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1.
A broadband seismic array of 7 stations was set up in the western Dabie Mountains (31°20′-31°50′N, 114°30′-115°E). Teleseismic events from May 2001 to November 2001 were collected and analyzed by radial receiver function to determine the S-wave velocity structure of the crust and uppermost mantle. The crustal thickness is 32-38 km beneath the array. The crust-mantle boundary appears as a gently north-dipping velocity discontinuity, but turns to be a velocity gradient beneath a station near the Qiliping shea...  相似文献   

2.
Wide-angle refraction and multichannel reflection seismic data show that oceanic crust along the Galápagos Spreading Center (GSC) between 97°W and 91°25′W thickens by 2.3 km as the Galápagos plume is approached from the west. This crustal thickening can account for ∼52% of the 700 m amplitude of the Galápagos swell. After correcting for changes in crustal thickness, the residual mantle Bouguer gravity anomaly associated with the Galápagos swell shows a minimum of −25 mGal near 92°15′W, the area where the GSC is intersected by the Wolf-Darwin volcanic lineament (WDL). The remaining depth and gravity anomalies indicate an eastward reduction of mantle density, estimated to be most prominent above a compensation depth of 50-100 km. Melting calculations assuming adiabatic, passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of ∼30°C. The associated thermal expansion and increase in melt depletion reduce mantle densities, but to a degree that is insufficient to explain the geophysical observations. The largest density anomalies appear at the intersection of the GSC and the WDL. Our results therefore require the existence of compositionally buoyant mantle beneath the GSC near the Galápagos plume. Possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the Galápagos Islands that is later transported to the GSC. Our estimate for the buoyancy flux of the Galápagos plume (700 kg s−1) is lower than previous estimates, while the total crustal production rate of the Galápagos plume (5.5 m3s−1) is comparable to that of the Icelandic and Hawaiian plumes.  相似文献   

3.
We present multichannel seismic reflection data collected over the Atlantis megamullion, at the eastern ridge-transform intersection of Atlantis fracture zone on the northern Mid-Atlantic Ridge, and over its conjugate crust. These data image for the first time the internal structure of a young, well-developed megamullion dome formed by tectonic extension across a long-lived oceanic detachment fault. The exposed, corrugated detachment-fault surface exhibits a sharp, coherent reflection that contrasts with less organized reflectivity of surrounding basaltic seafloor. At the termination of the megamullion the fault is imaged ∼13 km along-strike beneath a volcanic hanging-wall block at a sub-seafloor depth of 0.2-0.5 s two-way travel time, reaching north as far as 30°19′N. The eastward dipping of the fault beneath the hanging-wall block is estimated to be ∼6-14°. The corrugated fault surface is underlain by a continuous, strong, and relatively smooth reflection (D) at 0.2-0.25 s sub-bottom below the central axis of the dome. This reflection deepens up to 0.6 s sub-bottom beneath the western slope and it appears to intersect the seafloor on the eastern slope. We suggest that Atlantis massif formed by sequential slip on two different detachment faults that merged at depth, with breakaways as little as ∼2 km apart. The initial detachment is represented by reflection D, and the second corresponds to the presently exposed fault surface. In this interpretation, much of the sliver between the faults is interpreted to be strongly serpentinized peridotite with reduced seismic velocity; it lies in contact with less altered, higher-velocity mantle below the first detachment, resulting in the strong, smooth character of reflection D. Mantle rocks exposed in the megamullion indicate that the feature formed during a period of extreme tectonic extension and probably limited magmatism. In conjugate crust corresponding to termination of the megamullion, observed sub-bottom reflections are interpreted as base of seismic layer 2A. This layer is as thick as or thicker (∼570-900 m) than layer 2A in normal Atlantic crust, and it suggests that relatively normal crustal accretion occurred by the time the megamullion stopped forming.  相似文献   

