetic susceptibility (AMS), which in turn induces deviations of thermo-remanent magnetization (TRM) from the field direction. From the theories of multidomain TRM acquisition, it is shown that the TRM anisotropy tensor has its eigenvalue ratios ( T i ) related to the principal weak-field susceptibility ratios ( P i ) by the order of magnitude T i ≃ P 2 i . This relation has been experimentally verified on two sets of highly anisotropic rock samples. The exponent has been determined to be 1.94 in the samples from a Peruvian gabbro, and 1.81 in those from the granite of Flamanville (NW France). Accounting for experimental difficulties in determining the TRM anisotropy tensors, these exponents are judged to agree well with the expected one. It is therefore stressed that AMS measurements provide a good means of evaluating the magnetic field direction from deviated TRM directions, providing magnetic carriers are mainly multidomain magnetites. 相似文献
3.
Magnetic susceptibility ( χ ) variations and the behaviour of the ratio of susceptibility to saturation magnetization ( χ/J s ) along the loess/palaeosol section at Koriten (NE Bulgaria) are used to deduce climatic changes during the Pleistocene in southeastern Europe. A good correlation of susceptibility variations with the astronomically tuned oxygen isotope record from ODP site 677 enables us to propose a more precise dating of the upper part of the Bulgarian loess complex. Close correspondence between susceptibility and δ 1 8 O records demonstrates the global significance of the palaeoclimatic signal recorded, although differences in relative amplitudes of χ and χ/J s and δ 1 8 O create difficulties in making quantitative estimates of the climatic humidity in the past. The role of local factors affecting the palaeoclimatic mineral magnetic record deduced from the profile studied in Bulgaria is discussed. 相似文献
4.
The traveltime perturbation equations for the quasi-compressional and the two quasi-shear waves propagating in a factorized anisotropic inhomogeneous (FAI) media are derived. The concept of FAI media simplifies considerably these equations. In the FAI medium, the density normalized elastic parameters a ijkl ( X i ) can be described by the relation a ijkl ( X i ) = f 2 ( x i ) A ijkl , where A ijkl are constants, independent of coordinates x i and f 2 ( x i ) is a continuous smooth function of x i . The types of anisotropy ( A ijkl ) and inhomogeneity [ f ( x i )] are not restricted. The traveltime perturbations of individual seismic body waves ( q P , qS 1 and qS 2) propagating in the FAI medium depend, of course, both on the structural pertubations [δ f 2 ( x i )] and on the anisotropy perturbations (δ A ijkl ), but both these effects are fully separated. The perturbation equations for the time delay between the two qS -waves propagating in the FAI medium are simplified even more. If the unperturbed (background) medium is isotropic, the perturbation of the time delay does not depend on the structural perturbations (δ f 2 ( x i ) at all. This striking result, valid of course only in the framework of first-order perturbation theory, will simplify considerably the interpretation of the time delay between the two split qS -waves in inhomogeneous anisotropic media. Numerical examples are presented. 相似文献
5.
Summary. The Cordova gabbro of southern Ontario intrudes 1300 Myr old volcanic rocks of the Hastings Lowlands in the Grenville Structural Province. Three distinct vector magnetizations (A, B and C) have been isolated, using a combination of stable endpoints, subtracted vectors from orthogonal vector plots and converging remagnetization circles. The A magnetization, with mean direction D = 294° I =– 55.5° ( k = 42, α95 = 5.5°, N = 18 sites), is a high coercivity, high blocking temperature remanence recorded by 49 samples. The B magnetization was isolated in 33 samples and has a mean direction D = 305.5° I =– 1.5° ( k = 24, α95 , N = 11 sites). B has lower coercivities and blocking temperatures than A where the two are superimposed. The A and B palaeopoles, 151°E, 10.5°S ( dp = 6°, dm = 8°) and 165.5°E, 24°N ( dp = 5°, dm = 9.5°), fall on the Grenville Track around 900 and 820 Ma respectively. The A and B magnetizations thus date from uplift and cooling following the Grenvillian orogeny. The third magnetization, the C component, has been isolated in 23 samples. Its mean direction is D = 180° I = 27.5° ( k = 18, α95 = 10.5°, N = 12 sites). The C is a low coercivity, low blocking temperature overprint of A and B. Its palaeopole, 102°E, 31°N ( dp = 6.5°, dm = 12°), is unlike post-1300 Precambrian poles for cratonic North America but matches Silurian and late Ordovician poles. 40 Ar/39 Ar plateau ages of 446 and 447 Ma determined by Lopez-Martinez and York for plagioclases from one of the Cordova samples confirm this age assignment. The C magnetization therefore records a previously unrecognized mild thermal or hydrothermal event that occurred in Palaeozoic time, long after the Grenvillian orogeny. 相似文献
6.
