3D Crustal and Lithospheric Structures in the Southeastern Mediterranean and Northeastern Egypt

Crustal and lithospheric thicknesses of the southeastern Mediterranean Basin region were determined using 3D Bouguer and elevation data analysis. The model is based on the assumption of local isostatic equilibrium. The calculated regional and residual Bouguer anomaly maps were employed for highlighting both deep and shallow structures. Generally, the regional field in the area under study is considered to be mainly influenced by the density contrast between the crust and upper mantle. Use of the gravity and topographic data with earthquake focal depths has improved both the geometry and the density distribution in the 3-D calculated profiles. The oceanic-continental boundary, the basement relief, Moho depth and lithosphere-asthenosphere boundary maps were estimated. The results point to the occurrence of thick continental crust areas with a thickness of approximately 32 km in northern Egypt. Below the coastal regions, the thickness of crust decreases abruptly (transition zone). An inverse correlation between sediment and crustal thicknesses shows up from the study. Furthermore, our density model reveals the existence of a continental crustal zone below the Eratosthenes Seamount block. Nevertheless, the crustal type beneath the Levantine basin is typically oceanic; this is covered by sedimentary sequences more than 14 km thick. The modeled Moho map shows a depth of 28–30 km below Cyprus and a depth of 26–28 km beneath the south Florence Rise in the northern west. However, the Moho lies at a constant shallow depth of 22–24 km below the Levantine Basin, which indicates thinning of the crust beneath this region. The Moho map reveals also a maximum depth of about 33–35 km beneath both the northern Egypt and northern Sinai, both of which are of the continental crust. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thinning beneath south Cyprus (LAB ~90 km depth), followed by thickening beneath the Eratosthenes seamount, Florence Rise, Levantine Basin and reaching to maximum thickness below Cyprian Arc (LAB ~115–120 km depth), and further followed by thinning in the north African margin plate and north Sinai subplate (LAB ~90–95 km depth). According to our density model profiles, we find that almost all earthquakes in the study area occurred along the western and central segments of the Cyprian arc while they almost disappear along the eastern segment. The active subduction zone in the Cyprian Arc is associated with large negative anomalies due to its low velocity upper mantle zone, which might be an indication of a serpentinized mantle. This means that collision between Cyprus and the Eratosthenes Seamount block is marked by seismic activity. Additionally, this block is in the process of dynamically subsiding, breaking-up and being underthrusted beneath Cyprus to the north and thrusted onto the Levantine Basin to the south.     

中国及其邻区地球三维结构初始模型的建立

对人工地震测深及天然地震面波体波三维层折反演数据进行统一处理，建立了中国及其邻区地球三维结构初始模型．此模型图像表明，中国及其邻区地球各圈层横向变化明显．岩石圈及软流圈内速度分布主要反映这一区域自古生代以来板块及地块拼合模式．各主要板块或地块（塔里木、扬子、中朝、青藏、哈萨克斯坦、印度、印度支那）岩石圈增厚或有很深的地慢根，板块或地块间的造山带岩石圈减薄，软流圈速度降低。下地幔底部及核幔边界D″层出现高速异常，表明古太平洋及古特提斯洋俯冲板块因重力坍塌已进入地球深层，形成亚洲超级下降地幔柱。这一下降地幔柱引起地球表层物质向中亚、东亚地区集中，印度半岛、青藏高原、新疆、蒙古至贝加尔一带，成为全球岩石圈最大的汇聚场所．     

Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997–1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.     

