Km Of Mantle Research Articles

Some comments on the effects of lower-mantle anisotropy on SKS and SKKS phases

Anisotropy in the lowermost few 100 km of mantle, or D″ region, is indicative of deformation-induced alignment of crystals and/or inclusions of material, and as such offers insights into the dynamic nature of this region. Observations of shear-wave splitting in phases that transit this region provide constraints on such anisotropy. We investigate the effects of lower-mantle seismic anisotropy on SKS and SKKS phases through linked effective-medium modelling and ray-based waveform modelling. A mantle with vertical-transverse-isotropy (VTI) will not produce any splitting in such core phases. Instead we consider the effects of azimuthal-anisotropy due to aligned disk-shaped and tubular inclusions and aligned perovskite, periclase and columbite. Models are constructed subject to constraints imposed by observed anisotropy (<3%) and plausible variations in aggregate isotropic velocities (<± 2.5%). Melt-filled inclusions are much more effective in generating anisotropy than solid-filled inclusions and disk-shaped inclusions produce more anisotropy than tubular inclusions. In general the degree of splitting produced by most of the models is small, similar to that produced by the crust (<0.5 s). The exceptions are melt-filled vertically-aligned disk-shaped inclusions and horizontally aligned periclase, the former most likely in low-velocity regions, the latter in high-velocity regions. Both models produce splitting significant enough to mask the effects of upper-mantle anisotropy. Strong azimuthal variations in splitting and discrepancies in SKS and SKKS splitting are diagnostic of these anisotropic models.

An improved velocity model for the crust and upper mantle along the central mobile belt of the Newfoundland Appalachian orogen and its offshore extension

New modelling of wide-angle reflection-refraction data of the Canadian Lithoprobe East profile 91-1 along the central mobile belt of the Newfoundland Appalachian orogen reveals new features of the upper mantle, and establishes links in the crust and upper mantle between existing land and marine wide-angle data sets by combining onshore-offshore recordings. The revised model provides detailed velocity structure in the 30-34 km thick crust and the top 30 km of upper mantle. The lower crust is characterized by a velocity of 6.6-6.8 km/s onshore, increasing by 0.2 km/s to the northeast offshore beneath the sedimentary basins. This seaward increase in velocity may be caused by intrusion of about 4 km of basic rocks into the lower crust during the extension that formed the overlying Carboniferous basins. The Moho is found at 34 km depth onshore, rising to 30 km offshore to the northeast with a local minimum of 27 km. The data confirm the absence of deep crustal roots under the central mobile belt of Newfoundland. Our long-range (up to 450 km offset) wide-angle data define a bulk velocity of 8.1-8.3 km/s within the upper 20 km of mantle. The data also contain strong reflective phases that can be correlated with two prominent mantle reflectors. The upper reflector is found at 50 km depth under central Newfoundland, rising abruptly towards the northeast where it reaches a minimum depth of 36 km. This reflector is associated with a thin layer (1-2 km) unlikely to coincide with a discontinuity with a large cross-boundary change in velocity. The lower reflector at 55-65 km depths is much stronger, and may have similar origins to reflections observed below the Appalachians in the Canadian Maritimes which are associated with a velocity increase to 8.5 km/s. Our data are insufficient for discriminating among various interpretations for the origins of these mantle reflectors.

A global tomographic model of shear attenuation in the upper mantle

We present a global three‐dimensional model of shear attenuation in the upper mantle, based on the measurement of amplitudes of low‐frequency (100–300s) Rayleigh waves observed at stations of the Geoscope and Iris networks. Attenuation coefficients are measured on R1 and R2 paths using a method which minimizes the effects of focussing due to propagation in a three‐dimensional elastic Earth. Through a series of tests which, in particular, involve the computation of synthetic models of attenuation and focussing, we demonstrate that long wavelength lateral variations in attenuation in the first 400–500 km of the mantle can indeed be resolved. The model is obtained in a two‐step procedure. The first step consists in the computation of maps of Rayleigh wave attenuation at different periods, using an inversion method without a priori parametrisation, which involves the introduction of a correlation length, chosen here at 3000 km to optimize the trade‐off between resolution and variance in the model. In the second step, after corrections for shallow structure, an inversion with depth is performed, assuming lateral heterogeneity is confined to depths between 80 and 650 km. The resulting model presents lateral variations in Qβ that are correlated with tectonic features, in particular ridges and shields in the first 250 km of the upper mantle. Below that depth the pattern shifts and becomes correlated with the hotspot distribution, particularly so if the buoyancy strength of hotspots is taken into account. Two major low‐velocity zones appear to be located in the central pacific and beneath northern Africa, in the depth range 300–500 km. This pattern seems to continue at greater depth, but resolution becomes insufficient below 500 km to draw definitive conclusions. The smooth lateral variations retrieved are on the order of ±50% down to 400 km. We propose an interpretation in terms of plume/lithosphere/ridge interaction in the upper mantle, arguing for deflection of the bulk of hot upwelling material from plumes towards ridges, which may be occurring between 200 and 300 km depth.

