Lithosphere-asthenosphere System Research Articles

The lithosphere–asthenosphere system beneath Ireland from integrated geophysical–petrological modeling — I: Observations, 1D and 2D hypothesis testing and modeling

Modeling the continental lithosphere's physical properties, especially its depth extent, must be done within a self-consistent petrological–geophysical framework; modeling using only one or two data types may easily lead to inconsistencies and erroneous interpretations. Using the LitMod approach for hypothesis testing and first-order modeling, we show how assumptions made about crustal information and the probable compositions of the lithospheric and sub-lithospheric mantle affect particular observables, particularly especially surface topographic elevation. The critical crustal parameter is density, leading to ca. 600m error in topography for 50kgm−3 imprecision. The next key parameter is crustal thickness, and uncertainties in its definition lead to around ca. 4km uncertainty in LAB for every 1km of variation in Moho depth. Possible errors in the other assumed crustal parameters introduce a few kilometers of uncertainty in the depth to the LAB.We use Ireland as a natural laboratory to demonstrate the approach. From first-order arguments and given reasonable assumptions, a topographic elevation in the range of 50–100m, which is the average across Ireland, requires that the lithosphere–asthenosphere boundary (LAB) beneath most of Ireland must lie in the range 90–115km. A somewhat shallower (to 85km) LAB is permitted, but the crust must be thinned (<29km) to compensate.The observations, especially topography, are inconsistent with suggestions, based on interpretation of S-to-P receiver functions, that the LAB thins from 85km in southern Ireland to 55km in central northern Ireland over a distance of <150km. Such a thin lithosphere would result in over 1000m of uplift, and such rapid thinning by 30km over less than 150km would yield significant north–south variations in topographic elevation, Bouguer anomaly, and geoid height, none of which are observed. Even juxtaposing the most extreme probable depleted composition for the lithospheric mantle beneath southern Ireland against the most extreme fertile composition beneath northern Ireland only allows some 20km of LAB variation; any further variations would produce effects that are well beyond those observed.One model that satisfies almost all the extant data to first order includes a spinel-peridotite upper lithospheric mantle layer to 85km in southern Ireland and to 55km in northern Ireland, thinning over a lateral distance of 150km. Below this in southern Ireland is a garnet peridotite layer extending down to 115km, and in northern Ireland a refertilized layer down to 95km. The mid-lithospheric chemical discontinuity (MLD) at the base of the Spinel Peridotite zone may explain the observed discontinuity in S-to-P (Sp) receiver functions.

The lithosphere–asthenosphere system beneath Ireland from integrated geophysical–petrological modeling II: 3D thermal and compositional structure

The lithosphere–asthenosphere boundary (LAB) depth represents a fundamental parameter in any quantitative lithospheric model, controlling to a large extent the temperature distribution within the crust and the uppermost mantle. The tectonic history of Ireland includes early Paleozoic closure of the Iapetus Ocean across the Iapetus Suture Zone (ISZ), and in northeastern Ireland late Paleozoic to early Mesozoic crustal extension, during which thick Permo-Triassic sedimentary successions were deposited, followed by early Cenozoic extrusion of large scale flood basalts. Although the crustal structure in Ireland and neighboring offshore areas is fairly well constrained, with the notable exception of the crust beneath Northern Ireland, the Irish uppermost mantle remains to date relatively unknown. In particular, the nature and extent of a hypothetical interaction between a putative proto Icelandic mantle plume and the Irish and Scottish lithosphere during the Tertiary opening of the North Atlantic has long been discussed in the literature with diverging conclusions. In this work, the present-day thermal and compositional structure of the lithosphere in Ireland is modeled based on a geophysical–petrological approach (LitMod3D) that combines comprehensively a large variety of data (namely elevation, surface heat flow, potential fields, xenoliths and seismic tomography models), reducing the inherent uncertainties and trade-offs associated with classical modeling of those individual data sets. The preferred 3D lithospheric models show moderate lateral density variations in Ireland characterized by a slightly thickened lithosphere along the SW-NE trending ISZ, and a progressive lithospheric thinning from southern Ireland towards the north. The mantle composition in the southern half of Ireland (East Avalonia) is relatively and uniformly fertile (i.e., typical Phanerozoic mantle), whereas the lithospheric composition in the northern half of Ireland (Laurentia) seems to vary from moderately depleted to fertile, in agreement with mantle xenoliths erupted in northwestern Ireland.

Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume

The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere 'excess ice', but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere-asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere-asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.

Pattern of Stress-Strain Accumulation due to a Long Dipslip Fault Movement in a Viscoelastic Layered Model of The Lithosphere–Asthenosphere System

The process of stress accumulation near earthquake faults during the aseismic period in between two major seismic events in seismically active regions has become a subject of research during the last few decades. In the present paper a long dip -slip fault is taken to be situated in a viscoelastic layer over a viscoelastic half space representing the lithosphere-asthenosphere system. A movement of the dip-slip nature across the fault occurs when the accumulated stress due to various tectonic reasons, e.g., mantle convection etc., exceeds the local friction and cohesive forces across the fault. The movement is assumed to be slipping in nature, expressions for displacements, stresses and strains are obtained by solving the associated boundary value problem with the help of integral transformation and Green's function method. A detailed study of these expressions may give some ideas about the nature of stress accumulation in the system, which in turn will be helpful in formulating an earthquake prediction programme

Stress and Strain Accumulation Due to a Long Dip-Slip Fault Movement in an Elastic-Layer over a Viscoelastic Half Space Model of the Lithosphere-Asthenosphere System

Most of the earthquake faults in North-East India, China, mid Atlantic-ridge, the Pacific seismic belt and Japan are found to be predominantly dip-slip in nature. In the present paper a dip-slip fault is taken situated in an elastic layer over a viscoelastic half space representing the lithosphere-asthenosphere system. A movement of the dip-slip nature across the fault occurs when the accumulated stress due to various tectonic reasons e.g. mantle convection etc., exceeds the local friction and cohesive forces across the fault. The movement is assumed to be slipping in nature, expressions for displacements, stresses and strains are obtained by solving associated boundary value problem with the help of integral transformation and Green’s function method and a suitable numerical methods is used for computation. A detailed study of these expressions may give some ideas about the nature of stress accumulation in the system, which in turn will be helpful in formulating an earthquake prediction programme.

A shear wave velocity model of the European upper mantle from automated inversion of seismic shear and surface waveforms

SUMMARY We present a new, S-velocity model of the European upper mantle, constrained by inversions of seismic waveforms from broad-band stations in Europe and surrounding regions. We collected seismograms for the years 1990–2007 from all permanent stations in Europe for which data were available. In addition, we incorporated data from temporary experiments. Automated multimode inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations were then solved for seismic velocity perturbations in the crust and mantle with respect to a 3-D reference model with a realistic crust. We present two versions of the model: one for the entire European upper mantle and another, with the highest resolution, focused on the upper 200 km of the mantle beneath western and central Europe and the circum Mediterranean. The mantle lithosphere and asthenosphere are well resolved by both models. Major features of the lithosphere–asthenosphere system in Europe and the Mediterranean are indentified. The highest velocities in the mantle lithosphere of the East European Craton (EEC) are found at about 150 km depth. There are no indications for a deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved as well. The locations of kimberlites correlate with reduced S-wave velocities in the shallow cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the northwestern margin of the EEC and may indicate erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath western Europe and between the Tornquist–Teisseyre Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere–asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the circum Mediterranean is found in regions of shallow asthenosphere and close to changes in the depth of the lithosphere–asthenosphere boundary. Very low velocities at shallow upper-mantle depths are present from eastern Turkey towards the Dead Sea transform fault system and Sinai, beneath locations of recent volcanism. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massif Central, Sinai and the Dead Sea, the Canary Islands and Iceland.

