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1.
We constructed the S-wave velocity structure of the crust and uppermost mantle (10–100 km) beneath the North China based on the teleseismic data recorded by 187 portable broadband stations deployed in this region. The traditional two-step inversion scheme was adopted. Firstly, we measured the interstation fundamental Rayleigh wave phase velocity of 10–60 s and imaged the phase velocity distributions using the Tarantola inversion method. Secondly, we inverted the 1-D S-wave velocity structure with a grid spacing of 0.25° × 0.25° and constructed the 3-D S-wave velocity structure of the North China. The 3-D S-wave velocity model provides valuable information about the destruction mechanism and geodynamics of the North China Craton (NCC). The S-wave velocity structures in the northwestern and southwestern sides of the North–South Gravity Lineament (NSGL) are obviously different. The southeastern side is high velocity (high-V) while the northeastern side is low velocity (low-V) at the depth of 60–80 km. The upwelling asthenosphere above the stagnated Pacific plate may cause the destruction of the Eastern Block and form the NSGL. A prominent low-V anomaly exists around Datong from 50 to 100 km, which may due to the upwelling asthenosphere originating from the mantle transition zone beneath the Western Block. The upwelling asthenosphere beneath the Datong may also contribute to the destruction of the Eastern Block. The Zhangjiakou-Penglai fault zone (ZPFZ) may cut through the lithosphere and act as a channel of the upwelling asthenosphere. A noticeable low-V zone also exists in the lower crust and upper mantle lid (30–50 km) beneath the Beijing–Tianjin–Tangshan (BTT) region, which may be caused by the upwelling asthenosphere through the ZPFZ. 相似文献
2.
We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle (Pn and Sn). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust (Pcc) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average Pn velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor. 相似文献
3.
《Russian Geology and Geophysics》2014,55(7):892-906
Modeling of the seismic, thermal, and density structure of the Siberian craton lithospheric mantle at depths of 100-300 km has been performed along the superlong Meteorite and Rift seismic profiles. The 2D velocity sections reflect the specific features of the internal structure of the craton: lateral inhomogeneities, seismic-boundary relief at depths of ~ 100, 150, 240, and 300 km, velocities of 8.3-8.7 km/s, and the lack of low-velocity zone in the lower lithosphere. Mapping of the thermal state along the Meteorite and Rift profiles shows a significant temperature decrease in the cratonic mantle as compared with the average temperatures of the surrounding Phanerozoic mantle (> 300 °C) estimated from the global reference model AK135. Lateral temperature variations, reflecting the thermal anomalies in the cratonic keel, are observed at depths of < 200 km (with some decrease in temperature in the central part of the craton), whereas at depths of > 200 km, temperature variations are negligible. This suggests the preservation of residual thermal perturbations at the base of the lithosphere, which must lead to the temperature equalization in the transition zone between the lithosphere and the asthenosphere. Variations in chemical composition have a negligible effect on the thermal state but affect strongly the density structure of the mantle. The results of modeling admit a significant fertilization of matter at depths more than 180-200 km and stratification of the cratonic mantle by chemical composition. The thicknesses of chemical (petrologic) and thermal boundary layers beneath the Siberian craton are estimated. The petrologic lithosphere is localized at depths of ~ 200 km. The bottom of the thermal boundary layer is close to the 1450 °C isotherm and is localized at a depth of 300 km, which agrees with heat flow and seismic-tomography data. 相似文献
4.