Shear velocity structure of crust and uppermost mantle in China from surface wave tomography using ambient noise and earthquake data

We present a 3D model of shear velocity of crust and upper mantle in China and surrounding regions from surface wave tomography.We combine dispersion measurements from ambient noise correlation and traditional earthquake data.The stations include the China National Seismic Network,global networks,and all the available PASSCAL stations in the region over the years.The combined data sets provide excellent data coverage of the region for surface wave measurements from 8 to 120 s,which are used to invert for 3D shear wave velocity structure of the crust and upper mantle down to about150 km.We also derive new models of the study region for crustal thickness and averaged S velocities for upper,mid,and lower crust and the uppermost mantle.The models provide a fundamental data set for understanding continental dynamics and evolution.The tomography results reveal significant features of crust and upper mantle structure,including major basins,Moho depth variation,mantle velocity contrast between eastern and western North China Craton,widespread low-velocity zone in midcrust in much of the Tibetan Plateau,and clear velocity contrasts of the mantle lithosphere between north and southern Tibet with significant E–W variations.The low velocity structure in the upper mantle under north and eastern TP correlates with surface geological boundaries.A patch of high velocity anomaly is found under the eastern part of the TP,which may indicate intact mantle lithosphere.Mantle lithosphere shows striking systematic change from the western to eastern North China Craton.The Tanlu Fault appears to be a major lithosphere boundary.     

How many rifts are there in West Africa?

The West African Rift System has, for the last ten years, been thought to consist of five interconnected rifts extending from the Gulf of Guinea deep into the heart of Africa. Careful re-examination of the geophysical evidence makes it quite clear that there are only three interconnected rifts in West Africa; the Lower Benue Rift which extends to the northeast from the Gulf of Guinea to a triple junction near Chum, and the Gongola and Yola Rifts which extend to the north and east, respectively, from the Chum triple junction. These three rifts opened during the earlier part of the Mesozoic and were subsequently filled with Cretaceous sediments. The evidence for two further rifts, the Ati Rift and the Fort Archambault Rift which were thought to extend to the northeast and southeast, respectively, from a triple junction at the eastern end of the Yola Rift, does not stand up to re-examination.The “Ati Rift” was thought to follow a major linear positive gravity anomaly which had been mapped beneath the Quaternary sediments of the Chad Basin. The main gravity anomaly is separated from the Yola Rift by over 300 km and is probably due to a linear body of basic volcanic or volcano-clastic rocks associated with a suture of Pan-African age. Within the gap, between the main anomaly and the Yola Rift, there are three localised positive anomalies which relate to a gabbro of Precambrian age, a band of dense meta-sediments within the Basement Complex and an acid igneous complex of Palaeogene age. The anomaly as a whole is therefore a sequence of unrelated anomalies, none of which are due to features of Mesozoic age.The “Fort Archambault Rift” was thought to follow a major linear negative gravity anomaly which has been mapped beneath the Quaternary sediments of the Chad Basin. To a large extent the negative anomaly overlies the fosse de Baké-Birao (Baké-Birao Basin) which is itself part of a far larger structure that extends, parallel to the southern margin of the West African Rift System, from Douala on the Gulf of Guinea to Birao near the C.A.R. frontier with Sudan. The Douala-Birao Structure may possibly be loosely related to the West African Rift System in that it would appear to be a compressional structure formed at the same time as the Coniacian-Santonian phase of folding which is observed in the West African rifts. However, the two structures are clearly separated from each other and are quite different in character and to a lesser extent in age.     

利用远震P波层析成像研究巴颜喀拉地块东缘及邻区的深部结构

为了揭示巴颜喀拉地块东缘及邻区的壳幔速度结构差异,获取2017年九寨沟M_S7.0地震的深部构造背景,本文收集了2009年5月至2016年8月期间四川及邻区数字测震台网的203个地震台站所记录到的远震P波走时数据,应用有限频体波走时层析成像方法,反演得到了巴颜喀拉地块东缘及邻区50—600 km深度范围内的三维壳幔P波速度结构。反演结果表明:巴颜喀拉地块东缘及邻区的壳幔速度结构具有明显的横向不均匀性和分区特征,松潘—甘孜地槽褶皱系、西秦岭和祁连山褶皱系的整体速度异常较低,研究区东部具有克拉通性质的四川盆地西北缘和鄂尔多斯地块南缘则呈明显的高速异常。上地幔P波速度结构特征差异表明松潘—甘孜地块的抬升可能与地幔上涌有关,巴颜喀拉地块东缘九寨沟震区及周边50—250 km深度范围内的上地幔存在低速异常,在400—600 km地幔过渡带深度范围内表现为明显的高速异常特征。巴颜喀拉地块向东南方向运移受到东部高速、高强度的扬子克拉通地块对青藏高原物质东向挤出的强烈阻挡,而九寨沟震区处于松潘—甘孜地块重要的北东边界断裂交会处附近,应力容易在此集中,这些因素均可能是东昆仑断裂塔藏段与岷江断裂北段交会处附近发生九寨沟M_S7.0地震的深部动力学背景。      

