Structural geology of the Earth's interior

Geological Research Division, Scripps Institution of Oceanography, La Jolla, California 92093.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 09/1979; 76(9):4192-200. DOI: 10.1073/pnas.76.9.4192
Source: PubMed


Seismology is providing a more sharply focused picture of the Earth's internal structure that should lead to improved models of mantle dynamics. Lateral variations in seismic wave speeds have been documented in all major layers of the Earth external to its core, with horizontal scale lengths ranging from 10 to 10(4) km. These variations can be described in terms of three types of heterogeneity: compositional, aeolotropic, and thermobaric. All three types are represented in the lithosphere, but the properties of the deeper inhomogeneities remain hypothetical. It is argued that sublithospheric continental root structures are likely to involve compositional as well as thermobaric heterogeneities. The high-velocity anomalies characteristic of subduction zones-seismic evidence for detached and sinking thermal boundary layers-in some areas appear to extend below the seismicity cutoff and into the lower mantle or mesosphere. Mass exchange between the upper and lower mantles is implied, but the magnitude of the flux relative to the total mass flux involved in plate circulations is as yet unknown. Other observations, such as the vertical travel time anomalies seen in the western Pacific, may yield additional constraints on the flow geometries, but further documentation is necessary. Thermobaric heterogeneities associated with a thermal boundary layer at the base of the mantle could provide the explanation for some of the observations of heterogeneities in the deep mantle. The evidence for very small scale inhomogeneities (<50 km) in region D'' and for topography on the core-mantle interface motivate the speculation that there is a chemical boundary layer at this interface, as well as a thermal one.

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Available from: Thomas Jordan
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    • "The Earth's interior is divided into 5 layers: the crust, upper mantle, lower mantle, outer core, and inner core [1]. Seismic measurements show that the inner core is a solid sphere with a radius of 1,221.5 km, and that the outer core is a liquid spherical crust (plasma) around the inner core, with an external radius of 3,840.0 "
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    ABSTRACT: The theory accepted today for the origin of the Earth's magnetic field is based on convection currents created in the Earth's outer core due to the rotational motion of the planet Earth around its own axis. In this work, we show that the origin of the Earth's magnetic field is related to the gravitational mass of the outer core.
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    ABSTRACT: Multiple ScS travel times have been obtained by wave form cross correlation from seismograms digitally recorded by the High Gain Long Period (HGLP) and Seismic Research Observatory (SRO) networks. The surface projections of the paths corresponding to these data cross the western Pacific on oceanic crust greater than 100 m.y. old or traverse continental regions. The difference between the median ScS/sub n/--ScS/sub n-1/ residuals for all western Pacific paths and all continental paths is +5.2 s, in agreement with our World Wide Standardized Seismograph Network (WWSSN) data (Sipkin and Jordan, 1976). These results support the hypothesis that the average mantle shear velocity of old ocean basins is significantly less than that of old continental nuclei. The medians of both the oceanic and continental residuals for the HGLP and SRO data are more positive than those for the higher-frequency WWSSN data by amounts consistent with attenuative dispersion, which we take to be direct evidence for such dispersion. The residuals for paths crossing China have a median 2 s greater than the median for all continental paths, supporting the inference from dispersion studies that the upper mantle beneath China is characterized by anomalously low shear velocities. The residuals for western Pacific paths show lateral variations of 5 s or more not correlated in any systematic way with crustal ages along the paths. An analysis of these variations suggests that for horizontal scale lengths of the order of 10³ km the amplitude of lateral variability is greater along a SW-NE axis than along a SE-NW axis. Mesoscale heterogeneity in the western Pacific may thus consist of predominantly NW trending structures.
    No preview · Article · Jan 1980 · Journal of Geophysical Research Atmospheres
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    ABSTRACT: The ScSn phase-equalization and stacking algorithm of Jordan and Sipkin (1977) has been applied to an extensive set of HGLP and ASRO data to obtain regionalized estimates of Qs~s. Tests of the algorithm using synthetic data reveal no significant sources of bias. The low value of Qs~s previously obtained for the western Pacific (156 -+ 13) is corroborated by additional data, and Qs~s obser-vations in other regions correlate with variations in crustal age and tectonic type. A representative value for the ocean basins sampled by our data is 150, with the best estimates being somewhat lower (135 to 142) for younger oceanic regions and somewhat higher (155 to 184) for older regions. The two subduction zones sampled here, KuriI-Japan and western South America, are characterized by larger Qscs estimates than the ocean basins (197 -+ 31 and 266 --. 57, respectively), and the difference between them is qualitatively consistent with the contrasts in upper-mantle attenuation structure proposed by Sacks and Okada (1974). Continental regions are poorly sampled in this study because the signal-generated noise in the vicinity of the ScS, phases is generally larger for continental paths, but a representative value is inferred to be Qs~s --225. For paths crossing China, Qscs is observed to be lower (~180), providing additional evidence for a high-temperature upper mantle previously inferred from surface-wave and travel-time measurements. Our best estimate for the average Earth is Qscs --170 (__.20 per cent), which appears to be significantly lower than that predicted by normal mode data, suggesting some frequency dependence. Q~I correlates with ScS,-ScS~_ 1 travel time along a line given by Qscs (4.4 x 10-4)4 Ts~s + 4.88 x 10 -3, where &Tscs is the JB residual in seconds; this correlation favors a thermal control on the ATscs variations. It is inferred from the tectonic correlations that much, if not most, of the heterogeneity expressed in the Qs~s and & Tscs variations is confined to the upper mantle. Substantial differences in the attenuation structures underlying continents and oceans are implied. In fact, the average quality factor for the upper mantle beneath stable cratons may not be much less than that for the lower mantle.
    Full-text · Article · Jan 1980
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