Formation and growth of deep mantle plumes

Summary. If whole mantle convection occurs in the Earth's mantle, then the core–mantle boundary constitutes the lower boundary layer for mantle convection. This boundary layer appears to be unstable on a small scale, and thus may be a source of plumes of hot matter which penetrate the mantle and occasionally even the lithosphere (producing hot spots). A finite-amplitude numerical code is used to study the formation of such plumes and their growth through the mantle. The plumes are restricted to being two-dimensional sheets rather than cylinders. The initial conditions consist of a steadily convecting mantle, and plumes are produced by introducing a perturbation in the form of either a pulse or a steady stream of heat into the bottom of the mantle. Two main results are obtained: (1) A critical perturbation size has been found for a mantle with a Rayleigh number of 10⁷. Small perturbations produce plumes which fail to penetrate the mantle, and instead are swept up by the pre-existing convective pattern, while large perturbations succeed in penetrating the mantle and reaching the lithosphere. The critical perturbation size is shown empirically to be proportional to the effective bouyancy and to a factor related to the shape of the perturbation. A perturbed region 150km wide and 60 km deep should produce a successful plume when the temperature perturbation is 200K or more. (2) Deep mantle plumes appear to require on the order of 50–100Myr to penetrate the mantle; episodic plumes on shorter time-scales appear unlikely. A similar time is required for plumes forming in an initially static, uniform temperature mantle.     

Instabilities of fluid conduits in a flowing earth — are plates lubricated by the asthenosphere?

Summary A laboratory and theoretical study of the stability of conduits of buoyant fluid in a viscous shear flow has been conducted. The object of the study is to explain the formation of discrete islands in island chains such as the Hawaiian Emperor seamount chain, and to investigate a new method by which the variation of shear with depth in the mantle may be determined. The conduits were made by injecting oil into a more viscous oil of greater density. Initially a growing chamber of lower viscosity oil formed near the injector, but when the chamber got sufficiently large it rose as a buoyant spheroid. Behind this trailed a vertical cylindrical conduit through which fluid could continue to rise to the surface as long as the source continued. If the more viscous fluid was sheared laterally the conduit was gradually rotated to a more horizontal position. The diameter of the conduit increased with time due to a decreasing component of gravitational force along the axis of the conduit. When the conduit was tilted to more than 60° with the vertical, it began to go unstable by developing bumps which ultimately initiated a new chamber which rose to a new spot. In addition, if the Reynolds number of the conduit was greater than approximately ten, an axisymmetric wavy instability appeared in the walls of the conduit and the conduit had to be tilted less before a new chamber was initiated. If shear under the Pacific plate has to tilt buoyant mantle plumes to as much as 60° to form the relatively regular island chains associated with hot spots, most of the shear would be found in a zone with a vertical extent of less than 200 km.     

Lateral heterogeneity at the base of the mantle revealed by observations of amplitudes of PKP phases

Summary. Several studies have suggested that there are lateral heterogeneities in the velocity structure at or near the base of the mantle. Such heterogeneities can be studied through the analysis of amplitude ratios of core phases PKPAB and PKPDF for given earthquake-station pairs. In the epicentral distance range 155–175° these arrivals are well separated in time, and PKPAB has near-grazing incidence at the mantle—core boundary 80 that it is highly sensitive to lateral changes in velocity structure in that region, while PKPDF with its near-normal incidence is less sensitive. The observed amplitude ratios vary by well over an order of magnitude, but the size of the ratio is found to be correlated with the region of the core—mantle boundary sampled by the PKPAB rays: beneath the eastern Pacific and western Atlantic, for which abundant data are available, well-defined, contiguous regions (millions of square kilometres in extent) are found which correlated with predominantly larger or smaller amplitudes. We interpret this result as indicating differences in the degree to which the velocity structure of the lowermost mantle is heterogeneous.     