Forecasting the duration of volcanic eruptions: an empirical probabilistic model

Various xenoliths have been found in lavas of the 1763 (“La Montagnola”), 2001, and 2002–03 eruptions at Mt. Etna whose petrographic evidence and mineral chemistry exclude a mantle origin and clearly point to a cognate nature. Consequently, cognate xenoliths might represent a proxy to infer the nature of the high-velocity body (HVB) imaged beneath the volcano by seismic tomography. Petrography allows us to group the cognate xenoliths as follows: i) gabbros with amphibole and amphibole-bearing mela-gabbros, ii) olivine-bearing leuco-gabbros, iii) leuco-gabbros with amphibole, and iv) Plg-rich leuco gabbros. Geobarometry estimates the crystallization pressure of the cognate xenoliths between 1.9 and 4.1 kbar. The bulk density of the cognate xenoliths varies from 2.6 to 3.0 g/cm³. P wave velocities (V _P ), calculated in relation to xenolith density, range from 4.9 to 6.1 km/s. The integration of mineralogical, compositional, geobarometric data, and density-dependent V _P with recent literature data on 3D V _P seismic tomography enabled us to formulate the first hypothesis about the nature of the HVB which, in the depth range of 3–13 km b.s.l., is likely made of intrusive gabbroic rocks. These are believed to have formed at the “solidification front”, a marginal zone that encompasses a deep region (>5 km b.s.l.) of Mt. Etna’s plumbing system, within which magma crystallization takes place. The intrusive rocks were afterwards fragmented and transported as cognate xenoliths by the volatile-rich and fast-ascending magmas of the 1763 “La Montagnola”, 2001 and 2002–03 eruptions.     

南北地震带北段的远震P波层析成像研究

本文利用"中国地震科学台阵"探测项目在南北地震带北段布设的678个流动地震台站在2013年10月至2015年4月期间记录到的远震波形数据,经过波形互相关拾取到473个远震事件共130309条P波走时残差数据,通过远震层析成像研究获得了该区(30°N-44°N,96°E-110°E)下方0.5°×0.5°的P波速度扰动图像.结果显示,研究区下方P波速度结构显示强烈的不均一性和显著的分区、分块特征.岩石圈速度结构具有显著的东西差异:祁连、西秦岭和松潘甘孜地块组成的青藏东北缘地区显示明显的低速异常,而属于克拉通性质的鄂尔多斯地块和四川盆地则显示高速异常,表明东部克拉通块体对青藏高原物质的东向挤出起到了强烈的阻挡作用.阿拉善地块显示出弱高速和局部弱低速的异常并存的特征.阿拉善地块西部显示低速异常,而东部与鄂尔多斯相邻的地区显示高速异常,可能表明该地区的岩石圈的变形主要受到青藏高原东北缘的挤压作用.在鄂尔多斯和四川盆地之间的秦岭下方100~250 km深度上表现为明显的低速异常,表明该处可能存在软流圈物质的运移通道.鄂尔多斯北部的河套裂陷盆地下方在100~500 km深度内低速异常表现明显,说明该区有深部热物质上涌且至少来源于地幔过渡带.青藏东北缘上地幔显示低速异常且地幔过渡带中出现明显的高速异常,这种结构模式暗示了在青藏高原东北缘可能发生了岩石圈拆沉作用,而高速异常体可能是拆沉的岩石圈地幔.     

