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1.
This paper presents new data on the upper mantle characteristics, and on seismicity and volcanism in Kamchatka. It is shown that the seismic activity in the Pacific focal layer decreases sharply below that narrow line on which the foci of the active volcanoes are situated. A map of longitudinal wave velocitiesV p in the mantle upper layers under Kamchatka is given. The lowest values ofV p (7.3–7.6 km/sec) are found near the volcanic belt. The graphs Θ=lg (Es/Ep) (h) for the Kamchatka earthquakes indicate that Θmin at the depths of 120–250 km may be caused by a concentration of magmatic melts. A map of bodies (magma chambers?) screening S- and P-waves at the depths of 30–100 km under Kamchatka has been compiled. These bodies are mainly located under the belt of active volcanoes.  相似文献   

2.
Arrival times of P and S waves from local earthquakes in the Kamchatka area of the Kurile-Kamchatka Island Arc are used for calculating a spatial model of the elastic wave velocity distribution to a depth of 200 km. The lithosphere is shown to be strongly stratified in its velocity properties and laterally heterogeneous within the mantle wedge and seismic focal zone. A lower velocity layer (an asthenospheric wedge) is identified at depths of 70–130 km beneath the Eastern Kamchatka volcanic belt. The morphology of the Moho interface and the velocity properties of the crust are studied. The main tectonic structures of the region are shown to be closely interrelated with deep velocity heterogeneities. Regular patterns in the statistics of the earthquakes are analyzed in relation to variations in the elastic wave velocities in the focal layer. A mechanism of lithospheric block displacements along weakened zones of the lower crust and upper mantle is proposed.  相似文献   

3.
We consider results from modeling the crustal and upper mantle velocity structure in Kamchatka by seismic tomography and compare these with gravity data and present-day tectonics. We found a well-pronounced (in the physical fields) vertical and lateral variation for the upper mantle and found that it is controlled by fault tectonics. Not only are individual lithosphere blocks moving along faults, but also parts of the Benioff zone. The East Kamchatka volcanic belt (EKVB) is confined to the asthenospheric layer (the asthenosphere lens) at a depth of 70–80 km; this lens is 10–20 km thick and seismic velocity in it is lower by 2–4%. The top of the asthenosphere lens has the shape of a dome uplift beneath the Klyuchevskoi group of volcanoes and its thickness is appreciably greater; overall, the upper mantle in this region is appreciably stratified. A low-velocity heterogeneity (asthenolith) at least 100 km thick has been identified beneath the Central Kamchatka depression; we have determined its extent in the upper mantle and how it is related to the EKVB heterogeneities. Gravity data suggest the development of a rift structure under the Sredinnyi Range volcanic belt. The Benioff zone was found to exhibit velocity inhomogeneity; the anomalous zones that have been identified within it are related to asthenosphere inhomogeneities in the continental and oceanic blocks of the mantle.  相似文献   

4.
Viscosity is a fundamental property of the mantle which determines the global geodynamical processes. According to the microscopic theory of defects and laboratory experiments, viscosity exponentially depends on temperature and pressure, with activation energy and activation volume being the parameters. The existing laboratory measurements are conducted with much higher strain rates than in the mantle and have significant uncertainty. The data on postglacial rebound only allow the depth distributions of viscosity to be reconstructed. Therefore, spatial distributions (along the depth and lateral) are as of now determined from the models of mantle convection which are calculated by the numerical solution of the convection equations, together with the viscosity dependences on pressure and temperature (PT-dependences). The PT-dependences of viscosity which are presently used in the numerical modeling of convection give a large scatter in the estimates for the lower mantle, which reaches several orders of magnitude. In this paper, it is shown that it is possible to achieve agreement between the calculated depth distributions of viscosity throughout the entire mantle and the postglacial rebound data. For this purpose, the values of the volume and energy of activation for the upper mantle can be taken from the laboratory experiments, and for the lower mantle, the activation volume should be reduced twice at the 660-km phase transition boundary. Next, the reduction in viscosity by an order of magnitude revealed at the depths below 2000 km by the postglacial rebound data can be accounted for by the presence of heavy hot material at the mantle bottom in the LLSVP zones. The models of viscosity spatial distribution throughout the entire mantle with the lithospheric plates are presented.  相似文献   

