The heterogeneous upper mantle low velocity zone (original) (raw)

Fine scale heterogeneity in the Earth's mantle-observation and interpretation

2008

The upper mantle low velocity zone (LVZ) is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals. The zone is present below a relatively constant depth of 100 km in most continental parts of the world, both in cratonic areas with high average velocity and tectonically active areas with low average velocity. Evidence for the low velocity zone arises from controlled and natural source seismology, including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces, calculations of receiver functions, and absolute velocity tomography. The available data indicates a more pronounced reduction in seismic velocity and Q-value for S-waves than P-waves as well as high electrical conductivity in the LVZ. Seismic waves are strongly scattered by the zone, which demonstrates the existence of smallscale heterogeneity. The depth to the base of the LVZ is systematically shallower in cold, stable cratonic areas than in hot, active regions of the world. Because of its global occurrence below a relative constant depth of 100 km, the LVZ cannot be explained by metamorphic or compositional variation and rheological changes. Calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth, provided that the rocks contain water and carbon dioxide. The presence of, even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80-100 km depth. The seismic velocities and Q-values are significantly reduced of rocks, which are close to the solidus or contain small amounts of partial melt. Hence, the LVZ may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km. Assuming that the rocks contain only limited amounts of fluids, this mechanism may explain the low velocities, Q-values, and resistivity, as well as the intrinsic scattering, and the characteristic variation in thickness of the low velocity zone.

Localized velocity anomalies in the lower mantle

Geophysical Journal International, 1983

Two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000-2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of S, ScS, P and PcP travel times and amplitudes from deep earthquakes in Peru, Bolivia, Argentina and the Sea of Okhotsk. Comparison of the relative patterns of ScS-S differential travel times and S travel-time residuals across North American WWSSN and CSN stations for the different source regions provides baselines for interpreting which phases have anomalous times. A region of low S and P velocities is located beneath Northern Brazil and Venezuela at depths of 1700-2700 km. This region produces S-wave delays of up to 4 s for signals from deep Argentine events recorded at eastern North American stations. The localized nature of the anomaly is indicated by the narrow bounds in azimuth (15") and takeoff angle (13") of the arrivals affected by it. The long period S-waves encountering this anomaly generally show 30-100 per cent amplitude enhancement, while the short-period amplitudes show no obvious effect. The second anomaly is a high-velocity region beneath the Caribbean originally detected by Jordan & Lynn, who used travel times from deep Peruvian events. The data from Argentine and Bolivian events presented here constrain the location of the anomaly quite well, and indicate a possible short-and long-period S-wave amplitude diminution associated with it. When the travel-time data are corrected for the estimated effects of these two anomalies, a systematic regional variation in ScS-S station residuals is apparent between stations east of and west of the Rocky Mountains. One possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath North America.

Shear velocity structure at the base of the mantle

Journal of Geophysical Research, 2000

We inverted 4864 ScS-S and 1671 S¡ £ ¢¤-SKS residual travel times for shear wave speed anomalies at the base of the Earth's mantle. We applied ellipticity corrections, accounted for mantle structure outside of the basal layer using mantle tomography models, and used finite size sensitivity kernels. The basal layer thickness was set to 290 km; however, the data allow thicknesses between 200 and 500 km. The residuals were inverted using a spherical harmonic basis set of degree 30 for a model that both is smooth and has a small Euclidean norm, which limited spectral leakage of higher-order structures into low-order wavelengths. Hotspots dominantly overlay slow wave speed regions. Nonsightings of ultralow-velocity zones (ULVZs) most frequently appear in fast regions, suggesting that slow regions at the base of the mantle are associated with ULVZs. However, ULVZ sightings appear in both slow and fast regions. Recently active subduction zones do not correlate with velocity anomalies; however, the locations of subduction zones active prior to 90 Ma correlate extremely well with fast anomalies, implying that slabs descend as fast as 2 cm yr¥ through the lower mantle. The correlation continues through the historical subduction record to 180 Ma, suggesting that slabs remain in the deep mantle at least 90 Myr. Fast anomalies reach +2%, while slow anomalies extend to-5%. If we assume that the anomalies are thermal and anharmonic in origin and apply a wave speed/thermal anomaly conversion, the temperature deviations would be over-500 ¦ K (cold) in fastest regions and over +1000 ¦ K (hot) in the slowest regions, which would initiate plumes much hotter than those observed at the surface. Alternative explanations for the large anomalies are widespread partial melt or compositional differences in the lowermost mantle.