3D shear-wave velocity structure of the Eifel plume, Germany

The Quaternary Eifel volcanic fields, situated on the Rhenish Massif in Germany, are the focus of a major interdisciplinary project. The aim is a detailed study of the crustal and mantle structure of the intraplate volcanic fields and their deep origin. Recent results from a teleseismic P-wave tomography study reveal a deep low-velocity structure which we infer to be a plume in the upper mantle underneath the volcanic area [J.R.R. Ritter et al., Earth Planet. Sci. Lett. 186 (2001) 7-14]. Here we present a travel-time investigation of 5038 teleseismic shear-wave arrivals in the same region. First, the transverse (T) and radial (R) component travel-time residuals are treated separately to identify possible effects of seismic anisotropy. A comparison of 2044 T- and 2994 R-component residuals demonstrates that anisotropy does not cause any first-order travel-time effects. The data sets reveal a deep-seated low-velocity anomaly beneath the volcanic region, causing a delay for teleseismic shear waves of about 3 s. Using 3773 combined R- and T-component residuals, an isotropic non-linear inversion is calculated. The tomographic images reveal a prominent S-wave velocity reduction in the upper mantle underneath the Eifel region. The anomaly extends down to at least 400 km depth. The velocity contrast to the surrounding mantle is depth-dependent (from −5% at 31-100 km depth to at least −1% at 400 km depth). At about 170-240 km depth the anomaly is nearly absent. The resolution of the data is sufficient to recover the described features, however the anomaly in the lower asthenosphere is underestimated due to smearing and damping. The main anomaly is similar to the P-wave model except the latter lacks the ‘hole’ near 200 km depth, and both are consistent with an upper mantle plume structure. For plausible anhydrous plume material in the uppermost 100 km of the mantle, an excess temperature as great as 200-300 K is estimated from the seismic anomaly. However, 1% partial melt reduces the required temperature anomaly to about 100 K. The temperature anomaly associated with the deeper part of the plume (250 to about 450 km depth) is at least 70 K. However, this estimate is quite uncertain, because the amplitude of the shear-wave anomaly may be larger than the modelled one. Another possibility is water in the upwelling material. The gap at 170-240 km depth could arise from an increase of the shear modulus caused by dehydration processes which would not affect P-wave velocities as much. An interaction of temperature and compositional variations, including melt and possibly water, makes it difficult to differentiate quantitatively between the causes of the deep-seated low-velocity anomaly.     

Upper mantle heterogeneities in Africa deduced from Rayleigh wave dispersion

Rayleigh-wave group velocities have been measured along 11 paths on the African continent, north of the equator. The results lead to a division of this part of the continent into three regions: cratonic areas, non-cratonic areas (“mobile zones”), and a region located to the east of the meridian 31°E (Red Sea neighbourhood, Ethiopia, Djibouti). The highest velocities are found beneath the cratonic areas, at ～ 200 km depth (～ 5%). At greater depths, the “mobile zones” exhibit higher velocities than the cratonic regions, but the difference is not as marked (～ 2%) and the resolution is poor. Velocities in the eastern region are similar to those found under young oceanic regions.     