4.
We have produced a P-wave model of the upper mantle beneath Southeast (SE) Asia from reprocessed short period International Seismological Centre (ISC) P and pP data, short period P data of the Annual Bulletin of Chinese Earthquakes (ABCE), and long period PP-P data. We used 3D sensitivity kernels to combine the datasets, and mantle structure was parameterized with an irregular grid. In the best-sampled region our data resolve structure on scale lengths less than 150 km. The smearing of crustal anomalies to larger depths is reduced by a crustal correction using an a priori 3D model. Our tomographic inversions reveal high-velocity roots beneath the Archean Ordos Plateau, the Sichuan Basin, and other continental blocks in SE Asia. Beneath the Himalayan Block we detect high seismic velocities, which we associate with subduction of Indian lithospheric mantle. This structure is visible above the 410 km discontinuity and may not connect to the remnant of the Neo-Tethys oceanic slab in the lower mantle. Our images suggest that only the southwestern part of the Tibetan plateau is underlain by Indian lithosphere and, thus, that the upper mantle beneath northeastern Tibet is primarily of Asian origin. Our imaging also reveals a large-scale high-velocity structure in the transition zone beneath the Yangtze Craton, which could have been produced in multiple subduction episodes. The low P-wave velocities beneath the Hainan Island are most prominent in the upper mantle and transition zone; they may represent counter flow from the surrounding subduction zones, and may not be unrelated to processes beneath eastern Tibet.  相似文献   

5.
Two-dimensional crustal velocity models are derived from passive seismic observations for the Archean Karelian bedrock of north-eastern Finland. In addition, an updated Moho depth map is constructed by integrating the results of this study with previous data sets. The structural models image a typical three-layer Archean crust, with thickness varying between 40 and 52 km. P wave velocities within the 12–20 km thick upper crust range from 6.1 to 6.4 km/s. The relatively high velocities are related to layered mafic intrusive and volcanic rocks. The middle crust is a fairly homogeneous layer associated with velocities of 6.5–6.8 km/s. The boundary between middle and lower crust is located at depths between 28 and 38 km. The thickness of the lower crust increases from 5–15 km in the Archean part to 15–22 km in the Archean–Proterozoic transition zone. In the lower crust and uppermost mantle, P wave velocities vary between 6.9–7.3 km/s and 7.9–8.2 km/s. The average Vp/Vs ratio increases from 1.71 in the upper crust to 1.76 in the lower crust.The crust attains its maximum thickness in the south-east, where the Archean crust is both over- and underthrust by the Proterozoic crust. A crustal depression bulging out from that zone to the N–NE towards Kuusamo is linked to a collision between major Archean blocks. Further north, crustal thickening under the Salla and Kittilä greenstone belts is tentatively associated with a NW–SE-oriented collision zone or major shear zone. Elevated Moho beneath the Pudasjärvi block is primarily explained with rift-related extension and crustal thinning at ∼2.4–2.1 Ga.The new crustal velocity models and synthetic waveform modelling are used to outline the thickness of the seismogenic layer beneath the temporary Kuusamo seismic network. Lack of seismic activity within the mafic high-velocity body in the uppermost 8 km of crust and relative abundance of mid-crustal, i.e., 14–30 km deep earthquakes are characteristic features of the Kuusamo seismicity. The upper limit of seismicity is attributed to the excess of strong mafic material in the uppermost crust. Comparison with the rheological profiles of the lithosphere, calculated at nearby locations, indicates that the base of the seismogenic layer correlates best with the onset of brittle to ductile transition at about 30 km depth.We found no evidence on microearthquake activity in the lower crust beneath the Archean Karelian craton. However, a data set of relatively well-constrained events extracted from the regional earthquake catalogue implies a deeper cut-off depth for earthquakes in the Norrbotten tectonic province of northern Sweden.  相似文献   

6.
In order to investigate crustal structure beneath the eastern Marmara region, a seismic refraction survey was conducted across the North Anatolian Fault (NAF) zone in north west Turkey. Two reversed profiles across two strands of the NAF zone were recorded in the Armutlu Highland where a tectonically active region was formed by different continents. We used land explosions in boreholes and quarry blasts as seismic sources. A reliable crustal velocity and depth model is obtained from the inversion of first arrival travel times. The velocity-depth model will improve the positioning of the earthquake activities in this active portion of the NAF. A high velocity anomaly (5.6–5.8 km s−1) in the central highland of Armutlu block and the low velocity (4.90 km s−1) pattern north of Iznik Lake are the two dominant features. The crustal thickness is about 26 ± 2 km in the north and increases to about 32 ± 2 km beneath the central Armutlu block in the south. P-wave velocities are about 3.95 km s−1 to 4.70 km s−1 for the depth range between about 1 km and 5 km in the upper crust. The eastern Marmara region has different units of upper crust with velocities varying with depth to almost 8 km. The high upper crust velocities are associated with Armutlu metamorphic rocks, while the low velocity anomalies are due to unconsolidated sedimentary sequences. The western side of Armutlu block has complex tectonics and is well known for geothermal sources. If these sources are continuous throughout the portions of the crust, it may be associated with a granitic intrusion and deformation along the NAF zone. That is, the geothermal sources associated with the low velocity may be due to the occurrence of widespread shear heating, even shear melting. The presence of shear melting may indicate the presence of crustal fluid imposed by two blocks of the NAF system.  相似文献   

