Tomographic studies of the upper mantle beneath the Italian region*

Teleseismic P arrivals at seismological stations are inverted into a model of velocity perturbations down to a depth of about 470 km. Directionally independent average residuals, computed from steeply inciding waves, are transformed into a model of lithospheric thickness. Both models show a good correspondence with the main tectonic features of the Italian Peninsula. Positive velocity perturbations are observed beneath the Alps and in depths over 200 km also beneath the Po Basin. A high-velocity anomaly of the Tyrrhenian subduction is less pronounced, probably due to a directional dependence of P velocities in the mantle. Negative velocity perturbations indicate several low-velocity regions, e.g. beneath the Northern Apennines, the Sicily region and in the upper 100 km beneath the Po Basin. The amplitudes of velocity perturbations beneath the depth of 200 km are smaller on the average than those in the upper two layers. The whole region is characterized by large undulations of the lithosphere base which reaches depths from less than 60 km to more than 150 km. The most prominent lithospheric root beneath the Alps is a product of the collision between the European and the Adriatic plates while the lithospheric thickening beneath the Calabrian coast is likely to be connected with the eastern wing of the Tyrrhenian subduction. The dramatic changes of lithosphere thickness between the northern and the southern Apenninic arcs and northern Calabria as well as the thinnings at the western closure of the Po Basin, indicate important deep-seated boundaries of lithospheric blocks of autonomous geodynamic development.     

Structure of the crust and upper mantle beneath the central european rift system

The crustal structure of the central European rift system has been investigated by seismic methods with varying success. Only a few investigations deal with the upper-mantle structure. Beneath the Rhinegraben the Moho is elevated, with a minimum depth of 25 km. Below the flanks it is a first-order discontinuity, while within the graben it is replaced by a transition zone with the strongest velocity gradient at 20–22 km depth. An anomalously high velocity of up to 8.6 km/s seems to exist within the underlying upper mantle at 40–50 km depth. A similar structure is also found beneath the Limagnegraben and the young volcanic zones within the Massif Central of France, but the velocity within the upper mantle at 40–50 km depth seems to be slightly lower. Here, the total crustal thickness reaches only 25 km. The crystalline crust becomes extremely thin beneath the southern Rhônegraben, where the sediments reach a thickness of about 10 km while the Moho is found at 24 km depth. The pronounced crustal thinning does not continue along the entire graben system. North of the Rhinegraben in particular the typical graben structure is interrupted by the Rhenohercynian zone with a “normal” West-European crust of 30 km thickness evident beneath the north-trending Hessische Senke. A single-ended profile again indicates a graben-like crustal structure west of the Leinegraben north of the Rhenohercynian zone. No details are available for the North German Plain where the central European rift system disappears beneath a sedimentary sequence of more than 10 km thickness.     

The thermal structure of the Mediterranean upper mantle: a forward modelling approach

We develop and present an approach for gaining insight into the thermal structure of the upper mantle in a tectonically active region. As a starting point for the analysis we use regional geological reconstructions, based on data from surface geology, shallow seismics and paleomagnetism. For these reconstructions we determine the necessarily associated upper mantle processes, such as subduction. We then forwardly model the thermal structure of upper mantle corresponding with the geological reconstruction used. In this way we obtain an estimate of a very important characteristic – the temperature distribution of the upper mantle–that is independent from those estimates from the usual deep seismic methods. If sufficient detailed information on the seismological structure of the region is available, a quantitative test of our modelling results is possible, via the temperature dependence of seismic velocities. We apply our approach to the Mediterranean area, for which we will test the reconstruction published by Dercourt et al. (1986).     

Remnants of boninitic melts in the upper mantle beneath the central Pannonian Basin?

Summary We present a detailed textural and compositional study of two orthopyroxene-rich olivine websterites. One occurs as a vein in a harzburgite xenolith and the other is an individual xenolith, both found at Szentbékkálla in the Bakony–Balaton Highland Volcanic Field (central Pannonian Basin, western Hungary). The textural features of these orthopyroxene-rich rocks suggest that they crystallized from silicate melts to form veins in peridotite mantle rock. Their geochemical features, such as the presence of Al₂O₃-poor orthopyroxenes, Cr-rich spinels, and clinopyroxenes with U-shaped chondrite-normalized REE-patterns, indicate that the vein material formed from Mg-rich silicic (boninitic) melts at mantle depths. The olivine fabric investigation of both the veins and the wall-rock suggest that the development of the veins was followed by subsequent recrystallization during the Cenozoic evolution of the Carpathian–Pannonian region.     

