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1.
《Journal of South American Earth Sciences》1999,12(3):261-270
We have measured shear wave splitting at three temporary three-component short period stations that were deployed in southern Chile above the subducted Chile Rise spreading centre (Taitao Peninsula and environs). Subduction of the Chile Rise has been occurring beneath South America for at least the past 14 m.y. Previously published models of the ridge subduction posit the existence of ‘slab windows’, asthenosphere-filled gaps between subducted lithosphere segments of the spreading ridge, through which mantle might flow. Our preliminary results include two consistent fast polarization directions of splitting in the study region. Delay times between fast and slow split shear waves average around 1.0 s for all phases (ScS, PcS, SKS, and SKKS) that we measured. Fast-axis azimuths vary systematically among the three stations: near the coast, fast axes are parallel to the spreading ridge segments of the Chile Rise (approximately N-trending). This splitting fast-axis direction probably reflects either along-axis asthenospheric flow or results from the preferential attenuation effects of aligned pockets of melt at the subducted ridge segment. At one inland station above the slab window, we find two splitting fast-axis directions, one parallel to the subducted Chile Rise ridge segments, and a second trending NW–SE. We infer that upper mantle deformation in the vicinity of a well developed slab window is complicated and probably involves two superposed directions of upper mantle deformation. One of these directions (NW–SE) may indicate anomalous flow of asthenospheric mantle in the vicinity of the slab window gap. 相似文献
2.
华北克拉通从稳定到破坏的演化过程对有关地球动力学的经典理论提出了挑战,研究其独特的演化历史是固体地球科学研究的一项重要内容。上地幔矿物晶体的各向异性记录了上地幔发生构造变形的信息,研究上地幔地震波各向异性能够揭示现今和构造历史时期所发生的构造运动。本文总结了近年来作者在华北克拉通地区所进行的高密度、覆盖广泛的地震波横波分裂观测研究结果。横波分裂的快轴方向与绝对板块运动方向的不一致,以及横波分裂参数快速的空间变化特征表明了华北克拉通的SKS横波分裂主要反映上地幔的变形。观测结果表明：鄂尔多斯块体保留了克拉通较弱的各向异性特征,其西端体现了元古代克拉通拼合的变形特征; 中新生代华北克拉通破坏事件以不同的机制主导了华北克拉通中部和东部的上地幔变形,在东部地区北西-南东向的拉张应力作用使得快轴方向平行于拉张方向,而在中部则因受到较厚岩石圈的阻挡使得地幔流动改变了方向,因此造成了北东和北北东向的岩石圈拉张。 相似文献
3.
We conduct shear wave splitting measurements on waveform data from the Hi-net and the broadband F-net seismic stations in Kanto and SW Japan generated by shallow and intermediate-depth earthquakes occurring in the subducting Philippine Sea and Pacific slabs. We obtain 1115 shear wave splitting parameter pairs. The results are divided into those from the shallow (depth < 50 km) and the deep (depth > 50 km) events. The deep events beneath Kanto are further divided into PHS1 and PHS2 (upper and lower planes of the double seismic zone in the Philippine Sea slab, respectively), PAC1 and PAC2 (western and eastern Pacific slab, respectively) events. The results from the shallow events represent the crustal anisotropy, and their fast directions are more or less aligned in the σHmax directions, implying that the anisotropy is produced by the alignment of the vertical cracks in the crust induced by the compressive stresses. In Kanto, Kii Peninsula and Kyushu regions, the results from the deep events suggest a contribution from the mantle wedge anisotropy. Events from all groups beneath Kanto show NW, NE and EW fast directions. This complex pattern seems to be produced by the corner flows induced by both the WNW PAC plate subduction and the oblique NNW PHS slab subduction with the associated olivine lattice-preferred orientations (LPOs), and the anisotropy frozen in the PHS slab. The deep events beneath Kii Peninsula show NE and NW fast directions and may be produced by the corner flow produced by the NNW PHS slab subduction with the associated olivine LPOs. The NE directions might also be produced by the segregated melts in the thin layers parallel to the PHS slab subduction. The deep events beneath N Kyushu show NNW fast directions, which may result from the southeastward flow in the upper mantle inferred from the stresses in the upper plate. Results from the deep events beneath middle-south Kyushu show dominantly E–W fast directions, in both the fore- and back-arcs. They may be produced by the corner flow of the westward PHS slab subduction with the olivine LPOs. Because the source regions with multiple fast directions are not resolved in this study, further detailed analyses of shear wave splitting are necessary for a better understanding of the stress state, the induced mantle flow, and the melt-segregation processes. 相似文献
4.