7.
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.  相似文献   

8.
P-wave and S-wave receiver function analyses have been performed along a profile consisted of 27 broadband seismic stations to image the crustal and upper mantle discontinuities across Northeast China. The results show that the average Moho depth varies from about 37 km beneath the Daxing’anling orogenic belt in the west to about 33 km beneath the Songliao Basin, and to about 35 km beneath the Changbai mountain region in the east. Our results reveal that the Moho is generally flat beneath the Daxing’anling region and a remarkable Moho offset (about 4 km) exists beneath the basin-mountain boundary, the Daxing’anling-Taihang Gravity Line. Beneath the Tanlu faults zone, which seperates the Songliao Basin and Changbai region, the Moho is uplift and the crustal thickness changes rapidly. We interpret this feature as that the Tanlu faults might deeply penetrate into the upper mantle, and facilitate the mantle upwelling along the faults during the Cenozoic era. The average depth of the lithosphere-asthenosphere boundary (LAB) is ~80 km along the profile which is thinner than an average thickness of a continental lithosphere. The LAB shows an arc-like shape in the basin, with the shallowest part approximately beneath the center of the basin. The uplift LAB beneath the basin might be related to the extensive lithospheric stretching in the Mesozoic. In the mantle transition zone, a structurally complicated 660 km discontinuity with a maximum 35 km depression beneath the Changbai region is observed. The 35 km depression is roughly coincident with the location of the stagnant western pacific slab on top of the 660 km discontinuity revealed by the recent P wave tomography.  相似文献   

9.
The structure of the crust and the crust-mantle boundary in the Vogtland/West Bohemian region have been a target of several seismic measurements for the last 25 years, beginning with the steep-angle reflection seismic studies (DEKORP-4/KTB, MVE-90, 9HR), the refraction and wide-angle experiments (GRANU’95, CELEBRATION 2000, SUDETES 2003), and followed by passive seismic studies (receiver functions, teleseismic tomography). The steep-angle reflection studies imaged a highly reflective lower crust (4 to 6 km thick) with the Moho interpreted in a depth between 30 and 32 km and a thinner crust beneath the Eger Rift. The refraction and wide-angle reflection seismic studies (CELEBRATION 2000) revealed strong wide-angle reflections in a depth of 26–28 km interpreted as the top of the lower crust. Long coda of these reflections indicates strong reflectivity in the lower crustal layer, a phenomenon frequently observed in the Caledonian and Variscan areas. The receiver function studies detected one strong conversion from the base of the crust interpreted as the Moho discontinuity at a depth between 27 and 37 km (average at about 31 km). The discrepancies in the Moho depth determination could be partly attributed to different background of the methods and their resolution, but could not fully explain them. So that new receivers function modelling was provided. It revealed that, instead of a first-order Moho discontinuity, the observations can be explained with a lower crustal layer or a crust-mantle transition zone with a maximum thickness of 5 km. The consequent synthetic ray-tracing modelling resulted in the model with the top of the lower crust at 28 km, where highly reflective lower crustal layer can obscure the Moho reflection at a depth of 32–33 km.  相似文献   

10.
超慢速扩张西南印度洋中脊岩浆的集中供给在空间维度上表现为岩浆扩张段（NVR）与相邻的非转换断层不连续带（NTD）地壳结构的差异,而在时间维度上表现为离轴与沿轴地壳结构的差异.为了进一步揭示岩浆集中供给的时空分布特征,本文选取西南印度洋中脊热液区2010年海底地震仪深部探测中平行于洋中脊距轴部偏北约10 km的离轴测线d0d10,使用射线追踪正演和反演的方法,得到了NVR和NTD北侧离轴区域的地壳及上地幔P波速度结构,并与轴部速度结构进行了对比分析.研究结果表明：（1）NTD北侧离轴区域的地壳厚度约5.2 km,其厚度明显大于轴部NTD下方地壳厚度（~3.2 km）,由此推测洋脊轴部NTD区域形成的地壳在不断减薄;（2）NVR北侧离轴区域的地壳厚度约7.0 km,其厚度亦大于轴部NVR地壳厚度（~5.8 km）,表明在洋中脊演化过程中洋脊轴区域的岩浆供给在不断减少,其活动性在不断减弱.  相似文献   