Summary. Fifty-six orientated samples were collected from 13 sites on five dolerite dykes (between lat.14°23°N, long.77°43'E and lat.14°08'N, long. 77°49'E), which adjoin the south-western margin of the Cuddapah basin in Anantapur district, Andhra Pradesh. After af demagnetization, two dykes (five sites) striking ENE possess similar magnetic directions, (1) D = 57°, 1=-69° (K = 52, α95 = 7°) and (2) D = 71°, I = -72° ( K = 260, α95 = 5°). Again dykes (3) (three sites), and (4) (two sites) have similar strike (NE) and magnetic directions, D = 64°. I=-7 α( K = 142, α95 = 8°) and D = 53°, I =-8° (K = 142, α95 = 6°) and dyke (5) (two sites) striking NW shows D = 320°, I = -34° ( K = 68, α95 = 13°). Remanent directions estimated from total field magnetic anomaly data agree well with these results. Synthesis of these data with 10 other published palaeomagnetic studies of Precambrian dolerite dykes on the Indian peninsula, suggest that these three systems of dykes adjoining the Cuddapah basin had been emplaced prior to the basin formation perhaps representing the initial thermal event responsible for the basin development and also that there have been at least three separate periods of dyke emplacement on this shield. The radiometric data, however, are very sparse and these periods cannot be dated with confidence. 相似文献
7.
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
8.
Summary. A finite element method with uniform and variable resolution meshes is used to model very high Rayleigh number Ra thermal convection in a square box of infinite Prandtl number, Boussinesq fluid with constant viscosity and thermodynamic properties. Heating is either entirely from below or mostly from within and the boundaries are stress free. The variable mesh is coarse in the interior of the convection cell and it is fine in the very thin boundary layers and plumes surrounding the core. The highest resolution variable mesh has a dimensionless grid spacing of 0.027 in the core and 0.0017 in the boundary layers. The boundary layers contain about 10 mesh points even at the highest values of Ra considered and are thus highly resolved. The variable mesh approach is shown to yield reliable simulations of convection as long as the aspect ratio of the most elongated boundary layer elements is not too large; values of about 4 to 6 work well. This aspect ratio also measures the increase in resolution in the boundary layers as compared with the central core. Steady single-cell rolls are computed for bottom heating and Ra up to 5 × 105 times the marginal instability value of the Rayleigh number Racr . One and two-cell roll solutions are calculated for f = 1, 0.8 and 0.6, where f is the fraction of the heat escaping through the top of the box that is generated internally. The values of Racr for f = 1, 0.8 and 0.6 are 1296, 1024 and 864, respectively. The largest of Ra/Racr at which unicellular convection is stable (steady) are approximately 390, 610 and 970, for f = 1, 0.8 and 0.6. 相似文献
9.
