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1.
Pressures of Crystallization of Icelandic Magmas 总被引：1，自引：0，他引：1
Iceland lies astride the Mid-Atlantic Ridge and was createdby seafloor spreading that began about 55 Ma. The crust is anomalouslythick (20–40 km), indicating higher melt productivityin the underlying mantle compared with normal ridge segmentsas a result of the presence of a mantle plume or upwelling centeredbeneath the northwestern edge of the Vatnajökull ice sheet.Seismic and volcanic activity is concentrated in 50 km wideneovolcanic or rift zones, which mark the subaerial Mid-AtlanticRidge, and in three flank zones. Geodetic and geophysical studiesprovide evidence for magma chambers located over a range ofdepths (1·5–21 km) in the crust, with shallow magmachambers beneath some volcanic centers (Katla, Grimsvötn,Eyjafjallajökull), and both shallow and deep chambers beneathothers (e.g. Krafla and Askja). We have compiled analyses ofbasalt glass with geochemical characteristics indicating crystallizationof ol–plag–cpx from 28 volcanic centers in the Western,Northern and Eastern rift zones as well as from the SouthernFlank Zone. Pressures of crystallization were calculated forthese glasses, and confirm that Icelandic magmas crystallizeover a wide range of pressures (0·001 to 1 GPa), equivalentto depths of 0–35 km. This range partly reflects crystallizationof melts en route to the surface, probably in dikes and conduits,after they leave intracrustal chambers. We find no evidencefor a shallow chamber beneath Katla, which probably indicatesthat the shallow chamber identified in other studies containssilica-rich magma rather than basalt. There is reasonably goodcorrelation between the depths of deep chambers (> 17 km)and geophysical estimates of Moho depth, indicating that magmaponds at the crust–mantle boundary. Shallow chambers (<7·1 km) are located in the upper crust, and probablyform at a level of neutral buoyancy. There are also discretechambers at intermediate depths (11 km beneath the rift zones),and there is strong evidence for cooling and crystallizing magmabodies or pockets throughout the middle and lower crust thatmight resemble a crystal mush. The results suggest that themiddle and lower crust is relatively hot and porous. It is suggestedthat crustal accretion occurs over a range of depths similarto those in recent models for accretionary processes at mid-oceanridges. The presence of multiple stacked chambers and hot, porouscrust suggests that magma evolution is complex and involvespolybaric crystallization, magma mixing, and assimilation. KEY WORDS: Iceland rift zones; cotectic crystallization; pressure; depth; magma chamber; volcanic glass 相似文献
2.
The three-dimensional shear velocity lithospheric structure at depths from 0 to 70 km beneath the southern Baikal rift system and its surroundings has been imaged by inversion of P-to-SV receiver functions from 46 digital stations operated in two teleseismic international projects in southern Siberia and Mongolia. The receiver functions were determined from teleseismic P waveforms and inverted to obtain depth dependences of S velocities at each station which were related to tectonic structures. The computed vertical and horizontal sections of the 3D shear velocity model imaged a transition from relatively thin crust of the southern Siberian craton to thicker crust in the folded area south and southeast of Lake Baikal, with a local zone of thin crust right underneath the South Baikal basin. The velocity structure beneath the Baikal rift, the mountains of Transbaikalia, Mongolia, and the southern craton margin includes several low-velocity zones at different depths in the crust. Some of these zones may record seismic anisotropy associated with mylonite alignment along large thrusts. 相似文献
3.