5.
The paper presents a review and analysis of new seismic data related to the structure of the mantle beneath the East European platform. Analysis of observations of long-range profiles revealed pronounced differences in the structure of the lower lithosphere beneath the Russian plate and the North Caspian coastal depression. The highest P-velocities found at depths around 100 km are in the range 8.4–8.5 km s?1. Deep structure of the Baltic shield is different from the structures of both these regions. No evidence of azimuthal anisotropy in the upper mantle was found. A distribution of P-velocity in the upper mantle and in the transition zone consistent with accurate travel-time data was determined. The model involves several zones of small and large positive velocity gradients in the upper mantle, rapid increases of velocity near 400 and 640 km depths and an almost constant positive velocity gradient between the 400 and 640 km discontinuities. The depth of the 640 km discontinuity was determined from observations of waves converted from P to SV in the mantle.  相似文献   

6.
I present the results of statistical hypothesis testing of Grand’s (2002) global tomography model of three-dimensional shear velocity variations for the middle mantle underneath eastern and southern Africa. I apply an F test to evaluate the validity of a model where a tilted, slow-velocity anomaly in the deepest mantle under southern Africa, known as the African superplume, is continuous with a slow-velocity anomaly in the upper mantle under eastern Africa. This null hypothesis is tested against alternative hypotheses, in which various “obstruction volumes” in the middle mantle are constrained to zero perturbation from the one-dimensional reference velocity during the tomographic inversion. I find that there is an equal probability of accepting an alternative hypothesis with a thin “obstruction volume” at 850–1,000 km depth, whereas volumes at other depths are rejected. But the alternative hypothesis, where a connection is forced at 850–1,000 km depth, is rejected. I conclude that the African superplume rises to at least 1,150 km depth, and that the upper mantle slow-velocity anomaly continues from the surface to below the mantle transition zone. I interpret the “obstruction volume” as a weakening of the superplume in the middle mantle.  相似文献   

7.
The densities of silicate liquids with basic, picritic, and ultrabasic compositions have been estimated from the melting curves of minerals at high pressures. Silicate liquids generated by partial melting of the upper mantle are denser than olivine and pyroxenes at pressures higher than 70 kbar, and garnet is the only phase which is denser than the liquid at pressures from 70 kbar to at least 170 kbar. In this pressure range, garnet and some fraction of liquid separate from ascending partially molten diapirs. It is therefore suggested that aluminium-depleted komatiite with a high Ca/OAl2O3 ratio may be derived from diapirs which originated in the deep upper mantle at pressures from 70 kbar to at least 140 kbar (200–400 km in depth), where selective separation of pyropic garnet occurs effectively. On the other hand, aluminium-undepleted komatiite is probably derived from diapirs originating at shallower depths (< 200 km). Enrichment of pyropic garnet is expected at depths greater than 200 km by selective separation of garnet from ascending diapirs. The 200-km discontinuity in the seismic wave velocity profile may be explained by a relatively high concentration of pyropic garnet at depths greater than 200 km.  相似文献   