Anthony Arendt 《Geografiska Annaler: Series A, Physical Geography》1999,81(4):477-487
Broadband surface albedo measurements, made during the summer melt season at three weather stations on John Evans Glacier (79°40 ' N, 74°00 ' W), varied strongly with the solar zenith angle ( θ z ). Tests were carried out to assess the impact of diurnal variations in surface albedo on seasonal net shortwave radiation ( K * ) totals. Removing the diurnal signal from albedo measurements by daily averaging of hourly measurements, or by applying midday measurements to all hours of the day, changed K * by up to 16%. Ignoring measurements made at θ z & 75°, to account for measurement (cosine) error at high θ z , decreased K * by between 5 and 18%. Given the sensitivity of K * to diurnal patterns in surface albedo, experiments were carried out with two albedo models. One model accounted for albedo variations with θ z and one did not. The model driven by θ z , when implemented within a surface energy balance model for John Evans Glacier, produced better melt estimates. This suggests that diurnal variations in surface albedo should be accounted for in energy balance models of glacier melt. 相似文献
10.
David J. Dunlop Hironobu Hyodo Todd Knight Alan G. Steele 《Geophysical Journal International》1985,83(3):699-720
Summary. After thermal and alternating field (AF) cleaning, the characteristic high blocking temperature A component of natural remanent magnetization (NRM) of the Tudor gabbro of southern Ontario has a mean direction D = 326°, I =–46° ( k = 132, α95 = 4.8°, N = 8 sites). The corresponding palaeopole, 133°E, 12°N ( dp = 4°, dm = 6°), confirms the palaeopole 137°E, 17°N (α95 = 8.4°) reported earlier by Palmer & Carmichael, based on AF cleaning only. The A NRM has unblocking temperatures > 515–525°C which exceed the estimated 500°C peak temperature reached locally during ∼ 1050 Ma Grenvillian regional metamorphism. The A NRM therefore predates metamorphism and is probably a primary thermoremanence (TRM). The age of the Tudor NRM has previously been taken to be about 675 Ma, but recent 40 Ar/39 Ar dating by Baksi has shown that this is the time of post-metamorphic cooling to 200–250°C. Hornblendes record initial cooling of the intrusion to 590±20°C at 1110 Ma and this is the best estimate of the age of the A remanence. Successful Thellier-type palaeointensity determinations on 11 Tudor samples confirm that the A NRM is a TRM and indicate a palaeofield at this time of 18–27 μT, about 50–70 per cent of the present field intensity at 27° magnetic latitude. The anomalous Tudor A palaeopole, which lies well to the west of both 1000–800 Ma Grenvillian palaeopoles and 1100–1050 Ma poles from Interior Laurentia, is interpreted as recording divergence between Grenvillia and Interior Laurentia just before the Grenvillian orogeny, rather than a post-metamorphic extension of the apparent polar wander path as previously assumed. 相似文献
11.
Silvia Gonzalez Graham Sherwood Harald Böhnel Elisabeth Schnepp 《Geophysical Journal International》1997,130(1):201-219
13 lava flows of known age (ages from 14 C dating), which have been erupted in the last 30 000 years, have been studied to determine the palaeosecular variation of the geomagnetic field in Central Mexico. Samples were taken from two different monogenetic volcanic fields: the Michoacan-Guanajuato volcanic field (six sites) and the Chichinautzin Formation (seven sites), both part of the Transmexican Volcanic Belt. The lavas were studied in detail using rock magnetic methods (magnetic susceptibility at room temperature, low-temperature susceptibility behaviour, hysteresis loops, Curie temperatures), combined with reflected light microscopy, in order to deduce their magnetic mineralogy and the domain states of the magnetic minerals. The magnetic carriers are titanomagnetites, which show differing degrees of high-temperature deuteric oxidation, and seem to be predominantly pseudo-single domain (PSD), though in many cases are probably a mixture of domain states. Mean palaeomagnetic directions and palaeointensity values using Shaw and Thellier techniques were obtained using several specimens from each flow. Our data seem to indicate a sharp easterly swing in declination about 5000 years ago, which is also observed in lake sediments from Central Mexico. The calculated values of the virtual dipole moment (VDM) range from 3.1 to 14.9 × 1022 A m2 . Our data indicate that the virtual dipole moment seems to have increased gradually in magnitude over the last 30 kyr, with a peak at about 9000 years BP. These are features that have been observed in other parts of the globe and are probably caused by variations in the dipole part of the geomagnetic field. 相似文献
12.