Body-wave and surface-wave tomography, receiver-function imaging, and shear-wave splitting measurements have shown that seismic anisotropy and heterogeneity coexist in all parts of subduction zones, providing important constraints on the mantle flow and subduction dynamics. P-wave anisotropy tomography is a new but powerful tool for mapping three-dimensional variations of azimuthal and radial seismic anisotropy in the crust and mantle. P-wave azimuthal-anisotropy tomography has been applied widely to the Circum-Pacific subduction zones, Mainland China and North America, whereas P-wave radial-anisotropy tomography was applied to only a few areas including Northeast Japan, Southwest Japan and North China Craton. These studies have revealed complex anisotropy in the crust and mantle lithosphere associated with the surface geology and tectonics, anisotropy reflecting subduction-driven corner flow in the mantle wedge, frozen-in fossil anisotropy in the subducting slabs formed at the mid-ocean ridge, as well as olivine fabric transitions due to changes in water content, stress and temperature. Shear-wave splitting tomography methods have been also proposed, but their applications are still limited and preliminary. There is a discrepancy between the surface-wave and body-wave tomographic models in radial anisotropy of the mantle wedge beneath Japan, which is a puzzle but an intriguing topic for future studies. 相似文献
5.
The presence of two regional seismic networks in southeastern France provides us high-quality data to investigate upper mantle flow by measuring the splitting of teleseismic shear waves induced by seismic anisotropy. The 10 three-component and broadband stations installed in Corsica, Provence, and western Alps efficiently complete the geographic coverage of anisotropy measurements performed in southern France using temporary experiments deployed on geodynamic targets such as the Pyrenees and the Massif Central. Teleseismic shear waves (mainly SKS and SKKS) are used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 1.0 and 1.5 s have been observed at most sites, but some larger delay times, above 2.0 s, have been observed at some stations, such as in northern Alps or Corsica, suggesting the presence of high strain zones in the upper mantle. The azimuths of the fast split shear waves define a simple and smooth pattern, trending homogeneously WNW–ESE in the Nice area and progressively rotating to NW–SE and to NS for stations located further North in the Alps. This pattern is in continuity with the measurements performed in the southern Massif Central and could be related to a large asthenospheric flow induced by the rotation of the Corsica–Sardinia lithospheric block and the retreat of the Apenninic slab. We show that seismic anisotropy nicely maps the route of the slab from the initial rifting phase along the Gulf of Lion (30–22 Ma) to the drifting of the Corsica–Sardinia lithospheric block accompanied by the creation of new oceanic lithosphere in the Liguro–Provençal basin (22–17 Ma). In the external and internal Alps, the pattern of the azimuth of the fast split waves follows the bend of the alpine arc. We propose that the mantle flow beneath this area could be influenced or perhaps controlled by the Alpine deep penetrative structures and that the Alpine lithospheric roots may have deflected part of the horizontal asthenospheric flow around its southernmost tip. 相似文献
6.
