南海东北部及其邻近地区地壳上地幔P波速度结构

利用中国地震台网和ISC台站记录的P波到时数据,采用球坐标系有限差分地震层析成像方法反演了南海东北部及其邻近地区壳幔三维P波速度结构,并分析了不同地质单元的构造差异及其深部特征。结果表明:南海东北部表现出陆架地区的岩石层特性,属于华南大陆向海区的延伸,岩石层厚度较大,现今不存在大规模的地幔热流活动,推测大陆边缘张裂作用仅限于地壳内部而没有延伸进入上地幔,具有非火山型大陆边缘的深部特点。中央海盆附近上地幔P波速度明显降低,与海盆下方地幔热流活动密切相关。不同的速度异常特征表明:华南大陆暨台湾地区属于欧亚大陆的正常地壳或是与菲律宾海板块相互作用产生的增厚型地壳,冲绳海槽则是弧后扩张产生的减薄型地壳。滨海断裂带作为华南大陆高速异常和南海北部高速异常的分界,代表了一定地质时期华南地块和南海地块的拼合边界。断裂附近的上地幔低速异常揭示了闽粤沿海岩浆作用的深层动力机制。吕宋岛弧、马尼拉海沟、东吕宋海槽的速度异常与其所处的特殊构造位置有密切的关系,清晰地反映出岛弧俯冲带的地壳结构差异;台湾南部至吕宋岛弧的上地幔低速异常揭示了两个重要火山链的深部构造特征,北吕宋海脊下方100 km深度的条带状高速异常有可能代表了俯冲下沉的岩石层板片。     

S-wave velocity structure of the crust and upper mantle under southeastern China by surface wave dispersion analysis

Group velocity dispersion data of fundamental-mode Rayleigh and Love waves for 12 wave paths within southeastern China have been measured by applying the multiple-filter technique to the properly rotated three-component digital seismograms from two Seismic Research Observatory stations, TATO and CHTO. The generalized surface wave inversion technique was applied to these group velocity dispersion data to determine the S-wave velocity structures of the crust and upper mantle for various regions of southeastern China. The results clearly demonstrate that the crust and upper mantle under southeastern China are laterally heterogeneous. The southern China region south of 25°N and the eastern China region both have a crustal thickness of 30 km. The eastern Tibet plateau along the 100°E meridian has a crustal thickness of 60 km. Central China, consisting mainly of the Yangtze and Sino-Korean platforms, has a crustal thickness of 40 km. A distinct S-wave low-velocity layer at 10–20 km depth in the middle crust was found under wave paths in southeastern China. On the other hand, no such crustal low-velocity layer is evident under the eastern Tibet plateau. This low-velocity layer in the middle crust appears to reflect the presence of a sialic low-velocity layer perhaps consisting of intruded granitic laccoliths, or possibly the remnant of the source zone of widespread magmatic activities known to have taken place in these regions since the late Carboniferous.     

Moho depths and three-dimensional velocity structure of the crust and upper mantle beneath the Baikal region,from local tomography

We studied the 3D velocity structure of the crust and uppermost mantle beneath the Baikal region using tomographic inversion of ∼25,000 P and S arrivals from more than 1200 events recorded by 86 stations of three local seismological networks. Simultaneous iterative inversion with a new source location algorithm yielded 3D images of P and S velocity anomalies in the crust and upper mantle, a 2D model of Moho depths, and corrections to source coordinates and origin times. The resolving power of the algorithm, its stability against variations in the starting model, and the reliability of the final results were checked in several tests. The 3D velocity structure shows a well-pronounced low-velocity zone in the crust and uppermost mantle beneath the southwestern flank of the Baikal rift which matches the area of Cenozoic volcanism and a high velocity zone beneath the Siberian craton. The Moho depth pattern fits the surface tectonic elements with thinner crust along Lake Baikal and under the Busiyngol and Tunka basins and thicker crust beneath the East Sayan and Transbaikalian mountains and under the Primorsky ridge on the southern craton border.     

Rheological gravity model of the crust and upper mantle of Transbaikalia

Based on rheological interpretation of formalized gravity models, earlier known deep-seated structures in the Earth’s crust and mantle of Transbaikalia have been detailed and new ones discovered. The structures are asymmetric and transverse relative to the Baikal rift zone. Their presence explains the peculiar features of the Baikal rift, including the one-way southeasterly direction of horizontal displacement of tectonic masses and northwestern migration of the Earth’s crust extension processes. The prolonged history (more than 250 Ma) of the Baikal rift zone and Transbaikalia mountainous country involved gravity or rotational detachments of rigid tectonic slabs from the craton and their sliding along intracrustal and subcrustal decollement zones into the above-dome area of the Transbaikalia asthenolith.     

