Thermal and compositional structure of the subcontinental lithospheric mantle: Derivation from shear wave seismic tomography (original) (raw)

Supplementing ground truth data with shear wave velocity, seismic attenuation, and thermal structure of the continental lithosphere

Seismic Research Review, 2015

Ground truth data provide the opportunity to calibrate regional seismic velocity and Q (inverse attenuation) models. However, in many cases, available wave propagation data are too sparse to characterize seismic velocities and Q everywhere. It is therefore of interest to examine on a global basis the relationship between regional geology and heat flow versus the seismic properties (Vs and Qs) of the upper mantle. To better understand the propagation of seismic waves caused by nuclear explosions, we have developed new, theoretical models of the dissipation of energy in the crystalline rocks typical for the Earth's mantle. Using laboratory results, we suggest a temperature dependence of attenuation through the activation energy. We therefore compare maps of the thermal structure of the continental lithosphere with the inverse attenuation of seismic shear waves Qs and seismic velocity Vs as determined from surface wave dispersion and amplitudes. Our study is based on recently available global databases. We compare the values of Qs , Vs, and temperature T at the depths of 50, 100, and 150 km in the continental lithosphere. We find that qualitatively (by the sign of the anomaly) the maps of Qs closely correlate with lithospheric temperatures. The best correlation is observed for the depth of 100 km, where the resolution of the attenuation model is the highest. At this depth, the contour of zero attenuation anomaly approximately corresponds to the 1000 o C contour of lithospheric temperature, in agreement with laboratory data on a sharp change in seismic attenuation and shear velocities in upper mantle rocks at 900-1000 o C. The correlation between Vs and two other parameters (T and Qs), though present, is less distinct. We find that most cratonic regions show high lithospheric Vs, Qs and low T. Several prominent low Qs regions correlate with high lithospheric temperatures. We calculate that even if temperature variations in the lithosphere are the main cause of seismic velocity and attenuation variations, the relation between temperature and seismic properties is non-linear.

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 continental lithosphere: Reconciling thermal, seismic, and petrologic data (by Artemieva I.M.)

Lithos, 2009

(by Artemieva I.M.) The goal of the present study is to extract non-thermal signal from seismic tomography models in order to distinguish compositional variations in the continental lithosphere and to examine if geochemical and petrologic constraints on global-scale compositional variations in the mantle are consistent with modern geophysical data. In the lithospheric mantle of the continents, seismic velocity variations of a non-thermal origin (calculated from global Vs seismic tomography data [Grand S.P., 2002. Mantle shear-wave tomography and the fate of subducted slabs. Philosophical Transactions of the Royal Society of London. Series A, 360, 2475–2491.; Shapiro N.M., Ritzwoller M.H. 2002. Monte-Carlo inversion for a global shear velocity model of the crust and upper mantle. Geophysical Journal International 151, 1–18.] and lithospheric temperatures [Artemieva I.M., Mooney W.D., 2001. Thermal structure and evolution of Precambrian lithosphere: A global study. Journal of Geophysical Research 106, 16387–16414.] show strong correlation with tectono-thermal ages and with regional variations in lithospheric thickness constrained by surface heat flow data and seismic velocities. In agreement with xenolith data, strong positive velocity anomalies of non-thermal origin (attributed to mantle depletion) are clearly seen for all of the cratons; their amplitude, however, varies laterally and decreases with depth, reflecting either a peripheral growth of the cratons in Proterozoic or their peripheral reworking. These cratonic regions where kimberlite magmas erupted show only weakly positive compositional velocity anomalies, atypical for the “intact” cratonic mantle. A reduction in the amplitude of compositional velocity anomalies in kimberlite provinces is interpreted to result from metasomatic enrichment (prior or during kimberlite emplacement) of the cratonic mantle, implying that xenolith data maybe non-representative of the “intact” cratonic mantle.

Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity

Earth and Planetary Science Letters, 2012

Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.

Upper-mantle shear-wave structure under East and Southeast Asia from Automated Multimode Inversion of waveforms

Geophysical Journal International, 2015

We present a new Sv-velocity model of the upper mantle under East and Southeast Asia constrained by the inversion of seismic waveforms recorded by broad-band stations. Seismograms from earthquakes occurred between 1977 and 2012 are collected from about 4786 permanent and temporary stations in the region whenever and wherever available. Automated Multimode Inversion of surface and multipleS waveforms is applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations are then solved for the seismic velocity perturbations in the crust and upper mantle with respect to a three-dimensional (3-D) reference model and a realistic crust. Major features of the lithosphere-asthenosphere system in East and Southeast Asia are identified in the resulting model. At lithospheric depth, low velocities can be seen beneath Tibet, whereas high velocities are found beneath cratonic regions, such as the Siberian, North China, Yangtze,) Tarim, and Dharwarand cratons. A number of microplates are mapped and the interaction with neighbouring plates is discussed. Slabs from the Pacific and Indian Oceans can be seen in the upper mantle. Passive marginal basins and subduction zones are also properly resolved.