8.
—Recently a high-resolution tomographic model, the P1200, based on P-wave travel times was developed, which allowed for detailed imaging of the top 1200 km of the mantle. This model was used in diverse ways to study mantle viscosity structure and geodynamical processes. In the spatial domain there are lateral variations in the transition zone, suggesting interaction between the lower-mantle plumes and the region from 600 km to 1000 km. Some examples shown here include the continental region underneath Manchuria, Ukraine and South Africa, where horizontal structures lie above or below the 660 km discontinuity. The blockage of upwelling is observed under central Africa and the interaction between the upwelling and the transition zone under the slow Icelandic region appears to be complex. An expansion of the aspherical seismic velocities has been taken out to spherical harmonics of degree 60. For degrees exceeding around 10, the spectra at various depths decay with a power-law like dependence on the degree, with the logarithmic slopes in the asymptotic portion of the spectra containing values between 2 and 2.6. These spectral results may suggest the time-dependent nature of mantle convection. Details of the viscosity structure in the top 1200 km of the mantle have been inferred both from global and regional geoid data and from the high-resolution tomographic model. We have considered only the intermediate degrees (l = 12–25) in the nonlinear inversion with a genetic algorithm approach. Several families of acceptable viscosity profiles are found for both oceanic and global data. The families of solutions for the two data sets have different characteristics. Most of the solutions asociated with the global geoid data show the presence of asthenosphere below the lithosphere. In other families a low viscosity zone between 400 and 600 km depth is found to lie atop a viscosity jump. Other families evidence a viscosity decrease across the 660 km discontinuity. Solutions from oceanic geoid show basically two low viscosity zones one lying right below the lithosphere; the other right under 660-km depth. All of these results bespeak clearly the plausible existence of strong vertical viscosity stratification in the top 1000 km of the mantle. The presence of the second asthenosphere may have important dynamical ramifications on issues pertaining to layered mantle convection. Numerical modelling of mantle convection with two phase transitions and a realistic temperature- and pressure-dependent viscosity demonstrates that a low viscosity region under the endothermic phase transition can indeed be generated self-consistently in time-dependent situations involving a partially layered configuration in an axisymmetric spherical-shell model.  相似文献   

9.
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10.
The ∼0.2 mm/yr uplift of Hawaiian islands Lanai and Molokai and Hawaiian swell topography pose important constraints on the structure and dynamics of mantle plumes. We have formulated 3-D models of mantle convection to investigate the effects of plume-plate interactions on surface vertical motions and swell topography. In our models, the controlling parameters are plume radius, excess plume temperature, and upper mantle viscosity. We have found that swell height and swell width constraints limit the radius of the Hawaiian plume to be smaller than 70 km. The additional constraint from the uplift at Lanai requires excess plume temperature to be greater than 400 K. If excess plume temperature is 400 K, models with plume radius between 50 and 70 km and upper mantle viscosity between 1020 and 3×1020 Pa s satisfy all the constraints. Our results indicate that mantle plume in the upper mantle may be significantly hotter than previously suggested. This has important implications for mantle convection and mantle melting. In addition to constraining plume dynamics, our models also provide a mechanism to produce the observed uplift at Lanai and Molokai that has never been satisfactorily explained before.  相似文献   

11.
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12.
The presence of a phenomenological relationship between high velocity regions in the Benioff zone and sources of relatively strong earthquakes (M ≥ 6) was established for the first time from the comparison of such earthquakes with the velocity structure of central Kamchatka in the early 1970s. It was found that, in the region with P wave velocities of 8.1–8.5 km/s, the number of M ≥ 6 earthquakes over 1926–1965 was 2.5 times greater than their number in the region with velocities of 7.5–8.0 km/s. Later (in 1979), within the southern Kurile area, Sakhalin seismologists established that regions with V P = 7.3–7.7 km/s are associated with source zones of M = 7.0–7.6 earthquakes and regions with V P = 8.1–8.4 km/s are associated with M = 7.9–8.4 earthquakes. In light of these facts, we compared the positions of M = 7.0–7.4 earthquake sources in the Benioff zone of southern Kamchatka over the period 1907–1993 with the distribution of regions of high P velocities (8.0–8.5 to 8.5–9.0 km/s) derived from the interpretation of arrival time residuals at the Shipunskii station from numerous weak earthquakes in this zone (more than 2200 events of M = 2.3–4.9) over the period 1983–1995. This comparison is possible only in the case of long-term stability of the velocity field within the Benioff zone. This stability is confirmed by the relationship between velocity parameters and tectonics in the southern part of the Kurile arc, where island blocks are confined to high velocity regions in the Benioff zone and the straits between islands are confined to low velocity regions. The sources of southern Kamchatka earthquakes with M = 7.0–7.4, which are not the strongest events, are located predominantly within high velocity regions and at their boundaries with low velocity regions; i.e., the tendency previously established for the strongest earthquakes of the southern Kuriles and central Kamchatka is confirmed. However, to demonstrate more definitely their association with regions of high P wave velocities, a larger statistics of such earthquakes is required. On the basis of a direct correlation between P wave velocities and densities, the distributions of density, bulk modulus K, and shear modulus μ in the upper mantle of the Benioff zone of southern Kamchatka are obtained for the first time. Estimated densities vary from 3.6–3.9 g/cm3 in regions of high V P values to 3.0–3.2 g/cm3 for regions of low V P values. The bulk modulus K in the same velocity regions varies from (1.4–1.8) × 1012 to (0.8–1.1) × 1012 dyn/cm2, respectively, and the shear modulus μ varies from (0.8–1.0) × 1012 to (0.5–0.7) × 1012 dyn/cm2, respectively. Examination of the spatial correlation of the source areas of southern Kamchatka M = 7.0–7.4 earthquakes with the distribution of elastic moduli in the Benioff zone failed to reveal any relationship between their magnitudes and the moduli because of the insufficient statistics of the earthquakes used.  相似文献   