上地幔三维S波速度结构与大别苏鲁超高压变质带俯冲折返机制探讨

本文分析了中国东部的上地幔剪切波速度结构及其与超高压变质岩带之间关系的构造意义。结果表明,在华北块体下面150km深处的速度高于扬子块体的速度值。大别-苏鲁造山构造带下面存在着一条地震波速度变化带。苏鲁、山东半岛下面的速度分布与大别造山带下面的速度分布处于同一个速度等值区域上。横跨大别造山带的南北走向速度结构剖面上,在100km以上的地壳和上地幔区域,华北块体下与扬子块体下面的速度均略低平均值。100km以下,大别造山带南北两侧的扬子与华北块体下面的速度结构分布形态大相径庭。华北下面的波速高于扬子块体下面的波速。大别造山带下呈现速度异常,界线的南侧,有一个略低于零速度的负波速异常区,呈现由南向华北块体的下方斜冲形态,下冲角度大约为30°,其先端部位下冲深达300多公里,其外围零速度等值线的分布区,斜向下延伸超过400km。在速度结构变化分界线的北侧,一个零速度值的分布区带,呈现出从由100多公里深处从北向南朝地表面斜上冲形态。这些速度结构成像的几何形态可能意味着200Ma前大别超高压变质岩带的形成与演化的俯冲、折返的构造运动在上地幔和岩石圈中留下的“痕迹”。     

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.     

Rheology of Indian continental crust and upper mantle

A rheological model of the Indian shield has been constructed using the thermal structure derived from available surface heat flow and heat generation data and the flow properties of characteristic minerals and rocks like quartz, diabase and olivine which respectively represent the upper crust, lower crust and upper mantle. Lateral variations in the thicknesses of the brittle and ductile crust and of the brittle upper mantle have thus been obtained for different tectonic environments. Implications of these results to interpretation of the seismic structure of the Indian shield have been pointed out.     

Thermal structure of the crust and upper mantle of Romania

A synthesis of the heat-flow data for Romania enabled a study of the thermal regime of the crust and upper mantle to be made. This showed lateral thermal differences between various tectonic units. The thermal structure of the crust and upper mantle appears to be mainly the result of mantle convection and plate interaction in the studied area.     

Viscosity and dynamic processes in the crust and upper mantle

A mechanism responsible for the “range-basin” topography is suggested, on the assumption of a high (fluid)viscosity of the asthenosphere, and hence a significant difference between the densities on either side of the Benioff surface sufficient for a development of the vertical component of the pressure against the lithosphere.– V. P. Sokoloff.     

Tectonosphere of the Earth: upper mantle and crust interaction

The endogenic geological processes, which include tectonic, magmatic and metamorphic processes, form regular combinations called endogenic regimes. These regimes are: géosynclinal, orogenic, platform, rift, tectonic-magmatic activation (diwa), taphrogenic, plateau-basalt, oceanic basins and mid-oceanic ridges. The endogenic regimes are connected with the peculiarities of the structure, composition and state of the entire tectonosphere, i.e. not only of the crust but of the upper mantle as well.

Heat flow is a major factor controlling the type of the regime. The other conditions are the temperature distribution in the tectonosphere and the degree and type of penetrability of the tectonosphere to melts and fluids. There is a certain regular succession of regimes. The structural evolution of the tectonosphere and the transformation of the matter in it are in close relationship.

The main trend in the development of the tectonospheric material is directed towards geochemical depletion of the upper mantle by fractioning. At the initial stages, fractioning occurred mostly by degassing, and under these conditions the continental crust was formed, rich in non-compatible elements. At that stage the calc-alkaline magmas prevailed. As the upper mantle was depleted and began to lose its volatiles, the mechanism of fractioning changed: degassing was substituted for selective melting, and in this environment most of the tholeiitic magmas were formed. This change in magma composition and in fractioning mechanism was combined with the destruction of the continental crust and the formation of the oceanic crust. The diwa regime and the rifts were the first steps in the destruction of continental crust. The stages that followed were represented by taphrogenic regimes at various levels. These kinds of regimes were manifested in deep continental and marine depressions, compensated and not compensated by sediments.