Volcanism and mantle–crust evolution: The Etna case

Mount Etna is located in a particular region of convergence of African and Eurasian plates where intense post-collisional tectonics caused considerable uplift. However we present arguments supporting the hypothesis that volcanism and associated seismic activity would result from a local mantle uprise leading to a “horst”, probably linked to a deep-rooted hot spot. It ensued deformation and fracturing of the overlying crust with emission of aphyric tholeiitic basalts directly from their mantle source, and subsequent development of a “deep reservoir” (or complex of intrusions) at the top of a mantle diapir near 30 km depth. This is advocated by the appearance of porphyritic alkaline lavas whose mineral equilibria and differentiation processes are consistent with an 8–10 kbar pressure, and by the development of central volcanoes. The horst itself appears to have begun in the SW sector of the present volcanic area. Its uplift was greater westward, as seen from the trend of the terraces along the Simeto river, and became later obvious toward the SE. These differential movements produced fractures and faults which are to day evident in the southern area of Mt Etna. The growth of the horst then proceeded in a NE direction, following the regional tectonic lines and with a greater intensity along the side facing SE, crossed by the regional NNW–SSE line (Aeolian–Maltese escarpment).The seismicity and ground deformation registered over the last twenty years support the proposed model. Earthquakes are unfrequent in the lower southern and western areas of the volcano, whereas they are numerous and stronger to the north-east, in the summit area above 1600 m a.s.l., and in the eastern sector along the NW–SE faults and fractures. Finally, a digital elevation model recently published reveals the existence of two tectonic domains. The first one is associated with the horst and contains prevalently NE–SW oriented faults, whereas the second is mainly linked to regional tectonics with NNW–SSE and NW–SE faults and fractures.     

On the dynamics of extensional basin

Geological and geophysical data from the North China-Bohai Basin and “Basin and Range” Province were examined and compared. They are similar to each other in many respects. Surficial geological structures are characterized by a series of half-grabens with their one flank constituted by normal fault. Those extensional structures usually extend to a depth of 6–8 km. Therefore, the stress condition in the upper 8 km can be written as $$\sigma _2 > \sigma _x > \sigma _y$$ wherex, y denote the directions of maximum compression and maximum tension on the horizontal plane, whilez signifies the vertical direction. Some people think that this kind of stress condition exists through the entire crust in the extensional basin. However, the focal mechanisms of the earthquakes in the extensional basins with focal depths usually at 12–20 km are dominated by strike-slip faults. The stress condition in the focal regions can be expressed by $$\sigma _x > \sigma _z > \sigma _y .$$ Geodetic measurements conducted before and after the Tangshan earthquake in 1976 and the Xingtai earthquake in 1966 showed that both horizontal and vertical surficial deformations with magnitudes of a similar order occurred during the earthquakes. The surficial deformations during the earthquakes can be explained by a summation of the motions produced by both stress fields in the upper crust and the middle crust. Dynamical processes other than the homogeneous horizontal regional tectonic field are required to explain the vertical variation of the stress condition in the upper and middle crusts. Evidence from the seismic refractions, reflections and the three-dimensional seismic tomography from both local earthquakes and teleseismic events provide convincing evidence that magmatic intrusions from the uppermost mantle to the middle crust occur near the hypocenters of both the Tangshan and Xingtai earthquakes. The variation from the extensional stress regime at the upper crust to the compressional stress regime in the middle and lower crusts is considered to be the common feature in extensional basins. And the magmatic intrusions from the upper mantle to the middle crust observed in the extensional basin is suggested to be its genetic cause. Numerical simulations of magmatic intrusion from the uppermost mantle to the middle crust were studied. Both the intruded compression and the thermal stress due to magmatic intrusion were considered, also the viscoelasticity of the middle and lower crusts were assumed. The results successfully explain the vertical variation of the stress condition in the crust and the process producing an extensional basin.     

青藏高原中东部地壳和上地幔顶部P波层析成像

为获取青藏高原中东部地壳和上地幔顶部的精细结构,本文基于1万4 484条天然地震的P波（Pg和Pn）到时数据,对青藏高原中东部地壳和上地幔顶部进行P波三维速度结构层析成像,获取了该区域内地壳P波、上地幔顶部Pn波的速度结构和地壳厚度信息。层析成像结果显示,青藏高原中东部地壳P波速度范围为5.2—7.2 km/s,上地幔顶部Pn波速度范围为7.7—8.4 km/s,地壳厚度范围为48.0—68.6 km,地壳和上地幔顶部存在强烈的横向不均匀性,与地质块体分布有较好的对应关系。地壳P波速度结构显示,研究区中、下地壳分布有较大范围的低速区,上地壳与中下地壳P波分布存在明显的差异:羌塘地块和巴颜喀拉地块在上地壳主要表现为高速异常,随着深度增加逐渐表现为低速异常;而柴达木地块在上地壳主要表现为低速异常,下地壳则表现为高速异常;柴达木地块和拉萨地块在上地幔顶部表现为较高的Pn波速度,最高约为8.4 km/s,而在巴颜喀拉地块和羌塘地块东部,Pn波总体上表现为低速,最低约为7.7 km/s。研究区内地壳厚度的总体特征表现为南厚北薄,其中羌塘地块东部和拉萨地块的地壳较厚,而柴达木地块和巴颜喀拉地块东部的地壳相对较薄,羌塘地块西部存在局部的地壳变薄现象,反映了印度板块对欧亚板块北向俯冲作用下的岩石圈变形特征。      