The lithosphere-asthenosphere boundary observed with USArray receiver functions

Abstract. The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose, which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities, and is free from multiples, unlike P receiver functions. Only two major discontinuities are observed in the entire area down to about 300 km depth. These are the crust-mantle boundary (Moho) and a negative boundary, which we correlate with the lithosphere-asthenosphere boundary (LAB), since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70–80 km in large parts of westernmost North America. East of the Rocky Mountains, its depth is generally between 90 and 110 km. Regions with LAB depths down to about 140 km occur in a stretch from northern Texas, over the Colorado Plateau to the Columbia basalts. These observations agree well with tomography results in the westernmost USA and on the east coast. However, in the central cratonic part of the USA, the tomography LAB is near 200 km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100 km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) and Lekic and Romanowicz (2011) as a recently discovered mid-lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al., 2007).

Lithosphere density model in Italy: no hint for slab pull

Terra Nova, 23, 292–299, 2011AbstractThe lithosphere–asthenosphere system of the Italic region in terms of shear‐velocity and density distribution with depth is suitable to investigate the geodynamic context of the region. The velocity structure is obtained through nonlinear inversion of dispersion curves compiled from surface wave tomography on cells 1° × 1° and a smoothing optimization method to choose the representative cellular model, whose layering is used as fixed (a priori) information to obtain a density model by means of linear inversion of gravimetric data. Seismicity and heat flow are used as independent constraints in outlining both the crustal and the seismic lid thickness; the nonlinear moment tensor inversion of recent damaging earthquakes allows some insight in the ongoing kinematic processes. Asymmetry between west‐directed (Apennines) and east‐directed (Alps, Dinarides) subductions is a robust feature of the velocity model, while density model reveals that slabs are not denser than the ambient mantle, thus supplies no evidence for slab pull.

S-Wave Velocities of the Lithosphere–Asthenosphere System in the Caribbean Region

An overview of the S-wave velocity (V s) structural model of the Caribbean with a resolution of 2° × 2° is presented. New tomographic maps of Rayleigh wave group velocity dispersion at periods ranging from 10 to 40 s were obtained as a result of the frequency time analysis of seismic signals of more than 400 ray-paths in the region. For each cell of 2° × 2°, group velocity dispersion curves were determined and extended to 150 s by adding data from a larger scale tomographic study (Vdovin et al., Geophys. J. Int 136:324–340, 1999). Using, as independent a priori information, the available geological and geophysical data of the region, each dispersion curve has been inverted by the “hedgehog” non-linear procedure (Valyus, Determining seismic profiles from a set of observations (in Russian), Vychislitielnaya Seismologiya 4, 3–14. English translation: Computational Seismology (V.I. Keylis-Borok, ed.) 4:114–118, 1968), in order to compute a set of V s versus depth models up to 300 km of depth. Because of the non-uniqueness of the solutions for each cell, a local smoothness optimization has been applied to the whole region in order to choose a three-dimensional model of V s, satisfying this way the Occam's razor concept. Several known and some new main features of the Caribbean lithosphere and asthenosphere are shown on these models such as: the west directed subduction zone of the eastern Caribbean region with a clear mantle wedge between the Caribbean lithosphere and the subducted slab; the complex and asymmetric behavior of the crustal and lithospheric thickness in the Cayman ridge; the predominant oceanic crust in the region; the presence of continental type crust in Central America, and the South and North America plates; as well as the fact that the bottom of the upper asthenosphere gets shallower going from west to east.