《Gondwana Research》2014,25(3-4):849-864
We have imaged the lithospheric structure beneath the central and western North China Craton (NCC) with Rayleigh wave tomography. The Rayleigh waveforms of 100 teleseismic events recorded by 208 broadband stations are used to yield high-resolution phase velocity maps at 13 periods from 20 s to 143 s. A 3-D S-wave velocity model is constructed based on the phase velocity maps. Our S-wave velocity model is broadly consistent with the results of previous tomography studies, but shows more detailed variations within the lithosphere. The Trans-North China Orogen (TNCO) is generally characterized by low-velocity anomalies but exhibits great heterogeneities. Two major low-velocity zones (LVZs) are observed in the north and south, respectively. The northern LVZ laterally coincides with sites of Cenozoic magmatism and extends to depths greater than 200 km. We propose that a small-scale mantle upwelling is present, confined to the north of the TNCO. A high-velocity patch in the uppermost mantle is also observed between the two LVZs adjacent to the narrow transtensional zone of the Cenozoic Shanxi–Shaanxi Rift (SSR). We interpret this as the remnant of a cratonic mantle root. The Ordos Block in the western NCC is associated with high-velocity anomalies, similarly reflecting the existence of cratonic mantle root, but a discernible low-velocity layer is observed at depths of 100–150 km in this location. We interpret that this mid-lithospheric structure was probably formed by metasomatic processes during the early formation of the NCC. Based on the observations from our S-wave velocity model, we conclude that the current highly heterogeneous lithospheric structure beneath the TNCO is the result of multiphase reworking of pre-existing mechanically weak zones since the amalgamation of the craton. The latest Cenozoic lithospheric reworking is dominated by the far-field effects of both Pacific plate subduction and the India–Eurasia collision. 相似文献
5.
We have studied the structures of the Earth’s crust and upper mantle of the Asian continent using a representative sample of dispersion curves of group velocities of fundamental-mode Rayleigh and Love waves for more than 3200 seismic paths. Maps of distributions of variations in group velocities with periods of 10 to 250 s over a spherical surface were calculated by the 2D tomography method. The maps reflect the deep structure of the Earth’s crust and upper mantle of the study area and give a tentative idea of the horizontal distribution of the anisotropic properties of the mantle matter. The obtained data are confirmed by the calculations of the velocity profiles of SV- and SH-waves for the entire Asian continent and for its regions. Vertically, anisotropy is observed to the depths of ~ 250 km, with its maximum in the depth range from the bottom of the crust to 150 km. 相似文献
6.
The lithospheric structure of the western part of the Mediterranean Sea is shown by means of S-velocity maps, for depths ranging
from 0 to 35 km, determined from Rayleigh-wave analysis. The traces of 55 earthquakes, which occurred from 2001 to 2003 in
and around the study area have been used to obtain Rayleigh-wave dispersion. These earthquakes were registered by 10 broadband
stations located on Iberia and the Balearic Islands. The dispersion curves were obtained for periods between 1 and 45 s, by
digital filtering with a combination of MFT and TVF filtering techniques. After that, all seismic events were grouped in source
zones to obtain a dispersion curve for each source-station path. These dispersion curves were regionalized and after inverted
according to the generalized inversion theory, to obtain shear-wave velocity models for rectangular blocks with a size of
1° × 1°. The shear velocity structure obtained through this procedure is shown in the S-velocity maps plotted for several
depths. These maps show the existence of lateral and vertical heterogeneity. In these maps is possible to distinguish several
types of crust with an average S-wave velocity ranging from 2.6 to 3.9 km/s. The South Balearic Basin (SBB) is more characteristic
of oceanic crust than the rest of the western Mediterranean region, as it is demonstrated by the crustal thickness. We also
find a similar S-wave velocity (ranging from 2.6 km/s at the surface to 3.2 km/s at 10 km depth) for the Iberian Peninsula
coast to Ibiza Island, the North Balearic Basin (NBB) and Mallorca Island. In the lower crust, the shear velocity reaches
a value of 3.9 km/s. The base of the Moho is estimated from 15 to 20 km under Iberian Peninsula coast to Ibiza Island, continues
towards NBB and increases to 20–25 km beneath Mallorca Island. While, the SBB is characterized by a thinner crust that ranges
from 10 to 15 km, and a faster velocity. A gradual increase in velocity from the north to the south (especially in the upper
25 km) is obtained for the western part of the Mediterranean Sea. The base of the crust has a shear-wave velocity value around
of 3.9 km/s for the western Mediterranean Sea area. This area is characterized by a thin crust in comparison with the crustal
thickness of the eastern Mediterranean Sea area. This thin crust is related with the distensive tectonics that exists in this
area. The low S-wave velocities obtained in the upper mantle might be an indication of a serpentinized mantle. The obtained
results agree well with the geology and other geophysical results previously obtained. The shear velocity generally increases
with depth for all paths analyzed in the study area. 相似文献
7.