青藏高原各地体的地壳-上地幔结构及其东西向的变化特征

青藏高原是由多个地体拼合而成的,在印度板块向北俯冲的长期作用下,各地体被挤压,地壳缩短,高原隆升。尽管在北北东向挤压作用下发生了高原的近南北向的断裂活动,但各地体本身的结构整体上保持相对稳定,不仅地壳浅部的地层、岩石、古生物保持着各自的特征,而且深部Moho面的变化和岩石圈的特征也是相似的。青藏高原的相距500km以上的2条宽频地震探测剖面的接收函数结果证实:高喜马拉雅地体、特提斯喜马拉雅地体、冈底斯地体、羌塘地体和巴颜喀拉地体在东西方向上保持着相近的速度特征。这充分说明,印度板块向北俯冲与青藏高原碰撞,引发各地体碰撞造山与高原隆升是地壳和岩石圈的整体构造运动,高原各地体,至少高原腹地仍然保持着大致相同的深部结构,Moho面、岩石圈底界面的深度和产状变化不大。     

Hornblende lherzolite and the upper mantle

Hornblende lherzolite nodules from the Kirsh volcano, near Ataq, South Arabian Federation, are likely to have been derived from the upper mantle. The hornblende is a pargasitic variety, rich in sodium and chromium.     

Mica in the upper mantle

Despite several lines of indirect evidence, there has hitherto been little unambiguous evidence of a volatile bearing phase in the upper mantle. Mica has been found as a primary phase in several specimens of peridotite and one specimen of garnet lherzolite from the Lashaine volcano, northern Tanzania.     

Poisson's ratios of the upper and lower crust and the sub-Moho mantle beneath central Honshu, Japan

Poisson's ratios of the upper and lower crust and the sub-Moho mantle beneath central Honshu, Japan, are investigated using three independent methods that are based on S to P ratios of apparent velocities, the Wadati diagrams and an inversion of P and S arrivals. Shallow earthquakes at distances of 200—500 km from the Nagoya University Telemeter Network are used for the apparent velocity ratio method. Crustal and subcrustal earth-quakes under the network are used for the other two methods. The network consists of wide-band seismometers with three components which are particularly suitable for detecting S waves. The three different methods give a consistent result for Poisson's ratio σ, that is, (1) σ = 0.23 ± 0.01 in the upper crust, (2) σ = 0.26−0.28 in both the lower crust and in the sub-Moho mantle. The result indicates a sharp contrast in σ between the upper and the lower crust rather than at the Moho. The low σ in the upper crust can only be explained by the presence of a substantial amount of free quartz, indicating granitic rocks. A higher σ in the lower crust suggests that this portion is presumably less saturated in silica and may be even undersaturated, pointing to intermediate to mafic rocks. The sub-Moho σ is almost equal to the σ averaged over the entire upper mantle that has been estimated from the Wadati diagrams of deep shocks beneath Japan but is higher than those calculated from Pn and Sn velocities in oceanic and stable continental regions.     

Model for upper mantle below Malaita,Solomon Islands,deduced from chemistry of lherzolite and megacryst minerals

Clinopyroxene, orthopyroxene, and garnet megacrysts show consistent increase of Na and Ti, and decrease of Cr, with increasing Fe/Mg. Three groups of clinopyroxenes occur with increasing Fe/Mg: subcalcic diopside, lamellar intergrowth with ilmenite, and augite. Chemical relationships indicate simultaneous crystallization of garnet, orthopyroxene and sub-calcic diopside megacrysts, and pyroxene thermometry-barometry indicates a trend from 29 kb?1,230 ° C to 25 kb?1,080 ° C as crystallization proceeded to higher Fe/Mg. Ilmenite-pyroxene thermometry suggests a mean of 965 ° C for crystallization of the intergrowths, but calibration depends on crystal-chemical assumptions. Lherzolite assemblages fall into three groups: two garnet-bearing types which equilibrated at 31 kb?1,150 ° C and 22 kb?900 ° C, and a type bearing Al-rich spinel which probably crystallized below 20 kb. The minerals from the lherzolites have lower Fe/Mg than the megacrysts. The simplest model involves: (i) metamorphic equilibration of lherzolitic rocks to the local geotherm, (ii) local melting of lherzolite at P > 30 kb, (iii) sequential crystallization of megacrysts as the magma rose intermittently, (iv) generation of alnöitic magma at P > 32 kb, and (v) eruption to surface with transport of megacrysts and lherzolitic xenoliths. Garnet, olivine, orthopyroxene and clinopyroxene in these Malaita xenoliths have lower Na, Ti, and P relative to their equivalents from southern African kimberlites. Only clinopyroxene contains K (up to 270 ppmw), and no Na was found in olivine.     

