Deep Mantle Structure Research Articles

The structure of Earth's dynamic deep mantle and core-mantle boundary region

The structure of Earth's dynamic deep mantle and core-mantle boundary region

海底拡大系下の非対称電気伝導度構造

A comparative study of deep mantle structures beneath spreading centres revealed asymmetry of lithosphere and mantle. Multidisciplinary marine expeditions were conducted at two survey sites : one around a super-fast spreading ridge in the East Pacific Rise, and the other around a slow and stable ridge segment of the continental breakup region, the Gulf of Aden. At the former site, a Mantle ELectromagnetic and Tomography experiment (MELT) was conducted with a large number of seismometers and electromagnetometers deployed at the seafloor to image the shape and the geometry of partial melt bodies below the fast spreading centre. At the latter, a seafloor magnetotelluric array was deployed to obtain a twodimensional electrical conductivity structure beneath the continental breakup region, and the structure was compared with precise swath mapping and sea surface gravimetry. At both sites, the deep electrical conductivity structures showed remarkable asymmetry with respect to the ridge crest. Partial melt bodies that correspond to the seafloor-producing factory at the surface could not be observed right below the ridge crest, but they significantly shifted by several tens of kilometers from the ridge crest. The asymmetry also appeared in bathymetry, gravity and seismic structures. This implies that the mid-ocean ridges are created at the contacts of different oceanic mantles rather than right above the symmetric partial melt bodies.

The effect of D″ on PKP(AB−DF) travel time residuals and possible implications for inner core structure

The hypothesis that the deep inner core is anisotropic is based on PKP travel time observations at large distances and relies on a small number of very anomalous measurements for paths quasi-parallel to the Earth’s rotation axis. Here, we analyze a global dataset of PKP(AB−DF) travel times residuals, and discuss their significant dispersion (±2 s), and coherent large scale patterns. We show that the trends observed for quasi-equatorial paths are consistent with predictions from recent tomographic mantle models, when the latter are modified to account for strong heterogeneity at the base of the mantle under the Pacific Ocean and Africa, as documented in several recent studies. Likewise, for polar paths, we show that a large part of the signal could be explained by deep mantle structure. The effects of complex structure in the deep mantle on PKP(AB−DF) travel times should be carefully considered in order to reliably estimate the anisotropic structure of the central part of the inner core.

Constructing synthetics from deep earth tomographic models

Recent studies of deep mantle structure indicate strong heterogeneity. To conduct high-resolution waveform modelling of these structures, we have developed a new method to construct 2-D synthetics directly from block-style tomographic models. Unlike the WKBJ approximation, which utilizes rays overshooting and undershooting receivers, our method (WKM approximation) uses rays that arrive at the receiver. First, the ray paths from the 1-D layered reference model are used to localize each ray segment, where the anomalous velocities are applied by overlay, as in tomography. Next, new p_i (t_i) (p_i ray parameter, ti traveltime) are computed to satisfy Snell’s law along with their numerical derivative (δp/δt), which is used to construct a synthetic seismogram similar to the WKBJ method. As a demonstration of the usefulness of this method, we generated WKM synthetics for the D″ region of high velocities beneath Central America based on Grand’s tomography model. Reasonable fits to broad-band data are obtained by condensing his distributed anomalies into his lowermost mantle layer; such a 2-D model predicts synthetics containing a laterally varying S_cd triplication similar to observations.