Rrapo Ormeni 《Tectonophysics》2011,497(1-4):114-121
This paper describes the one-dimensional (1D) velocity model computed by VELEST in the SEISAN seismic analysis system, inverting re-picked P-wave and S-wave arrival times recorded during 2002–2006 by the Albanian, Montenegro, Thessalonica and Macedonia seismic networks. The re-picked data yield P-wave and S-wave velocities proved to be more suitable compared to bulletin data for this detailed inversion study. Seismic phases recorded by the Albania seismic network and integrated with data from the Montenegro, Thessalonica and Macedonia networks are used to prepare the Albanian seismic bulletin. Earthquake hypocenters from the Albanian bulletins have also location errors that are negligible for civil protection purposes, large scale seismotectonic analyses and more accurate hypocentral determinations which are necessary for detailed seismotectonic and geodynamic studies.It was noted that the smoothness of the velocity variation increased with depth. A velocity of 5.5 km/s was calculated for the upper crust, 6.1 km/s was calculated for the middle crust and 6.9 km/s was computed for the lower crust. P wave velocity was 7.85 km/s at depth of 50 km and for the upper mantle it is 8.28 km/s. Using the improved velocity model, the earthquakes which occurred in Albania in the past 5 years were able to be relocated, achieving constrained hypocentral determinations for events in Albania. The interpretation of the 1 D velocity models infers interesting features of the deep structure of Albania. These results represent an important step towards more detailed seismotectonic analyses. 相似文献
8.
We present a 3D S-velocity model for the crust and upper mantle of the South China Sea and the surrounding regions, constrained from the analysis of over 12,000 of fundamental Rayleigh wave dispersion curves between 10 s and 150 s periods. The lateral resolution was found to vary from 2° to 4° with the increasing period over the study region. A robust scheme of Debayle and Sambridge allowed us to conduct the tomographic inversion efficiently for massive datasets. Group velocity maps varying with period show lateral heterogeneities, well related to the geological and tectonic features in the study region. The 3D S-velocity model was constructed from the 1D structure inversion of the tomographic group velocity dispersion curves at each node. The obtained average crustal structure is similar to the PREM model, while the average mantle velocity is typically lower than the global average. The complicated 3D structures reveal three prominent features correlated with geological divisions: sea basin regions, island and arc regions, and continental regions. The derived crustal and lithospheric thicknesses range from ∼15 to >50 km and from ∼60 to >140 km, respectively, with the thinnest in the South China Sea, the thickest in eastern Tibet and the Yangtze Block, and the medium in the South China Fold Belt, Indochina, and island arc regions. Our results further confirm that (1) a Mesozoic subduction zone, which is interpreted as the tectonic weak zone during the Paleogene, exists along the South China margin; (2) the influence of the Indochina extrusion along the Red River Fault is limited for the South China Sea region; (3) there is a slab remnant of the proto-South China Sea beneath Borneo. New findings suggest that the Mesozoic subduction zone should be built into any evolution model for the region, as well as the other two major tectonic boundaries of the Red River Fault and proto-South China Sea subduction zone. 相似文献
9.
We explore the variations of Rayleigh-wave phase-velocity beneath the East China Sea in a broad period range (5–200 s). Rayleigh-wave dispersion curves are measured by the two-station technique for a total of 373 interstation paths using vertical-component broad-band waveforms at 32 seismic stations around the East China Sea from 6891 global earthquakes.The resulting maps of Rayleigh-wave phase velocity and azimuthal anisotropy provide a high resolution model of the lithospheric mantle beneath the East China Sea. The model exhibits four regions with different isotropic and anisotropic patterns: the Bohai Sea, belonging to the North China Craton, displays a continental signature with fast velocities at short periods; the Yellow Sea, very stable unit associated with low deformation, exhibits fast velocities and limited anisotropy; the southern part of the East China Sea, with high deformation and many fractures and faults, is related to slow velocities and high anisotropic signature; and the Ryukyu Trench shows high-velocity perturbations and slab parallel anisotropy. 相似文献
10.