Subgeosynclinal tectonics of the upper mantle

The concept of tectonogen, a deep-seated tectonic mechanism (originating presumably in the Gutenberg layer), a narrow inclined zone of the maximum flow of energy from the mantle to the surface ("energovod" = "energagogue") is developed by analysis of deep-seated earthquakes, relations between island arcs and folded zones, positions of magmatic hearths, and other evidence. Evolution of the tectonogen is related to evolution of geosynclines, geoisotherms, types and composition of magmas, seismic and volcanic phenomena, migration of the sources of earthquakes, origin of the zone of deep fractures, and others. — IGR Staff.     

Redox state of the upper mantle

The oxygen fugacity condition of equilibration has been carefully determined from a spinel lherzolite from Mongolia, olivine xenocrysts from chrome pyrope-bearing peridotite nodules from kimberlites of Yakutia, and basaltic samples from ocean floor, iron arcs and the continental areas. These indicate that the spinel lherzolites occurring within alkali basalts from Mongolia, equilibrated under an \(f_{O_2 } \) condition similar to that of WM buffer. The diamond and chrome pyrope-bearing peridotites from the kimberlite pipes equilibrated between IW and WM buffers. Some of the ilmenite-bearing peridotite crystallized under \(f_{O_2 } \) conditions similar to that between WM and QFM buffers and chondrites equilibrated below the QFI buffer. It is concluded that during geochemical processes in the upper mantle the \(f_{O_2 } \) conditions vary broadly, and are similar to that between FMQ and IW buffers. There is a dramatic change in the composition of the kimberlitic fluid, which is CH₄-bearing at an early stage, but is in equilibrium with H₂O and CO₂ at a later stage. This is related to mass transfer of fluids from the lower part of the mantle with a low oxidation state to the upper part having a higher \(f_{O_2 } \) condition.     

Grain-size distribution and rheology of the upper mantle

The physical processes that govern the grain size of rocks in the upper mantle are examined. The analysis is based on the experimental data on creep, recrystallization, and grain growth in dunites and on a theoretical model for the thermomechanical structure of the cooling moving lithosphere. The grain size of rocks is shown to be determined by the in situ stress only at the deeper part where the temperature is high enough to allow significant strain rate. Above this depth, the microstructures record the thermomechanical history of rocks rather than the in situ stress.In the case of the oceanic lithosphere where the thermomechanical history is best known, the following features of grain-size distribution are found. At the uppermost mantle, where the amount of grain growth is limited, the grain size is determined by the initial value and the growth rate, and, where the effect of grain growth dominates, it increases with depth. When the amount of grain growth becomes large and the grain size reaches the steady state size corresponding to the ambient stress while the rock is hot enough to deform, the grain size is then determined by the applied stress. This grain size is, however, frozen, when the rock gets cool and the strain rate becomes too small to induce any further dynamic recrystallization. Thus, at the intermediate depth region, the grain size records the fossil (frozen) stress at which the microstructures of rock have been frozen. Since the frozen stress increases with age, the grain size in this depth interval decreases with depth. Finally, the grain size below this level reflects the in situ stress, and increases with depth, its extent being dependent on the nature of return flow in the deep mantle.Thus the grain size versus depth relation may show a sigmoid curve. The qualitative features of this curve may be similar also in the case of the continental lithosphere, if a similar thermal event (i.e., the intrusion of hot material and subsequent cooling) occurs. The results are quite consistent with the observed depth variation of olivine grain size in peridotite nodules (Avé Lallemant et al., 1980). The present model suggests that the depth of minimum grain size (65 and 150 km at the continental rift zone and the shield region respectively) corresponds to that where the mechanical properties of the upper mantle change from elastic to ductile at tectonic stress levels (~ 1 MPa) and in the geological time scale. This result leads to a new definition of the thickness of lithosphere in terms of its rheological properties. This thickness is about twice as large as that inferred from the flexure of lithosphere but approximately equal to seismic thickness. The model suggests the importance of grain growth as well as dynamic recrystallization and plastic flow in determining the texture of upper mantle rocks and therefore seismic anisotropy.     