The shear-wave velocity structure of the lithosphere–asthenosphere system in the Iberian area and surroundings

The S-wave velocity model, to the depth of 300 km for the Iberian area and surroundings, is obtained with the non-linear inversion of Rayleigh wave tomographic data in the period range from 5 to 150 s. The three-dimensional model of the region is assembled from 74 juxtaposed one-dimensional cellular structures, sized 1° × 1°, by means of a local smoothing optimisation. The S-wave velocity model shows the almost constant velocity (~4.40 km/s) in the upper mantle beneath the Iberian Massif, the well-developed low-velocity zones in the Valencia Trough and under northwest Africa, and specific features of the upper mantle layering in the zone of the Iberian range. The thickness of the crust varies between 25 and 35 km with exceptions in the zones of the Valencia Trough (11–18 km) and Pyrenees (35–45 km). Our Vs models are well correlated with the geological structure along Transmed I and evidence several features of the lithosphere–asthenosphere system in the Betic–Rif domain: quite distinct Iberian, Alboran and Meseta crustal domains; relatively thin, well-developed lithosphere under the Rif system; and clearly defined boundary between the upper and lower asthenosphere. The cellular velocity model of the Betics, where ultrapotassic lamproitic magmatism is observed, has features similar to those of other three areas in the western Mediterranean where ultrapotassic lamproitic magmatism of different age is observed. The crust and upper mantle models in these four areas can be correlated with the age of the magmatism and the local heat flow, and outline how, starting from a naturally quite undifferentiated mantle where magma genesis is currently in progress, the differentiation in lithosphere and asthenosphere can take place in time.

MAGNUS--A Seismological Broadband Experiment to Resolve Crustal and Upper Mantle Structure beneath the Southern Scandes Mountains in Norway

The 4-D evolution of topography is a complex scientific topic in geosciences that requires interdisciplinary efforts. In Europe, increased attention toward this research topic was originally coordinated by the TOPO-EUROPE initiative (Cloetingh et al. 2007), which has led to a continent-wide research program under the auspices of the European Science Foundation. One project within that framework is titled “TopoScandiaDeep— the Scandinavian Mountains: Deep Processes,” which aims at developing a self-consistent geophysical model for the lithosphere-asthenosphere system under southern Norway and at understanding the mechanisms that led to mountain-building far away from the plate boundary. A major component of the project is the analysis of recently acquired, passive seismological data from the MAGNUS ( MA ntle investi G ations of N orwegian U plift S tructures) experiment. The temporary network covered the high topography of southern Norway, which is, in the absence of compressional tectonics, a typical example of a particular geoscientific problem that is still not entirely understood—the origin of high topography along passive continental margins ( e.g. , Japsen and Chalmers 2000). Along the Northeast-Atlantic continental margin, at the western rim of the Fennoscandian shield, this topography is expressed through the Scandes mountain range, the second largest mountain range in Europe (Figure 1). It extends over a total length of more than 2,000 km along the entire Norwegian coast and has a peak topography of up to 2,500 m. With a few exceptions, such as the alpine formation in Sunnmore (central Norway), the topography is rather smooth, to a large extent dominated by subplanar (paleic) surfaces at elevations above 500 m. Contrary to the central European Alps, whose orogeny is in principle well understood as a classic collision belt, the evolution of the Scandes and its high topography remains an unsolved and highly debated issue. In particular the vicinity of …