华北克拉通火成碳酸岩时空分布和锶钕同位素特征及其地质意义 总被引：2，自引：0，他引：2
已有的研究得出：华北克拉通火成碳酸岩在时间上主要形成于古元代末—中元古代初、早中生代和晚中生代3个时段,反映了华北克拉通演化历史上3个拉张阶段。在空间上,前两个拉张阶段形成的火成碳酸岩分布在华北克拉通的北缘和南缘;后一个拉张阶段形成的火成碳酸岩分布在克拉通的中部和东部。华北克拉通火成碳酸岩的钕同位素主要特征是εNd(t )全为负值,而且随时间由老到新负值趋于降低。在εNd(t )-I Sr图解上,其投影点都落到了第3和第4象限,除莱芜—淄博地区火成碳酸岩外,其它都在富集地幔演化线附近,而且从老到新富集程度越来越高。推测华北克拉通火成碳酸岩形成的深部动力学机制可能是：在热地幔柱活动或周边板块向华北板块俯冲之后华北克拉通构造环境转变为伸展拉张环境,因压力减小而引起地幔部分熔融形成的碱性基性—超基性岩浆或碱性中性岩浆沿区域深断裂上侵,又经分异作用冷凝结晶形成杂岩体,火成碳酸岩是这些碱性岩浆演化晚阶段的主要产物。部分火成碳酸岩可能是从上地幔低程度部分熔融形成的碳酸岩浆直接上侵冷凝结晶而成。 相似文献
7.
We integrated a systematic sedimentary data into a regional Early Mesozoic stratigraphic framework which demonstrated a detailed picture of spatiotemporal variations in basin deposition and formation in the North China Craton. The Early Mesozoic basin sedimentary evolution is utilized to interpret polyphase tectonism and to unravel the craton deformation. The Late Triassic, nearly WNW-trending, giant intracratonic Ordos basin was widely distributed across most of North China Craton, with a southern wedge-top depozone along the northern East Qilian–Qinling orogenic belt and a northwestern rift depozone along the Helanshan. The continuous subsidence and deposition within the basin were dominantly related to the thrust load of the East Qilian–Qinling belt and inferred mantle flow effects associated with paleotethys plate subduction, and the rift in the northwestern Ordos was driven by nearly north-vergent compression of the eastern North Qilian–North Qinling active margins with the stable North China Craton. This intracratonic Ordos basin formation initiated the deformation of the North China Craton. Formation of the Jurassic NNE-trending walled intracratonic Ordos basin and the broken flexural basins indicates the North China Craton underwent the second, even more abroad nearly NNE trending crustal deformation, with lithosphere thickening in the eastern part of the North China Craton, and dynamic subsidence in the west, which may have been driven by nearly northwestward subduction of the Izanagi plate and the eastward extrusion and underthrusting of the western North China Craton crustal basement. 相似文献
8.
Structure of the Upper Mantle and Transition Zone Beneath the South China Block Imaged by Finite Frequency Tomography 总被引：1，自引：0，他引：1
We applied the finite frequency tomography method to S wave data recorded by 350 broadband stations beneath the South China Block(SCB) and its surroundings from earthquakes occurring between July 2007 and July 2010,to better understand upper mantle deformation.Differential travel-times in the pair of stations with appropriate weighting for each station are used in the inversion.Our results are consistent with previous tomography that show a high velocity anomaly beneath the Sichuan basin and a high velocity anomaly in the transition zone beneath the Yangtze Craton.However,the resolution of mantle heterogeneity provides new insight into the tectonic framework of subduction of Burmese lithosphere in the west part of the study region and subduction of oceanic lithosphere in the east.In the subduction realm,west of 107°E,a significant fast S-wave anomaly is located on the southeast of Sichuan Basin.East of 107°E,and two narrow and discontinuous fast S-wave anomalies occur at a depth of 400-600 km beneath the middle of the South China block overlain by the pronounced low S-wave anomalies at a depth of 100 and 400 km.If the fast anomalies located in the mantle transition zone represent stagnant slabs,their fragmented nature may suggest that they could be produced by different episodes of subduction beneath western Pacific island and the above slow velocity anomaly may associated with the back-arc regions of ongoing subduction.In addition,tomography also reveals an anomalously high S-wave velocity continental root extends eastward to a depth 400 km beneath the eastern Sichuan Basin.This anomaly may be related to eastern extrusion of Indian lithosphere associated with the collision of India and Eurasia.Moreover,our results also show large slow anomalies beneath the Red River fault region connected to deeper anomalies beneath the South China Fold Belt and South China Sea.AH these observations are consistent with the scenario that the South China block has been built by both of subduction of Paleopacific plate and eastward subduction of Burma microplate. 相似文献
9.