Huajian Yao Pierre Gouédard John A. Collins Jeffrey J. McGuire Robert D. van der Hilst 《Comptes Rendus Geoscience》2011,343(8-9):571-583
Inter-station Green's functions estimated from ambient noise studies have been widely used to investigate crustal structure. However, most studies are restricted to continental areas and use fundamental-mode surface waves only. In this study, we recover inter-station surface (Scholte-Rayleigh) wave empirical Green's function (EGFs) of both the fundamental- and the first-higher mode using one year of continuous seismic noise records on the vertical component from 28 ocean bottom seismographs deployed in the Quebrada/Discovery/Gofar transform faults region on the East Pacific Rise. The average phase-velocity dispersion of the fundamental mode (period band 2–30 s) and the first-higher mode (period band 3–7 s) from all EGFs are used to invert for the 1-D average, shear-velocity structure in the crust and uppermost mantle using a model-space search algorithm. The preferred shear-velocity models reveal low velocities (4.29 km/s) between Moho and 25 km depth below sea-surface, suggesting the absence of a fast uppermost mantle lid in this young (0–2 Myr) oceanic region. An even more pronounced low-velocity zone, with shear velocities ～3.85 km/s, appears at a depth between 25–40 km below sea-surface. Along with previous results, our study indicates that the shear velocity in the uppermost oceanic mantle increases with increasing seafloor age, consistent with age-related lithospheric cooling. 相似文献
11.
Friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions. We investigated the influence of in situ effective stresses and temperatures on the frictional properties of (simulated) fault gouges, prepared either from Nankai ODP material or illite shale, at sliding velocities approaching those relevant for earthquake nucleation and SSEs. Biaxial (double direct shear) experiments were performed at room temperature, normal stresses of 5–30 MPa, and sliding velocities of 0.16–18 μm/s. All materials exhibited velocity strengthening under these conditions, along with an increase in the friction coefficient and slip hardening rate with increasing normal stress. Illite gouge showed increased velocity strengthening towards higher normal stresses. The effect of temperature was investigated by means of ring shear experiments on illite gouge at 200–300 °C, an effective normal stress of 170 MPa, a pore-fluid pressure of 100 MPa and sliding velocities of 1–100 μm/s. These experiments showed a transition from velocity strengthening to velocity weakening at ∼250 °C. Our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and SSEs at low effective normal stresses. 相似文献
12.
《Journal of African Earth Sciences》2011,61(5):328-336
The ∼500,000 km2 Saharan Metacraton in northern Africa (metacraton refers to a craton that has been mobilized during an orogenic event but that is still recognisable through its rheological, geochronological and isotopic characteristics) is an Archean–Paleoproterozoic cratonic lithosphere that has been destabilized during the Neoproterozoic. It extends from the Arabian–Nubian Shield in the east to the Trans-Saharan Belt in the west, and from the Oubanguides Orogenic Belt in the south to the Phanerozoic cover of North Africa. Here, we show that there are high S-wave velocity anomalies in the upper 100 km of the mantle beneath the metacraton typical of cratonic lithosphere, but that the S-wave velocity anomalies in the 175–250 km depth are much lower than those typical of other cratons. Cratons have possitive S-wave velocity anomalies throughout the uppermost 250 km reflecting the presence of well-developed cratonic root. The anomalous upper mantle structure of the Saharan Metacraton might be due to partial loss of its cratonic root. Possible causes of such modification include mantle delamination or convective removal of the cratonic root during the Neoproterozoic due to collision-related deformation. Partial loss of the cratonic root resulted in regional destabilization, most notably in the form of emplacement of high-K calc-alkaline granitoids. We hope that this work will stimulate future multi-national research to better understand this part of the African Precambrian. Specifically, we call for efforts to conduct systematic geochronological, geochemical, and isotopic sampling, deploy a reasonably-dense seismic broadband seismic network, and conduct systematic mantle xenoliths studies. 相似文献
13.