Dynamic models of continental rifting with melt generation

Active or passive continental rifting is associated with thinning of the lithosphere, ascent of the asthenosphere, and decompressional melting. This melt may percolate within the partially molten source region, accumulate and be extracted. Two-dimensional numerical models of extension of the continental lithosphere–asthenosphere system are carried out using an Eulerian visco-plastic formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component (crust–mantle) and two-phase (solid–melt) system. Temperature-, pressure-, and stress-dependent rheologies based on laboratory data for granite, pyroxenite and olivine are used for the upper and lower crust, and mantle, respectively. Rifting is modelled by externally prescribing a constant rate of widening with velocities between 2.5 and 40 mm/yr. A typical extension experiment is characterized by 3 phases: 1) distributed extension, with superimposed pinch and swell instability, 2) lithospheric necking, 3) continental break up, followed by oceanization. The timing of the transition from stages 1) to 2) depends on the presence and magnitude of a localized perturbation, and occurs typically after 100–150 km of total extension for the lithospheric system studied here. This necking phase is associated with a pronounced negative topography (“rift valley”) and a few 100 m of rift flanks. The dynamic part of this topography amounts to about 1 km positive topography. This means, if rifting stops (e.g. due to a drop of external forces), immediate additional subsidence by this amount is predicted. Solidification of ascended melt beneath rift flanks leads to basaltic enrichment and underplating beneath the flanks, often observed at volcanic margins. After continental break up, a second time-dependent upwelling event off the rift axis beneath the continental margins is found, producing further volcanics. Melting has almost no or only a small accelerating effect on the local extension value ( β-value) for a constant external extension rate. Melting has an extremely strong effect on the upwelling velocity within asthenospheric wedge beneath the new rift. This upwelling velocity is only weakly dependent on the rifting velocity. The melt induced sublithospheric convection cell is characterized by downwelling flow beneath rift flanks. Melting increases the topography of the flanks by 100–200 m due to depletion buoyancy. Another effect of melting is a significant amplification of the central subsidence due to an increase in localized extension/subsidence. Modelled magma amounts are smaller than observed for East African Rift System. Increasing the mantle temperature, as would be the case for a large scale plume head, better fits the observed magma volumes. If extension stops before a new ocean is formed, melt remains present, and convection remains active for 50–100 Myr, and further subsidence is significant.

Carbonate metasomatism and CO 2 lithosphere–asthenosphere degassing beneath the Western Mediterranean: An integrated model arising from petrological and geophysical data

We present an integrated petrological, geochemical, and geophysical model that offers an explanation for the present-day anomalously high non-volcanic deep (mantle derived) CO 2 emission in the Tyrrhenian region. We investigate how decarbonation or melting of carbonate-rich lithologies from a subducted lithosphere may affect the efficiency of carbon release in the lithosphere–asthenosphere system. We propose that melting of sediments and/or continental crust of the subducted Adriatic–Ionian (African) lithosphere at pressure greater than 4 GPa (130 km) may represent an efficient mean for carbon cycling into the upper mantle and into the exosphere in the Western Mediterranean area. Melting of carbonated lithologies, induced by the progressive rise of mantle temperatures behind the eastward retreating Adriatic–Ionian subducting plate, generates low fractions of carbonate-rich (hydrous-silicate) melts. Due to their low density and viscosity, such melts can migrate upward through the mantle, forming a carbonated partially molten CO 2-rich mantle recorded by tomographic images in the depth range from 130 to 60 km. Upwelling in the mantle of carbonate-rich melts to depths less than 60–70 km, induces massive outgassing of CO 2. Buoyancy forces, probably favored by fluid overpressures, are able to allow migration of CO 2 from the mantle to the surface, through deep lithospheric faults, and its accumulation beneath the Moho and within the lower crust. The present model may also explain CO 2 enrichment of the Etna active volcano. Deep CO 2 cycling is tentatively quantified in terms of conservative carbon mantle flux in the investigated area.

Relationships between magmatism and lithosphere-asthenosphere structure in the Western Mediterranean and implications for geodynamics

Shear-wave (V S ) tomography along transects across the Western-Central Mediterranean area reveals heterogeneous lateral and vertical physical characteristics in the lithosphere-asthenosphere system (LAS). A 50 km thick low velocity layer (LVL), with V S ∼ 4.0–4.2 km/sec, typical of low rigidity fluid-bearing mantle material, is observed at a depth of about 70–120 km from offshore Provence, to Sardinia and the Central Tyrrhenian Sea. This LVL, enclosed between higher velocity mantle rocks, rises to a depth of less than 30 km below the recent and active volcanoes of Central Italy and the Southern Tyrrhenian Sea, where a maximum in the heath flow is observed. The LVL is absent beneath Southeastern France and the northern border of the African foreland.