本文在讨论中国东部现有构造演化模式基础上，着重从华北与华南地块之间板块尺度的动力学过程剖析秦岭－大别中生代造山带构造演化，以及大别－苏鲁超高压变质地体的形成和折返过程。晚二叠世－中三叠世华南地块向华北地块持续挤压，陆壳大规模俯冲导致超高压变质作用的产生，而华北地块在晚三叠世至早侏罗世发生快速逆时针旋转，使得这一地区上地幔深度的超高压变质地体快速折返至下地壳。由于超高压变质地体侵位后，地壳结构、构造的差异，在南北地块的进一步挤压下，造成中国东部晚侏罗世－早白垩世郯庐断裂带的巨大左行剪切。 相似文献
10.
华北东部大陆地幔橄榄岩组成、年龄与岩石圈减薄 总被引：16，自引：1，他引：15
对比分析了华北东部地块陆下岩石圈地幔橄榄石Mg#值和单斜辉石的REE配分形式。报道了汉诺坝和鹤壁橄榄岩中不同产状硫化物的激光MC-ICPMS原位Re-Os年龄和信阳橄榄岩中锆石的U-Pb年龄和信阳橄榄岩锆石的U-Pb年龄。在这些资料基础上,进一步讨论了华北东部岩石圈中、新生代时的减薄机制。原位分析在揭示岩石圈深部过程的细节上,有比全岩分析更大的优越性,并揭示出了在华北深部有中元古代(14亿年)和新元古代(7～8亿年)热活动的记录。岩石圈拆沉作用不能很好地解释古老难熔地幔、过渡型地幔和新生饱满地幔并存的事实;同时,单纯的熔体-橄榄岩相互作用也难以解释中、新生代岩石圈的减薄过程和新生地幔单斜辉石中出现强烈LREE亏损现象,即历史复杂的克拉通岩石圈向历史明显简单的“大洋型”地幔的转换。因此,华北东部岩石圈减薄包括地幔伸展、熔-岩作用、侵蚀置换等复杂过程。这些过程可能包括:(1)早中生代时,扬子地块向北俯冲碰撞所引起华北岩石圈的熔/流体交代富集作用、地幔伸展和受扰动软流圈物质上涌并侵蚀被改造了的岩石圈;(2)晚中生代—古近纪,因太平洋俯冲的热扰动致使软流圈物质进一步的强烈侵蚀作用引起岩石圈的巨大减薄;(3)晚第三纪以来的软流圈热沉降作用所带来的小幅度岩石圈增厚过程。岩石圈先大幅减薄、后小幅增厚实现了最终的地幔置换和岩石圈整体的减薄过程。喷发时代为100Ma的阜新玄武岩所捕获的橄榄岩主体是饱满的,说明华北东部部分地区在此之前曾有过地幔置换作用。 相似文献
11.
大洋板块俯冲带地震波各向异性及剪切波分裂的成因机制 总被引：1，自引：1，他引：0
大洋板块俯冲带是许多重要地质作用(例如脱水、部分熔融、岩浆和地震活动)发生的场所.对位于俯冲带之上的地震台站所检测到的不同剪切波的数据解析,可以获得源于上覆板块、地幔楔、俯冲板块和板下地幔的地震波各向异性的关键信息.本文系统总结了世界各地大洋俯冲带的剪切波分裂样式,对目前国际上流行的大洋俯冲带的地震波各向异性的主要成因... 相似文献
12.
Mohamed K.Salah 《地学前缘(英文版)》2012,3(5):681-696
Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting.Waveform data generated by 95 teleseismic events recorded at 17 broadband stations deployed in the western Mediterranean region are used in the present study.Although the station coverage is not uniform in the Iberian Peninsula and northwest Africa,significant variations in the fast polarization directions and delay times are observed at stations located at different tectonic domains.Fast polarization directions are oriented predominantly NW-SE at most stations which are close to the plate boundary and in central Iberia;being consistent with the absolute plate motion in the region.In the northern part of the Iberian Peninsula,fast velocity directions are oriented nearly E—W;coincident with previous results.Few stations located slightly north of the plate boundary and to the southeast of Iberia show E—W to NE-SW fast velocity directions,which may be related to the Alpine Orogeny and the extension direction in Iberia.Delay times vary significantly between 0.2 and 1.9 s for individual measurements,reflecting a highly anisotropic structure beneath the recording stations.The relative motion between Africa and Iberia represents the main reason for the observed NW-SE orientations of the fast velocity directions.However,different causes of anisotropy have also to be considered to explain the wide range of the splitting pattern observed in the western Mediterranean region.Many geophysical observations such as the low Pn velocity,lower lithospheric Q values,higher heat flow and the presence of high conductive features support the mantle How in the western Mediterranean,which may contribute and even modify the splitting pattern beneath the studied region. 相似文献
13.
