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: 9.81). 09/1979; 76(9):4192-200. DOI: 10.1073/pnas.76.9.4192
Source: PubMed

ABSTRACT 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.


Available from: Thomas Jordan, May 29, 2015
  • [Show abstract] [Hide abstract]
    ABSTRACT: This is the final installment in a four-part sequence that examines the nature of mantle layering required by the multiple-ScS phases and internal reflections observed within the reverberative interval of SH-polarized seismograms. In this paper, long-period, SH-polarized, multiple-ScS phases reflected once from a mantle discontinuity (first-order reverberations) are used to search for abrupt shear-wave impedance contrasts in the lower mantle. Beneath the geographic regions sampled, the depth interval of 1000–2600 km (Bullen's region D′) appears free of any distinct, radial layering, in agreement with the majority of recent seismic models and the notion of near-adiabatic compression of a compositionally homogeneous lower mantle. To pass undetected, discontinuities in D′ must be either small (SH-polarized reflection coefficient R0
    11/1991; 96(B12). DOI:10.1029/91JB02163
  • Source
    [Show abstract] [Hide abstract]
    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.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Major developments in earth structure in the last four years have been concentrated in the description of the earth's lateral heterogeneity: the regions that are heterogenous and the per cent variation of velocity and density in each region. Most studies find that lateral variation is concentrated in the upper 400 and lower 200 km. of the mantle. A radially symmetric earth model has been defined that represents the best average fit to seismic data in a broad frequency band, sampling many regions. P and S velocity is found to increase in zones of 50 km. or less at 400 and 650 to 700 km. depth. The model is transversely anisotropic in the upper mantle. It possesses a vertical axis of symmetry such that the elastic constants are different for vertical propagation than for horizontal and intermediate angles of propagation. The real earth generally exhibits azimuthal anisotropy as well, but the azimuthal anisotropy cannot be resolved by a global average of data. The nature and magnitude of the anisotropy agrees with that found in ultramafic samples of the upper mantle. In attenuation, models of intrinsic attenuation have included the dispersive properties of intrinsic anelasticity and constructed relaxation models consistent with an observed frequency dependence of Q in the body wave band. There has been progress in mapping the scattering properties of the lithosphere. Attenuation due to scattering in the crust and lithosphere has been shown to have strong effects on the amplitudes of seismic waves at local and teleseismic distances.
    07/1983; 21(6). DOI:10.1029/RG021i006p01277