On the seismic structure of Indian upper-mantle regions

Several velocity models on upper mantle regions of the world have been postulated during the last two decades. There has been a broad agreement amongst seismologists that upper mantle has got two transition zones, though the models differ in detail. These zones have been found to occur around ‘400 km’ and ‘650 km’ depth ranges with varying thicknesses of the zones. A limited number of such studies have been made on the upper mantle structure of the Indian subcontinent. High positive velocity gradients were reported to exist around the above depth range. Evidence for lateral heterogeneities has also been found. We address some problems like refinement of Indian upper mantle velocity models specially after considering the effect of scattering and attenuation on the short periodP-waves. The study of proper positioning of the cusps of the travel-time branches and their extension is essential as well. In our opinion, analysis of such problems would help in the better understanding of the nature of propagation of seismic waves and mechanism of earthquakes. Complexity of seismic signatures observed is another major problem and may also be taken into consideration.     

The persistent mantle plume myth

Seismology, thermodynamics and classical physics—the physics associated with the names of Fourier, Debye, Born, Grüneisen, Kelvin, Rayleigh, Rutherford, Ramberg and Birch—show that ambient shallow mantle under large long-lived plates is hundreds of degrees hotter than in the passive upwellings that fuel the global spreading ridge system, that potential temperatures in mantle below about 200 km generally decrease with depth and that deep mantle low shear wave-speed features are broad, sluggish and dome-like rather than narrow and mantle-plume-like. The surface boundary layer of the mantle is more voluminous and potentially hotter than regions usually considered as sources for intraplate volcanoes. This means that the ‘mantle plume’ explanation for Hawaii and large igneous provinces is unnecessary. In isolated systems, heated from within and cooled from above, upwellings are passive and large, which suggests that tomographic features, and upwellings, are responses to plate tectonics, spreading and subduction, at least until melting introduces a small intrinsic buoyancy at shallow depths. Melting anomalies, or ‘hotspots,’ are side-effects of plate tectonics and are fed primarily by shear-driven processes in the boundary layer (BL), not by deep buoyant upwellings. A dense basal melange (BAM) component further stabilises the lower boundary layer of the mantle. Mid-ocean ridges and associated broad passive depleted mantle (DM) upwellings probably originate in the transition region. Deeper mantle upwellings are broad domes that stay in the lower mantle.     

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).     

Garnet‐controlled very low velocities in the lower mantle transition zone at sites of mantle upwelling

Deep mantle plumes and associated increased geotherms are expected to cause an upward deflection of the lower–upper mantle boundary and an overall thinning of the mantle transition zone between about 410 and 660 km depth. We use subsequent forward modelling of mineral assemblages, seismic velocities, and receiver functions to explain the common paucity of such observations in receiver function data. In the lower mantle transition zone, large horizontal differences in seismic velocities may result from temperature‐dependent assemblage variations. At this depth, primitive mantle compositions are dominated by majoritic garnet at high temperatures. Associated seismic velocities are expected to be much lower than for ringwoodite‐rich assemblages at undisturbed thermal conditions. Neglecting this ultralow‐velocity zone at upwelling sites can cause a miscalculation of the lower–upper mantle boundary on the order of 20 km.     

Detecting a magnesiowüstite phase transition in the lower mantle by inversion of geomagnetic data

Global geomagnetic data are inverted for detecting a high-conductivity layer at depths of 1500–2000 km to test the hypothesis of a magnesiowüstite phase transition in the lower mantle. We present the results of processing of both synthetic and global data—average monthly values of the geomagnetic field from 1920 to 2009. The inverted global data are consistent with the possible existence of a high-conductivity layer at great depths in the lower mantle.     

下地幔及核幔边界结构及地球动力学

新一代高分辨率下地幔及核幔边界的地震层析成像,改变了我们对全球构造模式及地球动力过程的认识。古海洋岩石圈板片一直俯冲到下地幔底部,其残留体在核幔边界积累,并支持了地幔整体对流模式。位于核幔边界上的D″层有着十分复杂而精细的结构。紧靠核幔边界的地幔一侧发现了超低速层(ULVZ),它们可能是D″层内的局部熔融物,是引起地表热点的上升地幔柱的源头。     

Evidence of a partial melt zone beneath Stromboli volcano (Italy) from inversion of teleseismic receiver functions

Teleseismic body waves from broadband seismic stations are used to investigate the crustal and uppermost mantle structure of Stromboli volcano through inversion of the receiver functions (RFs). First, we computed RFs in the frequency domain using a multiple-taper spectral correlation technique. Then, the non-linear neighbourhood algorithm was applied to estimate the seismic shear wave velocity of the crust and uppermost mantle and to define the main seismic velocity discontinuities. The stability of the inversion solution was tested using a range of initial random seeds and model parameterizations. A shallow Moho, present at depth of 14.8 km, is evidence of a thinned crust beneath Stromboli volcano. However, the most intriguing and innovative result is a low S velocity layer in the uppermost mantle, below 32 km. The low S velocity layer suggests a possible partial melt region associated with the volcanism, as also recently supported by tomographic studies and petrological estimations.     