A global isostatic gravity model of the Earth

The long-wavelength features of the external gravity field of the Earth contain the gravitational signal from deep-seated lateral mass and density inhomogeneities sustained by dynamic Earth mantle processes. To interpret the observed gravity field with respect to mantle dynamics and structures, it is essential first to remove the lithosphere-induced anomalous gravitational potential, which is generated by the topographic surface load and its isostatically compensating masses. Based upon the most recent global compilation of crustal thickness and density data and the age distribution of cooling oceanic lithosphere, residual topography and gravity are calculated by subtracting the ‘known’ crustal and oceanic lithosphere compensating masses and gravitational effects from the surface fields. Empirical admittances between residual topography and gravity are then computed to estimate the effective depths of the remaining compensating masses, which are not explained by the initial data and model assumptions. This additional compensation is eventually placed by adjusting the density in the uppermost mantle between the Moho and, on average, 70 km depth, with a maximum of 118 km under Tibet. The lithospheric mass distribution is used in a subsequent forward computation to create a global model of the lithosphere-induced gravitational potential. The resulting isostatic model is considered to be valid for spatial wavelengths longer than 500 km. The isostatic lithosphere model field, expressed in terms of both gravity and geoid heights, is subtracted from the observed free-air gravity field to yield a global set of 1° × 1° isostatic gravity disturbances and from a satellite-derived long-wavelength geoid to yield the isostatic residual geoid. The comparison of residual (mantle) gravity, residual topography and isostatic corrected gravity allows us to identify the main characteristics of the underlying mantle; for example, dynamic support by mantle flow of the North Atlantic topographic high. Applying the isostatic correction, the overall pattern of the geoid becomes smoother and the most pronounced features, which are separated in the observed geoid, tend to get connected to larger structures. These results stress the importance of separation of the lithospheric gravitational impact for a correct interpretation of the external gravity field, even in its very long-wavelength constituents. Also, the isostatic corrected geoid spectrum reveals a stronger decrease in power from degree 3 to degree 4 and degree 5 to degree 6, which is in accordance with seismological models of deep-mantle structures.

About this title

The coexistence in space and time of growing mountain belts and actively extending basins poses a number of yet unsolved questions in terms of mechanics. This problem is particularly crucial in the Mediterranean regions, where all Cenozoic basins opened in the iflternal zones of mountain belts.TheTyrrhenian Sea opened in the back-arc region of the Apennines, the Aegean Sea in the back-arc domain of the Hellenides and Hellenic arc, the Pannonian Basin behind the Carpathians and the Alboran Sea between the Betics and the Rif. In some examples, such as the Tyrrhenian Sea and the Aegean Sea, extension is ongoing while peripheral compression and convergence are active.The Alboran Sea and Pannonian Basin are now in a compression stage. Several models have been proposed to explain this coexistence of compression and extension: slab retreat during subduction process, detachments of a deep lithospheric root under the internal zones leading to radial extension and peripheral compression and slab detachments. This volume brings together contributions from geologists and geophysicists in the quest to solve the complex dynamic problem posed by the Mediterranean region. It presents a wealth of new data on various topics centred on the Mediterranean region from the deep mantle structure to the detailed geometry of sedimentary basins. This book results from the Integrated Basins Studies Project, which was funded by the European Commission and which involved the collaboration of over 200 researchers across Europe.

Dynamic topography, plate driving forces and the African superswell

Discovering the connection between processes observed to occur at the surface of the Earth and its internal dynamics remains an essential goal in the Earth sciences. Deep mantle structure, as inferred from seismic tomography or subduction history, has been shown to account well for the observed surface gravity fieldand motions of tectonic plates1,2,3. But the origin of certain large-scale features, such as the anomalous elevation of the southern and eastern African plateaux, has remained controversial. Whereas the average elevation of most cratons is between 400 and 500 m, the southern African plateau stands more than 1 km above sea level, with the surrounding oceans possessing a residual bathymetry in excess of 500 m (ref. 4). Global seismic tomography studies have persistently indicated the existence of a large-scale low-velocity anomaly beneath the African plate5,6,7,8,9,10 and here we show that mantle flow induced by the density variations inferred from these velocity anomalies can dynamically support the excess elevation of the African ‘superswell’. We also find that this upwelling mantle flow—which is most intense near the core–mantle boundary—constitutes a significant driving force for tectonic plates in the region.

Marine magnetotellurics for petroleum exploration Part I: A sea‐floor equipment system

Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s. For this reason the marine MT method traditionally has been used only at periods of 103 to 105 s to probe deep mantle structure; rarely does a sea‐floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for marine controlled‐source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea‐floor magnetic records can be replaced by land recordings, producing an effective sea‐surface MT response. Field trials of such equipment in 1-km-deep water produced good‐quality MT responses at periods of 3 to 1000 s; in shallower water, responses to a few hertz can be obtained. Using an autonomous sea‐floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.