Taphrogenic regimes are advancing from the east and west onto the Eurasian continent: in the east they form marginal seas and cause subsidence of the eastern parts of the Chinese platform; in the west they produce collapses of the crust in the Mediterranean area.

The major crisis occurred between the Palaeozoic and Mesozoic and since that time the process of substitution of the continental crust by the oceanic crust has proceeded over increasingly large territories.

The evolution of the tectonosphere, instigated by the changes in its matter, was further complicated by temporal and spatial irregularities in deep heat escape, which caused the alternation of excited and quiescent endogenic regimes (tectonomagmatic periodicity) and their co-existence. The combination of all these phenomena creates the structural inhomogeneity of the Earth's crust at any stage of its history.     

The crust and upper mantle of the Sulu UHPM belt

All results from integrated geophysical investigations in the Sulu region are summarized in this paper, trying to reconstruct the Sulu UHPM processes. New seismic S-wave tomographic results suggest a velocity-abnormal zone occurs beneath the Sulu crust, revealing detailed upper mantle structures that high-velocity lumps within the abnormal zone are sequentially distributed beneath the bottom of the asthenosphere. These high-velocity lumps might represent delaminated eclogites or residuals of the subducted oceanic plate. Based on integrated interpretation of the geophysical data, we propose a working model for tectonic reconstruction of the Sulu UHPM processes, which can explain the crust and upper mantle structures of the area. The involved tectonic processes are related to north-eastward escaping of the Sulu terrane, subduction and delamination cycles of the Dabie-Sulu oceanic plate, and post-orogenic lithospheric thinning and magma underplating. The UHPM rocks are believed to have syn-subduction delaminated down to the bottom of the asthenosphere during 245-180 Ma, and the delamination process seemed smooth and nearly continuous without extensive violence.     

Seismic velocity model of the crust and uppermost mantle around the Mirnyi kimberlite field in Siberia

We present the first detailed seismic velocity models of the crust and uppermost mantle around the Mirnyi kimberlite field in Yakutia, Siberia. We have digitized vintage seismograms that were acquired in 1981 and 1983 by use of Taiga analogue seismographs along two perpendicular seismic profiles. The 370-km long, northwest striking profile I across the kimberlite pipe was covered by 41 seismographs, which recorded seismic signals from 21 chemical shots along the line, including one off-end shot. The perpendicular, 340-km long profile II across profile I ca. 30 km to the south of the Mirnyi kimberlite field was covered by 45 seismographs, which recorded seismic signals from 22 chemical shots, including four off-end shots. Each shot involved detonation of between 1.5 and 6.0 tons of TNT, distributed in individual charges of 100–200 kg in shallow water (< 2 m deep). The data is of high quality with high signal/noise ratio to the farthest offsets. We present the results from two-dimensional ray tracing, forward modelling.Both velocity models show normal cratonic structure of the ca. 45-km-thick crust with only slight undulation of the Moho. However, relatively small seismic velocity is detected to 25-km depth in a ca. 60-km wide zone around the kimberlite pipe, surrounded by elevated velocity (> 6.3 km/s) in the upper crust. The lower crust has a relatively constant velocity of 6.8–6.9 km/s. It appears relatively unaffected by the presence of the kimberlite field. Extremely large P-wave velocity (> 8.7 km/s) of the sub-Moho mantle is interpreted along profile I, except for a 70-km wide zone with a “normal” Pn velocity of 8.1 km/s below the kimberlite. Profile II mainly shows Pn velocities of 8.0–8.2 km/s, with unusually large velocity (> 8.5 km/s) in two, ca. 100-km wide zones, at its southwestern end, one zone being close to the kimberlite field. The nature of these exceptionally large, sub-Moho mantle velocities is not yet understood. The difference in velocity in the two profile directions indicates anisotropy, but the effect of unusual rock composition, e.g. from a high concentration of garnet, cannot be excluded.     