A two-dimensional model of the crust and uppermost mantle for the western Siberian craton and the adjoining areas of the Pur-Gedan basin to the north and Baikal Rift zone to the south is determined from travel time data from recordings of 30 chemical explosions and three nuclear explosions along the RIFT deep seismic sounding profile. This velocity model shows strong lateral variations in the crust and sub-Moho structure both within the craton and between the craton and the surrounding region. The Pur-Gedan basin has a 15-km thick, low-velocity sediment layer overlying a 25-km thick, high-velocity crystalline crustal layer. A paleo-rift zone with a graben-like structure in the basement and a high-velocity crustal intrusion or mantle upward exists beneath the southern part of the Pur-Gedan basin. The sedimentary layer is thin or non-existent and there is a velocity reversal in the upper crust beneath the Yenisey Zone. The Siberian craton has nearly uniform crustal thickness of 40–43 km but the average velocity in the lower crust in the north is higher (6.8–6.9 km/s) than in the south (6.6 km/s). The crust beneath the Baikal Rift zone is 35 km thick and has an average crustal velocity similar to that observed beneath the southern part of craton. The uppermost mantle velocity varies from 8.0 to 8.1 km/s beneath the young West Siberian platform and Baikal Rift zone to 8.1–8.5 km/s beneath the Siberian craton. Anomalous high Pn velocities (8.4–8.5 km/s) are observed beneath the western Tunguss basin in the northern part of the craton and beneath the southern part of the Siberian craton, but lower Pn velocities (8.1 km/s) are observed beneath the Low Angara basin in the central part of the craton. At about 100 km depth beneath the craton, there is a velocity inversion with a strong reflecting interface at its base. Some reflectors are also distinguished within the upper mantle at depth between 230 and 350 km. 相似文献
4.
What triggers Koyna region earthquakes? Preliminary results from seismic tomography digital array 总被引：1，自引：0，他引：1
S. S. Rai Sunil K. Singh P. V. S. S. Rajagopal Sarma D. Srinagesh K. N. S. Reddy K. S. Prakasam Y. Satyanarayana 《Journal of Earth System Science》1999,108(1):1-14
The cause for prolific seismicity in the Koyna region is a geological enigma. Attempts have been made to link occurrence of
these earthquakes with tectonic strain as well as the nearby reservoirs. With a view to providing reliable seismological database
for studying the earth structure and the earthquake process in the Koyna region, a state of the art digital seismic network
was deployed for twenty months during 1996–97. We present preliminary results from this experiment covering an area of 60
× 80 km2 with twenty seismic stations. Hypocentral locations of more than 400 earthquakes confined to 11×25 km2 reveal fragmentation in the seismicity pattern — a NE — SW segment has a dip towards NW at approximately 45°, whilst the
other two segments show a near vertical trend. These seismic segments have a close linkage with the Western Ghat escarpment
and the Warna fault. Ninety per cent of the seismicity is confined within the depth range of 3–10 km. The depth distribution
of earthquakes delimits the seismogenic zone with its base at 10 km indicating a transition from an unstable to stable frictional
sliding regime. The lack of shallow seismicity between 0 and 3 km indicates a mature fault system with well-developed gouge
zones, which inhibit shallow earthquake nucleation. Local earthquake travel time inversion for P- and S-waves show ≈ 2% higher
velocity in the seismogenic crust (0–10 km) beneath the epicentral tract relative to a lower velocity (2–3%) in the adjoining
region. The high P- and S-wave velocity in the seismogenic crust argues against the presence of high pressure fluid zones
and suggests its possible linkage with denser lithology. The zone of high velocity has been traced to deeper depths (≈ 70
km) through teleseismic tomography. The results reveal segmented and matured seismogenic fault systems in the Koyna region
where seismicity is possibly controlled by strain build up due to competent lithology in the seismic zone with a deep crustal
root. 相似文献
5.
西藏及其周围地区地壳、地幔地震层析成像——印度板块大规模俯冲于西藏高原之下的证据 总被引：20，自引：1，他引：20
文中介绍有关西藏—喜马拉雅碰撞带的一项地震层析成像研究。根据一个用天然地震数据产生的全球波速模型 ,印度板块有可能以近水平状俯冲于整个西藏高原之下至 16 5～ 2 6 0km深度。西藏岩石圈具有低波速地壳和高波速下岩石圈 (75～ 12 0km深 )。在 12 0～ 16 5km深度范围 ,西藏岩石圈与俯冲的印度板块之间有一层低速软流圈物质。高原中部从地表到 310km深处有一低速体 ,说明地幔物质有可能穿过俯冲板块的脆弱部位上隆。这些结果以及野外实测的地壳缩短值说明高原的抬升得助于印度板块的近水平俯冲。我们推论俯冲印度板块的升温上浮以及上覆软流层的存在是造成西藏高原高海拔抬升以及内部地表仍相对平坦的主要原因。2 0 0 1年 1月 2 6日在印度西部发生的毁灭性大地震有可能是俯冲应力在印度板块后缘薄弱处引发的岩石圈大断裂。 相似文献
6.