Magnetotelluric Phase Anisotropy above Extensional Structures of the Neogene Pannonian Basin

Neogene extensional forces generated narrow rift zones (deep subbasins) in the Pannonian Basin, among others one of the deepest, the 7 km deep Békés graben. Above this structure strong magnetotelluric (MT) phase anisotropy (phase-deviation in two orthogonal directions) has been observed indicating the upwelling of the partially molten asthenosphere. This upwelling is certainly the first to prove the validity of the deep mantle structure of Buck's extensional narrow rift model by an electromagnetic method. A geotectonic relation will also be pointed out between the causes of MT phase anisotropy in cratonic and orogenic belts.

Two mantle-plume components in Hawaiian picrites inferred from correlated Os–Pb isotopes

OCEAN island basalts (OIBs) are thought to result from the melting of plumes rising from a boundary layer in the mantle, such as the core–mantle boundary or that between the upper and lower mantle at 660km depth1,2. OIBs display considerable compositional heterogeneity, generally attributed to mixing between components such as depleted mantle, subducted oceanic crust, continental lithospheric mantle and primitive mantle3–8. Although the existence of multiple endmembers in the OIB array is now well established, the relationships between crustal recycling, plume volcanism and deep mantle structure and composition are controversial. We present isotope data for picritic lavas from seven volcanic centres, sampling 2 million years of Hawaiian plume activity9, and show that the osmium and lead isotopic compositions form remarkably linear, negative arrays. These correlations are indicative of binary mixing, but are difficult to explain by combining recycled oceanic crust with depleted mantle. We argue that mixing between two distinct mantle components within the Hawaiian plume provides a better explanation of the isotopic variations of the picrites.

Lunar Interior Exploration by Japanese Lunar Penetrator Mission, LUNAR-A.

The Institute of Space and Astronautical Science (ISAS), Japan, plans to undertake a lunar mission, named as LUNAR-A, which is to be launched in 1997. The scientific objective of the mission is to explore the lunar interior using seismometry and heat-flow measurement to obtain a better understanding of the origin and evolution of the moon. The M-V, the newest version of the Mu series launch vehicles, will be used to send a 53 kg spacecraft into a lunar transfer orbit. Three penetrators containing ultra-sensitive seismometers and heat-flow probes will be deployed from a spacecraft onto the lunar surface, and will constitute a seismic and heat-flow measurement network of a much larger span than the Apollo ALSEP network. The seismic observations are expected to provide key data on the size of the lunar core and its physical properties, as well as data on deep lunar mantle structure. The heat flow measurements at three penetrator landing sites will also provide important data on thermal structure and concentrations of heat-generating elements in the moon. Combining these data, it is expected that we will be able to obtain much stronger geophysical constraints on the origin and evolution of the moon than has been previously obtained.

The KRISP 90 seismic experiment—a technical review

On the basis of a preliminary experiment in 1985 (KRISP 85), a seismic refraction/wide-angle reflection survey and a teleseismic tomography experiment were jointly undertaken to study the lithospheric structure of the Kenya rift down to depths of greater than 200 km. This report serves as an introduction to a series of subsequent papers and will focus on the technical description of the seismic surveys of the main KRISP 90 effort. The seismic refraction/wide-angle reflection survey was carried out in a 4-week period in January and February 1990. It consisted of three profiles: one extending along the rift valley from Lake Turkana to Lake Magadi, one crossing the rift at Lake Baringo, and one located on the eastern flank of the rift proper. A total of 206 mobile vertical-component seismographs, with an average station interval of about 2 km, recorded the energy of underwater and borehole explosions to distances of up to about 550 km. During the teleseismic survey an array of 65 seismographs was deployed to record teleseismic, regional and local events for a period of about 7 months from October 1989 to April 1990. The elliptical array spanned the central portion of the rift, with Nakuru at its center, and covered an area about 300 × 200 km, with an average station spacing of 10–30 km. Major scientific goals of the project were to reveal the detailed crustal and upper-mantle structure under the Kenya rift, to study the relationship between deep crustal and mantle structure and the development of sedimentary basins and volcanic features within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system, and to answer fundamental questions such as the mode and mechanism of continental rifting.