11.
Receiver functions of teleseismic waveforms recorded at four Hungarian permanent broadband stations have been analyzed using semilinearized and stochastic inversion methods to estimate the crustal thickness and S wave velocity structure in the Pannonian Basin. The results of both inversion methods agree well with the crustal thicknesses obtained by previous seismic refraction and reflection studies in the regions which are densely covered with seismic lines (28 and 27 km in westernmost and southern Hungary, respectively) and suggest a thicker crust compared to what was known before beneath the Transdanubian and Northern Ranges (34 and 33 km, respectively). The comparison of the one-dimensional shear wave velocity models derived by the different inversion methods shows that, in case of simple, smoothly varying structures, the results do not differ significantly and can be regarded as absolute velocities. Otherwise, the recovered velocity gradients agree, but there are differences in the absolute velocity values. The back-azimuthal variations of both radial and tangential receiver functions are interpreted as dipping structure and as waves sampling different geological areas. The signature of the deep structure on low-frequency receiver functions suggests a strong velocity contrast at the 670-km discontinuity. The vanishing 410-km boundary may be attributed to the remnant of a subducted oceanic slab with increased Poisson’s ratio in the transition zone.  相似文献   

12.
We invert S-wave velocities for the 3D upper-mantle temperatures, in which the position with a temperature crossing the 1300℃ adiabat is corresponding to the top of the seismic low velocity zone. The temperatures down to the depth of 80 km are then calculated by solving steady-state thermal conduction equation with the constraints of the inverted upper-mantle temperatures and the surface temperatures, and then surface heat flows are calculated from the crustal temperatures. The misfit between the calculated and observed surface heat flow is smaller than 20% for most regions. The result shows that, at a depth of 25 km, the crustal temperature of eastern China (500―600℃) is higher than that of western China (<500℃). At a depth of 100 km, temperatures beneath eastern and southeastern China are higher than the adiabatic temperature of 1300℃, while that beneath west China is lower. The Tarim craton and the Sichuan basin show generally low temperature. At a depth of 150 km, temperatures beneath south China, eastern Yangtze craton, North China craton and around the Qiangtang terrane are higher than the adiabatic temperature of 1300℃, but is the lowest beneath the Sichuan basin and the regions near the Indian-Eurasian collision zone. At a depth of 200 km, very low temperature occurs beneath the Qinghai-Tibet Plateau and the south to the Tarim craton.  相似文献   

13.
The lower crust of magmatically active rifts is usually too hot and ductile to allow seismicity. The Icelandic mid-Atlantic rift is characterized by high heat flow, abundant magmatism generating up to 25–30 km thick crust, and seismicity within the upper 8 km of the crust. In a 20-seismometer survey in July-August 2006 within the northern rift zone around the Askja volcano we recorded ~1700 upper-crustal earthquakes cutting off at 7–8 km depth, marking the brittle-ductile boundary. Unexpectedly, we discovered 100 small-magnitude (ML <1.5) earthquakes, occurring in swarms mostly at 14–26 km depth within the otherwise aseismic lower crust, and beneath the completely aseismic middle crust. A repeat survey during July-August 2007 yielded more than twice as many lower-crustal events. Geodetic and gravimetric data indicate melt drainage from crustal magma chambers beneath Askja. We interpret the microearthquakes to be caused by melt moving through the crust from the magma source feeding Askja. They represent bursts of magma motion opening dykes over distances of a few meters, facilitated by the extensional setting of the active rift zone.  相似文献   