1927年古浪8级大震及其周边地区三维地壳P波速度结构特征

本文联合利用甘肃及周边测震台网记录的古浪及周边地区4592次地震的P波绝对到时资料和相对到时资料,采用双差地震层析成像方法反演了古浪震源区高分辨率的三维P波速度精细结构.结果显示,浅部P波速度分布与地表地质之间具有很好的对应关系.皇城—双塔断裂带在6 km以上深度表现为高速异常带,而在6~15 km逐渐转换为明显的低速特征,之后再次转换为高速体.震区下部在10~20 km深度有一个尺度约200 km²的低速异常体,地震发生时破裂首先在该低速体发生,与主震空间位置非常吻合.主震区的岩石结构主要由奥陶纪变质砂岩、石英岩和加里东期的花岗岩等坚硬岩体组成.这种坚硬岩体对应的P波速度结构为高速体,有利于能量积累.武威盆地在20 km以上深度表现为明显的低速异常,在25 km深度之下,整体显示为高速体,表现出稳定块体的特征.表明武威盆地中下地壳和上地幔顶部已插入到冷龙岭隆起带之下.震区小震重新定位发现,皇城—双塔断裂带东、西两段表现出不同的力学运动性质,西段以逆冲运动为主,地震主要发生在断裂的下盘.而东段地震却主要发生在上盘,断层活动以局部拉张为主.我们还首次发现在皇城—双塔断裂带的中段与主破裂呈垂直方向存在有在主震发生时新产生的一条共轭断层,基于小震的断层面参数反演显示该断裂是一高倾角运动性质以右旋为主兼具正断的断裂.     

Estimating Curie Point Depth and Heat Flow Map for Northern Red Sea Rift of Egypt and Its Surroundings, from Aeromagnetic Data

In this study, we aim to map the Curie point depth surface for the northern Red Sea rift region and its surroundings based on the spectral analysis of aeromagnetic data. Spectral analysis technique was used to estimate the boundaries (top and bottom) of the magnetized crust. The Curie point depth (CPD) estimates of the Red Sea rift from 112 overlapping blocks vary from 5 to 20 km. The depths obtained for the bottom of the magnetized crust are assumed to correspond to Curie point depths where the magnetic layer loses its magnetization. Intermediate to deep Curie point depth anomalies (10–16 km) were observed in southern and central Sinai and the Gulf of Suez (intermediate heat flow) due to the uplifted basement rocks. The shallowest CPD of 5 km (associated with very high heat flow, ~235 mW m^?2) is located at/around the axial trough of the Red Sea rift region especially at Brothers Island and Conrad Deep due to its association with both the concentration of rifting to the axial depression and the magmatic activity, whereas, beneath the Gulf of Aqaba, three Curie point depth anomalies belonging to three major basins vary from 10 km in the north to about 14 km in the south (with a mean heat flow of about 85 mW m^?2). Moreover, low CPD anomalies (high heat flow) were also observed beneath some localities in the northern part of the Gulf of Suez at Hammam Fraun, at Esna city along River Nile, at west Ras Gharib in the eastern desert and at Safaga along the western shore line of the Red Sea rift. These resulted from deviatoric tensional stresses developing in the lithosphere which contribute to its further extension and may be due to the opening of the Gulf of Suez and/or the Red Sea rift. Furthermore, low CPD (with high heat flow anomaly) was observed in the eastern border of the study area, beneath northern Arabia, due to the quasi-vertical low-velocity anomaly which extends into the lower mantle and may be related to volcanism in northern Arabia. Dense microearthquakes seem to occur in areas where the lateral gradients of the CPD are steep (e.g. entrance of the Gulf of Suez and Brothers Island in the Red Sea). These areas may correspond to the boundaries between high and low thermal regions of the crust. Thus, the variations in the microseismic activity may be closely related to thermal structures of the crust. Indeed, shallow cutoff depths of seismicity can also be found in some geothermal areas (e.g. western area of Safaga city along the Red Sea coastal region and at Esna city along the River Nile). These facts indicate that the changes in the thickness of the seismogenic layer strongly depend on temperature. Generally, the shallow Curie point depth indicates that some regions in our study area are promising regions for further geothermal exploration particularly in some localities along the River Nile, Red Sea and Gulf of Suez coastal regions.     

