ISSN:0094-8276    

DOI:10.1029/GL016i007p00601

年代:1989

数据来源: WILEY

摘要:

Transverse component bodywave ground displacements from deep focus earthquakes exhibit complexities which are not accounted for by standard radially symmetric Earth models. This anomalous behavior is examined using 123 broadband teleseismic SH displacement recordings, reconstituted from short‐ and long‐period or broadband seismograms for 11 events in the Kurile subduction zone. The greatest complexity, involving an extra arrival between the S and ScS phases at epicentral distances greater than 72°, has a strong distance dependence and little azimuthal variation with respect to the source region, favoring an interpretation as a deep mantle triplication rather than a near‐source slab diffr

ISSN:0094-8276    

DOI:10.1029/GL016i007p00605

年代:1989

数据来源: WILEY

摘要:

Experiments with the laser‐heated diamond cell show that (Mg,Fe)SiO3perovskite reacts chemically with liquid iron at pressures greater than 70 GPa and temperatures above 3700 K. X‐ray diffraction analyses of quenched samples demonstrate that the reaction products include SiO2stishovite and iron alloys (Fe,Mg)xO and FexSiy, at the interface between the Fe and silicate. Our results suggest that similar chemical reactions occur at the Earth's core‐mantle boundary, forming chemical heterogeneities composed of silicate‐rich and iron alloy‐rich regions in the D′ layer. This scenario is consistent with seismological o

ISSN:0094-8276    

DOI:10.1029/GL016i007p00609

年代:1989

数据来源: WILEY

摘要:

Numerical calculations of fluid dynamos are used to constrain the probable origin of toroidal flows in the outer core. Azimuthal drift of the core fluid relative to the mantle is the sum of three components: (1) uniform differential rotation of the core relative to the mantle produced by electromagnetic torques on the mantle; (2) cylindrical flow driven by electromagnetic torques in the core, and (3) buoyantly driven thermal wind flow produced by lateral variations of temperature and composition. Dynamo calculations indicate that electromagnetic torques on the mantle contribute to long‐term drift, but are not likely to be its primary cause. Zonal flow driven by electromagnetic torques in the core and thermal wind flows driven by zonal temperature gradients are both sufficiently energetic to account for azimuthal flows with angular velocities of 0.1–0.5° yr−1, as inferred from frozen flux analyses of secular var

ISSN:0094-8276    

DOI:10.1029/GL016i007p00613

年代:1989

数据来源: WILEY

摘要:

Numerical models of three‐dimensional, thermal convection in highly viscous spherical shells with different combinations of internal and basal heating consistently have upwelling concentrations in the form of cylindrical plumes and downwelling in planar sheets. As the proportion of internal heating increases, the number of upwelling plumes increases, and downwelling sheets become more vigorous and time‐dependent. With any amount of basal heating, the entire convective pattern, during its evolution, is anchored to the upwelling plumes. As the proportion of internal heating increases, the heat flow carried by the upwelling plumes remains a large fraction of the basal heat flow. Downwelling sheets carry only a minor fraction (approximately 30%) of the basal heat flow (even when the shell is entirely heated from below), but they advect almost all of the internally generated heat. The relatively large number of plumes in the Earth's mantle (inferred from hotspots), the possibility that downwelling slabs are vigorous enough to penetrate the lower mantle, and the small fraction of terrestrial surface heat flow carried by plumes all suggest that the mantle is predominantly heated from wit

ISSN:0094-8276    

DOI:10.1029/GL016i007p00617

年代:1989

数据来源: WILEY

摘要:

The seismically observed topography on the core‐mantle boundary (CMB) and the velocities of the surface tectonic plates together constitute important observational constraints on viscous flow models of the Earth's mantle. We show here that the CMB deflections inferred by Morelli and Dziewonski (1987) may be explained in terms of a simple model of whole‐mantle convection and in the process we provide a new method for inferring the penetration depths of subducted sl

ISSN:0094-8276    

DOI:10.1029/GL016i007p00621

年代:1989

数据来源: WILEY

摘要:

Numerical models of upper mantle convection incorporating both plates and a substantial low viscosity layer show that long‐wavelength topography is only moderately reduced compared with a model without the low viscosity layer. In particular, topography due to hot upwelling from a putative hot boundary layer at 670 km depth is still large. In the case of a migrating spreading center, the symmetry and square‐root‐of‐age dependence of seafloor topography would be destroyed. These results strengthen the case against mantle models in which heat transport across the 670 km seismic discontinuity is predominantly by conduction. A substantial mass flux through 670 km is

ISSN:0094-8276    

DOI:10.1029/GL016i007p00625

年代:1989

数据来源: WILEY

摘要:

Two‐dimensional, time‐dependent calculations show that the thermal and flow structures can be influenced by the lower‐mantle thermal‐chemical instabilities. The style of the core‐mantle boundary (CMB) deformation in thermal‐chemical convection is different from that obtained in thermal convection. The amplitude of CMB hills is reduced greatly from the depth‐dependence of thermal expansivity, found in recent high‐press

ISSN:0094-8276    

DOI:10.1029/GL016i007p00629

年代:1989

数据来源: WILEY

摘要:

Numerical models of mantle convection using the Boussinesq, extended Bousinessq and anelastic‐liquid approximation are compared. For steady state solutions there is good quantitative agreement between the results if they are scaled in a proper way. Time‐dependent extended Boussinesq and anelastic‐liquid flows show only qualitative agreement, the main difference being a distortion of timescale. Compressibility induces an asymmetry in the structure of upper and lower boundary layers that cannot be observed in Boussinesq f

ISSN:0094-8276    

DOI:10.1029/GL016i007p00633

年代:1989

数据来源: WILEY

摘要:

Numerous small‐wavelength elongated features are visible both on filtered geoid maps and on topography maps of the South Atlantic Ocean. A number of them are oriented along the directions of absolute plate motion between 80 Ma B.P. and 30 Ma B.P.: N50°E for the African plate and N63°W for the South American plate. Magmatic traces left over fixed convective plumes more numerous than the “classical” hot‐spots best explain these observations. Other traces, especially in the area of the Sandwich subduction zone and on the African plate are oriented in a N50°W direction and seem to indicate large‐scale deformation of the oc

ISSN:0094-8276    

DOI:10.1029/GL016i007p00637

年代:1989

数据来源: WILEY