We propose a genetic algorithm (GA) search procedure for waveform modeling of local crustal earthquakes for optimal one-dimensional (1-D) crustal velocity model. Both waveforms and travel-time data are used for the structure determination. The use of travel times in model evaluation improves the waveform modeling performance in the sense of computation speed and accuracy. We applied this method to broadband waveforms of a local crustal earthquake (M 4.2) in Northeast Japan. P-wave velocities of the crustal model are found to be 4.95 ± 0.30, 5.9 ± 0.02, and 6.51 ± 0.20 km/s for a surface layer, upper crust and lower crust, respectively. The surface layer thickness and the Conrad and Moho depths are found to be 3.01 ± 0.8, 17.77 ± 0.4 and 34.59 ± 1.0 km, respectively. For epicentral distances <200 km, our synthetic waveforms match the observed ones generally well. Early arrivals are mainly observed at stations near the Pacific coast in the forearc area having a thinner crust. In contrast, delayed arrivals appear at stations near the volcanic front and back-arc areas where low-velocity anomalies exist due to the effect of the Pacific slab dehydration and the hot upwelling flows in the mantle wedge. In general, our results agree well with the main tectonic setting of the study area, which confirms the reliability of the proposed approach. Despite a 1-D velocity model is too simple to represent the complex crustal structure, it is still required for the conventional routine analysis of seismology, such as earthquake location and source parameter studies. The current approach is considered as a step toward the genetic full waveform modeling for the 3-D velocity model estimation. 相似文献
14.
《Russian Geology and Geophysics》2015,56(6):885-892
Based on the analysis of experimental data on the viscosity of mafic to ultramafic magmatic melts with the use of our structure-chemical model for the calculation and prediction of the viscosity of magmas, we have first predicted that diamond-carryihg kimberlite magma must ascend from mantle to crust with considerable acceleration. The viscosity of kimberlite magma decreases by more than three times during its genesis, evolution, and ascent from mantle to crust despite the significant decrease in the temperature of the ascending kimberlite magma (~ 150 °C) and its partial crystallization and degassing. In the case of partial melting (< 1 wt.%) of carbonated peridotite in the mantle at depths of 250-350 km, high-viscosity (~ 35 Pas) kimberlite melts can be generated at ~ 8.5 GPa and ~ 1350 °C, the water content in the melt being up to ~ 8 wt.%, C(OH-) = 0-2 wt.%, and C(H2O) = 0-6 wt.%. On the other hand, during the formation of kimberlite pipes, dikes, and sills, the viscosity of near-surface kimberlite melts is much lower (~ 10 Pa s) at ~ 50 MPa and 1200 °C, the volume contents of crystals (Vcr) and the fluid phase (bubbles) (Vfl) are 35 and 5 vol.%, respectively, and the water content in magma, C(OH-), is 0.5 wt.%. On the contrary, the viscosity of basaltic magmas increases by more than two orders of magnitude during their ascent from mantle to crust. The basaltic magmas which can be generated in the asthenosphere at depths of ~ 100 km have the minimum viscosity (up to ~ 2.3 Pas) at ~ 4.0 GPa, 1350 °C, C(OH-) - 3 wt.%, and C(H2O) - 5 wt.%. However, at the final stage of evolution (e.g., during volcanic eruptions), the viscosity of basaltic magma is considerably higher (600 Pa s) at ~ 10 MPa, 1180 °C, Vcr - 30 vol.%, Vf - 15 vol.%, and C(OH-) - 0.5 wt.%. 相似文献
15.