Upper lithospheric seismic velocity structure across the Pripyat Trough and the Ukrainian Shield along the EUROBRIDGE'97 profile

We present new results on the structure resulting from Palaeoproterozoic terrane accretion and later formation of one of the aulacogens in the East European Platform. Seismic data has been acquired along the 530-km-long, N–S-striking EUROBRIDGE'97 traverse across Sarmatia, a major crustal segment of the East European Craton. The profile extends across the Ukrainian Shield from the Devonian Pripyat Trough, across the Palaeoproterozoic Volyn Block and the Korosten Pluton, into the Archaean Podolian Block. Seismic waves from chemical explosions at 18 shot points at approximately 30-km intervals were recorded in two deployments by 120 mobile three-component seismographs at 3–4 km nominal station spacing. The data has been interpreted by use of two-dimensional tomographic travel time inversion and ray trace modelling. The high data quality allows modelling of the P- and S-wave velocity structure along the profile. There are pronounced differences in seismic velocity structure of the crust and uppermost mantle between the three main tectonic provinces traversed by the profile: (i) the Pripyat Trough is a ca. 4-km-deep sedimentary basin, fully located in the Osnitsk–Mikashevichi Igneous Belt in the northern part of the profile. The velocity structure is typical for a Precambrian craton, but is underlain by a ca. 5-km-thick lowest crustal layer of high velocity. The development of the Pripyat Trough appears to have only affected the upper crust without noticeable thinning of the whole crust; this may be explained by a rheologically strong lithosphere at the time of formation of the trough. (ii) Very high seismic velocity and Vp/Vs ratio characterise the Volyn Block and Korosten Pluton to a depth of 15 km and probably also the lowest crust. The values are consistent with an intrusive body of mafic composition in the upper crust that formed from bimodal melts derived from the mantle and the lower crust. (iii) The Podolian Block is close to a typical cratonic velocity structure, although it is characterised by relatively low seismic velocity and Vp/Vs ratio. A pronounced SW-dipping mantle reflector from Moho to at least 70 km depth may represent the Proterozoic suture between Sarmatia and Volgo–Uralia, the structure from terrane accretion, or a later shear zone in the upper mantle. The sub-Moho P-wave seismic velocity is high everywhere along the profile, with the exception of the area above the dipping reflector. This velocity change further supports a plate tectonic origin of the dipping mantle reflector. The profile demonstrates that structure from Palaeoproterozoic plate tectonic processes are still identifiable in the lithosphere, even where younger metamorphic equilibration of the crust has taken place.     

P-wave tomography and origin of the Changbai intraplate volcano in Northeast Asia

We present the first seismic image of the upper mantle beneath the active intraplate Changbai volcano in Northeast Asia determined by teleseismic travel time tomography. The data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations. Our results show a columnar low-velocity anomaly extending to 400-km depth with a P-wave velocity reduction of up to 3%. High velocity anomalies are visible in the mantle transition zone, and deep-focus earthquakes occur at depths of 500–600 km under the region, suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly by global tomography. These results suggest that the intraplate Changbai volcano is not a hotspot like Hawaii but a kind of back-arc volcano related to the deep subduction and stagnancy of the Pacific slab under Northeast Asia.     

A multi‐system geochronology in the Ad‐3 borehole,Pannonian Basin (Hungary) with implications for dating volcanic rocks by low‐temperature thermochronology and for interpretation of (U–Th)/He data

Independent geochronological and thermal modelling approaches are applied to a biostratigraphically exceptionally well‐controlled borehole, Alcsútdoboz‐3 (Ad‐3), in order to constrain the age of Cenozoic geodynamic events in the western Pannonian Basin and to test the efficacy of the methods for dating volcanic rocks. Apatite fission track and zircon U–Pb data show two volcanic phases of Middle Eocene (43.4–39.0 Ma) and Early Oligocene (32.72 ± 0.15 Ma) age respectively. Apatite (U–Th)/He ages (23.8–14.8 Ma) and independent thermal and subsidence history models reveal a brief period of heating to 55–70 °C at ~17 Ma caused by an increased heat‐flow related to crustal thinning and mantle upwelling. Our results demonstrate that, contrary to common perception, the apatite (U–Th)/He method is likely to record ‘apparent’ or ‘mixed’ ages resulting from subsequent thermal events rather than ‘cooling’ or ‘eruption’ ages directly related to distinct geological events. It follows that a direct conversion of ‘apparent’ or ‘mixed’ (U‐Th)/He ages into cooling, exhumation or erosion rates is incorrect.     