Seismotectonics and analysis of earthquakes from the Hindukush region

Summary. Over 80 earthquakes, exclusively from the Hindukush focal region, which were recorded at the Gauribidanur seismic array (GBA) have been used in this study. These events have similar epicentral distances and a narrow azimuthal range from GBA but varying focal depths from 10 to 240 km. A fault plane dipping steeply (75°) in the north-west direction and striking N 66° E has been investigated on the basis of the spatial distribution of earthquakes in two vertical planes through 68° E and 32° N. Short period P -wave recordings up to 30 s were processed using the adaptive cross-correlation filtering technique. Slowness and azimuthal anomalies were obtained for first arrivals. These anomalies show positive as well as negative bias and are attributed to a steep velocity gradient in the upper mantle between the 400–700 km depth range where the seismic rays have their maximum penetration. Relative time residuals between the stations of GBA owe their origin very near to the surface beneath the array. A search of the signals across the array revealed that most of the events occurring at shallower depths had complex signatures as compared to the deeper events. The structure near the source region, complicated source functions and the scattering confined to the crust—upper mantle near source are mainly responsible for the complexity of the Hindukush earthquakes as the transmission zone of the ray tubes from turning point to the recording station is practically the same.     

Numerical models of mantle convection with secular cooling

A 2-D time-dependent finite-difference numerical model is used to investigate the thermal character and evolution of a convecting layer which is cooling as it convects. Two basic cooling modes are considered: in the first, both upper and lower boundaries are cooled at the same rate, while maintaining the same temperature difference across the layer; in the second, the lower boundary temperature decreases with time while the upper boundary temperature is fixed at 0°C. The first cooling mode simulates the effects of internal heating while the second simulates planetary cooling as mantle convection extracts heat from, and thereby cools, the Earth's core. The mathematical analogue between the effects of cooling and internal heating is verified for finite-amplitude convection. It is found that after an initial transient period the central core of a steady but vigorous convection cell cools at a constant rate which is governed by the rate of cooling of the boundaries and the viscosity structure of the layer. For upper-mantle models the transient stage lasts for about 30 per cent of the age of the Earth, while for the whole mantle it lasts for longer than the age of the Earth. Consequently, in our models the bulk cooling of the mantle lags behind the cooling of the core-mantle boundary. Models with temperature-dependent viscosity are found to cool in the same manner as models with depth-dependent viscosity; the rate of cooling is controlled primarily by the horizontally averaged variation of viscosity with depth. If the Earth's mantle cools in a similar fashion, secular cooling of the planet may be insensitive to lateral variations of viscosity.     

An Analytic Model For A Mantle Plume Fed By A Boundary Layer

Summary. An approximate analytical solution for flow in a mantle plume of constant radius, viscosity, and density contrast is obtained in cylindrical coordinates. the differential equations for vertical velocity of the mantle surrounding the plume and for topography are homologous to the equation for flexure of an elastic plate. Although the model is too simple to be fully applicable to the Earth, one can conclude that the vertical velocity in the mantle changes significantly away from plumes, that the viscosity of the plume is important for controlling flow rate, and that the long-wavelength geoid anomalies are sensitive to the viscosity of the surrounding mantle. the first induced upwelling away from a plume is quite weak and unlikely to control the spacing of plumes.     

Regional 3-D heterogeneities by waveform inversion – application to the Atlantic area

The waveform inversion method described in Woodhouse & Dziewonski (1984) was modified to retrieve regional scale 3-D heterogeneities by using the minor arc part of seismograms. The lateral heterogeneities are expanded horizontally into blocks (10°× 10°) and radially into Legendre polynomials up to order 3 (0–670 km), and thus the results show much fine details of lateral variation than previous global scale studies. We assumed that the heterogeneities produce the perturbation of eigenfrequencies which are the minor arc average of local eigenfrequency shift. We applied the method to the upper mantle beneath the Atlantic Ocean and its environments. Care was taken about the weighting of the data set. We found that the fit of each seismogram became better when the weighting of each seismogram is proportional to the inverse of initial data residuals. Resolution is good in the triangular region surrounded by South America, Europe, and North America. Resolution is not good in the South Atlantic because of the poor path coverage. Depth resolution is not clear, because of the use of Legendre polynomials, though the results suggest a broad half-width of the order of 200 km or more. We found some similarities between previous global studies and our results. For example, low velocities beneath the East Pacific Rise, Chile Rise and Azores triple junction and a high velocity Canadian shield are obtained. However, there are also differences; the high-velocity zone beneath the Brazilian shield at shallow depth is not a prominent feature in this study. Instead, we found a somewhat unexpected feature near the Romanche and Vema fracture zones where shallow positive anomalies exist. Smoothed results calculated by the spherical harmonic expansion are also shown for the purpose of comparison with global studies.     