Based on passive seismic interferometry applied to ambient seismic noise recordings between station pairs belonging to a small-scale array, we have obtained shear wave velocity images of the uppermost materials that make up the Dead Sea Basin. We extracted empirical Green’s functions from cross-correlations of long-term recordings of continuous data, and measured inter-station Rayleigh wave group velocities from the daily correlation functions for positive and negative correlation time lags in the 0.1–0.5 Hz bandwidth. A tomographic inversion of the travel times estimated for each frequency is performed, allowing the laterally varying 3-D surface wave velocity structure below the array to be retrieved. Subsequently, the velocity-frequency curves are inverted to obtain S-wave velocity images of the study area as horizontal depth sections and longitude- and latitude-depth sections. The results, which are consistent with other previous ones, provide clear images of the local seismic velocity structure of the basin. Low shear velocities are dominant at shallow depths above 3.5 km, but even so a spit of land with a depth that does not exceed 4 km is identified as a salt diapir separating the low velocities associated with sedimentary infill on both sides of the Lisan Peninsula. The lack of low speeds at the sampling depth of 11.5 km implies that there are no sediments and therefore that the basement is near 10–11 km depth, but gradually decreasing from south to north. The results also highlight the bowl-shaped basin with poorly consolidated sedimentary materials accumulated in the central part of the basin. The structure of the western margin of the basin evidences a certain asymmetry both whether it is compared to the eastern margin and it is observed in north–south direction. Infill materials down to ∼8 km depth are observed in the hollow of the basin, unlike what happens in the north and south where they are spread beyond the western Dead Sea shore. 相似文献
16.
Based on the Crust2.0 model and the topography data of Chinese continent and its adjacent regions, a three-dimensional finite element model is constructed in terms of the spherical coordinate system. In our numerical model, the average annual ground temperature from 195 meteorological stations and temperature of upper mantle derived from the seismic velocities are adopted as the top and bottom boundary conditions, respectively. The observed thermal conductivity and heat production from P wave velocity based on empirical formula are employed in our numerical model as well. The comparison between the calculated and observed surface heat flow proved that our results are reliable. The temperature beneath the Precambrian cratons is lower than that of other areas for 100–300 °C also. The typical temperature rang at the Moho is estimated to be 800–1000 °C beneath the Tibetan plateau and 500–700 °C beneath the Precambrian cratons (such as Indian plate, Sichuan basin, South China, North China and Tarim), respectively. The thermal state in the eastern part of Sino-Korean craton at the depth deeper than 60 km indicates that it was destructed. The thermal structure in center of Tibetan plateau (especially beneath Qiangtang area) supports the proposed flow of lower crustal or upper mantle material to the east. Generally, the distribution of volcanoes in Chinese continent is consistent with the high temperature areas in the crust or upper mantle. There are many obvious thermal transition zones across the orogenic belts. The thermal transition zone between eastern and western parts in the crust of Chinese continent is consistent with the north–south seismic zone. 相似文献
17.
An integrated interpretation of multi-channel seismic reflection, gravity and magnetic datasets belonging to northern most part of the 85°E Ridge in the Mahanadi offshore is carried out to study the crustal structure and mode of its emplacement. The basement structure map of the ridge reveals that it is 130–150 km wide and is composed of an eastern high which appears as a continuous, broad and smooth topographyand the western high characterized by several steep isolated highs. The seismic velocities reported for the first time over the ridge indicate several sedimentary sequences ranging in velocities between 1.6 and 4.0 km/s above the acoustic basement top. The salient aspects of the sedimentary velocities are; a low velocity layer (2.6–3.2 km/s) within the Cretaceous sequence in the intervening depressions encompassing the flank region, and a regionally widespread higher velocity layer (3.5–3.8 km/s) belonging to the Eocene–Oligocene section overlying the ridge. A layer having a velocity of 4.2–4.7 km/s probably made of volcanoclastic rocks is observed immediately below the acoustic basement. The sediment isopach maps presented here for three major horizons are used to compute the 3-D sediment gravity effect to obtain a crustal Bouguer anomaly map of the region. Detailed analysis of the gravity and magnetic anomaly maps clearly demonstrates the continuity of ridge up to the Mahanadi coast at Chilka Lake. Seismically constrained gravity and magnetic models indicate that the ridge is composed of volcanic material that was emplaced on continental crust in the shelf-slope areas and over the oceanic crust in the deep offshore areas. The modeled crustal structure below the ridge further indicates volcanic emplacement of the ridge on a relatively younger lithosphere. We propose two alternative models for the emplacement of the ridge. 相似文献
18.