Eastward subduction of the Indian plate beneath the Indo-Myanmese arc revealed by teleseismic P-wave tomography

The deep structure of the eastward-subducting Indian plate can provide new information on the dynamics of the India-Eurasia collision. We collected and processed waveform data from temporary seismic arrays (networks) on the eastern Tibetan Plateau, seismic arrays in Northeast India and Myanmar, and permanent stations of the China Digital Seismic Network in Tibet, Gansu, Qinghai, Yunnan, and Sichuan. We combined these data with phase reports from observation stations of the International Seismological Center on the Indian plate and selected 124,808 high-quality P-wave relative travel-time residuals. Next, we used these data to invert the 3-D P-wave velocity structure of the upper mantle to a depth of 800 km beneath the eastern segment of the arcuate Himalayan orogen, at the southeastern margin of the Tibetan Plateau. The results reveal a high-angle, easterly dipping subducting plate extending more than 200 km beneath the Indo-Myanmese arc. The plate breaks off at roughly 96°E; its fragments have passed through the 410-km discontinuity (D410) into the mantle transition zone (MTZ). The MTZ beneath the Tengchong volcanic area contains a high-velocity anomaly, which does not exceed the Red River fault to the east. No other large-scale continuous subducted plates were observed in the MTZ. However, a horizontally spreading high-velocity anomaly was identified on the D410 in some regions. The anomaly may represent the negatively buoyant 90°E Ridge plate or a thickened and delaminated lithospheric block experiencing collision and compression at the southeastern margin of the Tibetan Plateau. The Tengchong volcano may originate from the mantle upwelling through the slab window formed by the break-off of the subducting Indian continental plate and oceanic plate in the upper mantle. Low-velocity upper mantle materials on the west side of the Indo-Myanmese arc may have supplemented materials to the Tengchong volcano.     

Crustal thickness, discontinuity depth, and upper mantle structure beneath southern Africa: constraints from body wave conversions

The technique of receiver function analysis is applied to the study of crustal and upper mantle structures beneath the Kaapvaal craton in southern Africa and its surroundings. Seismic data were recorded by the seismic array of 82 sites deployed from April 1997 to April 1999 across southern Africa, as well as a dense array of 32 sites near Kimberley, in operation from December 1998 to June 1999. Arrival times for phases converted at the Moho are used to determine crustal thickness. The Moho depth in the south–western section of the craton was found to vary between 37 and 40 km, except for one station that recorded a depth of 43 km (SA23). Farther north along the western block of the craton (into Botswana) the depth increases up to 43 km. The depth increases even further in the north–eastern section of the craton, where results vary from 40 to 52 km. Just north of the Kaapvaal craton, in the neighbouring Zimbabwe craton, the crustal thickness drops significantly. The results obtained there varied from 36 to 40 km. For the Kimberley area, using the dense array, the Moho depth was found to be 37.3 km. Arrivals of the Ps and Ppps phases were used to determine the Poisson’s ratio in the region. This was found to be 0.26±0.01. Arrivals of phases from the 410 and 660 km mantle discontinuities are used to interpret the relative positions of these discontinuities, as well as for comparison of mantle temperatures and seismic velocities in the region with global averages. In the Kimberley area the 410 and 660 km discontinuities were found at their expected depth, implying that mantle temperatures in the region are close to the global average. The seismic velocities above the ‘410’ were found up to 5% faster than the averages from the global iasp91 model, which is fast even by Precambrian standards. In other sections of the Kaapvaal craton, the velocities are also faster than global averages, but not as fast as beneath Kimberley. In these sections, the ‘410’ is also slightly elevated, while the ‘660’ is depressed, which implies a slightly lower mantle temperature relative to the global average. Beneath the Kaapvaal craton we find evidence suggesting the presence of a zone with a reduced wavespeed gradient at an upper bound of approximately 300 km, which may mark the lower chemical boundary of the craton.     

