摘要:

Burg's maximum entropy spectral analysis is used in order to study monthly and annual mean values of absolute elements of the geomagnetic field at a number of world observatories. After removal of a parabolic trend, which accounts for most of the secular variation of internal origin, the residual variations clearly exhibit a worldwide character. A test study of monthly mean values at three sample observatories is first performed; earlier conclusions on both the origin and the geometry of the annual and semiannual variations are discussed. Annual mean values from approximately 30 observatories are next analyzed; most of the power found in the spectra, for periods between 2 and approximately 20 yr, is clearly related to the solar cycle (peaks around 10–12 yr and the first two harmonics). No peaks that can be related to phenomena of internal origin are found. The accuracy and resolving power of these data concerning the problem of the conductivity distribution in the deeper mantle are found to be rather disappointin

ISSN:0148-0227    

DOI:10.1029/JB081i017p02941

年代:1976

数据来源: WILEY

摘要:

Three components of remanent magnetization have been isolated by af and thermal demagnetization of highly metamorphosed dioritic and gabbro‐anorthositic rocks from Haliburton County, Ontario, in the Grenville structural province of the Canadian Precambrian Shield. The predominant A component, a normal magnetization, defines a paleoinagnetic pole at 142.5°E, 35.6°S (α95= 6.3°, k = 37.6), near southeast Australia, in agreement with published poles from widely separated areas in the Grenville Province. Paleopoles for the normal B and reversed C magnetizations fall much farther north, at 172.3°E, 24.5°N (α95= 15.7°, k = 9.4) and 167.4°E, 3.0°S (α95= 6.6°, k = 53.5), respectively. The C pole is close to secondary poles of other studies, but the B pole has not been detected elsewhere within the Grenville Province. It lies among poles for late Keweenawan rocks of the Lake Superior area. The A and B components are frequently superimposed in individual samples.The three components have distinctive coercivity and blocking temperature spectra, which we interpret as recording mineralogy and physical conditions (temperature and duration of heating) at different stages of slow cooling following high‐grade regional metamorphism of the Grenville Province. They constitute a record, from a single locality, of apparent polar wander between about 1200 and 950 m.y. The A magnetization was acquired at high temperatures, the B at much lower temperatures. Although both B and C magnetizations nay have been set during subsidiary thermal pulses, a single‐stage heating‐cooling model of Grenville metamorphism is not ruled out. The proposed age sequence is A‐C‐B, a sequence most naturally explained by convergence of separate Grenville and North American plates. However, one‐plate models which would incorporate the A‐C‐B sequence into a ‘Grenville Loop’ in the North American po

ISSN:0148-0227    

DOI:10.1029/JB081i017p02951

年代:1976

数据来源: WILEY

摘要:

Heat flow measurements in the eastern Pacific now total over 800, a sufficient number to permit the analysis of their distribution within a wide range of age zones. Major results are that (1) the midocean ridge crestal regions have lower heat flow than that expected from conductive cooling models of the lithospheric plate, (2) an increase in heat flow is observed some distance away from the crest in a zone which appears to delineate a transition from dominantly convective to conductive heat transfer, and (3) the heat flow in crust older than this transition zone closely approximates that predicted by conductive cooling models for oceanic lithospheric plates. The transition from low to high heat flow occurs when 150–200 m of sediment is deposited over basement, apparently preventing convective transfer of heat from the oceanic crust to seawater. Thus the general shape of the empirical heat flow versus age curve for the East Pacific Rise (EPR) agrees with that from the Galapagos Spreading Center, but the age of the transition zone occurs in 5‐ to 6‐m.y.‐old crust on the Galapagos Spreading Center, whereas it occurs in 10‐ to 15‐m.y.‐old sea floor on the EPR. Proximity to the equatorial high sedimentation region causes the deposition of a thick layer of sediment much more quickly on the Galapagos Spreading Center than on the East Pacific Rise. Quantitatively, the degree of heat transfer by convection appears to correlate inversely with the ratio of sediment thickness to topog

ISSN:0148-0227    

DOI:10.1029/JB081i017p02968

年代:1976

数据来源: WILEY

摘要:

Fourier analyses were made of the geomagnetic horizontal annual means from 20 observatories and the vertical component annual means from 17 observatories from 1900 to 1972 for most of the observatories. Curves synthesized from the Fourier coefficients for periods from approximately 4 to 13 yr have patterns easily explained in terms of a westward‐flowing ring current that generally increases as the sunspot numbers increase but lags behind the sunspot numbers by a year or two. External spherical harmonic coefficients should be included in models of the earth's magnetic field. By using the horizontal component curves, synthesized as described above, for seven selected observatories and the vertical component curves for six selected observatories, values of both internal and external first‐order zonal spherical harmonic coefficients were determined for a 22‐yr span. The amplitude and phase of the ratio of these coefficients are consistent with an earth having a conductivity of 33 mhos/m from the center of the earth to a radius of 0.9REsurrounded by a nonconducting shell to the su

ISSN:0148-0227    

DOI:10.1029/JB081i017p02990

年代:1976

数据来源: WILEY

摘要:

One of the major problems in the use of heat flow data in a quantitative manner has been the variability of closely spaced measurements. It is suggested that this variability is directly related to hydrothermal circulation in an ‘effectively permeable’ oceanic crust. As a consequence, only where this crust is sealed from the seawater by an extensive and thick layer of impermeable sediment can reliable estimates be made of the flux at depth. Heat flow data from such areas are analyzed. It is shown that the scatter in the data is low and that in most areas the mean heat flow through a province of known age is close to that predicted by plate models which account for the increase in depth with increasing age of the ocean floor. From this analysis it is argued that these heat flow means are a reliable estimate of the flux at depth. In order to be able to use heat flow measurements to investigate the age of the crust, the thickness of the lithospheric plate, upper mantle convection, and local anomalies, it is necessary to substantiate this analysis and to find an adequate explanation of the low mean heat flow observed at the center of the sediment bulge in the equatorial Paci