Fabien Deschamps Stéphane Guillot Marguerite Godard Catherine Chauvel Muriel Andreani Kéiko Hattori 《Chemical Geology》2010,269(3-4):262-277
The Tso Morari serpentinites in the Ladakh area, northwest Himalaya, originated from the forearc mantle overlying the northward subducting Neo-Tethys lithosphere and the margin of the Indian continent. The serpentinites are characterized by high concentration of fluid-mobile elements (FME: As, Sb, B, Li, and U) compared to ophiolitic or abyssal serpentinites. The Pb isotopic compositions of serpentinites show influence of the subducted Indian continental lithosphere. Trace element concentrations of antigorite determined in situ with Laser Ablation High Resolution Inductively Coupled Mass Spectrometer (LA-HR-ICP-MS) show high contents of FME including Pb, in contrast to the spatially associated iron oxides. Rare earth elements (REE) and compatible elements, such as Sc and Co, remained immobile during the hydration, allowing the identification of the primary minerals (olivine or orthopyroxene) from which serpentine formed. Serpentinized olivine displays higher Sb and As concentrations (up to 1000 × PM) than serpentinized orthopyroxenes that are enriched in Pb, Cs and Li (2 to up to 10 × PM).We propose that the observed FME distribution in two types of serpentine reflect the differential incorporation of FME during the downward movement of the serpentinite along the subduction plane. At temperature lower than 400 °C, at shallow depths, olivine is preferentially serpentinized and incorporates elements that are fluid soluble at low temperatures, such as Sb and As. Above 400 °C, orthopyroxene is hydrated and incorporates Pb, Cs, Li and possibly Ba. Boron and U are incorporated in both types of serpentine suggesting that they are released from slabs at temperatures around 300–400 °C. The serpentine acts as a sink for water, but also for FME and transports them to deeper and hotter levels in the mantle, down to the isotherm 600–650 °C where dehydration occurs. 相似文献
19.
《Journal of African Earth Sciences》2011,59(5):820-828
The shear velocity structure beneath the Virunga volcanic area was estimated by using an average solution in the time domain inversion of stacked teleseismic receiver functions provided by two seismic broadband stations KUNENE (KNN) and KIBUMBA (KBB). These two stations are 29 km apart and located at the eastern and western escarpment of the Western Rift Valley of Africa in the Virunga area, respectively. The velocity model was presented as P-wave velocity models. From these models, the crust mantle transition zone beneath the area sampled by KNN and KBB in the Virunga area was determined at depth from about 36 to 39 km and 30 to 41 km, respectively. A low velocity zone was observed below stations KNN and KBB at depths between 20–30 km and 18–28 km, respectively, and with average velocity 5.9 km/s and 6.0 km/s. This low velocity zone may probably related to a magma chamber or a melt-rich sill. The models show also high velocity material (6.8–7.4 km/s) lying beneath stations KNN and KBB at depths 3–20 km and 3–10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. The result obtained in this study was applied to the determination of epicentres during the period prior to the 27 November 2006 Nyamuragira eruption. This eruption was preceded by a swarm of hybrid volcanic earthquakes with clear P-waves onset. Using the receiver function model was found to improve the location of events. The located events correlate well with the location of the eruptive site and data provided by the INSAR observations of surface deformation associated with eruption. 相似文献
20.
In view of the major advancement made in understanding the seismicity and seismotectonics of the Indian region in recent times, an updated probabilistic seismic hazard map of India covering 6–38°N and 68–98°E is prepared. This paper presents the results of probabilistic seismic hazard analysis of India done using regional seismic source zones and four well recognized attenuation relations considering varied tectonic provinces in the region. The study area was divided into small grids of size 0.1° × 0.1°. Peak Horizontal Acceleration (PHA) and spectral accelerations for periods 0.1 s and 1 s have been estimated and contour maps showing the spatial variation of the same are presented in the paper. The present study shows that the seismic hazard is moderate in peninsular shield, but the hazard in most parts of North and Northeast India is high. 相似文献