Shear-wave attenuation tomography of the lithosphere–asthenosphere system beneath the Mediterranean region

3-D tomographic images of the shear-wave attenuation structure of the lithosphere–asthenosphere system beneath the Mediterranean region are derived from the inversion of anelastic attenuation coefficients obtained for the fundamental mode of nearly 3100 Rayleigh wavetrains recorded at very-broad-stations placed in the Mediterranean region. The two-station method is used to compute 124 interstation path-averaged attenuation coefficients within the 15–120 s period range. An adaptation to attenuation coefficients of the Yanovskaya–Ditmar formulation, specifically devoted to path-averaged surface-wave dispersion data and body waves, permits to obtain the local attenuation coefficients and their uncertainties for all the period range, at every node of a grid covering the studied region. Maps of stretching, azimuths and mean radii of the resolution ellipses are also obtained for every period. All of these parameters are strongly related to the spatial path-coverage, data quality and data coherency. A local attenuation dispersion curve is defined then in each grid node and the stochastic inversion of these curves yields 1-D models of inverse shear-wave intrinsic quality factor ( Q β − 1 ). The shear-wave velocity models obtained for every grid node in an elastic tomography developed in a previous work for the same domain are taken as reference models for the inversion process. The kriging contouring algorithm is used to represent the resulting 3-D Q β − 1 structure by means of horizontal and vertical cross-sections down to 160 km depth. The main patterns of this anelastic structure are mostly in agreement with the structural and tectonic features obtained in the reference shear-velocity tomography. The local attenuation maps manifest notable lateral changes for periods of 20 and 40 s, whereas more moderate spatial variation is observed for longer periods. The 3-D anelastic models show that, at shallow levels, the attenuation in the western Mediterranean is usually slightly higher than in the central-eastern part. The area from Corsica–Sardinia Islands to Italy presents high Q β − 1 values of 40.0–70.0 × 10 − 3 down to 75 km depth, whereas Q β − 1 does not exceed 20.0 × 10 − 3 in the rest of the Mediterranean basin, except for the south-eastern part, where slightly higher values are obtained (20.0–30.0 × 10 − 3 ). Those high Q β − 1 values suggest the presence of asthenospheric material at shallow levels, which would be related to distensive tectonic processes responsible for the recent opening of extensional basins, as the Tyrrhenian Sea and the Ligurian–Provençal basin. In the eastern Mediterranean, the vertical cross-sections at different latitudes and longitudes reveal at deep levels some structural patterns consistent with signs of subduction, which are linked to the convergence between the African and Eurasian plates.

Mantle fabric of western Bohemian Massif (central Europe) constrained by 3D seismic P and S anisotropy

New teleseismic data from a dense network of temporary and permanent seismic stations were analyzed to investigate large-scale fabric of the lithosphere–asthenosphere system beneath the western part of the Variscan Bohemian Massif. We study three-dimensional orientation and strength of seismic anisotropy and model a fabric of the mantle lithosphere from shear-wave splitting and directional terms of relative P residuals. Fast shear-wave polarizations have mostly E–W orientation and split-delay times δt are around 1.2 s, on average. Small but systematic changes of the fast S split polarizations show a difference at about 20° to 30° between the Teplá-Barrandian (TBU)/Moldanubian (MD) tectonic units and the adjacent part of the Saxothuringian (ST), and about 40° between two parts of the ST, separated by a suture identified in the upper crust by different authors and by other methods. On the other hand, P-velocity anisotropy clearly indicates three different orientations of fossil olivine fabrics with inclined axes of symmetry in the mantle lithospheres of three tectonic units (ST, TBU, MD). The seeming contradiction between the results of the two independent data sets can be explained by different 3D anisotropic models: one with hexagonal ‘slow’ symmetry axis b and divergently dipping ( a, c) foliations (ST, MD) and the other with hexagonal or orthorhombic symmetry with dipping high-velocity lineation a (TBU). These 3D self-consistent anisotropic models are compatible for both the P- and S-anisotropy observables. Our results suggest that a directionally varying constituent of the anisotropic signal is “frozen” in the mantle lithosphere. Regional changes in seismic anisotropy thus map tectonic boundaries, which cut the whole lithosphere, though with shifted crustal and mantle parts (TBU). Depending on symmetry and orientation of fabrics of the lithosphere domains, lateral changes are reflected in either P-velocity anisotropy and/or shear-wave polarizations. About half of the shear-wave split-time delays with predominantly E–W polarization azimuths represent a constant anisotropic signal which we associate with an olivine preferred orientation due to present-day flow in the asthenosphere. Our study emphasizes the importance of combining different methods of analysis and using complementary data sets in the three-dimensional analysis of mantle fabrics.

