Inhomogeneities in the Earth's Mantle

Toksöz, M. Nafi; Chinnery, Michael A.; Anderson, Don L.

doi:10.1111/j.1365-246x.1967.tb02145.x

Cited by 69 publications

(12 citation statements)

References 35 publications

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“…The first is the change from the bending of oceanic to continental lithosphere, and this takes place immediately. Continental shield lithosphere may have a thickness of about 140 km [Toks6z et al, 1967] or a thickness about twice that of old oceanic plates [Forsyth, 1973]. Thus it is considerably more resistant to bending and shearing.…”

Section: Mechanics Of Continental Convergencementioning

confidence: 99%

Thermal and mechanical models of continent-continent convergence zones

Bird¹,

Toksöz²,

Sleep³

1975

J. Geophys. Res.

Self Cite

177

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The thermal regimes of continent-continent convergence zones are modelled by a finite difference technique, assuming that there is some subduction of continental crust. Gravity and heat flow profiles are generated from the thermal models. Subducted crust and slab remain cool except at the upper surface where frictional heating is important. Crustal rocks may be metamorphosed or melted by friction while the plates are converging or by radioactive self-heating more than 30 m.y. after the plates have stopped. In the former case, melting requires frictional shear stresses between 500 and 2000 bars at a plate velocity of 5 cm/yr. At lower velocities the upper limit of frictional stress is greater. The total work performed in continental subduction may be minimized if the angle of underthrusting becomes more shallow, changing the location of subduction. A model for the geometry of oceanic and continental slabs in the Zagros Mountains is presented which satisfies gravity, heat flow, seismic, and geologic,constraints. Continental underthrusting is beginning along a new fracture at the edge of the Persian Gulf, isolating the Arabian continental shelf from the subduction process. Slippage along this fault is Pleistocene and probably does not exceed 30 km. Subduction of continental crust at the crush zone is probably insignificant. o KM 200 , ß ß ß \ ß

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Section: Mechanics Of Continental Convergencementioning

confidence: 99%

Thermal and mechanical models of continent-continent convergence zones

Bird¹,

Toksöz²,

Sleep³

1975

J. Geophys. Res.

Self Cite

177

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“…Estimates of the maximum depth to which lateral differences in the earth structure persist have been increasing throughout the last two or three decades: first came the evidence of distinctly different structures of the continental and oceanic crusts; then surface wave data and, in particular, pure path dispersion data indicated that the differences may persist to depths as great as 600 or 700 km [cf. Toks& et al, 1967;Jordan, 1975]. Hales et al [1968] suggested that azimuthal differences in observed teleseismic travel times of P and S waves to North American stations [Cleary and Hales, 1966;Doyle and Hales, 1967] may be an expression ol a departure of the earth structure from the ellipsoidal configuration predicted by the hydrostatic theory.…”

Section: Introductionmentioning

confidence: 99%

Large-scale heterogeneities in the lower mantle

Dziewoński¹,

Hager²,

O’Connell³

1977

J. Geophys. Res.

416

194

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Coefficients of a spherical harmonic expansion, up to angular order 3, of velocity anomalies in five shells within the earth's mantle were obtained from an analysis of nearly 700,000 P wave travel time residuals. The results for depths less than 1100 km are unreliable; on the basis of tests and numerical experiments we infer that lateral heterogeneities in this depth interval are dominated by velocity perturbations of lateral dimensions less than 5000 km. Relatively large wavelength features were resolved below 1500‐km depth; the average perturbation level increases in the lowermost mantle. The region between 1100 and 1500 km may represent a transition zone with respect to the dimensions of anomalies. We present statistical evidence for a negative correlation between the long wavelength gravity anomalies observed at the surface and those computed from velocity anomalies at depths greater than 1100 km under the assumption of proportionality between velocity and density perturbations. The proportionality coefficient Δυ/Δρ has been determined by using two different methods: the values are −4.45 and −6.02 (km/s)/(g/cm3). Only minor changes in the velocity distribution are needed to satisfy the long wavelength gravity field exactly. Possible origins of the correlation are (1) sinking of eclogite‐rich material into the lower mantle from regions of lithospheric subduction, (2) chemical plumes of light high‐velocity material originating near the core‐mantle boundary, (3) temperature differences and perturbations of the core‐mantle boundary and the earth's surface associated with mantle‐wide convection, or (4) static chemical heterogeneities in a nonconvecting mantle. The first three alternatives, all involving flow in the lower mantle, may be complementary but act on a different time scale. There appears to be a westward or northwestward translation of some anomalies with respect to the pattern obtained for the innermost shell. In particular, the direction of translation of a large negative anomaly under the Pacific is in agreement with the sense of motion of the Pacific plate. We must caution the reader, however, that this is a highly speculative interpretation. If correct, it would favor the convective hypotheses.

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“…In this way they extracted three significantly different dispersion curves from the great circle measurements of the Love wave phase velocities; these were used subsequently to derive shear velocity distributions (Toksoz, Chinnery & Anderson 1967). The experiment of Toksoz and Anderson has recently been repeated on a larger body of data by Kanamori (1970) who also investigated regional variations of Rayleigh wave dispersion.…”

Section: Introductionmentioning

confidence: 99%

On Regional Differences in Dispersion of Mantle Rayleigh Waves

Dziewoński

1971

Geophys J Int

140

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Rayleigh waves generated by the Peru-Bolivian border earthquake of 1963 August 15 have been analysed at 35 WWSSN stations in the period range belween 150 and 300 seconds. Thirty-one of these stations were grouped into six narrow azimuth windows as a result of an analysis of observed phase velocities. In each of these windows the phase velocity dispersion shows similar characteristics, and the composition along the paths (in terms of the shield, oceanic and tectonic fractions of a path) was approximately the same. An error analysis indicates that differences in phase velocities obtained for individual windows are statistically meaningful, thus proving that the Rayleigh wave dispersion measured on world-circling paths shows regional variations. These six areal windows cover a third of the Earth's surface.Group velocities were measured for the six azimuthal windows by summing the auto-correlograms of individual seismograms for stations within each azimuth range. A statistical test has shown that the measurements of phase and group velocities are mutually consistent proving for the first time that the lateral inhomogeneities in crust and mantle can also be detected by measurements of group velocities for world-circling paths. The simultaneous least-squares condition for both phase and group velocity observations for each window was used to derive a polynomial approximation for the frequency of free oscillations as a function of order number measured as the excess over a reference model. ' Pure path ' phase and group velocities derived according to the approach of Toksoz and Anderson indicate substantial differences in dispersion for each basic type of mantle. Pure path phase velocities derived in this report differ considerably from the corresponding results of an analysis by Kanamori, whereas world-wide averages are very similar for both reports.Group velocities can be separated much more effectively than phase velocities, especially at the longest periods. This effect may be explained by world-wide variations of depths to the so-called 400 km and 650 km discontinuities. These variations appear to be unrelated to the regionaiization adapted in this paper.

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Inhomogeneities in the Earth's Mantle

Cited by 69 publications

References 35 publications

Thermal and mechanical models of continent-continent convergence zones

Thermal and mechanical models of continent-continent convergence zones

Large-scale heterogeneities in the lower mantle

On Regional Differences in Dispersion of Mantle Rayleigh Waves

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