Upper mantle heterogeneities beneath Central Europe

Summary. An inversion of ISC travel-time data from selected earthquakes in the distance range 30°-90° to 53 stations in Central Europe has been used to model velocity down to 600 km depth. The model explains 0.1–0.2s of the residuals, as for other array studies, leaving 0.5 s unexplained as noise. The uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology. This may be due to deep-seated thermal anomalies or, in some areas, to delays introduced by passage of the rays through sedimentary cover. The deeper anomalies are smaller and unrelated to those in the lithosphere, which suggests that the asthenosphere is decoupled from the rigid lithosphere. The structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity. The models show some suggestion of a high velocity slab trending from east to west beneath the Alps.     

A lower mantle S-wave triplication and the shear velocity structure of D"

Summary. A lower mantle S-wave triplication detected with short- and long-period WWSSN and CSN recordings indicates a substantial shear velocity discontinuity near 280 km above the core–mantle boundary. The triplication can be observed in rotated SH seismograms from intermediate and deep focus events throughout the distance range from 70° to 95°. Three distinct source region–receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication, with slight systematic shifts in the triplication indicating up to 40 km variations in the depth of the discontinuity. Modelling of the observations with synthetic seismograms produced with the Cagniard de Hoop and reflectivity methods constrains the shear velocity increase to be 235 ± 0.25 per cent, comparable to upper mantle discontinuities. Short-period observations indicate that the velocity increase may be a sharp first-order discontinuity, or may extend over a transition zone no more than 50 km thick. The shear velocity gradient below the discontinuity, within the D" layer, is not well-constrained by the SH data, but slightly positive or near zero velocity gradients are consistent with the long-period amplitude ratios of ScSH/SH .     

Localized velocity anomalies in the lower mantle

Summary. 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 take-off 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 and 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.     

Seismic Modelling of Lateral Heterogeneity At the Base of the Mantle

Summary. Results from several recent studies suggest that there are lateral heterogeneities of up to a few per cent in the lowermost 150–200 km of the mantle (Bullen's D " region). Inferred anomaly sizes span the range from less than 50 km to greater than 1000 km.
In this study differences in the velocity structure among regions at the base of the mantle were inferred from an analysis of amplitude ratios of PKPAB and PKPDF for given earthquake-station pairs at distances greater than 155° (Sacks, Snoke & Beach). We distinguish two kinds of regions: A (anomalous) regions in which the mean, median and spread in AB/DF amplitude ratios are significantly higher (> 50 per cent) than for a reference radial earth model and N (normal) regions in which the distribution of the amplitude ratios is as expected.
The AB branch has near-grazing incidence to the core and therefore maximum sensitivity to velocity structure compared to the near-normal incident DF phases. Using an iterative, forward-modelling approach, we have determined general characteristics of the velocity structure for regions at the base of the mantle which can produce amplitude-ratio distributions similar to those for an A region. Agreement between model and data is obtained over the period range from 0.5 s to greater than 10 s using a laterally heterogeneous model for the D " region. the model consists of cells which are 200 km in lateral extent with velocity variations of up to ±1 per cent. This structure is modulated by a region-wide (1000km) perturbation which increases smoothly from zero at the edges of the region to a negative 1 per cent at the centre. Small cells (∼40 km) cannot produce anomalously large amplitude, long-period AB arrivals, and larger cells (∼1000km) cannot match the observed scatter. the ∼200 km scale anomalies could be small-scale convection cells confined to the D " region.     