Upper mantle beneath the Eger Rift (Central Europe): plume or asthenosphere upwelling?

SUMMARY We present the first results of a high-resolution teleseismic traveltime tomography and seismic anisotropy study of the lithosphere–asthenosphere system beneath the western Bohemian Massif. The initial high-resolution tomography down to a depth of 250 km did not image any columnar low-velocity anomaly which could be interpreted as a mantle plume anticipated beneath the Eger Rift, similar to recent findings of small plumes beneath the French Massif Central and the Eifel in Germany. Alternatively, we interpret the broad low-velocity anomaly beneath the Eger Rift by an upwelling of the lithosphere–asthenosphere transition. We also map lateral variations of seismic anisotropy of the mantle lithosphere from spatial variations of P-wave delay times and the shear wave splitting. Three major domains characterised by different orientations of seismic anisotropy correspond to the major tectonic units—Saxothuringian, Moldanubian and the Tepla-Barrandian—and their fabrics fit to those found in our previous studies of mantle anisotropy on large European scales.

Geophysical and petrological modelling of the structure and composition of the crust and upper mantle in complex geodynamic settings: The Tyrrhenian Sea and surroundings

Information on the physical and chemical properties of the lithosphere–asthenosphere system (LAS) can be obtained by geophysical investigation and by studies of petrology–geochemistry of magmatic rocks and entrained xenoliths. Integration of petrological and geophysical studies is particularly useful in geodynamically complex areas characterised by abundant and compositionally variable young magmatism, such as in the Tyrrhenian Sea and surroundings. A thin crust, less than 10 km, overlying a soft mantle (where partial melting can reach about 10%) is observed for Magnaghi, Vavilov and Marsili, which belong to the Central Tyrrhenian Sea backarc volcanism where subalkaline rocks dominate. Similar characteristics are seen for the uppermost crust of Ischia. A crust about 20 km thick is observed for the majority of the continental volcanoes, including Amiata–Vulsini, Roccamonfina, Phlegraean Fields–Vesuvius, Vulture, Stromboli, Vulcano–Lipari, Etna and Ustica. A thicker crust is present at Albani – about 25 km – and at Cimino–Vico–Sabatini — about 30 km. The structure of the upper mantle, in contrast, shows striking differences among various volcanic provinces. Volcanoes of the Roman region (Vulsini–Sabatini–Alban Hills) sit over an upper mantle characterised by V s mostly ranging from about 4.2 to 4.4 km/s. At the Alban Hills, however, slightly lower V s values of about 4.1 km/s are detected between 60 and 120 km of depth. This parallels the similar and rather homogeneous compositional features of the Roman volcanoes, whereas the lower V s values detected at the Alban Hills may reflect the occurrence of small amounts of melts within the mantle, in agreement with the younger age of this volcano. The axial zone of the Apennines, where ultrapotassic kamafugitic volcanoes are present, has a mantle structure with high-velocity lid ( V s ∼ 4.5 km/s) occurring at the base of a 40-km-thick crust. Beneath the Campanian volcanoes of Vesuvius and Phlegraean Fields, the mantle structure shows a rigid body dipping westward, a feature that continues southward, up to the eastern Aeolian arc. In contrast, at Ischia the upper mantle contains a shallow low-velocity layer ( V s = 3.5–4.0 km/s) just beneath a thin but complex crust. The western Aeolian arc and Ustica sit over an upper mantle with V s ∼ 4.2–4.4 km/s, although a rigid layer ( V s = 4.55 km/s) from about 80 to 