Test of the upper mantle low velocity layer in Siberia with surface waves

Tectonophysics, 2006

The existence of the upper mantle low velocity layer (LVL) below 100 km depth in cratonic areas is tested with surface waves dispersion curves. Given the ambient noise we find that a pronounced LVL (80 km thick and 2% velocity reduction or 40 km thick and 5% velocity reduction) can be distinguished from a constant velocity model by comparison of the fundamental mode group velocities, whereas a thin LVL (less than 40 km thick) with small velocity contrast (less than 2%) cannot be resolved. The fundamental modes of Love and Rayleigh waves have similar properties and, in general, the phase velocity differences are smaller than the standard error. Phase velocity alone cannot discriminate between the models, and the group velocity is in general more sensitive to the velocity structure than the phase velocity. The higher modes at short periods could potentially determine a LVL but in reality it is difficult to obtain sufficiently accurate measurements. We invert the synthetic dispersion curves by the non-linear Hedgehog inversion method. A pronounced LVL (more than 40 km thick and with a strong velocity contrast of about 5%) is detectable by the non-linear inversion but for a thin LVL with a strong velocity contrast it is not possible to resolve both velocity and thickness. In the inversions all solutions include a LVL for models with a pronounced LVL, whereas the solution space includes models with and without a LVL for models with a zero or positive gradient velocity-depth structure. We invert also real data with travel path across the Siberian craton with the Hedgehog method. Almost all solutions include a LVL in the depth range of 80-150 km with a velocity contrast up to 2% to the surrounding intervals. Hence, the LVL appears to be a common feature of the Siberian upper mantle, although a constant velocity at the same depth range cannot be totally excluded. Despite low resolution at large depth, a pronounced asthenospheric LVL below a depth of about 225 km is a constant characteristic of the set of solutions.

Shear wave velocity, seismic attenuation, and thermal structure of the continental upper mantle

Geophysical Journal International, 2004

Seismic velocity and attenuation anomalies in the mantle are commonly interpreted in terms of temperature variations on the basis of laboratory studies of elastic and anelastic properties of rocks. In order to evaluate the relative contributions of thermal and non-thermal effects on anomalies of attenuation of seismic shear waves, Q −1 s , and seismic velocity, V s , we compare global maps of the thermal structure of the continental upper mantle with global Q −1 s and V s maps as determined from Rayleigh waves at periods between 40 and 150 s. We limit the comparison to three continental mantle depths (50, 100 and 150 km), where model resolution is relatively high.

The shear-wave velocity gradient at the base of the mantle

Journal of Geophysical Research, 1983

The relative amplitudes and travel have been directed toward obtaining global times of ScS and S phases are utilized to place averages, and the degree of lateral variation in constraints on the shear-wave velocity gradient D" properties remains an open question. above the core-mantle boundary. A previously A conflicting result was found by Mitchell and reported long-period ScSH/SH amplitude ratio Helmberger [1973], who utilized the relative minimum in the distance range 65 ø to 70 ø is shown amplitudes and timing of long-period ScS and S to be a localized feature, apparently produced by phases to constrain the S-wave velocity gradient an amplitude anomaly in the direct S phase, and in D". They found a minimum in the ScSH/SH therefore need not reflect the velocity gradient amplitude ratio near 68 ø , which was attributed to at the base of the mantle. The amplitude ratios low amplitudes of the ScS arrivals. Unable to that are free of this anomaly are consistent with explain this feature by models with negative or calculations for the JB model or models with mild near-zero shear velocity gradients in D", they positive or negative velocity gradients in the proposed models with positive S-wave velocity lowermost 200 km of the mantle. ScSV arrivals gradients above the CMB. These positive are particularly sensitive to the shear velocity gradients extended over 40 to 70 km above the structure just above the core-mantle boundary. core, reaching velocities at the CMB as high as The apparent arrival time of the peak of ScSV is 7.6 to 7.8 km/s. These models can explain the as much as 4 s •reater than that of ScSH in the observed amplitude ratio behavior, as well as an distance range 75 v to 80 ø for Sea of Okhotsk apparent difference observed in the arrival times events recorded in North America. This can be of transversely and radially polarized ScS. explained by interference effects produced by a Mitchell and Helmberger also proposed a low Q$ localized high velocity layer or strong positive zone in D", or finite outer core rigidity, to S wave velocity gradient in the lowermost 20 km explain the baseline of the ScSH/SH amplitude of the mantle. A velocity increase of about 5% ratios. While the majority of their data was for is required to explain the observed shift between deep South American events recorded in North ScSV and ScSH. This thin, high velocity layer America, they did analyze one deep Sea of Okhotsk varies laterally, as it is not observed in event for which the radial and transverse ScS similar data from Argentine events. Refined arrival times were not different, which suggested estimates of the outermost core P velocity lateral variations in the D" velocity structure. structure are obtained by modeling SKS signals in In this paper we extend the analysis of ScS the distance range 75 ø to 85 ø ß and S phases using an enlarged data set in order to understand the discrepancy between the ß SES LHC BOZ ß SCB ON • •RCD AAM• DUG• •GOLFLO• N45*W ß WWSSN Stations ß CSN Stotions X Deep Argentine Events WES SCP