Spatial and temporal analysis of global seismological data 1964–2005 reveals a distinct teleseismic earthquake activity producing
a columnar-like formation in the continental wedge between the Krakatau volcano at the surface and the subducting slab of
the Indo-Australian plate. These earthquakes occur continuously in time, are in the body-wave (m
b) magnitude range 4.5–5.3 and in the depth range 1–100 km. The Krakatau earthquake cluster is vertical and elongated in the
azimuth N30°E, suggesting existence of a deep-rooted fault zone cutting the Sunda Strait in the SSW-NNE direction. Possible
continuation of the fault zone in the SW direction was activated by an intensive 2002/2003 aftershock sequence, elongated
in the azimuth of N55°E. Beneath the Krakatau earthquake cluster, an aseismic gap exists in the Wadati-Benioff zone of the
subducting plate at the depths 100–120 km. We interpret this aseismic gap as a consequence of partial melting inhibiting stress
concentration necessary to generate stronger earthquakes, whereas the numerous earthquakes observed in the overlying lithospheric
wedge beneath the volcano probably reflect magma ascent in the recent plumbing system of the Krakatau volcano. Focal depth
of the deepest events (~100 km) of the Krakatau cluster constrains the location of the primary magma generation to greater
depths. The ascending magmatic fluids stress fault segments within the Sunda Strait fault zone and change their friction parameters
inducing the observed tectonic earthquakes beneath Krakatau. 相似文献
7.
Nigel Harris Alison Hunt Ian Parkinson Andrew Tindle Magisuren Yondon Samantha Hammond 《Contributions to Mineralogy and Petrology》2010,160(1):67-81
Garnet-bearing mantle xenoliths have been recovered from Quaternary alkali basalts, both within and peripheral to the Hangay
dome of central Mongolia. Microfabric analysis and thermobaromery, combining empirical thermobarometers and the self-consistent
dataset of THERMOCALC, indicate that garnet websterites from the Shavaryn-Tsaram volcanic centre at the dome core were formed
in the spinel-lherzolite upper mantle at pressures of 17–18 kbars and temperatures of 1,070–1,090°C, whereas garnet lherzolites
were derived from greater depths (18–20 kbars). Garnet lherzolites from the Baga Togo Uul vents near the dome edge were formed
at 18–22 kbars under significantly cooler conditions (960–1,000°C). These xenoliths reveal reaction coronas of (1) orthopyroxene,
clinopyroxene, plagioclase and spinel mantling garnets; (2) spongy rims of olivine replacing orthopyroxene and (3) low-Na,
low-Al clinopyroxene replacing primary clinopyroxene. Trace-element abundances indicate that clinopyroxene from these coronas
is in chemical equilibrium with the host magma. The thermobarometric and textural data suggest that lherzolite xenoliths from
both sites were derived from depths of 60–70 km and entrained in magma at 1,200–1,300°C. The average rate of ascent, as determined
by olivine zoning, lies in the range 0.2–0.3 m s−1. The contrast in thermal profiles of the upper mantle between the two sites is consistent with a mantle plume beneath the
Hangay dome with elevated thermal conditions beneath the core of the dome being comparable to estimates of the Pleistocene
geotherm beneath the Baikal rift. 相似文献
8.