An upper mantleP-wave velocity model for eastern and southern Africa

AP-wave velocity model for the upper mantle beneath eastern and southern Africa is proposed. The top 250 km of the model is characterized by relatively low velocities similar to those deduced for the upper mantle beneath the western United States of America. At greater depths, the velocities gradually change to normal mantle values.     

汶川MS8.0级地震余震分布及周边区域P波三维速度结构研究

利用川滇地区长期积累的地震走时观测资料和汶川地震余震观测资料对汶川地震震源区及周边区域地壳和上地幔P波三维速度结构进行了研究.结果表明,浅部P波速度分布与地表地质之间具有很好的对应关系.龙门山断裂带在20 km以上深度表现为高速异常带,彭灌杂岩体和宝兴杂岩体为局部高速异常区.龙门山断裂带中上地壳的局部高速异常体对汶川地震的余震分布具有明显的控制作用.在余震带南端,余震全部发生在与宝兴杂岩体对应的高速异常体的东北侧;在余震带的中段,与彭灌杂岩体对应的高速异常体在一定程度上控制了余震的分布;在余震带的东北端,宁强-勉县一带的高速异常体可能阻止了余震进一步向东北扩展.龙门山断裂带中上地壳的P波高速异常表明介质具有相对较高的强度,在青藏高原物质向东挤出过程中起到了较强的阻挡作用,有利于深部能量积累.在30 km深度之下,扬子地块具有明显的高速特征,其前缘随深度增加向青藏高原方向扩展,在下地壳和上地幔顶部已达到龙门山断裂带以西.     

华北地区上地幔温度及岩石圈厚度分布研究

上地幔温度是控制地幔流变性和动力学过程的关键参数之一.本文利用高分辨率S波地震层析成像速度结果,基于岩石温度与地震波速度的关系,研究了华北地区上地幔50~300 km深度范围内的温度分布和"热"岩石圈厚度.为了验证结果的可靠性,本文用计算的上地幔60 km深度处的温度作为底面约束,得到了相应的地表热流.计算地表热流与观测地表热流之间符合程度较好,相对误差大部分都在地表热流观测误差范围之内.通过对上地幔的温度分布进行分析,我们研究发现:(1)在上地幔浅部,温度与地表构造之间有很好的对应关系.在小于170 km的深度上,温度呈现东高西低的分布态势.温度较高的区域集中在东部的河淮盆地、渤海湾盆地、华北平原和中部陆块的交界处、西部鄂尔多斯高原北缘的银川―河套地堑以及阴山地区,同时,这些地区的岩石圈厚度也相应较薄,大约为80~100 km;(2)西部的鄂尔多斯高原是整个华北地区岩石圈地幔温度最低的地区,比东部地区低200~400 ℃,岩石圈厚度相应最厚,平均岩石圈厚度达到140~150 km,最厚处超过160 km.(3)在170 km以下的软流圈地幔部分,温度分布发生反转,西部温度高于东部,表明东、西部陆块在地质历史时期经历了不同的深部地幔动力学过程.