Deep crust and mantle structures related to rifting and basin formation in Skagerrak; new results from reprocessing of deep seismic profiles

(1992). Deep crust and mantle structures related to rifting and basin formation in Skagerrak; new results from reprocessing of deep seismic profiles. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 114, No. 2, pp. 245-247.

Mantle Convection

A wide range of geophysical and geochemical observations pertaining to convection in the earth's mantle and the dynamics of the tectonic plates is discussed. It is inferred that the dominant mode of mantle convection is a plate-scale flow and that the plates are an integral part of this flow. Upwelling buoyant plumes, that cause volcanic hotspots, are inferred to comprise a secondary model of convection arising from a relatively weak thermal boundary layer at the base of the mantle. The balance of a large range of evidence weighs against the transition zone being a barrier to flow, the strongest evidence coming from seafloor topography and the gravity field. We infer a significant viscosity increase, by perhaps two to three orders of magnitude, through the depth of the mantle, with a large part of this increase occurring through the transition zone. This viscosity increase can account for the low velocities of hotspots relative to plates and the apparent lag between surface plate configuration and deep mantle structure, as probed by seismology and the gravity field. The viscosity increase also enhances the survival of chemical heterogeneities in the mantle and produces an increase in heterogeneity and mean residence time with depth. There may also be inefficient gravitational settling of old subducted oceanic crust near the bottom of the mantle, which might account for seismological complications in the D" layer at the bottom of the mantle and for trace elements exceeding bulk-earth concentrations in hotspot sources. With ridges sampling the top of the mantle and plumes sampling the bottom, these features offer explanations for the main geochemical characteristics of, and differences between, mid-ocean ridge basalts and oceanic island (hotspot) basalts.

Investigation of deep slab structure using long‐period S waves

Travel times and amplitudes of long‐period SH, ScSH, sSH, and sScSH phases from several deep focus earthquakes in the northwest Pacific are analyzed for evidence of lithospheric slab penetration into the lower mantle. Inclusion of amplitude observations in the analysis provides constraints on lateral velocity gradients present in the deep slabs which are not resolvable using travel times alone. Travel time and amplitude residual spheres are presented for two deep focus events with good azimuthal coverage, but while interesting patterns are present, quantitative analysis is precluded by the lack of accurate methods for calculating synthetic long‐period seismograms for three‐dimensional slab models. Therefore, we focus our analysis on a two‐dimensional approximation to the downdip geometry of the Kurile and Japan slabs, which allows comparison of a much larger data set with accurate two‐dimensional synthetics, as well as the use of sS and sScS travel time and amplitude patterns as empirical corrections for deep mantle structure. While the limited azimuth range required for this approximation reduces the diagnostic capability of the data, it also increases our confidence that the corrected data are most sensitive to the near‐source region. The sS and sScS observations indicate that the downdip shear wave travel time pattern previously attributed to a deep slab extension is primarily caused by broad‐scale lower mantle heterogeneity. Once corrected for this structure, the S wave observations do not support a simple, undeformed slab steepening in dip and penetrating deep into the lower mantle beneath the Kurile and Japan Islands, as proposed by Jordan (1977) and Creager and Jordan (1984, 1986). Rather, the observations support shorter and/or broader slab models man those previously hypothesized: models which can probably be reconciled with P wave data. Further analysis of the complete three‐dimensional patterns will be required for more precise resolution of the penetration depths of these slabs, if, indeed, any deep slab heterogeneity is actually seismically detectable.

Lagging mantle convection, the geoid and mantle structure

In numerical models of convection incorporating migration of a simulated subduction zone, the main descending flow lags far behind the migrating trench, and a geoid low is associated with the main descending flow. This provides physical plausibility for the suggestion by Chase and Sprowl that present very long-wavelength (degree 2–4) geoid lows are associated with Mesozoic trench locations, and suggests further that the present long-wavelength geoid, deep mantle structure and hotspot distribution may be straightforward consequences of plate evolution since the Upper Paleozoic.