日本列岛下太平洋俯冲板块的精细结构 总被引：1，自引：0，他引：1
尽管许多学者对日本列岛下的太平洋俯冲板块做了大量的研究,但板块内部的结构(比如板块厚度,板块内地震波速度随深度的变化以及洋壳的俯冲情况等)仍然不太清楚。利用日本地区密集台网收集到的中深和深发地震到时数据来探讨上述问题。采用三维射线追踪正演模拟法,首先利用333个远震计算得到了日本地区太平洋板块的厚度为85km;然后利用3283个地震(震源深度大于40km)的130227条P波到时进一步研究板块内部的精细结构。结果显示,沿深度方向6个地层段(间隔100km)内的速度扰动值分别为5.5%,4.0%,3.5%,2.5%,2.0%和6.0%,在40～500km范围内速度扰动随深度的增加而减小,这与温度随深度的变化情况相一致。当深度大于500km时,速度扰动突然增大到6.0%,分析认为该异常可能由发生在东亚大陆边缘下方的深发地震无法精确定位导致的。最后利用40～500km深度范围内的近震测试得到日本东北和北海道地区下方洋壳俯冲的深度均为110km,平均厚度分别为7.5km和5km,相对于一维模型的速度扰动分别为1%和-3%。这说明洋壳在俯冲到110km以深时,由于受温度和压力的影响,逐渐脱水、变质,直至与板块融合。通过分析震源与洋壳的位置关系,本研究认为北海道地区比东北地区下方的俯冲洋壳可能含有更多的流体(比如水),导致两地区洋壳内的速度相差如此之大。此外,因为日本南部与洋壳对应的区域多为海洋,观测台站较少,所以本研究无法测试得到该区域内的洋壳俯冲情况。 相似文献
9.
Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography 总被引：1，自引：0，他引：1
We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-VP/VS zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-VP/VS zone; however, VP/VS at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-VP/VS zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-VP/VS zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity. 相似文献
10.
《International Geology Review》2012,54(12):1129-1144
Groups of grabens in west Anatolia have contrasting E-W and NE-SW orientations and are the subject of debate as to their relative ages and relationships. We investigated the E-W-trending Gediz graben and its neighboring NE-SW-trending Gördes, Demirci, and Selendi grabens, which form an important graben system representative of the region. We studied gravity data from one profile and magnetotelluric (MT) data from two profiles, 73 km and 93 km long. The data supports the hypothesis that the Gediz graben was superimposed onto the (older) NE-SW grabens. 2D gravity and MT modelling revealed an undulating graben floor, varying in depth between 500 and 3000-4000 m (gravity-MT); within the graben two apparent basins 3–4 and 1.5-2.5 km deep (gravity-MT) are separated by a subsurface horst. The residual gravity map appears to indicate the continuation of NE-SW grabens from north of Gediz graben to beyond its southern border. The MT model revealed three main zones of varying thickness within the crust. The britde upper crust comprises two zones: sedimentary fill (apparent resistivity 15-50 ohm.m) and Menderes massif basement (200 ohm.m). The third zone is highly conductive lower crust (10 ohm.m), identified by our MT modeling at an average depth of 10 km. This conductive layer was considered in conjunction with two other regional features, high heat flow values and shallow earthquake focal depths. A heat flow map shows a very high average value of 108 mWm?2 for west Anatolia and 120-300 mWm?2 for the Gediz graben area specifically, compared with the world average of 80 mWm?2. Seismological records showing shallow earthquake focal depths together with the high conductivity zone were taken to indicate a partially melted, viscoelastic lower crust. 相似文献
11.
The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array
of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either
encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths
between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently
and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the
western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the
seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions
fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal
basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction
of the Indian-EuroAsian plate convergence in the eastern Himalaya. 相似文献
12.
中国东部岩石圈热状态与流变学强度特征 总被引：10，自引：1，他引：9
根据均衡原理制约的地热计算得到中国东部岩石圈的温度分布状态,以40、70、100km和莫霍面深度等温线图以及600°C、1100°C等温面深度的形式表示.同时计算了以1350°C等温面深度表示的中国东部的热岩石圈厚度.结果显示:在扬子克拉通西部四川盆地之下存在160～200km厚的岩石圈根,但在整个华北克拉通之下缺失岩... 相似文献
13.