Structure, mineralogy and dynamics of the lowermost mantle

Mineralogy and Petrology, 2010

The 2004-discovery of the post-perovskite transition initiated a vigorous effort in high-pressure, high-temperature mineralogy and mineral physics, seismology and geodynamics aimed at an improved understanding of the structure and dynamics of the D"-zone. The phase transitions in basaltic and peridotitic lithologies under pT-conditions of the lowermost mantle can explain a series of previously enigmatic seismic discontinuities. Some of the other seismic properties of the lowermost mantle are also consistent with the changes in physical properties related to the perovskite (pv) to post-perovskite (ppv) transition. After more than 25 years of seismic tomography, the lowermost mantle structure involving the sub-Pacific and sub-African Large Low Shear-Velocity Provinces (LLSVPs) has become a robust feature. The two large antipodal LLSVPs are surrounded by wide zones of high Vs under the regions characterized by Mesozoic to recent subduction. The D" is further characterized by a negative correlation between shear and bulk sound velocity which could be partly related to an uneven distribution of pv and ppv. Ppv has higher VS and lower \( V_{\Phi } \) (bulk sound speed) than pv and may be present in thicker layers in the colder regions of D". Seismic observations and geodynamic modelling indicate relatively steep and sharp boundaries of the 200-500 km thick LLSVPs. These features, as well as independent evidence for their long-term stability, indicate that they are intrinsically denser than the surrounding mantle. Mineral physics data demonstrate that basaltic lithologies are denser than peridotite throughout the lowermost mantle and undergo incremental densification due to the pvppv- transition at slightly shallower levels than peridotite. The density contrasts may facilitate the partial separation and accumulation of basaltic patches and slivers at the margins of the thermochemical piles (LLSVPs). The slopes of these relatively steep margins towards the adjacent horizontal core-mantle boundary (CMB) constitute a curved (concave) thermal boundary layer, favourable for the episodic generation of large mantle plumes. Reconstruction of the original positions of large igneous provinces formed during the last 300 Ma, using a paleomagnetic global reference frame, indicates that nearly all of them erupted above the margins of the LLSVPs. Fe/Mg-partitioning between pv, ppv and ferropericlase (fp) is important for the phase and density relations of the lower mantle. Electronic spin transition of Fe2+ and Fe3+ in the different phases may influence the Fe/Mg-partitioning and the radiative thermal conductivity in the lowermost mantle. The experimental determination of the \( {K_D}{^{Fe/Mg}_{pv/fp}}\left[ { = {{\left( {Fe/Mg} \right)}_{pv}}/{{\left( {Fe/Mg} \right)}_{fp}}} \right] \) and \( {K_D}{^{Fe/Mg}_{ppv/fp}} \) is technologically challenging. Most studies have found a \( {K_D}{^{Fe/Mg}_{pv/fp}} \) of 0.1-0.3 and a higher Fe/Mg-ratio in ppv than in pv. The experimental temperature is important, with the partitioning approaching unity with increasing temperature. Although charge-coupled substitutions of the trivalent cations Al and Fe3+ seem to be important in both pv and ppv (especially in basaltic compositions), the complicating crystal-chemistry effects of these cations are not fully clarified. The two anti-podal thermochemical piles as well as the thin ultra-low velocity zones next to the CMB may represent geochemically enriched reservoirs that have remained largely isolated from the convecting mantle through a major part of Earth history. The existence of such “hidden” reservoirs have previously been suggested in order to account for the imbalance between the inferred composition of the geochemically accessible convecting mantle and the observed heat flow from the Earth and chondritic models for the bulk Earth.

A Shear Velocity Discontinuity in the Lower Mantle

Geophysical Research Letters, 1983

A lower mantle S-wave triplication is detected using short-and long-period SH seismograms in the distance range 70 ø to 95 ø . Modeling of the observations with synthetic seismograms indicates that the tripliGation is produced by a 2.75 • 0.25% shear velocity increase about 280 km above the core-mantle boundary. The SH data from intermediate and deep focus events for three distinct source ragion-receiver array combinations show generally consistent travel time and Lay and Helmberger: A Lower Mantl• Discontinuity localized strong positive velocity gradients exist at