Earthquake hazard in Northeast India — A seismic microzonation approach with typical case studies from Sikkim Himalaya and Guwahati city

A comprehensive analytical as well as numerical treatment of seismological, geological, geomorphological and geotechnical concepts has been implemented through microzonation projects in the northeast Indian provinces of Sikkim Himalaya and Guwahati city, representing cases of contrasting geological backgrounds — a hilly terrain and a predominantly alluvial basin respectively. The estimated maximum earthquakes in the underlying seismic source zones, demarcated in the broad northeast Indian region, implicates scenario earthquakes of M _W 8.3 and 8.7 to the respective study regions for deterministic seismic hazard assessments. The microzonation approach as undertaken in the present analyses involves multi-criteria seismic hazard evaluation through thematic integration of contributing factors. The geomorphological themes for Sikkim Himalaya include surface geology, soil cover, slope, rock outcrop and landslide integrated to achieve geological hazard distribution. Seismological themes, namely surface consistent peak ground acceleration and predominant frequency were, thereafter, overlaid on and added with the geological hazard distribution to obtain the seismic hazard microzonation map of the Sikkim Himalaya. On the other hand, the microzonation study of Guwahati city accounts for eight themes — geological and geomorphological, basement or bedrock, landuse, landslide, factor of safety for soil stability, shear wave velocity, predominant frequency, and surface consistent peak ground acceleration. The five broad qualitative hazard classifications — ‘low’, ‘moderate’, ‘high’, ‘moderate high’ and ‘very high’ could be applied in both the cases, albeit with different implications to peak ground acceleration variations. These developed hazard maps offer better representation of the local specific seismic hazard variation in the terrain.     

Crustal accretion beneath the Koyna coastal region (India) and Late Cretaceous geodynamics

In 1967 a major earthquake in the Koyna region attracted attention to the hitherto considered stable Indian shield. The region is covered by a thick pile of Deccan lava flows and characterized by several hidden tectonic features and complex geophysical signatures. Although deep seismic sounding studies have provided vital information regarding the crustal structure of the Koyna region, much remains unknown. The two available DSS profiles in the region have been combined along the trend of Bouguer gravity anomalies. Unified 2-D density modelling of the Koyna crust/mantle suggests a ca. 3 km thick and 40 km wide high velocity/high density anomalous layer at the base of the crust along the coastline. The thickness of this anomalous layer decreases gradually towards the east and ahead of the Koyna gravity low the layer ceases to be visible. Based on the seismic and gravity data interpretation in the geodynamical/rheological boundary conditions the anomalous layer is attributed to igneous crustal accretion at the base of the crust. It is suggested that the underplated layer is the imprint of the magmatism caused by the deep mantle plume when the northward migrating Indian plate passed over the Reunion hotspot.     

Intrinsic versus extrinsic seismic anisotropy: Surface wave phase velocity inversion

The precise determination and interpretation of anisotropy are relatively difficult because the apparent anisotropy is usually a mixture of intrinsic and extrinsic anisotropy, which might partly hide the true properties of the medium investigated. The artificial anisotropy can be due to the fact that seismic waves do not ‘see’ the real details of a medium, but a ‘filtered’ (or ‘upscaled’) version of the Earth model. This can be due to a bad quality of the data coverage, to limited frequency band effects, or to errors in the approximate theory. With the limitation to layered Earth models, through comparisons of the results of the homogenization method with those of the periodic isotropic two-layered model as an analytical solution, we illustrate that the Backus theory for the long wavelength equivalent effect can be extended to calculate the extrinsic anisotropy, due to upscaling effects at discontinuities for the general isotropic layered model, when its spatial scale is much less than or equal to the seismic wavelength. We find that the extrinsic radial S-wave anisotropy produced by the vertical heterogeneities in the upper mantle of the Earth can be as large as 3% (about 30% extrinsic anisotropy of the 10% radial anisotropy). To better recover information from seismic data, we propose a surface wave phase velocity inversion method based on the first-order perturbation theory. We show that resolution at discontinuities can be improved by adding overtone modes of surface wave data. For more general layered models, the homogenization method could be considered, which can flexibly adapt the scale of the model to seismic wavelengths. However, the periodic isotropic two-layered model can also help to analytically quantify the amount of extrinsic radial, and possibly azimuthal anisotropy produced by the tilted fine layering.     

