摘要:

The thermal wind equations, in which the Coriolis force is balanced by pressure gradients and horizontal density gradients rather than by Lorentz forces, are used to describe patterns of magnetic field drift associated with core fluid motions near the core‐mantle boundary (CMB). The advection of magnetic field may be due in part to the flow driven by such horizontal temperature gradients, just as East‐West air flow is driven by North‐South temperature gradients in the Earth's atmosphere. It is argued that this flow may be concentrated in a shell near the CMB, and the horizontal temperature gradients are expected to be directly proportional to horizontal gradients in CMB topography, the lowest harmonics of which are approximately constrained in seismology. Part of the zonal drift is then associated with the 1=2, m=0 harmonic of CMB topography, and anticyclones are attached to topographic highs (thermal highs). Comparison of our derived flow pattern with those determined purely by magnetic field observations provides tentative support for our

ISSN:0094-8276    

DOI:10.1029/GL017i010p01473

年代:1990

数据来源: WILEY

摘要:

The high‐pressure mineral assemblage produced from a natural peridotite sample (xMg = 0.897) has been synthesized in the laser‐heated diamond cell and identified by x‐ray diffraction at 54 GPa. Orthorhombic perovskite, (Mg,Fe)SiO3, magnesiowüstite, (Mg,Fe)O, and cubic perovskite, CaSiO3, are observed in estimated molar ratios of 1.00 : 0.46 : 0.08 (1.00 : 0.21 : 0.09 by volume) respectively, with no evidence of other phases being present. The lattice parameters measured at 54 GPa yield a density of 4.97(±0.03) Mg/m3for the high pressure assemblage at 300 K, in good agreement with prior calculations of density for candidate lower mantle asse

ISSN:0094-8276    

DOI:10.1029/GL017i010p01477

年代:1990

数据来源: WILEY

摘要:

The perovskite form of Mg0.981, Fe0.028Cr0.011Si0.979O3, quenched from 26 GPa and 2173 K in a uniaxial split‐sphere‐type high‐pressure apparatus, was studied at the SUNY X3A beamline at the National Synchrotron Light Source. With a Si (111) double‐crystal monochromator and a single crystal of perovskite measuring 29×29×43 μm, intensities of 127 independent reflections were collected with sin θ/λ3.5σI. This is the first‐reported single‐crystal diffraction study of a Mg‐silicate perovskite containing a significant amount of Fe or other cations. The unit cell parameters obtained through the refinement of 30 reflections are:a= 4.7823(3),b= 4.9331(3),c= 6.9026(8) Å, V = 162.84(2) Å3. The result of structural refinement in space groupPbnmindicates that Fe and a small amount of Cr substitute for Mg rather than for Si, contrary to the result of Jackson et al. (1987) who reported Fe in the octahedral site of a perovskite synthesized at 50 GPa and 2000 K in a diamond‐anv

ISSN:0094-8276    

DOI:10.1029/GL017i010p01481

年代:1990

数据来源: WILEY

摘要:

Reductions of four independent blocks of Pioneer Venus Orbiter Doppler radio tracking data have produced very consistent determinations of the GM of Venus (the product of the universal gravitational constant and total mass of Venus). These estimates have uncertainties that are significantly smaller than any values published to date. The value of GM is also consistent with previously published results in that it falls within their one‐sigma uncertainties. The value of 324858.60 ±0.05 km3/sec2is our best estima

ISSN:0094-8276    

DOI:10.1029/GL017i010p01485

年代:1990

数据来源: WILEY

摘要:

On a rotating, aspherical earth, coupling between spheroidal and toroidal seismic free oscillations is manifested in the time domain as a slow alternation of the oscillation envelope between the transverse and the radial and/or vertical motion components. Using a narrow‐band smooth envelope estimator, we have observed this behavior in data from theMS= 8.1 Macquarie Ridge earthquake of 5/23/89. Rotational coupling is the principal cause of mixed‐type coupling forf<4.2 mHz. However, envelope perturbations consistent with mixed‐type coupling are observed for0S38‐0T36and0S37‐0T35, which do not couple through rotation, and couple only weakly through smooth upper mantle velocity models. Such envelope perturbations can bias estimates of modalQ, and suggest that scattering may play a role in the attenuation of long‐period su