ISSN:0148-0227    

DOI:10.1029/JB081i017p02997

年代:1976

数据来源: WILEY

摘要:

Two‐dimensional numerical calculations of hydrothermal circulation in permeable oceanic crust have been carried out. The effects of Rayleigh number, impermeable and permeable upper boundaries, and permeability variations with depth have been investigated. Flow and temperature fields as well as surface heat flux distributions are presented. Spatial distributions of surface heat flux are compared with observations at the Galapagos spreading center. The hydrothermal circulation alters the spatial distribution of surface heat flow but not its mean value. It is concluded that the oceanic crust has a permeability of about 4.5×10−12cm2. With a permeable upper boundary the mass of seawater circulating through the crust equals the mass of the crust in about 2.1 m.y. A corresponding typical circulation time is estimated to be about 4 y

ISSN:0148-0227    

DOI:10.1029/JB081i017p03007

年代:1976

数据来源: WILEY

摘要:

Twelve strain relief measurements were made at five sites on Potsdam sandstone northeast of the Adirondack Mountains, New York, to test Sbar and Sykes's (1973) suggestion that the upper crust in a large area of eastern North America is currently in a state of compression with the maximum principal stress (σ1) east‐northeast. In situ strain was relieved by overcoring and detected by using strain gages bonded to the sandstone at the surface. The average of nine measurements at four sites in the Late Cambrian Keeseville member of the Potsdam sandstone indicates that compressive stresses were relieved upon overcoring with an average direction of maximum expansion of N78°W. These nine measurements also indicate that stress can be consistent in orientation over areas of tens of square kilometers. The average of three measurements at one site in the Nicholville member 11 km from the other four sites indicates a maximum expansion oriented N18°E. At several sites, residual strain was recovered when specimens free of boundary loads were overcored. We suggest that the residual strain measured in the Nicholville member may be related to a late Proterozoic (Hadrynian) or Early Cambrian stress f

ISSN:0148-0227    

DOI:10.1029/JB081i017p03013

年代:1976

数据来源: WILEY

摘要:

The thermal diffusivity of Teflon, sodium chloride, quartz, and silica glass was measured at 40° C to pressures of 35, 18, 30, and 36 kbar, respectively. A transient line source method was modified for use in a piston‐cylinder high‐pressure cell. Pressure gradients were determined by experiments with bismuth foils. The pressure dependence of the thermal diffusivity at 40°C for the substances studied may be represented as follows (κ in square centimeters per second,Pin kilobars): for the low‐pressure phases of Teflon, Teflon I‐II, P<5.5 kbar, κ = 0.0012 + 3.6 × 10−5P; for the high‐pressure phase, Teflon III, 5.5 kbar

ISSN:0148-0227    

DOI:10.1029/JB081i017p03018

年代:1976

数据来源: WILEY

摘要:

The method proposed by Mooney and Steg (1969) for obtaining the dilatational dependence of the Grüneisen parameter from data on the pressure dependence of the thermal conductivity (or, equivalently, thermal diffusivity) is critically examined and applied to thermal diffusivity data for sodium chloride and quartz. The values obtained are γ0′(≡(dγ/dΔ)|Δ=0) = 3.0 for sodium chloride and γ0′ = 2.0 for quartz. Corresponding values of the parameterq(≡γ0′/γ0) are 1.9 and 2.8, in reasonable agreement with values obtained by other methods. It is suggested that this method be further investigated as a means of obtaining γ0′ andqfrom thermal data. A model for the lattice thermal conductivity of the mantle to the core boundary is presented. The model suggests that increases in conductivity with pressure due to lattice conduction processes in the mantle are less than 2.0%/kbar or 0.7%/km. Under conditions of normal geothermal gradient in the crust and upper mantle the increase in lattice conductivity due to the pressure effect will be substantially less than the decrease due to the temperature effect. A minimum value of lattice conductivity is attained in the region of the olivine‐spinel phase change, 400 km. The lattice conductivity may increase by a factor of 3 at the depth of the spinel‐postspinel phase change owing to the high conductivity of the dense oxide phases. The lattice contribution to the thermal conductivity at the mantlecore bound

ISSN:0148-0227    

DOI:10.1029/JB081i017p03025

年代:1976

数据来源: WILEY

摘要:

By using sound wave velocity data the entropy changes and dP/dT of 10 polymorphic transitions and solid‐solid reactions have been calculated according to Debye theory. These values show systematic deviations from experimentally determined values of the same parameters. This data set allows the application of a correction factor to calculate values for cases where experimental determinations of the phase diagram or entropies are not available but where sound wave velocities are available or can be estimated for all phases. When applied to γMg2SiO4and γMg2GeO4disproportionation, the estimated values of dP/dT are +0.016 and +0.027, respectively. The entropy expressions indicate that negative dP/dT for a polymorphic transition implies either anomalously low sound velocities in the high‐pressure phase as compared to the low‐pressure phase or non‐Debye contributions to the specific heat. This is not necessarily the case for disproportionation reactions, as illustrated by γFe2SiO4an

ISSN:0148-0227    

DOI:10.1029/JB081i017p03031

年代:1976

数据来源: WILEY