Deep structure of Kamchatka according to the results of MT sounding and seismic tomography

The key features in the distribution of geoelectric and velocity heterogeneities in the Earth’s crust and the upper mantle of Kamchatka are considered according to the data of deep magnetotelluric sounding and seismotomography. Their possible origin is discussed based on the combined analysis of electric conductivity and seismic velocity anomalies. The geoelectric model contains a crustal conducting layer at a depth of 15–35 km extending along the middle part of Kamchatka. In the Central Kamchatka volcanic belt, the layer is close to the ground surface to a depth of 15–20 km, where its conductivity considerably increases. Horizontal conducting zones with a width of up to 50 km extending into the Pacific Ocean are revealed in the lithosphere of eastern Kamchatka. The large centers of current volcanism are confined to the projections of the horizontal zones. The upper mantle contains an asthenospheric conducting layer that rises from a depth of 150 km in western Kamchatka to a depth of 70–80 km beneath the zone of current volcanism. According to the seismotographic data, the low- and high-seismic-velocity anomalies of P-waves that reflect lateral stratification, which includes the crust, the rigid part of the upper mantle, the asthenospheric layer in a depth range of ~70–130 km, and a high-velocity layer confined to a seismofocal zone, are identified on the vertical and horizontal cross sections of eastern Kamchatka. The cross sections show low-velocity anomalies, which, in the majority of cases, correspond to the high-conductivity anomalies caused by the increased porosity of rocks saturated with liquid fluids. However, there are also differences that are related to the electric conductivity of rocks depending on pore channels filled with liquid fluids making throughways for electric current. The seismic velocity depends, to a great extent, on the total porosity of the rocks, which also includes isolated and dead-end channels that can be filled with liquid fluids that do not contribute to the electric-current transfer. The data on electric conductivity and seismic velocity are used to estimate the porosity of the rocks in the anomalous zones of the Earth’s crust and the upper mantle that are characterized by high electric conductivity and low seismic velocity. This estimate serves as the basis for identifying the zones of partial melting in the lithosphere and the asthenosphere feeding the active volcanoes.     

Enregistrements sismologiques de l'explosion sur le site de l'usine AZF (Toulouse,France)Seismological records of an explosion at the AZF chemical complex in Toulouse,France

This study presents a detailed analysis of the seismic records of a strong explosion that occurred on 21 September 2001 at a chemical complex located south of Toulouse, France, and provoked important damages. The explosion, which is equivalent to a 3.4 magnitude earthquake, has been recorded at most of the stations of the National Seismological Network, as well as at a station under test at the ‘Observatoire Midi-Pyrénées’, 4.2 km away from the epicentre. The main seismic phases are interpreted using the known crustal structures, and a modelling with synthetic seismograms is performed. To cite this article: A. Souriau et al., C. R. Geoscience 334 (2002) 155–161.     

The role of boundary conditions in numerical models of subduction zone dynamics

Numerical studies of subduction zone dynamics on a regional scale usually use a limited spatial extent for their models and therefore need to define boundary conditions on all model edges. These boundary conditions imply a choice for the mechanical and thermal state of the surrounding regions which may influence the evolution of the model system. We investigate the role of the surface and lateral boundary conditions for simple mechanical subduction models using a standard finite element method. We find that subduction is only possible if the slab can decouple from the surface. This decoupling can be achieved by a true free surface, a free-slip surface with a weak crust for the subducting plate, or a free-slip surface with a layer of low viscosity, low density material (‘sticky air’) between the model surface and the crust. Models of slab dynamics that employ a free-slip surface reproduce trench migration, slab sinking velocities and slab geometry of models with a free surface, as long as they use either a weak crust, which can be viscous, viscoelastic and/or brittle, or a ‘sticky air’ layer. The vertical topography will however not be reproduced for free-slip models without a ‘sticky air’ layer. For ocean–ocean convergent models we find that the application of inflow boundary conditions at the edges of the subducting or overriding lithosphere controls trench motion and the geometry of the subducting slab. Inflow on the overriding side causes trench retreat and a slab resting on the lower mantle, whereas inflow restricted to the subducting side can cause trench advance and a slab which folds on the lower mantle.     