ISSN:0094-8276    

DOI:10.1029/GL017i010p01489

年代:1990

数据来源: WILEY

摘要:

We show that a global distribution of small scale heterogeneities in the lower mantle induces both frequency splitting and amplitude scatter of low angular order modes. These effects can be used in order to map the global distribution of small scale structures in the mantle, such as plumes. Two independent datasets, consisting respectively of lower mantle modes interaction coefficients and of observed amplitudes of core modes, can then be used to constrain the low angular degrees of the spatial distribution and the mean size of heterogeneities, allowing us to constrain the mantle structure at different scales. If these heterogeneities are assumed to be plumes associated with hotspot activity, the mean size of plumes obtained using the two datasets is then in good agreement with that obtained from geodetical or numerical convection studies.

ISSN:0094-8276    

DOI:10.1029/GL017i010p01493

年代:1990

数据来源: WILEY

摘要:

Three‐dimensional numerical models are used to study the planform of thermal convection in rectangular boxes, imposing uniform velocity at the surface. This problem is relevant to the structure of mantle convection under moving tectonic plates. Different planforms can be obtained depending on the Rayleigh number, viscosity structure and length of the plate. If there is a thin low viscosity layer in the uppermost part of the model, the solution consists of narrow ascending plumes below the divergent edge of neighboring plates as well as descending sheets below lines of plate convergence. If the experimental box is long, well developed drifting columnar blobs arise from the unstable lower boundary layer, giving the solution an oscillatory characte

ISSN:0094-8276    

DOI:10.1029/GL017i010p01497

年代:1990

数据来源: WILEY

摘要:

All available Jurassic, Triassic, and late Paleozoic paleomagnetic poles from the Colorado Plateau and stable North America are used to estimate the rotation of the Colorado Plateau relative to stable North America since Jurassic time. The paleomagnetic poles differ from those of our prior study [Bryan and Gordon, 1986] through the addition of seven recently published poles, the deletion of one pole, and the replacement of two others. We also improve on our prior analysis by omitting Cretaceous poles. From the systematic offset between plateau poles and stable North American poles we estimate rotation of the Colorado Plateau to be(95% confidence limits) clockwise, ∼1° larger than we found before. The null hypothesis of no systematic difference between plateau poles and stable North American poles can be rejected with a formally high level of confidence (99.99%). The hypothesis that the systematic differences between plateau and off‐plateau poles is as large as 10°, as proposed by several workers using fewer poles, can be rejected at an equally high level of confi

ISSN:0094-8276    

DOI:10.1029/GL017i010p01501

年代:1990

数据来源: WILEY

摘要:

This paper can be considered as a continuation of the work by Carrion and Carneiro (1989), where a generalized approach to linearized inversion of geophysical data was developed. Their method allows one to incorporate virtually any constraints in the inversion and reformulate the problem in the dual space of Langrangian multipliers (see also Carrion, 1989a). The constrained tomography makes traveltime inversion robust: it automatically rejects “bad data” which correspond to solutions beyond the chosen constraints and allows one to start inversion with an arbitrary chosen initial model.In this paper, I will derive basic formulas for constrained tomographic imaging that can be used in such areas of geophysics as global mapping of the earth interior, exploration geophysics, etc. The method is fast: an example that will be shown in the paper took only 6 min. of VAX CPU time. Had the conventional least‐squares matrix inversion been used it would have taken more than 10 hours of the CPU time to solve the same pr

ISSN:0094-8276    

DOI:10.1029/GL017i010p01505

年代:1990

数据来源: WILEY

摘要:

We present a nonlinear approach for reconstruction of discontinuities in geological environment (earth's crust, say). The advantage of the proposed method is that it is not limited to a Born approximation (small angles of propagation and weak scatterers). One can expect significantly better images since larger apertures including wide angle reflection arrivals can be incorporated into the imaging operator. In this paper, we treat only congressional body waves: shear and surface waves are considered as noise.

ISSN:0094-8276    

DOI:10.1029/GL017i010p01509

年代:1990

数据来源: WILEY