Summary. Palaeomagnetic investigations were carried out on nine lava flows around the Dhar region, which constitute the northern part of the Deccan traps. The stability of remanent magnetism of these specimens was analysed by an alternating magnetic field, thermal demagnetization and memory tests. Six flows exhibited characteristic components of magnetization, with a mean direction of D =143°, I =+46° ( K = 107.1, α95 =5.5°). This gives a VGP located at 29° N, 67° W (δp =4.5°, δm = 7.0°). The lower site with normal magnetization and the upper five sites with reverse magnetization indicate a geomagnetic field reversal during the initial phases of Deccan volcanism in the Early Tertiary period. A rapid northward migration of about 18° in latitude and a simultaneous anticlockwise rotation of 37° is calculated for the subcontinent. 相似文献
13.
N. Abrahamsen J. R. Wilson P. Thy N. Ø. Olesen K. H. Esbensen 《Geophysical Journal International》1979,59(2):231-248
Summary. Palaeomagnetic results are presented from the c . 160 km2 Caledonian synorogenic layered Fongen-Hyllingen gabbro complex (of probable late Silurian age) located about 75 km SE of Trondheim, Norway, in the allochthonous Seve-Kdli Nappe Complex. A total of 80 oriented samples from eight sites in the northern part of the gabbro were investigated. After detailed af demagnetization two stable high coercivity components emerge: one with a well defined NW direction with D =325°, I =−21° (α95 =8°, N =8), and another, less well defined, probably younger, SW direction with D = 237°, I = 6° (α95 = 9°, N = 8). Correction for dip of these two directions gives D = 329°, I =−7° (α95 = 10°) and D = 238°, I =−11° (α95 = 12°), respectively. The corresponding pole positions are P 1 : 19° N, 225° E and P 2 : 19° S, 308° E, respectively. The reversed pole -P 2 of the SW direction lies close to other NW European palaeomagnetic poles of Caledonian, Upper Silurian-Lower Devonian age. However, the dominant pole PI is far away from these, and could be due to a late Caledonian geomagnetic excursion of considerable duration; or it could record a c . 90° rotation around a vertical axis of a crustal block within the Scandinavian Caledonides. Block rotation could have been related to nappe translation, although geological observations do not at present appear to support the occurrence of such an event. 相似文献
14.
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
15.
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
16.
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
17.
S. A. Vincenz M. Jele&#;ska K. Aiinehsazian K. Birkenmajer 《Geophysical Journal International》1984,78(3):751-773
Summary. Two late Mesozoic dolerite sills, situated near Agardhbukta on the east coast of Vestspitsbergen and dated radiométrically at 100 ± 4 Myr BP, have been sampled in five localities and subjected to detailed mineralogical and rock magnetic studies to determine the direction and origin of their magnetization. Although the sills lie outside the Tertiary orogenic belt, one locality (no. 4) has undergone strong hydrothermal alteration and a small part of another locality (no. 3) has also been affected. A conventional procedure based on examination of Zijderveld diagrams, applied to specimens demagnetized by alternating fields and thermally, yielded similar remanence directions at all five localities, except at the altered part of locality 3. Using a least squares computer méthod of analysis of step demagnetization data, comparable directions were isolated from all localities, including the altered part of locality 3. Except in this last case, all directions were reversed. The adjusted mean direction obtained from this analysis is D = 159.0°, I = 62.2°, α95 = 9.0° yielding a palaeomagnetic pole situated at 225.0°, 54.3°N comparable with pole positions obtained from other late Mesozoic igneous rocks on Spitsbergen and distinct from palaeopoles derived from Mesozoic rocks in North America and Eurasia. This suggests that during the late Mesozoic Svalbard existed as a semi-independent microplate. 相似文献
18.