Effects of melt percolation on highly siderophile elements and Os isotopes in subcontinental lithospheric mantle: A study of the upper mantle profile beneath Central Europe

The effects of melt percolation on highly siderophile element (HSE) concentrations and Re-Os isotopic systematics of subcontinental lithospheric mantle are examined for a suite of spinel peridotite xenoliths from the 4 Ma Kozákov volcano, Bohemian Massif, Czech Republic. The xenoliths have previously been estimated to originate from depths ranging from ∼32 to 70 km and represent a layered upper mantle profile. Prior petrographic and lithophile trace element data for the xenoliths indicate that they were variably modified via metasomatism resulting from the percolation of basaltic melt derived from the asthenosphere. Chemical and isotopic data suggest that lower sections of the upper mantle profile interacted with melt characterized by a primitive, S-undersaturated composition at high melt/rock ratios. The middle and upper layers of the profile were modified by more evolved melt at moderate to low melt/rock ratios. This profile permits an unusual opportunity to examine the effects of variable melt percolation on HSE abundances and Os isotopes.Most HSE concentrations in the studied rocks are significantly depleted compared to estimates for the primitive upper mantle. The depletions, which are most pronounced for Os, Ir and Ru in the lower sections of the mantle profile, are coupled with strong HSE fractionations (e.g., Os_N/Ir_N ratios ranging from 0.3 to 2.4). Platinum appears to have been removed from some rocks, and enriched in others. This enrichment is coupled with lithophile element evidence for the degree of percolating melt fractionation (i.e., Ce/Tb ratio).Osmium isotopic compositions vary considerably from subchondritic to approximately chondritic (γ_Os at 5 Ma from -6.9 to +2.1). The absence of correlations between ¹⁸⁷Os/¹⁸⁸Os and indicators of fertility, as is common in many lithospheric mantle suites, may suggest significant perturbation of the Os isotopic compositions of some of these rocks, but more likely reflect the normal range of isotopic compositions found in the modern convecting mantle. Osmium isotopic compositions correspondingly yield model Re-depletion (T_RD) ages that range from essentially modern to ∼1.3 Ga.Our data provide evidence for large-scale incompatible behavior of HSE during melt percolation as a result of sulfide dissolution, consistent with observations of prior studies. The degree of incompatibility evidently depended on melt/rock ratios and the degree of S-saturation of the percolating melt. The high Pt contents of some of these rocks suggest that the Pt present in this pervasively metasomatized mantle was controlled by a phase unique to the other HSE. Further, high Os concentrations in several samples suggest deposition of Os in a minority of the samples by melt percolation. In these rocks, the mobilized Os was characterized by similar to the ¹⁸⁷Os/¹⁸⁸Os ratios in the ambient rocks. There is no evidence for either the addition of Os with a strongly depleted isotopic composition, or Os with suprachondritic isotopic composition, as is commonly observed under such circumstances.     

Ferropericlase—a lower mantle phase in the upper mantle

Experiments on compositions along the join MgO–NaA³⁺Si₂O₆ (A=Al, Cr, Fe³⁺) show that sodium can be incorporated into ferropericlase at upper mantle pressures in amounts commonly found in natural diamond inclusions. These results, combined with the observed mineral parageneses of several diamond inclusion suites, establish firmly that ferropericlase exists in the upper mantle in regions with low silica activity. Such regions may be carbonated dunite or stalled and degassed carbonatitic melts. Ferropericlase as an inclusion in diamond on its own is not indicative of a lower mantle origin or of a deep mantle plume. Coexisting phases have to be taken into consideration to decide on the depth of origin. The composition of olivine will indicate an origin from the upper mantle or border of the transition zone to the lower mantle and whether it coexisted with ferropericlase in the upper mantle or as ringwoodite. The narrow and flat three phase loop at the border transition zone—lower mantle together with hybrid peridotite plus eclogite/sediments provides an explanation for the varying and Fe-rich nature of the diamond inclusion suite from Sao Luiz, Brazil.     