The shear-wave velocity structure in the upper mantle beneath Eurasia

We develop an approach that allows us to invert for the mantle velocity structure within a finely parametrized region as a perturbation with respect to a low-resolution, global tomographic model. We implement this technique to investigate the upper-mantle structure beneath Eurasia and present a new model of shear wave velocity, parametrized laterally using spherical splines with ∼2.9° spacing in Eurasia and ∼11.5° spacing elsewhere. The model is obtained from a combined data set of surface wave phase velocities, long-period waveforms and body-wave traveltimes. We identify many features as narrow as few hundred kilometres in diameter, such as subducting slabs in eastern Eurasia and slow-velocity anomalies beneath tectonically active regions. In contrast to regional studies in which these features have been identified, our model encompasses the structure of the entire Eurasian continent. Furthermore, including mantle- and body-wave waveforms helped us constrain structures at depths larger than 250 km, which are poorly resolved in earlier models. We find that up to +9 per cent faster-than-average anomalies within the uppermost ∼200 km of the mantle beneath cratons and some orogenic regions are separated by a sharp gradient zone from deeper, +1 to +2 per cent anomalies. We speculate that this gradient zone may represent a boundary separating the lithosphere from the continental root, which might be compositionally distinct from the overlying lithosphere and remain stable either due to its compositional buoyancy or due to higher viscosity compared with the suboceanic mantle. Our regional model of anisotropy is not significantly different from the global one.     

Correlation between seismic anisotropy and Bouguer gravity anomalies in Tibet and its implications for lithospheric structures

As a baseline measurement for understanding the Himalayan–Tibetan orogen, a product of continent–continent collision between India and Eurasia, we analyse digital seismic data in order to constrain the seismic anisotropy of the Indian shield. Based on spatially sparse data that are currently available in the public domain, there is little shear-wave birefringence for SKS phases under the Indian shield, even though it is part of a fast-moving plate in the hotspot frame of reference. If most of the northern Indian mantle has little transverse anisotropy, the onset of significant anisotropy under Tibet marks the northern terminus of intact Indian lithosphere that is thrusting under the Himalayan–Tibetan orogen. Beyond this terminus, tectonic fabric such as that associated with the deforming lithospheric mantle of Eurasia must be present in the upper mantle. Along the profile from Yadong to Golmud, the only profile in Tibet where a number of shear-wave birefringence data are available, the amount of birefringence shows two marked increases, near 30° and 33°N, between which a local high in Bouguer gravity anomaly is observed. Such a correlation between patterns of shear-wave birefringence and gravity anomalies is explained by the juxtaposition of Indian lithosphere against the overlying Eurasian lithosphere: while the Eurasian lithospheric mantle appears only to the north of 30°N, the Indian lithospheric mantle extends northwards to near 33°N.     

Shear and compressional velocity models of the mantle from cluster analysis of long-period waveforms

We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique.
This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23° to 100°) and SS and PP (from 50° to 170°). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the core–mantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.     

Seismic P-wave anisotropy in the subcrustal lithosphere of north-west Australia

Summary. Reduced P_n travel times from the Archaean Pilbara Craton of north-west Australia show a strong correlation with azimuth, which could be used as evidence of anisotropy. However, the azimuthal correlation could also be explained by a southerly dip of between 1 and 2° on the crust–mantle boundary, although the models from several reversed seismic profiles across the craton suggest a smaller dip.
A time-term analysis of the P_n date yielded several models. The preferred solution, in which the dip on the crust–mantle boundary is similar to that in the models from the reversed profiles, has approximately 2 per cent anisotropy in the uppermost mantle, with the direction of maximum velocity 30° east of north. One possible cause of the anisotropy is that olivine crystals were aligned by syntectonic recrystallization and/or power law creep in the tensional environment caused at the base of the lithosphere by flexure during loading of the lithosphere by the strata of the Hamersley Basin which overlies the Pilbara Craton.
A seismic discontinuity occurs about 15 km below the crust–mantle boundary under the craton. A qualitative analysis of all available seismic data suggests that the velocity below the boundary is probably also anisotropic, with the direction of maximum velocity between north and 40° west of north. The direction of minimum velocity below the sub-Moho boundary correlates loosely with the direction of basement lineaments in the Proterozoic Capricorn Orogenic Belt to the south of the craton, suggesting that the anisotropy under the boundary may be younger than that immediately under the crust/mantle boundary. This is consistent with the notion that the Archaean lithosphere was thinner than the present lithosphere.     

Seismic evidence for a sharp lithospheric base persisting to the lowermost mantle beneath the Caribbean

Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.     