Summary. Attenuation of earthquake intensities with epicentral distance was studied by analysing the intensity data for 39 earthquakes in the United States. Attenuation of MM intensity ( I ) with distance (Δ km) follows a simple relation of the type log I = log I 0 - m Δ, where I 0 is the intensity at the epicentre and m is a constant. Slope m is found to be inversely proportional to the square of the focal depth. Intensity attenuation pattern in the United States in general can be represented by a unified relation I/I 0 = exp [-(0.8999/ h 2 + 0.0014)Δ] where 16km ≤ h ≤ 60km. Intensities were calculated with the help of this equation and a good agreement with the observed intensities were found. A comparative study has also been made between the attenuation relations applicable to India and the United States. 相似文献
19.
Vincenzo Cuomo Vincenzo Lapenna Maria Macchiato Carmine Serio & Luciano Telesca 《Geophysical Journal International》1999,139(3):889-894
We explore the inner dynamics of daily geoelectrical time series measured in a seismic area of the southern Apennine chain (southern Italy). Autoregressive models and the Higuchi fractal method are applied to extract maximum quantitative information about the time dynamics from these geoelectrical signals. First, the predictability of the geoelectrical measurements is investigated using autoregressive models. The procedure is based on two forecasting approaches: the global and the local autoregressive approximations. The first views the data as a realization of a linear stochastic process, whereas the second considers the data points as a realization of a deterministic process, which may be non-linear. Comparison of the predictive skills of the two techniques allows discrimination between low-dimensional chaos and stochastic dynamics. Our findings suggest that the physical systems governing electrical phenomena are characterized by a very large number of degrees of freedom and can be described only with statistical laws. Second, we investigate the stochastic properties of the same geoelectrical signals, searching for scaling laws in the power spectrum. The spectrum fits a power law P ( f )∝ f −α , with the scaling exponent α a typical fingerprint of fractional Brownian processes. In this analysis we apply the Higuchi method, which gives a linear relationship between the fractal dimension D Σ and the spectral power law scaling index α : D Σ =(3− α )/2. This analysis highlights the stochastic nature of geoelectrical signals recorded in this seismic area of southern Italy. 相似文献
20.
We report on a detailed palaeomagnetic study of the Miocene Farellones volcanic formation in the Chilean Andes near Santiago (two sections, 37 sites, about 400 orientated cores). Petrological observations show evidence of low-grade metamorphism increasing downwards through the volcanic sequence. Optical observations of opaque minerals and magnetic experiments suggest that in many cases maghemitization is associated with hydrothermal alteration. However, thermal demagnetization data indicate that the low-grade metamorphism did not significantly modify the direction of the primary remanent magnetization recorded at the time of emplacement of the volcanic lava flows. Four intervals of polarity with two intermediate palaeodirections were observed in the ~650-m-thick composite section. According to the dispersion of flow average directions, palaeosecular variation was slightly larger than that observed in general during the Upper Cenozoic. The site mean directions obtained in this study differ significantly from the expected Miocene direction. Clockwise rotations of up to 20° of small blocks are probably associated with the deformation of the Andean Cordillera since middle Miocene times. Geomagnetic palaeointensity data were obtained, using the Thellier method, on 24 samples from eight distinct lava flows. The flow mean VDM varies from 1.4 to 4.0 × 1022 A m−2 . Altogether, our data seem to suggest the existence of a relatively low geomagnetic field undergoing large fluctuations. Although a linear relationship was observed between the natural remanent magnetization and the thermal remanent magnetization acquired during the Thellier–Thellier experiments, undetected chemical alteration of the magnetic minerals during hydrothermalism may also explain the unusually low palaeointensity obtained. 相似文献