The seismic anisotropy of the mantle is studied based on the data of S and ScS waves from earthquakes occurred in the mantle transition zone over the period of 2007–2013 and recorded by seismic stations in the continental margin of Asia, on Sakhalin Island, and in the southern part of the Kamchatka Peninsula. The measurements of the azimuths of polarization of the fast S and ScS waves in the continental margin of Asia show that they are predominantly oriented in the E–SE directions. Based on the distribution of the shear wave splitting parameters, the symmetry of the medium can be described in terms of a transversely isotropic model with a horizontal symmetry axis and may correspond to horizontal flow in the upper mantle beneath the Amur Plate. The fast azimuths of polarization of ScS wave, which were determined to be of N–NE directions in the northern area of Sakhalin Island and in the continental part of Asia, may correspond to an inclined flow under the conditions of oblique subduction and complex geometry of the downgoing Pacific Plate. In the south of the Kamchatka Peninsula, the S- and ScS-wave azimuths of polarization from the M 8.4 Sea of Okhotsk earthquake are determined to be oriented along the direction of the Pacific Plate motion. The fast-S-wave azimuths of polarization from the aftershocks of the Sea of Okhotsk earthquake and from other large events of 2008–2009 are determined to be nearly parallel to the motion trend of the Pacific Plate, but orthogonal to it for the events of 2008–2009. On the basis of the distribution of azimuths of polarization of the fast S waves, the symmetry of the medium can be described in terms of a transversely isotropic model with the symmetry axis inclined orthogonally to the plane of downgoing plate and oriented westward orthogonally to the trench strike. 相似文献
14.
略论华北地块北缘显生宙三类不同的造山作用 总被引：3，自引：0，他引：3
华北地块北缘显生宙发育3种不同类型的造山作用。古生代,华北地块北缘处于古亚洲洋构造域,造山作用属陆缘俯冲-碰撞型,形成以EW向至NEE向为主的褶皱、逆冲推覆构造及韧性剪切带等构造类型。造山机制与古亚洲洋板块向南俯冲-碰撞导致近SN向构造动力的挤压作用密切相关。中生代,华北地块北缘处于西滨太平洋构造域,造山作用以陆内挤压型为主,形成以NE-NNE向与近EW向为主的多期不同方向的褶皱、逆冲断裂、推覆构造、韧性剪切带及局部地区的固态塑性流变构造等构造类型;造山动力以古太平洋(或Izanagi)板块西向俯冲导致NW-NWW向强烈挤压力为主。新生代,华北地块北缘虽仍属西滨太平洋构造域,但造山作用以陆内伸展型为主,裂谷作用与陆内伸展构造居主导地位,褶皱变形微弱,张性-张扭性断裂活动显著,形成现今盆-山构造地貌格局;造山动力以NW-NWW向主张应力为主。造山类型的两次重大转换分别发生于早、中三叠世与晚白垩世。 相似文献
15.
华北岩石圈减薄与克拉通破坏研究的主要学术争论 总被引：114，自引：39，他引：75
华北岩石圈减薄是近十年来国内外研究的热门课题,但关于岩石圈减薄的具体时间、减薄幅度、空间分布范围、机制及其构造控制因素,多有争论.本文对上述问题进行了全面的回顾与讨论,内容包括:(1)是克拉通破坏还是岩石圈减薄;(2)古生代以来华北克拉通岩石圈地幔时代与壳幔解耦;(3)岩石圈减薄与克拉通破坏发生的时间;(4)岩石圈减薄的垂向幅度;(5)克拉通破坏在中国东部的空间分布;(6)克拉通破坏机制;(7)克拉通破坏的地球动力学原因;(8)华北克拉通破坏在世界上的特殊性等.文后,汇编了近10余年来华北岩石圈减薄与克拉通破坏研究的重要文献. 相似文献
16.