Shear wave splitting as a diagnostic of variable anisotropic structure of the upper mantle beneath central Fennoscandia

We analyze splitting of shear waves recorded during the SVEKALAPKO passive seismic experiment in south-central Finland to study fabrics of the mantle lithosphere of the Precambrian region and thus to bring information into a debate on existence of plate tectonics or its forms in the early stage of continent formation. Geographical variations of the splitting parameters and their distinct dependence on direction of wave propagation through the upper mantle allow us to identify six domains of the central Fennoscandian mantle lithosphere, including the Proterozoic–Archean transition, and to model their fabrics by joint inversion of body wave anisotropic parameters. Fabrics of the Archean mantle lithosphere can be approximated by a peridotite aggregate with lineation a dipping to the NE. On the other hand, anisotropy of the Proterozoic mantle lithosphere is weaker and we model its fabric by the (a, c) foliations dipping to the SE. We present a 3D self-consistent anisotropic model of the Proterozoic and Archean upper mantle along the SW-NE profile in the south-central Finland. Boundaries of inter-growing wedges of the Proterozoic and Archean mantle lithospheres explain the longitudinal and shear wave propagation and polarization, mantle xenolith ages, surface wave tomography and location of the upper mantle reflectors. We interpret the six anisotropic domains as fragments of mantle lithosphere retaining an old fossil olivine fabric which was created before these micro-continents assembled.     

The crust and upper mantle of continents

Models of continental crust and of geosynclinal processes, in their historical perspective, and generalized views on composition and structure of the tectonosphere are presented and discussed, particularly in reference to the local inversion stage, regional metamorphism, and granitization in geosynclines. Because of the known variations of the tectonosphere, depending on its position (e.g., under geosynclines, platforms, or zones of tectonic activation), it stands to reason that it varies also depending on the stage of the evolution of the overlying zones. -- V.P. Sokoloff.     

Structure of the crust and upper mantle in Canada

The seismic probing of the crust and upper mantle in Canada started in 1938 and since then has involved many government and university groups using a wide variety of techniques. These have included simple profiling with both wide and narrow station spacing, areal time-term surveys, detailed deep reflection experiments, very long-range refraction studies and the analysis of surface wave dispersion between stations of the Canadian Standard Network.

A review of the published interpretation leads to the general conclusion that:

1. (1) P_n-velocities vary from a value possibly as low as 7.7 km/sec under Vancouver Island to 8.6 km/sec and higher in the extreme eastern part of the shield and some parts of the Atlantic coast.

2. (2) Large areas of Canada have a crustal thickness of 30–40 km, with Vancouver Island, the southwestern Prairies, the Lake Superior basin and parts of the eastern shield of Quebec being thicker. No continental area in Canada is known to have a crust thinner than 29 km.

3. (3) The Riel discontinuity — a deep intra-crustal reflector and sometime refractor, is widely reported in the Prairies and Manitoba. It is not seen to the north in the vicinity of Great Slave Lake, nor in the Hudson Bay, Lake Superior and Maritime regions, nor in the interior of British Columbia. It may be present in some areas of the eastern shield.

4. (4) As experiments have become more detailed, crustal structures of greater complexity have been revealed. The concept that crustal structure becomes simpler with increasing depth is apparently unfounded.

Long-range refraction studies suggest that the Gutenberg P-wave low-velocity channel is poorly developed under the Canadian Shield. The analysis of the dispersion of surface waves, however, suggests that the channel is better developed for S-waves, and is present throughout the country. The lid of the channel is deepest under the central shield and shallowest under the Cordillera.     

Links between the structure of the mantle lithosphere and morphology of the Cheb Basin (Eger Rift, central Europe)

The Cheb Basin (CHB), located in the western part of the Eger Rift (ER) and the western Bohemian Massif, is characterized by earthquake swarms, neotectonic crust movements and emanations of CO₂ dominated gases of mantle origin. Deep structure of the region can be characterized as junction of three domains of mantle lithosphere with different olivine fabrics revealed by consistent orientations of seismic anisotropy. The domains represent mantle components of the major tectonic units (micro-plates): Saxothuringian (ST), Teplá-Barrandian (TB) and Moldanubian (MD), which were assembled during the Variscan orogeny. The ST-TB boundary, reactivated during the Cenozoic extension, controlled the position and development of the ER and the CHB. We show that the CHB originated above the rejuvenated mantle suture between the ST and TB. Though the basin is located within the ST crust domain, which is thrust over the mantle junction, it is the mantle suture that controls the CHB shape and its development through the allochthonous ST crust. The seismically active Mariánské Lázně Fault limits the basin against the uplifted block of the Erzgebirge Crystalline Complex. The most subsided parts of the ER and CHB developed above the centre of the mantle transition, whereas a well expressed morphology developed above its flanks. Our study documents a long memory of the mantle lithosphere assembly inherited from the Variscan orogeny. It is possible that other continental regions also contain some of intra-plate basins that originated above healed palaeo-plate mantle boundaries.