Upper-mantle structure of the Baltic Shield below the Swedish National Seismological Network (SNSN) resolved by teleseismic tomography

Upper-mantle structure under the Baltic Shield is studied using non-linear high resolution teleseismic P -phase tomography. Observed relative arrival-time residuals from 52 teleseismic earthquakes recorded by the Swedish National Seismological Network (SNSN) are inverted to delineate the structure of the upper mantle. The network consists of 47 (currently working) three-component broad-band stations located in an area about 450 km wide and 1450 km long. In order to reduce complications due to possible significant three-dimensionality of Earth structure, events chosen for this study lay close to in-line with the long-axis of the array  (±30°)  . Results indicate P -wave velocity perturbations of ±3 per cent down to at least 470 km below the network. The size of the array allows inversion for structures even at greater depths, and lateral variations of velocity at depths of up to 680 km appear to be resolved. Below the central part of the array (60°–64° N), where ray coverage is best, the data reveals a large region of relatively low velocity at depths of over about 300 km. At depths less than about 250–300 km, the models include a number of features, including an apparent slab-like structure dipping gently towards the north.     

New seismological constraints on differential rotation of the inner core from Novaya Zemlya events recorded at DRV, Antarctica

Novaya Zemlya nuclear test records at the seismic station DRV, Antarctica, are analysed in order to obtain further constraints on a possible differential rotation of the inner core with respect to the mantle. These data allow the sampling of the inner core along a nearly polar path in very stable conditions over more than two decades, from 1966 to 1990. The PKP (BC)– PKP (DF) traveltime residuals, which reflect the inner-core anisotropy and/or heterogeneities sampled along the path, exhibit a great stability through time. A computation of the residuals that are expected for various differential rotation rates and the same rotation axis as the mantle has been performed using the worldwide residual catalogue of Engdahl et al . (1997) for summary rays that include the time as an additional parameter in data stacking. Comparison of data and predictions shows that an eastward differential rotation with a rate as large as 3°  yr⁻¹, as suggested by some authors, is not possible, but an eastward rotation at 1°  yr⁻¹ or lower cannot be rejected.     

A boundary layer model for mantle convection with surface plates

Summary. A simple, analytical model for mantle convection with mobile surface plates is presented. Our aim is to determine under what conditions free convection can account for the observed plate motions, and to evaluate the thermal structure of the mantle existing under these conditions. Boundary layer methods are used to represent two-dimensional cellular convection at large Rayleigh and infinite Prandtl numbers. The steady-state structure consists of cells with isentropic interiors enclosed by thermal boundary layers. Lithospheric plates are represented as upper surfaces on each cell free to move at a uniform speed. Buoyancy forces are concentrated in narrow rising and decending thermal plumes; torques imparted by these plumes drive both the deformable mantle and overlying plate. Solutions are found for a comprehensive range of cell sizes. We derive an expression for the plate speed as a function of its length, the mantle viscosity and surface heat flux. Using mean values for these parameters, we find that thermal convection extending to 700 km depth can move plates at 1 cm yr^-1, while convection through the whole mantle can move plates at 4–5 cm yr^-1. Analysis of the steady-state temperature field, for the case of heating from below, shows that the upper thermal boundary layer develops a complex structure, including an 'asthenosphere' defined by a local maximum in the geotherm occurring at depths of 50–150 km.     

Regional vertical tectonics in the Eastern Mediterranean

Summary. New gravity observations from a systematic survey of the Eastern Mediterranean Sea and from a reconnaissance land survey in Central and Western Turkey have been compiled with existing data. Lack of sufficient geological and geophysical information precludes an analysis of the local anomalies or crustal structure; however, implications of the topography and gravity field at long wavelengths have been examined. Negative free-air anomalies characterize almost the entire Eastern Mediterranean basin and positive anomalies predominate in Turkey and the Aegean Sea. The change in sign coincides with the northern boundary of the African plate, and the wavelength and amplitude of the gravity variation are of the order of 1000 km and 100 mgal respectively. The lithosphere is probably unable to support such anomalies because the implied shear stresses are too large. The source of the anomalies is concluded to be in the asthenosphere where the low finite strength of material suggests that some sort of flow must exist to maintain the stresses. A good correlation is observed between the gravity and topography at wavelengths greater than 300 km; and the relationship is the same as that observed in the North Atlantic and the Central Pacific, as well as that computed for simple models of mantle convection. The gravity and topography of the Eastern Mediterranean can be explained in terms of flow in the upper mantle. This is the first region of subsidence for which this interpretation has been made.