致密岩石纵横波波速各向异性的比较研究

通过对板岩、千枚岩、糜棱岩和变质砂岩等4种岩石在平行和垂直层理、板理的3个正交方向上的纵、横波波速试验,提出了致密岩石纵波波速的各向异性与横波波速的各向异性普遍存在着一致性和差异性两种特征。一致性特征主要表现为致密岩石的横波各向异性随纵波各向异性的增强而增强;差异性特征主要表现为在平行和垂直层理、板理的两个方向上,纵波波速的各向异性指数大于横波,亦即纵波波速的各向异性比横波更为显著。从横观各向同性弹性介质的波速方程出发,通过具体的算例对这两种特征进行了较好的理论分析。     

Average structure of the crust and upper mantle in East Africa

An average crust-mantle model has been derived for the East African Rift system based on a number of presently available seismic data. The inversion of experimentally determined spectral transfer ratios of long-period body waves recorded at stations AAE (Addis Ababa, Ethiopia), NAI (Nairobi, Kenya) and LWI (Lwiro, Zaïre) requires at least a two-layer crust. Except for station AAE. the observed P-wave delays can be accounted for by differences in the deduced crustal structure. Phase- and group-velocity measurements of Rayleigh waves along the path AAE-NAI provide additional information on the gross structure of the crust and upper mantle. Only a well-developed asthenosphere channel can explain the observed surface-wave dispersion. It is shown that the average model MS-71 permits a satisfactory interpretation of all the data presented in this paper.     

Resistivity pattern of crust and upper mantle in Southern Urals

A resistivity model of the southern Urals to depths of 120 km was obtained by numerical simulation of natural- and controlled-source EM soundings at 160 kHz to 4⋅10⁻⁴ Hz. The structure of crust and upper mantle was imaged along a transect running ∼800 km across the East European Platform, the Ural foredeep, and the Ural mountains. The new data on geology and tectonics of the southern Urals enlarge the knowledge gained through URSEIS-95 reflection profiling along one of best representative cross-orogen profiles. We discovered a large conductor traceable to depths at least 100–120 km at the junction between the East European Platform and the Ural foredeep. It indicates that the Ural foredeep originated in a weak tectonic zone at the platform edge. The Ural orogen is imaged as a nearly bivergent structure to depths of 70–80 km producing a mosaic pattern of conductors rooted deep beneath the Magnitogorsk greenstone province and the granitic belt of the central East Ural uplift where it is 150 km wide at a depth of ∼120 km. We interpret the discovered deep roots in the context of the geological history of the Urals.     

The structure of the crust and upper mantle to depths of 320 km beneath the Kaapvaal craton, from P wave arrivals generated by regional earthquakes and mining-induced tremors

Travel times from earthquakes recorded at two seismic networks were used to derive an average P wavespeed model for the crust and upper mantle to depths of 320 km below southern Africa. The simplest model (BPI1) has a Moho depth of 34 km, and an uppermost mantle wavespeed of 8.04 km/s, below which the seismic wavespeeds have low positive gradients. Wavespeed gradients decrease slightly around 150 km depth to give a ‘knee’ in the wavespeed-depth model, and the wavespeed reaches 8.72 km/s at a depth of 320 km. Between the Moho and depths of 270 km, the seismic wavespeeds lie above those of reference model IASP91 of Kennett [Research School of Earth Sciences, Australian National University, Canberra, Australia (1991)] and below the southern African model of Zhao et al. [Journal of Geophysical Research 104 (1999) 4783]. At depths near 300 km all three models have similar wavespeeds. The mantle P wavespeeds for southern Africa of Qiu et al. [Geophysical Journal International 127 (1996) 563] lie close to BPI1 at depths between 40 and 140 km, but become lower at greater depths. The seismic wavespeeds in the upper mantle of model BPI1 agree satisfactorily with those estimated from peridotite xenoliths in kimberlites from within the Kaapvaal craton.The crustal thickness of 34 km of model BPI1 is systematically lower than the average thickness of 41 km computed over the same region from receiver functions. This discrepancy can be partly explained by an alternative model (BPI2) in which there is a crust–mantle transition zone between depths of 35 and 47 km, below which seismic wavespeed increases to 8.23 km/s. A low-wavespeed layer is then required at depths between 65 and 125 km.     

A granite window to the lower electrical crust and upper mantle

The structure and properties of the deep crust and upper mantle can be investigated using magnetotelluric observations. Near-surface and upper crustal complexities may distort or limit the capability of the data to adequately resolve deep structure. Granite batholiths have been regarded as windows into the lower crust in the context of seismic reflection data although the granite bodies themselves are not usually detected. Magnetotelluric data from SW England are here used to demonstrate that, in addition to imaging the internal structure and base of a granite, the batholith itself provides a suitable environment for the effective estimation of the resistivity structure to lower crustal and upper mantle depths.