Mantle downwelling beneath the Australian-Antarctic discordance zone: evidence from geoid height versus topography

The Australian-Antarctic discordance zone (AAD) is an anomalously deep and rough segment of the Southeast Indian Ridge between 120° and 128°E. A large, negative (deeper than predicted) depth anomaly is centered on the discordance, and a geoid low is evident upon removal of a low-order geoid model and the geoid height-age relation. We investigate two models that may explain these anomalies: a deficiency in ridge-axis magma supply that produces thin oceanic crust (i.e. shallow Airy compensation), and a downwelling and/or cooler mantle beneath the AAD that results in deeper convective-type compensation. To distinguish between these models, we have calculated the ratio of geoid height to topography from the slope of a best line fit by functional analysis (i.e. non-biased linear regression), a method that minimizes both geoid height and topography residuals. Geoid/topography ratios of 2.1 ± 0.9 m/km for the entire study area (38°–60°S, 105°–140°E), 2.3 ± 1.8 m/km for a subset comprising crust ≤ 25 Ma, and 2.7 ± 2.0 m/km for a smaller area centered on the AAD were obtained. These ratios are significantly larger than predicted for thin oceanic crust (0.4 m/km), and 2.7 m/km is consistent with downwelling convection beneath young lithosphere. Average compensation depths of 27, 29, and 34 km, respectively, estimated from these ratios suggest a mantle structure that deepens towards the AAD. The deepest compensation (34 km) of the AAD is below the average depth of the base of the young lithosphere ( ∼ 30 km), and a downwelling of asthenospheric material is implied. The observed geoid height-age slope over the discordance is unusually gradual at −0.133 m/m.y. We calculate that an upper mantle 170°C cooler and 0.02 g/cm 3 denser than normal can explain the shallow slope. Unusually fast shear velocities in the upper 200 km of mantle beneath the discordance, and major-element geochemical trends consistent with small amounts of melting at shallow depths, provide strong evidence for cooler temperatures beneath the AAD.

Crust‐mantle whispering gallery phases: A deterministic model of teleseismic Pn wave propagation

Teleseismic Pn waves are modeled as a sum of whispering galley waves, which propagate in a waveguide composed of a simple high velocity mantle lid underlain by a low velocity zone. This model is able to account for those Pn wave propagation properties that are not dominated by scattering, i.e., their apparent velocity and lack of long period energy. Pn apparent velocity data constrain the P wave velocity gradient in the upper 100 km of mantle to be low; dvp/dz&lt;0.001 s−1. The whispering gallery rays are shown to have significant amplitude (when compared to the direct P wave) and to have spectra that rapidly fall off at long periods. Yet this model cannot account for certain details of the observed Pn amplitude spectra. Most important among these is the spectral ratio of Pn to P, which the model underestimates by a factor of 10. Nevertheless, the model presents a useful framework for understanding some characteristics of Pn wave propagation and provides an estimate of the distribution of energy with depth in the Pn waves.

Petrologic properties of the upper 670 km of the earth's mantle; geophysical implications

The relative distribution of Fe 2+ and Mg 2+ between mantle minerals is calculated for various mantle thermal models and is used to estimate compressional and shear velocity and density profiles to depths of 670 km using ultrasonic, equation of state, and thermodynamic data for olivines, pyroxenes, garnets, and spinels. Except for uncertainties, arising from the lack of a quantitative description of the low-velocity zone, and inadequate elasticity data for Ca-bearing minerals, especially the clinopyroxenes, the calculated seismic profiles for pyrolite and lherzolite upper mantles are virtually identical. Varying the total iron content ratio, Fe t = Fe/(Fe + Mg) from 0.1 to 0.2, brackets the seismic observation Important constraints to the composition and thermal profile of the mantle underlying continental regions are obtainable by fitting theoretical models to the detailed V p -profile of Helmberger and Wiggins. This profile, although possibly inaccurate in the vicinity of the low-velocity zone, is well determined below 350 km. Values of Fe t = 0.15 − 0.5 with a pyroxene-garnet (pyrope-almandine, majorite, solid-solution) content of 25 ± 10% and a relatively cool geotherm (1630–1650°C, 570–670 km) are implied from the position of the α (olivine) to α + β to β (modified spinel) transition, at a depth centered at ∼420 km, the position of the β to β + γ to γ (spinel) transition, centered at ∼500 km as well as the absolute value of the P-wave velocity. Because of the low resolution of the V S - and ρ-profiles, presently offered by seismic techniques, the concomitant V S - and ρ-profiles are useful as predictive, rather than comparitive, tools. If, as has been suggested by Anderson and co-workers, substantial enrichment of iron occurs with increasing depth in the mantle, this study demonstrates that for at least the continental regions, such an increase, if it occurs, would take place at, or below the 670-km discontinuity in the mantle. Distribution coefficients, based on thermochemical data, predict that within the upper 100 km of the mantle, successively greater relative iron contents will be present in the order: orthopyroxene, clinopyroxene, olivine, and garnet. The distribution of Fe 2+ between orthopyroxene and the coexisting mantle phases is calculated (using the Mössbauer data of Virgo and Hafner for the M 1, M 2 intersite energy in the formulation of Grover and Orville). The relative enrichment of iron in orthopyroxene (relative to clinopyroxene) in the upper 100 km decreases rapidly with depth. Below 100 km, orthopyroxene composition becomes insensitive to the assumed intersite energy. In the upper 100 km Fe 2+ is markedly concentrated in the garnet phase. For a total mantle value of Fe t of 0.15 the garnet will have a composition corresponding to Fe/(Fe + Mg) = 0.30. At a depth of about 100–150 km the Fe 2+ content of the garnet decreases and approaches that of the bulk mantle. Below this depth range, the relative content of iron in the garnet again increases.