Deep structure under Yellowstone National Park U.S.A.: A continental “hot spot”

Abstract In order to understand the origin of long-lived loci of volcanism (sometimes called “hot spots”) and their possible role in global tectonic processes, it is essential to know their deep structure. Even though some work has been done on the crustal, upper-mantle, and deep-mantle structure under some of these “hot spots”, the picture is far from clear. In an attempt to study the structure under the Yellowstone National Park U.S.A., which is considered to be such a “hot spot”, we recorded teleseisms using 26 telemetered seismic stations and three groups of portable stations. The network was operated within a 150 km radius centered on the Yellowstone caldera, the major, Quaternary volcanic feature of the Yellowstone region. Teleseismic delays of about 1.5 sec are found inside the caldera, and the delays remain high over a 100 km wide area around the caldera. The spatial distribution and magnitude of the delays indicate the presence of a large body of low-velocity material with horizontal dimensions corresponding approximately to the caldera size (40 km × 80 km) near the surface and extending to a depth of 200–250 km under the caldera. Using ray-tracing and inversion techniques, it is estimated that the compressional velocity inside the anomalous body is lower than in the surrounding rock by about 15% in the upper crust and by 5% in the lower crust and upper mantle. It is postulated that the body is partly composed of molten rock with a high degree of partial melting at shallow depths and is responsible for the observed Yellowstone volcanism. The large size of the partially molten body, taken together with its location at the head of a 350 km zone of volcanic propagation along the axis of the Snake River Plain, indicates that the volcanism associated with Yellowstone has its origin below the lithosphere and is relatively stationary with respect to plate motion. Using our techniques, we are unable to detect any measurable velocity contrast in the mantle beneath the low-velocity body, and, hence, we are unable to determine whether the Yellowstone melting anomaly is associated with a deep heat source or with any deep phenomenon such as a convection plume, chemical plume, or gravitational anchor.

Studies of the deep mantle structures by electromagnetic soundings

Studies of the deep mantle structures by electromagnetic soundings

On the postulated Hawaiian plume with emphasis on the limitations of seismic arrays for detecting deep mantle structure

Ray tracing through simple three-dimensional velocity models clearly demonstrates that a localized region of mantle inhomogeneity beneath Hawaii is not the source of large dt/dΔ anomalies. Upper mantle inhomogeneity beneath western North America is shown to be the likely source of most anomalous seismic data recorded there from events in the western Pacific Ocean.

Effect of scattering onPcP/Pamplitude ratios at Lasa from 40° to 84° distance

Attempts to study the core-mantle boundary have traditionally been frustrated by the large scattered amplitude ratios of PcP to P phases. An analysis of these phases at the Large-Aperture Seismic Array (Lasa) shows that two sources of severe scattering are the crust and upper mantle structure under the receiver and the low-velocity zone near the source. Measured amplitudes at the 21 Lasa subarray sites show that P and PcP amplitude patterns from the same event are generally uncorrelated. As a result, PcP/P amplitude ratios have large standard deviations across Lasa, typically 50% of the mean PcP/P ratios. However, P patterns for two events several degrees apart are highly correlated, as are PcP patterns. Thus amplitude fluctuations across Lasa are caused by near-receiver scattering rather than deep mantle or core structure. Steered beams were formed of P and PcP phases for 35 events in the distance range 40°–84° with depths from 0 to 609 km. The PcP/P ratios obtained from these beams show a clear trend from ∼0.4 at a depth of 0 km for explosion data to ∼0.15 at depths greater than 200 km for all distances. This depth effect cannot be explained by laterally homogeneous low-Q models of the upper mantle. When the events are normalized to Lasa mb = 6.0, the resulting PcP amplitudes show no clear depth dependence. It is thus suggested that variations in P amplitude cause most of the scatter in PcP/P ratios obtained from beamed data. A horizontally laminated low-velocity zone consisting of alternating hard and soft layers could produce the observed variation of PcP/P amplitude ratios with depth, although other scattering mechanisms cannot be eliminated.