O. P. Pandey Nimisha Vedanti Ravi P. Srivastava 《Journal of the Geological Society of India》2017,90(3):289-300
Complexity in the earth’s crustal structure plays an important role in governing earth’s thermal and geodynamic behavior. In the present study, an attempt has been made taking insights from our recent geological, geochemical, petrophysical and geophysical findings from specially drilled deep boreholes, to understand the lithospheric thermal evolution of the highly complex western India, which forms the core region of the Deccan large igneous province. This region was severely affected by the Deccan volcanic eruptions 65 Ma ago, which resulted in a totally degenerated, reworked and exhumed mafic crust, which presently contains several Tertiary basins with proven hydrocarbon reserves. Our detailed case study from the disastrous 1993 Killari earthquake (Mw 6.3) region, apart from some other geotectonically important localities like seismically active 2001 Bhuj and 1967 Koyna earthquake regions together with Tertiary Cambay graben, indicate that the western part of India, is perhaps one of the warmest segments of the earth. It is characterized by an average high mantle heat flow and Moho temperatures of about 43 mW/m2 (range: 31-65 mW/m2) and 660°C (range: 540-860°C) respectively. Estimated thickness of the lithosphere beneath these areas varies from as low as about 45 km to 100 km. Consequently, melting conditions in certain segments are expected at extremely shallow depths due to asthenospheric swell, like northern part of Cambay basin and Bhuj seismic zone beneath which only about half of original crystalline crust now remains due to sub-crustal melting and massive exhumation of deeper crustal layers. Sustained thermal heating and rise of isotherms appear to have resulted in substantial enhancement of hydrocarbon generation and maturation processes in Tertiary sediments. The present study highlights the need of an integrated geological, geochemical and geophysical study, if reasonably accurate deep crustal thermal regime is to be investigated. 相似文献
14.
THE CAUSE OF FORMATION OF THE LAYERS WITH LOW VELOCITY AND HIGH ELECTRICAL CONDUCTIVITY IN WESTERN TIBET 相似文献
15.
Lower crustal intrusions beneath the southern Baikal Rift Zone: Evidence from full-waveform modelling of wide-angle seismic data 总被引：1，自引：0，他引：1
The Cenozoic Baikal Rift Zone (BRZ) is situated in south-central Siberia in the suture between the Precambrian Siberian Platform and the Amurian plate. This more than 2000-km long rift zone is composed of several individual basement depressions and half-grabens with the deep Lake Baikal at its centre. The BEST (Baikal Explosion Seismic Transect) project acquired a 360-km long, deep seismic, refraction/wide-angle reflection profile in 2002 across southern Lake Baikal. The data from this project is used for identification of large-scale crustal structures and modelling of the seismic velocities of the crust and uppermost mantle. Previous interpretation and velocity modelling of P-wave arrivals in the BEST data has revealed a multi layered crust with smooth variation in Moho depth between the Siberian Platform (41 km) and the Sayan-Baikal fold belt (46 km). The lower crust exhibits normal seismic velocities around the rift structure, except for beneath the rift axis where a distinct 50–80-km wide high-velocity anomaly (7.4–7.6 ± 0.2 km/s) is observed. Reverberant or “ringing” reflections with strong amplitude and low frequency originate from this zone, whereas the lower crust is non-reflective outside the rift zone. Synthetic full-waveform reflectivity modelling of the high-velocity anomaly suggests the presence of a layered sequence with a typical layer thickness of 300–500 m coinciding with the velocity anomaly. The P-wave velocity of the individual layers is modelled to range between 7.4 km/s and 7.9 km/s. We interpret this feature as resulting from mafic to ultra-mafic intrusions in the form of sills. Petrological interpretation of the velocity values suggests that the intrusions are sorted by fractional crystallization into plagioclase-rich low-velocity layers and pyroxene- and olivine-rich high-velocity layers. The mafic intrusions were probably intruded into the ductile lower crust during the main rift phase in the Late Pliocene. As such, the intrusive material has thickened the lower crust during rifting, which may explain the lack of Moho uplift across southern BRZ. 相似文献
16.