Anelasticity of the Crust and Upper Mantle Beneath North and Central India from the Inversion of Observed Love and Rayleigh Wave Attenuation Data

Pure and Applied Geophysics, 1991

The fundamental mode Love and Rayleigh waves generated by earthquakes occurring in Kashmir, Nepal Himalaya, northeast India and Burma and recorded at Hyderabad, New Delhi and Kodaikanal seismic stations are analysed. Love and Rayleigh wave attenuation coefficients are obtained at time periods of 15 I00 seconds, using the spectral amplitude of these waves for 23 different paths along northern (across Burma to New Delhi) and central (across Kashmir, Nepal Himalaya and northeast India to Hyderabad and Kodaikanal) India. Love wave attenuation coefficients are found to vary from 0.0003 to 0.0022 km 1 for northern India and 0.00003 km I to 0.00016 km-i for central India. Similarly, Rayleigh wave attenuation Coefficients vary from 0.0002km-~ to 0.0016 km-~ for northern India and 0.00001 kin-~ to 0.0009 km-~ for central India. Backus and Gilbert inversion theory is applied to these surface wave attenuation data to obtain Q~-~ models for the crust and uppermost mantle beneath northern and central India. Inversion of Love and Rayleigh wave attenuation data shows a highly attenuating zone centred at a depth of 20-80 km with low Q for northern India. Similarly, inversion of Love and Rayleigh wave attenuation data shows a high attenuation zone below a depth of 100 kin. The inferred low Q value at mid-crustal depth (high attenuating zone) in the model for northern India can be by underthrusting of the Indian plate beneath the Eurasian plate which has caused a low velocity zone at this shallow depth. The gradual increase of Q~ ~ from shallow to deeper depth shows that the lithosphere asthenosphere boundary is not sharply defined beneath central India, but rather it represents a gradual transformation, which starts beneath the uppermost mantle. The lithospheric thickness is 100 km beneath central India and below that the asthenosphere shows higher attenuation, a factor of about two greater than that in the lithosphere. The very low Q can be explained by changes in the chemical constitution taking place in the uppermost mantle.

Seismic, petrological and geodynamical constraints on thermal and compositional structure of the upper mantle: global thermochemical models

2011

Mapping the thermal and compositional structure of the upper mantle requires a combined interpretation of geophysical and petrological observations. Based on current knowledge of material properties, we interpret available global seismic models for temperature assuming end-member compositional structures. In particular, we test the effects of modelling a depleted lithosphere, which accounts for petrological constraints on continents. Differences between seismic models translate into large temperature and density variations, respectively, up to 400 K and 0.06 g cm −3 at 150 km depth. Introducing lateral compositional variations does not change significantly the thermal interpretation of seismic models, but gives a more realistic density structure. Modelling a petrological lithosphere gives cratonic temperatures at 150 km depth that are only 100 K hotter than those obtained assuming pyrolite, but density is ∼0.1 g cm −3 lower. We determined the geoid and topography associated with the density distributions by computing the instantaneous flow with an existing code of mantle convection, STAG-YY. Models with and without lateral variations in viscosity have been tested. We found that the differences between seismic models in the deeper part of the upper mantle significantly affect the global geoid, even at harmonic degree 2. The range of variance reduction for geoid due to differences in the transition zone structure (i.e. from 410 to 660 km) is comparable with the range due to differences in the whole mantle seismic structure. Since geoid is dominated by very long wavelengths (the lowest five harmonic degrees account for more than 90 per cent of the signal power), the lithospheric density contrasts do not strongly affect its overall pattern. Models that include a petrological lithosphere, however, fit the geoid and topography better. Most of the long-wavelength contribution that helps to improve the fit comes from the oceanic lithosphere. The signature of continental lithosphere worsens the fit, even in simulations that assume an extremely viscous lithosphere. Therefore, a less depleted, and thus less buoyant, continental lithosphere is required to explain gravity data. None of the seismic tomography models we analyse is able to reproduce accurately the thermal structure of the oceanic lithosphere. All of them show their lowest seismic velocities at ∼100 km depth beneath mid-oceanic ridges and have much higher velocities at shallower depths compared to what is predicted with standard cooling models. Despite the limited resolution of global seismic models, this seems to suggest the presence of an additional compositional complexity in the lithosphere.

Shear velocity structure of the laterally heterogeneous uppermost mantle beneath the Indian region

The shear velocity structure of the Indian lithosphere is mapped by inverting regionalized Rayleigh wave group velocities in time periods of 15-60 s. The regionalized maps are used to subdivide the Indian plate into several geologic units and determine the variation of velocity with depth in each unit. The Hedgehog Monte Carlo technique is used to obtain the shear wave velocity structure for each geologic unit, revealing distinct velocity variations in the lower crust and uppermost mantle. The Indian shield has a high-velocity (4.4-4.6 km/s) upper mantle which, however, is slower than other shields in the world. The central Indian platform comprised of Proterozoic basins and cratons is marked by a distinct low-velocity (4.G4.2 km/s) upper mantle. Lower crustal velocities in the Indian lithosphere generally range between 3.8 and 4.0 km/s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities, respectively. A remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the Indian plate. The South Burma region along the eastern subduction front of the Indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle. High velocities (~4.8 km/s) are also observed in the upper mantle beneath the Ninetyeast ridge in the northeastern Indian Ocean.