14.
The Qinling–Dabie–Sulu orogenic belt in east-central China is the largest high and ultrahigh pressure (HP and UHP) metamorphic zone in the world. The Dabie Mountains are the central segment of this orogenic belt between the North China and Yangtze cratons. This work studies the nature of the crustal structure beneath the Dabie orogenic belt to better understand the orogeny. To do that, we apply ambient noise tomography to the Dabie orogenic belt using ambient noise data from 40 stations of the China National Seismic Network (CNSN) between January 2008 and December 2009. We retrieve high signal noise ratio (SNR) Rayleigh waves by cross-correlating ambient noise data between most of the station pairs and then extract phase velocity dispersion measurements from those cross-correlations using a spectral method. Taking those dispersion measurements, we obtain high-resolution phase velocity maps at 8–35 second periods. By inverting Rayleigh wave phase velocity maps, we construct a high-resolution 3D shear velocity model of the crust in the Dabie orogenic belt.The resulting 3D model reveals interesting crustal features related to the orogeny. High shear wave velocities are imaged beneath the HP/UHP metaphoric zones at depths shallower than 9 km, suggesting that HP/UHP metaphoric rocks are primarily concentrated in the upper crust. Underlying the high velocity HP/UHP metamorphic zones, low shear velocities are observed in the middle crust, probably representing ductile shear zones and/or brittle fracture zones developed during the exhumation of the HP/UHP metamorphic rocks. Strong high velocities are present beneath the Northern Dabie complex unit in the middle crust, possibly related to cooling and crystallization of intrusive igneous rocks in the middle crust resulting from the post-collisional lithosphere delamination and subsequent magmatism. A north-dipping Moho is revealed in the eastern Dabie with the deepest Moho appearing beneath the Northern Dabie complex unit, consistent with the model of Triassic northward subduction of the Yangtze Craton beneath the North China Craton.  相似文献   

15.
本文利用宽频流动台阵记录的远震波形资料和接收函数波动方程叠后偏移方法,获得了华北克拉通东北部边界及其邻近地区的地壳和地幔转换带的间断面结构图像.结果显示研究区域的地壳厚度存在显著的横向变化:以南北重力梯度带为界,西北部的兴蒙造山带地壳较厚(~40 km),东南部的燕山带、松辽盆地和辽东台隆地壳明显较薄(30~35 km).这有可能反映,研究区南北重力梯度带两侧地壳在中-新生代区域构造伸展过程中经历了不同程度的改造和减薄.地幔转换带成像结果显示,研究区410 km和660 km间断面结构存在横向差异.经度121°E-122°E之间,上地幔底部出现双重间断面,深度分别为660 km和690 km.经度122.5°E以东(北黄海地区),410 km间断面有5~20 km幅度的下沉,660 km间断面有5~15 km幅度的抬升;该地区地幔转换带厚度相对全球平均偏薄10~20 km,指示着该地区较热的上地幔底部温度环境.我们认为太平洋俯冲板块可能停滞在研究区119°E-122°E经度范围的地幔转换带中,但未延伸至118°E以西;而俯冲板块在124°E以东可能局部穿透了上地幔底部而进入下地幔,同时引起小尺度的地幔对流,导致北黄海地区下地幔物质的上涌.  相似文献   

16.
Abstract : The Hidaka metamorphic belt consists of an island-arc assembly of lower to upper crustal rocks formed during early to middle Paleogene time and exhumed during middle Paleogene to Miocene time. The tectonic evolution of the belt is divided into four stages, D0rs, D1, D2rs, and D3, based on their characteristic deformation, metamorphism, and igneous activity. The premetamorphic and igneous stage (D0) involves tectonic thickening of an uppermost Cretaceous and earliest Tertiary accretionary complex, including oceanic materials in the lower part of the complex. D1 is the stage of prograde metamorphism with increasing temperatures at a constant pressure during an early phase, and with a slight decrease of pressure at the peak metamorphic phase, accompanying flattening of metamorphic rocks and intrusions of mafic to intermediate igneous rocks. At the peak, incipient partial melting of pelitic and psammitic gneisses took place in the amphibolite–granulite facies transition zone, the melt and residuals cutting the foliations formed by flattening. In the deep crust, large amounts of S-type tonalite magma formed by crustal anatexis, intruded into the granulite facies gneiss zone and also into the upper levels of the metamorphic sequence during the subsequent stage. During D1 stage, mafic and intermediate magmas supplied and transported heat to form the arc-type crust and at the same time, the magmatic underplating caused extensional doming of the crust, giving rise to flattening and vertical uplifting of the crustal rocks. D2 stage is characterized by subhorizontal top-to-the-south displacement and thrusting of lower to upper crustal rocks, forming a basal detachment surface (décollement) and duplex structures associated with intrusions of S-type tonalite. Deformation structures and textures of high-temperature mylonites formed along the décollement, as well as the duplex structures, show that the D2 stage movement occurred under a N-S trending compressional tectonic regime. The depth of intra-crustal décollement in the Hidaka belt was defined by the effect of multiplication of two factors, the fraction of partial melt which increases downward, and the fluid flux which decreases downward. The crustal décollement, however, might have extended to the crust-mantle boundary and/or to the lithosphere and asthenosphere boundary. The subhorizontal movement was transitional to a dextral-reverse-slip (dextral transpression) movement accompanied by low-temperature mylonitization with retrograde metamorphism, the stage defined as D3. The crustal rocks from the basal décollement to the upper were tilted eastward on the N–S axis and exhumed during the D3 stage. During D2 and D3 stages, the intrusion of crustal acidic magmas enhanced the crustal deformation and exhumation in the compressional and subsequent transpressional tectonic regime.  相似文献   