根据均衡原理制约的地热计算得到中国西部及邻区岩石圈的温度分布状态,以40、100km和莫霍面深度等温线图的形式表示,同时计算了以1 350℃等温面深度表示的中国西部及邻区的热岩石圈厚度。结果显示:中国大陆西北部地区、哈萨克斯坦东部地区以及上扬子地块、蒙古中西部地区和青藏高原中部的深部地温较低,青藏高原北部、东部以及天山褶皱带中部的深部地温高。在中国西部及邻区范围内,岩石圈厚度在180km以上的地区包括准噶尔盆地,塔里木盆地核心部位,西藏东部、中部以及祁连山地区。上扬子地块(四川盆地)岩石圈厚度为160km或更多,蒙古中西部地区以及哈萨克斯坦东部地区的岩石圈厚度为140～180km。青藏高原东部边缘和藏北地区以及天山中部吉尔吉斯伊塞克湖地区的岩石圈厚度较薄(<140km)。地热计算得到的结果与地震层析成像研究结果之间相互吻合。采用湿的上地幔流变学模型的计算结果表明,青藏高原及其东部边缘、天山褶皱带中部和蒙古中西部地区的岩石圈流变学强度模型为"奶油蛋糕(crèmebrlée)"型,其强度剖面显示强地壳而弱地幔的特点;上扬子地块(四川盆地)、准噶尔盆地、塔里木盆地和哈萨克斯坦东部地区岩石圈流变学强度模型为"果冻三明治(jelly sandwich)"型。 相似文献
17.
《Russian Geology and Geophysics》2007,48(2):204-220
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. 相似文献
18.
Deep Background of Wenchuan Earthquake and the Upper Crust Structure beneath the Longmen Shan and Adjacent Areas 总被引：5，自引：0，他引：5
LI Qiusheng GAO Rui WANG Haiyan ZHANG Jisheng LU Zhanwu LI Pengwu GUAN Ye HE Rizheng 《《地质学报》英文版》2009,83(4):733-739
Abstract: By analyzing the deep seismic sounding profiles across the Longmen Shan, this paper focuses on the study of the relationship between the upper crust structure of the Longmen Shan area and the Wenchuan earthquake. The Longmen Shan thrust belt marks not only the topographical change, but also the lateral velocity variation between the eastern Tibetan Plateau and the Sichuan Basin. A low-velocity layer has consistently been found in the crust beneath the eastern edge of the Tibetan Plateau, and ends beneath the western Sichuan Basin. The low-velocity layer at a depth of ~20 km beneath the eastern edge of the Tibetan Plateau has been considered as the deep condition for favoring energy accumulation that formed the great Wenchuan earthquake. 相似文献
19.
By analyzing the deep seismic sounding profiles across the Longmen Shan,this paper focuses on the study of the relationship between the upper crust structure of the Longmen Shan area and the Wenchuan earthquake.The Longmen Shan thrust belt marks not only the topographical change,but also the lateral velocity variation between the eastern Tibetan Plateau and the Sichuan Basin.A lowvelocity layer has consistently been found in the crust beneath the eastern edge of the Tibetan Plateau, and ends beneath the ... 相似文献
20.
S. S. Rai P. V. S. S. Rajagopala Sarma K. S. Prakasam V. K. Rao 《Journal of Earth System Science》1996,105(4):431-439
The deep crustal structure of eastern Dharwar craton has been investigated through τ-p extremal inversion of P-wave travel times from a network of seismographs recording quarry blasts. Travel times have been
observed in the distance range 30–250 km in a laterally homogeneous lithospheric segment Main features of the inferred velocity-depth
relationship include: (a) 29 km thick combined upper and middle crust velocity varying from 6 km/s to 7 km/s, with no observable
velocity discontinuity in this depth range; (b) a lower crust (∼ 29–41 km) with velocity increasing from 7.0 to 7.3 km/s;
(c) an average upper mantle velocity of 8.1 km/s; and (d) presence of a 12 km thick high velocity crustal layer (7.4 – 7.8
km/s) in the depth range 41–53 km, with a distinct velocity gradient marking a velocity increase of 0.4 km/s. The anomalous
53 km thick crust is viewed as a consequence of magmatic underplating at the base of the crust in the process of cratonization
of the eastern Dharwar craton during late Archaean. The underplated material reflects here with the velocity of 7–3 to 7–8
km/s below the depth of 40 km. Our proposition of magmatic underplating is also supported by the presence of large scale I-granitoid,
a product of partial melting of the upper mantle material. 相似文献