13.
Major tectonic units of Spain have been investigated by deep seismic sounding experiments since 1974 to determine crustal structures and to delineate their differences. These areas are the central part of the Hercynian Meseta, and the Alpine chains: the Betic Cordillera in the south, including the Balearic promontory and the Alboran Sea, and the Pyrenees in the north.The main features of the crust and the upper mantle along a NNE-SSW cross-section from the Pyrenees to the Alboran Sea are described.The crust under the Meseta is typical of Hercynian areas found elsewhere in Europe, with an average thickness of 31 km, whereas the two Alpine regions are characterized by very large lateral inhomogeneities, such as rapid thickening of the crust to 50 and 40 km under the Pyrenees and the Betics, respectively. The deep-reaching E-W-trending North Pyrenean fault has a throw of 10–15 km at the base of the crust. A Pn velocity of 8.1 km s?1 is found under the entire Iberian Peninsula.In the Alboran Sea, strongly varying thicknesses of sediments, shallow variable depths to the Moho (～ 13 km under the Alboran ridge), and strong variations of Pn velocity between 7.5 and 8.2 km s?1 have been found.  相似文献   

14.
We have produced a P-wave model of the upper mantle beneath Southeast (SE) Asia from reprocessed short period International Seismological Centre (ISC) P and pP data, short period P data of the Annual Bulletin of Chinese Earthquakes (ABCE), and long period PP-P data. We used 3D sensitivity kernels to combine the datasets, and mantle structure was parameterized with an irregular grid. In the best-sampled region our data resolve structure on scale lengths less than 150 km. The smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3D model. Our tomographic inversions reveal high-velocity roots beneath the Archean Ordos Plateau, the Sichuan Basin, and other continental blocks in SE Asia. Beneath the Himalayan Block we detect high seismic velocities, which we associate with subduction of Indian lithospheric mantle. This structure is visible above the 410 km discontinuity and may not connect to the remnant of the Neo-Tethys oceanic slab in the lower mantle. Our images suggest that only the southwestern part of the Tibetan plateau is underlain by Indian lithosphere and, thus, that the upper mantle beneath northeastern Tibet is primarily of Asian origin. Our imaging also reveals a large-scale high-velocity structure in the transition zone beneath the Yangtze Craton, which could have been produced in multiple subduction episodes. The low P-wave velocities beneath the Hainan Island are most prominent in the upper mantle and transition zone; they may represent counter flow from the surrounding subduction zones, and may not be unrelated to processes beneath eastern Tibet.  相似文献   

15.
《Journal of Geodynamics》2008,45(3-5):149-159
Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO2 and 3He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Teplá-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.  相似文献   