17.
台湾海峡大容量气枪震源海陆联测初探   总被引：2，自引：0，他引：2       下载免费PDF全文
本文利用在我国台湾海峡采用大容量气枪震源开展海陆联测获得的广角地震测线HX9, 采用二维射线追踪法反演得到了HX9剖面的地壳二维速度结构和地壳界面形态, 初步探明了福建—台湾海峡海陆过渡带的深部构造． 结果表明: HX9剖面的地壳内存在两个速度间断面, 即C界面和莫霍面, 其中: C界面为上、 下地壳的分界面, 是一个小的速度不连续面, 速度变化值达0.08—0.16 km/s; 而地壳底部的莫霍面则有较大的速度反差, 变化值达1.02—1.29 km/s, 莫霍面上、 下的速度分别为6.75—6.97 km/s和8.00—8.07 km/s． 沿剖面的地壳界面形态总体起伏不大, 陆域上、 下地壳的厚度和界面变化趋势均相似, 从陆域到海域呈微倾斜变化趋势, 表现为减薄陆壳的特征． 莫霍面陆域埋深约为31.6 km, 向福建东南沿海逐渐减薄至27.4 km左右．   相似文献   

18.
Crustal structure of Dabieshan orogenic belt   总被引：2，自引：0，他引：2  
The crustal structures ofP velocity and density on the deep seismic sounding profile across the Ilabieshan orogenic belt are presented. There is a 5-km-thick crustal “root” between the Yuexi and Xiaotian where the elevation is highest on the profile. An apparent Moho offset of 4. 5 km beneath the Xiaotian-Mozitan fault marks the paleo-suture of the Triassic collision. A high-velocity anomaly zone at the depth below 3 km beneath the ultra-high pressure (UHP) zone may be correlated to the higher content of UHP metamorphic rocks. Project supported by the National Natural Science Foundation of China and the Joint Earthquake Science Foundation.  相似文献   

19.
In the absence of convincing magnetic anomaly information, topographic profiles have been used to infer the tectonic history of the East Pacific Rise between 5°S and 20°S. Profiles projected at right angles to the rise crest show a sharp drop in elevation at roughly the same distance on either side of the crest. Profiles to the east of the rise also show a second topographic high at 95°W. Comparison of these profiles with empirical depth versus age curves for the North Pacific suggests that this rise, the Galapagos Rise, is the fossil East Pacific Rise which terminated close to 6 mybp by the spreading center jumping 900 km to the west. The extreme youthfulness of the present East Pacific Rise, the step structure of its flanks, and the similarity in age of the top of this step and the crest of the Galapagos Rise substantiate this interpretation. This jump coincided with a similar readjustment involving the Mathematicians ridge at 5° to 20°N and the opening of the Gulf of California.  相似文献   

20.
Recent seismological studies have presented evidence for the existence of a layer with ultra-low seismic velocities at the core-mantle boundary at ca. 2900 km depth. We report high-amplitude, high-frequency, and laterally coherent seismic arrivals from three nuclear explosions in Siberia. With recording station intervals of 15 km, the seismic phases are readily correlated and show the presence of a thin, ultra-low velocity zone in a region where it was not previously reported. The duration and complexity of the arrivals are inconsistent with a simple core-mantle boundary and require a hitherto unidentified, kilometre-scale, fine structure in the ultra-low velocity zone. The observations may be explained by a ca. 7 km thick, two-layer, ultra-low velocity zone with exceptional low velocities, which indicate the presence of high percentages of melt (>15%), in particular in the lower part of the zone. Waveform variation implies lateral change in the thickness and physical properties of the ultra-low velocity zone with a wavelength of less than 100 km.  相似文献   

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