150 km occurs beneath the western Aeolian arc. In Sardinia, no significant differences in the LAS structure are detected from north to south. The petrological–geochemical signatures of Italian volcanoes show strong variations that allow us to distinguish several magmatic provinces. These often coincide with mantle sectors identified by V s tomography. For instance, the Roman volcanoes show remarkable similar petrological and geochemical characteristics, mirroring similar structure of the LAS. The structure and geochemical-isotopic composition of the upper mantle change significantly when we move to the Stromboli–Campanian volcanoes. The geochemical signatures of Ischia and Procida volcanoes are similar to other Campanian centres, but Sr–Pb isotopic ratios are lower marking a transition to the backarc mantle of the Central Tyrrhenian Sea. The structural variations from Stromboli to the central (Vulcano and Lipari) and western Aeolian arc are accompanied by strong variations of geochemical signatures, such as a decrease of Sr-isotope ratios and an increase of Nd-, Pb-isotope and LILE/HFSE ratios. The dominance of mafic subalkaline magmatism in the Tyrrhenian Sea basin denotes large degrees of partial melting, well in agreement with the soft characteristics of the uppermost mantle in this area. In contrast, striking isotopic differences of Plio-Quaternary volcanic rocks from southern to northern Sardinia does not find a match in the LAS geophysical characteristics. The combination of petrological and geophysical constraints allows us to propose a 3D schematic geodynamic model of the Tyrrhenian basin and bordering volcanic areas, including the subduction of the Ionian–Adria lithosphere in the southern Tyrrhenian Sea, and to place constraints on the geodynamic evolution of the whole region.

P- andS-velocity images of the lithosphere-asthenosphere system in the Central Andes from local-source tomographic inversion

SUMMARY About 50 000 P and S arrival times and 25 000 values of t* recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P, S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp, Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.

The Lithosphere-Asthenosphere System in the Calabrian Arc and Surrounding Seas – Southern Italy

A fairly detailed structural model of the lithosphere-asthenosphere system (thickness, S- and P-wave velocities of the crust and of the uppermost mantle layers) has been defined in the Calabrian Arc region (Southern Tyrrhenian Sea, Calabria and the northwestern part of the Ionian Sea) in Southern Italy using seismic data from literature as a priori constraints of the nonlinear inversion of surface-wave data. The main features identified by this study are: (1) A very shallow (less then 10 km deep) crust-mantle transition in the Southern Tyrrhenian Sea and a very low vs just below a very thin lid, in correspondence of the submarine volcanic bodies Magnaghi, Marsili and Vavilov, while the vs in the lid is quite high in the area that separates Marsili from Magnaghi-Vavilov; (2) a shallow and very low vs layer in the uppermost mantle in the areas of the Aeolian Islands, Vesuvius, Phlegraean Fields and Ischia, which represents their shallow-mantle magma source; (3) a thickened continental crust and lithospheric doubling in Calabria; (4) a crust about 25-km thick and a mantle velocity profile versus depth consistent with the presence of a continental rifted lithosphere, now thermally relaxed, in the investigated part of the Ionian Sea; (5) the subduction towards northwest of the Ionian lithosphere below the Southern Tyrrhenian Sea; (6) the subduction of the Adriatic/Ionian lithosphere underneath the Vesuvius and Phlegraean Fields.