16.
Locations of the Eger Rift, Cheb Basin, Quaternary volcanoes, crustal earthquake swarms and exhalation centers of CO2 and 3He of mantle origin correlate with the tectonic fabric of the mantle lithosphere modelled from seismic anisotropy. We suggest that positions of the seismic and volcanic phenomena, as well as of the Cenozoic sedimentary basins, correlate with a “triple junction” of three mantle lithospheres distinguished by different orientations of their tectonic fabric consistent within each unit. The three mantle domains most probably belong to the originally separated microcontinents – the Saxothuringian, Teplá-Barrandian and Moldanubian – assembled during the Variscan orogeny. Cenozoic extension reactivated the junction and locally thinned the crust and mantle lithosphere. The rigid part of the crust, characterized by the presence of earthquake foci, decoupled near the junction from the mantle probably during the Variscan. The boundaries (transitions) of three mantle domains provided open pathways for Quaternary volcanism and the ascent of 3He- and CO2-rich fluids released from the asthenosphere. The deepest earthquakes, interpreted as an upper limit of the brittle–ductile transition in the crust, are shallower above the junction of the mantle blocks (at about 12 km) than above the more stable Saxothuringian mantle lithosphere (at about 20 km), probably due to a higher heat flow and presence of fluids.  相似文献   

17.
Siberian traps are the result of huge basalt eruptions which took place about 250 Ma ago over a vast territory of Siberia. The genesis of Siberian traps is attributed to a mantle plume with a center in the region of Iceland or beneath the central Urals in terms of their present coordinates. The eruption mechanism is associated with delamination—replacement of the mantle lithosphere by the deep magma material. The receiver function analysis of the records from the Norilsk seismic station (NRIL) allows comparing these hypotheses with the factual data on the depth structure of the region of Siberian traps. The S-wave velocity section place the seismic lithosphere/asthenosphere boundary (LAB) at a depth of 155–190 km, commensurate with the data for the other cratons. The mantle lithosphere has a high S-wave velocity characteristic of cratons (4.6–4.8 km/s instead of the typical value 4.5 km/s). The seismic boundary, which is located at a depth around 410 km beneath the continents is depressed by ~10 km in the region of the NRIL station. The phase diagram of olivine/wadsleyite transformation accounts for this depression by a 50–100°С increase in temperature. At the depths of 350–400 km, the S-wave velocity drops due to partial melting. A new reduction in the S-wave velocities is observed at a depth of 460 km. The similar anomalies (deepening of the 410-km seismic boundary and low shear wave velocity at depths of 350–400 and 460–500 km, respectively) were previously revealed in the other regions of the Meso-Cenozoic volcanism. In the case of a differently directed drift of the Siberian lithosphere and underlying mantle at depths down to 500 km, these anomalies are barely accountable. In particular, if the mantle at a depth ranging from 200 to 500 km is fixed, the anomalies should be observed at the original locations where they emerged 250 Ma ago, i.e. thousands of km from the Siberian traps. Our seismic data suggest that despite the low viscosity of the asthenosphere, the mantle drift at depths ranging from 200 to 500 km is correlated with the drift of the Siberian lithospheric plate. Furthermore, the position of the mantle plume beneath the Urals is easier to reconcile with the seismic data than its position beneath Iceland because of the Siberian traps being less remote from the Urals.  相似文献   