俯冲物质深地幔循环——地球动力学研究的一个新方向

地球上发生的各种地壳运动,大规模的火山喷发,不同深度不同规模的地震活动,规模宏大的山脉和高原的形成,以及地球历史上发生的大陆漂移运动,都被认为与板块构造活动密切相关.但这些运动的动力源究竟来自何方?如何去发现和证明它们的存在以及从理论上去认识和解释,是当今地球科学面临的巨大挑战,也是今后很长一段时间内地球科学的前沿和热...     

地球内部物理和演化的几个核心论题：I.地球内部结构和物理性质

本文综述研究地球内部结构和物理特性的几种常见规方法和主要研究结果,并首重讨论地球物理状态方程,地震成象,综合反演,高温高压实验和有关对比研究方法,均匀各向同性球对球模型仍不失其参考意义,但最新研究结果表明,地球内部状态是非均匀和各向异性的,横向不均匀对称地球模型仍不其参考意义,但最新研究结果表明,地球内部状态是非均匀和各向异性的,横向不均匀性主要表现在上地幔部分,下地幔和液态外核似乎比较均匀,但核－幔边界过渡带(D″)可能代表一个内含非均匀化学边界的热边界层,其形态起伏和横向变化影响地球模壁的球对称性。3-D地震成象实质上反映地震波建与温度异常的关系．而温度变化又会引起密度异常,因而密度变化是控制地幔对流的关键参数之一。     

Asymmetry in oceanic crustal structure of the South China Sea basin and its implications on mantle geodynamics

ABSTRACT

We investigated the oceanic crustal structure and lithospheric dynamics of the South China Sea (SCS) basin through a comprehensive analysis of residual gravity anomaly and bathymetry combined with seismic constraints and interpretation from geodynamic modelling. We first calculated the residual mantle Bouguer anomaly (RMBA) of the oceanic crustal regions of the SCS by removing from free-air gravity anomaly the predicted gravitational attractions of water-sediment, sediment-crust, and crust-mantle interfaces, as well as the effects of lithospheric plate cooling, using the latest crustal age constraints including IODP Expedition 349 and recent deep-tow magnetic surveys. We then calculated models of the gravity-derived crustal thickness and calibrated them using the available seismic refraction profiles of the SCS. The gravity-derived crustal thickness models correlate positively with seismically determined crustal thickness values. Our analysis revealed that the isochron-averaged RMBA are consistently more negative over the northern flank of the SCS basin than the southern conjugate for magnetic anomaly chrons C8n (~25.18 Ma) to C5Dn (~17.38 Ma), implying warmer mantle and/or thicker crust over much of the northern flank. Computational geodynamic modelling yielded the following interpretations: (1) Models of asymmetric and variable spreading rates based on the relatively high-resolution deep-tow magnetic analysis would predict alternating thicker and thinner crust at the northern flank than the southern conjugate, which is inconsistent with the observed systematically thicker crust on the northern flank. (2) Models of episodic southward ridge jumps could reproduce the observed N-S asymmetry, but only for crustal age of 23.6–20 Ma. (3) Southward migration of the SCS ridge axis would predict slightly thinner crust at the northern flank, which is inconsistent with the observations. (4) Models of higher mantle temperatures of up to 25–50°C or >2% less depleted mantle sources on the northern flank could produce large enough anomalies to explain the observed N-S asymmetries.     

Why do continents break-up parallel to ancient orogenic belts?

The frequently observed parallelism between rifts and the preexisting orogenic fabric of continents suggests that the inherited tectonic fabric of the lithosphere influences the rupture of continents. We propose that the existence of a pervasive fabric in the lithospheric mantle induces an anisotropic strength in the lithosphere, that guides the propagation of continental rifts. Subcrustal mantle mechanical anisotropy is supported by (i) the anisotropic strength of olivine, (ii) an ubiquitous tectonic fabric in exposed mantle rocks, and (iii) measurements of seismic and electrical anisotropy. During major episodes of continent assembly, a pervasive deformation of the lithosphere induces a lattice-preferred orientation of olivine in mantle rocks. Later on, this crystallographic fabric is ‘frozen-in’ and represents the main source of shear wave splitting. This olivine fabric may entail a mechanical anisotropy in the lithospheric mantle. During subsequent tectonic events, especially during rifting, mechanical anisotropy may control the tectonic behaviour of the lithosphere