Seismic Travel Time Evidence for Lateral Inhomogeneity in the Deep Mantle

Julian, Bruce R.; Sengupta, Mrinal K.

doi:10.1038/242443a0

Cited by 88 publications

(36 citation statements)

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“…For the lower mantle, the assumption of ' random ' geometry might traditionally have been regarded as very reasonable, but recent observations suggesting significant lateral variations in lower mantle properties (Kanasewich et al 1973;Julian & Sengupta 1973, for example) and suggestions that the lower mantle might be undergoing largescale motions (Morgan 1971) in turn suggest that the lower mantle might be far from a random mixture. It was shown by Hill (1952) that the averages of elastic moduli proposed by Reuss (1929), who assumed uniform stress, and by Voigt (1928), who assumed uniform strain, were in fact lower and upper bounds, respectively, on the elastic moduli of a mixture of perfectly elastic components.…”

Section: Equations Of State Of Mixturesmentioning

confidence: 97%

Limits on the Constitution of the Lower Mantle

Davies

1974

Geophysical Journal International

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The limits on the constitution of the lower mantle which can be inferred from currently available data are investigated. The density and elasticity of the mantle are compared to those of various model mineral assemblages at zero pressure and at high pressures, the former case using extrapolations of mantle properties. The models are based on equations of state of MgO (periclase), SiO, (stishovite) and FeO (wiistite), and of the Twin Sisters and Mooihoek dunite high-pressure phases. The bounds on the elastic properties of mixtures are discussed. The properties of hypothetical phases denser than their isochemical simple oxide mixtures are estimated relative to those of the oxide mixtures. The uncertainties in all quantities are discussed and the interdependence of some inferred mantle characteristics is considered. The inferred silica content, temperature and phase assemblage are strongly interdependent. The inferred FeO content depends to a small extent on the phase assemblage and temperature corrections and more strongly on the presence of iron in the ' low spin ' electronic state. Models which best fit the data used range between (1) an oxide mixture, or phase assemblage with similar properties, with pyroxene composition and 9 mole per cent FeO, and (2) a phase assemblage about 5 per cent denser than an isochemical simple oxide mixture with olivine composition and 6 mole per cent FeO. Combining all uncertainties and interdependences allows the FeO content to vary between about 3 and 14 mole per cent, and the temperature to vary beyond the independently estimated bounds of 20GO"K and 6000°K. The possibilities of an isentropic temperature gradient, chemical homogeneity, or some iron enrichment in the lower mantle relative to the upper mantle are allowed, but not required, by the data.

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Section: Equations Of State Of Mixturesmentioning

confidence: 97%

Limits on the Constitution of the Lower Mantle

Davies

1974

Geophysical Journal International

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“…The travel times at large distances show little azimuthal dependence (Jeffreys 1962), and it has been inferred that the lateral heterogeneity of this region is small, at least in comparison with the upper mantle. Recent studies using more refined data indicate, however, that the lower mantle may be laterally variable (Alexander & Phinney 1966;Chinnery 1969;Kanesewich et al 1972;Julian & Sengupta 1973;Jordan 1972).…”

Section: The Lower Mantle (Region D)mentioning

confidence: 99%

Earth Structure from Free Oscillations and Travel Times

Jordan

Anderson

1974

Geophysical Journal International

208

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SummaryAn extensive set of reliable gross Earth data has been inverted to obtain a new estimate of the radial variations of seismic velocities and density in the Earth. The basic data set includes the observed mass and moment of inertia, the average periods of free oscillation (taken mainly from the Dziewonski-Gilbert study), and five new sets of differential travel-time data. The differential travel-time data consists of the times of PcP-P and ScS-S , which contain information about mantle structure, and the times of P'AB -P'DF and P'Bc -PIDF, which are sensitive to core structure.A simple but realistic starting model was constructed using a number of physical assumptions, such as requiring the Adams-Williamson relation to hold in the lower mantle and core. The data were inverted using an iterative linear estimation algorithm. By using baseline-insensitive differential travel times and averaged eigenperiods, a considerable improvement in both the quality of the fit and the resolving power of the data set has been realized. The spheroidal and toroidal data are fit on the average to 0.04 and 0.08 per cent, respectively. The final model, designated model B 1, also agrees with Rayleigh and Love wave phase and group velocity data.The ray-theoretical travel times of P waves computed from model B1 are about 0.8s later than the 1968 Seismological Tables with residuals decreasing Model B1 is characterized by an upper mantle with a high, 4.8 km s-l, S, velocity and a normal, 3.33 g ~m -~, density. A low-velocity zone for S is required by the data, but a possible low-velocity zone for compressional waves cannot be resolved by the basic data set. The upper mantle transition zone contains two first-order discontinuities at depths of 420 km and 671 km. Between these discontinuities the shear velocity decreases with depth. The radius of the core, fixed by PcP -P times and previous mode inversions, is 3485 km, and the radius of the inner core-outer core boundary is 1215 km. There are no other first-order discontinuities in the core model. The shear velocity in the inner core is about 3.5 km s-'.

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“…Julian and Sengupta (84) have observed that the scatter in P-wave travel times from deep-focus events increases substantially beyond 700, implying a greater Basal heterogeneity (78,(84)(85)(86)(87)(88)(89)(90)(91)(92)(93)(94)(95) lateral variability in the mantle below a depth of 2000 km. They estimated that this heterogeneity is characterized by horizontal scale lengths of approximately 1000 km and velocity fluctuations of 1% or more.…”

Section: Fig 2 Generalized Tectonic Map Of the Globe Between 750n Amentioning

confidence: 99%

Structural geology of the Earth's interior

Jordan

1979

Proc. Natl. Acad. Sci. U.S.A.

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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 104 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 hy--ntheticaL It is argued that sublithospheric continental root structures are likely to invoke 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 betweenthe 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. Thermo6aric 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.The colors are chocolate, purple, lavender, and magenta, of many tones and shades. If it were not for this powerful coloring, which discloses every band and layer with emphasis, and each with a habit peculiar to itself, we could not venture to assert so much about them as we have done. For we have been reading geology from miles away from our rocks. But what are miles in this Brobdingnagian country! -Capt. C. E. Dutton, describing the Paleozoic sequence at the head of the Grand Canyon (1).The U.S. Geological Survey was founded by pioneers whose gaze was set on the grand scales of geology exposed in the American West. Confronted by a record laid starkly bare in these arid wastelands, they could not help but sense the movings within our planet. They vocalized their perceptions with bold new words: J. W. Powell gave us diastrophism to comprise the various processes of crustal deformation, which G. K. Gilbert (2) divided into orogenic (mountain-making) and epierogenic (continent-making); Dutton (3) defined isostasy to connote the tendency toward equality of total lithostatic loads, fully recognizing its control on vertical motions. The intervening century has witnessed considerable development of these concepts. From studies of Dutton's isostasy came the modern notions of lithosphere and asthenosphere. We now interpret the lith...

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Seismic Travel Time Evidence for Lateral Inhomogeneity in the Deep Mantle

Cited by 88 publications

References 8 publications

Limits on the Constitution of the Lower Mantle

Limits on the Constitution of the Lower Mantle

Earth Structure from Free Oscillations and Travel Times

Structural geology of the Earth's interior

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