18.
The mantle convection model with phase transitions, non-Newtonian viscosity, and internal heat sources is calculated for two-dimensional (2D) Cartesian geometry. The temperature dependence of viscosity is described by the Arrhenius law with a viscosity step of 50 at the boundary between the upper and lower mantle. The viscosity in the model ranges within 4.5 orders of magnitude. The use of the non-Newtonian rheology enabled us to model the processes of softening in the zone of bending and subduction of the oceanic plates. The yield stress in the model is assumed to be 50 MPa. Based on the obtained model, the structure of the mantle flows and the spatial fields of the stresses σxz and σxx in the Earth’s mantle are studied. The model demonstrates a stepwise migration of the subduction zones and reveals the sharp changes in the stress fields depending on the stage of the slab detachment. In contrast to the previous model (Bobrov and Baranov, 2014), the self-consistent appearance of the rigid moving lithospheric plates on the surface is observed. Here, the intense flows in the upper mantle cause the drift and bending of the top segments of the slabs and the displacement of the plumes. It is established that when the upwelling plume intersects the boundary between the lower and upper mantle, it assumes a characteristic two-level structure: in the upper mantle, the ascending jet of the mantle material gets thinner, whereas its velocity increases. This effect is caused by the jump in the viscosity at the boundary and is enhanced by the effect of the endothermic phase boundary which impedes the penetration of the plume material from the lower mantle to the upper mantle. The values and distribution of the shear stresses σxz and superlithostatic horizontal stresses σxx are calculated. In the model area of the subducting slabs the stresses are 60–80 MPa, which is by about an order of magnitude higher than in the other mantle regions. The character of the stress fields in the transition region of the phase boundaries and viscosity step by the plumes and slabs is analyzed. It is established that the viscosity step and endothermic phase boundary at a depth of 660 km induce heterogeneities in the stress fields at the upper/lower mantle boundary. With the assumed model parameters, the exothermic phase transition at 410 km barely affects the stress fields. The slab regions manifest themselves in the stress fields much stronger than the plume regions. This numerically demonstrates that it is the slabs, not the plumes that are the main drivers of the convection. The plumes partly drive the convection and are partly passively involved into the convection stirred by the sinking slabs.  相似文献   

19.
Stress in the lithosphere: Inferences from steady state flow of rocks   总被引：9，自引：0，他引：9  
Mechanical data and flow processes from steady state deformation experiments may be used to infer the state of stress in the lithosphere and asthenosphere. Extrapolations of flow equations to a representative geologic strain rate of 10–14/sec. for halite, marble, quartzite, dolomite, dunite and enstatolite are now warranted because the steady state flow processes in the experiments are identical to those in rocks and because the geotherms are reasonably well established. More direct estimates are obtained from free dislocation densities, subgrain sizes and recrystallized grain sizes all of which are functions only of stress. Using the last of these techniques, we have estimated stress profiles as a function of depth from xenoliths in basalts and kimberlites, whose depths of equilibration were determined by pyroxene techniques, from four different areas of subcontinental and suboceanic upper mantle. The results are similar and indicate stress differences of about 200 to 300 bars at 40 to 50 km, decaying to a few tens of bars at depths betow 100 km. These stresses are reasonable and are in accord with extrapolations of the mechanical data provided that allowance is made for a general increase in strain rate and decrease in viscosity with depth.  相似文献   

20.
A self-consistent approach is proposed for the investigation of the thermal conditions, chemical composition, and internal structure of the upper mantle of the Earth. Using this approach, the thermal state of the lithospheric mantle beneath the Siberian Craton (SC) is reconstructed from P velocities, taking into account the phase transitions, anharmonicity, and the effects of anelasticity. The velocities of seismic waves are more sensitive to temperature than to the composition of the mantle rocks, which allows the velocity models to be effectively used for reconstruction of the thermal regime of the mantle. The temperature at depths 100–300 km is reconstructed by inversion of the Kraton and Kimberlit superlong seismic profiles for compositions of the garnet harzburgite, lherzolite, and intermediate composition of garnet peridotite. The averaged temperature in the normal continental mantle is reconstructed by inversion of the IASP91 reference model for depleted and fertile substance. One-dimensional models and two-dimensional thermal fields undergo a substantial fall in temperature (～300–600°C) beneath the Siberian Craton as compared to the temperatures of the continental mantle and paleotemperatures inferred from the thermobarometry of xenoliths. Temperature profiles of the Siberian Craton deduced from seismic data lie between the conductive geotherms of 32.5–40.0 mW/m2 and below the P(H)-T values obtained for low- and high-temperature xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlite pipes. The thickness of the thermal lithosphere estimated from the intersection with the potential adiabat is 300–320 km, which is consistent with the data on heat flows and seismotomographic observations. This provides grounds for the assumption that the low-temperature anomalies (thermal roots of continents) penetrate down to a depth of 300 km. The analysis of the sensitivity of seismic velocity and density to the variations in temperature, pressure, and chemical and phase composition of petrological models shows that recognition of fine differences in chemical composition of the lithospheric rocks by seismic methods is impossible.  相似文献   

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