青藏高原东部及其邻区力学耦合的岩石圈变形模式 总被引：1，自引：0，他引：1
根据青藏高原东部及其邻区布设的143个宽频带固定和流动地震台站的远震记录的SKS波分裂分析获得了各台站的快波偏振方向和快慢波之间的时间延迟。SKS分裂分析结果总体上反映了高原东部的上地幔物质流动方向,即高原内部表现为环绕喜马拉雅东构造结的顺时针旋转。在造山运动过程中有关岩石圈地壳和地幔力学耦合的造山变形方式,用从GPS和第四纪断裂滑动速率数据确定的地面变形场和由地震波各向异性数据推断的地幔变形场联合分析来定量求得。在青藏高原东部和云南、四川等地区新近快速增加的GPS和SKS波分裂观测数据,提供了对青藏高原岩石圈地幔实际变形方式的检验。这些新的数据不仅加强了高原内部力学耦合岩石圈的证据,而且也解释了高原外部相同的耦合特征。文中引入简单剪切变形和纯剪切变形的概念,用于解释高原内外不同的耦合变形特征。青藏高原和周围区域力学耦合岩石圈的垂直连贯变形有两个方面的大陆动力学含义:第一,岩石圈垂直强度剖面被一个重要的条件所约束,即要求与重力势能变化相关的应力能够从地壳传递到地幔;第二,青藏高原各向异性的空间变化反映了一个岩石圈变形的大尺度模式,以及从高原内部的简单剪切变形向高原外部的纯剪切变形的过渡带。文中提出的力学耦合岩石圈变形模型与当前已有的多种造山运动变形模型具有不同的变形含义,因此,地幔变形在青藏高原隆升过程中起主要作用。 相似文献
17.
Seismic anisotropy structure beneath the southeastern Mediterranean from shear-wave splitting 总被引：1，自引：0，他引：1
Mohamed K. Salah 《Arabian Journal of Geosciences》2013,6(6):1717-1730
Analysis of seismic anisotropy in the crust and the uppermost mantle gives lots of information about the ambient mantle flow, stress state, and the dynamic processes inside the Earth. Thus, seismic anisotropy and its main distinctive features beneath the southeastern Mediterranean region are studied through the analysis of teleseismic shear-wave splitting observed at six broadband seismic stations belonging to the GEOFON and the MedNet. Although the number of the recording stations is small; a total of 495 splitting parameters are obtained, which revealed significant variations in the observed fast polarization directions beneath the study area. The stations in northern Egypt and Cyprus show fast velocity directions oriented roughly N–S to NNE–SSW, coincident with many previous results. A slightly different splitting pattern comprising NE–SW fast polarization directions is observed in the stations located along the Dead Sea fault in the southeastern Mediterranean; which are consistent with the current strike-slip motion between Africa and Arabia. In addition, NW–SE fast polarization directions are recognized in the latter group. The observed delay times vary greatly but their averages lie between 0.35 and 1 s. Although large-scale mechanisms, such as the absolute plate motion of Africa and Arabia towards Eurasia and the differential motion between Arabia and Africa can be invoked to predominantly explain the origin of anisotropic features, we suggest that density-driven flow in the asthenosphere is a possible additional cause of the wide range of the splitting pattern observed beneath some stations. 相似文献
18.
Shear wave splitting measurements in South Kamchatka during the 3-year period (1996–1998) in which the Kronotsky Earthquake (M=7.7, December 5, 1997) occurred are used to determine anisotropic parameters of the subduction zone and shear wave splitting variations with time. The local small seismic events recorded at the Petropavlovskaya IRIS station (PET) were analyzed. The dominant azimuths of the fast shear wave polarizations for the 3-year period are defined within N95±15°E, which are consistent with the general Pacific Plate motion direction. Modeling of fast shear wave polarizations shows that HTI model with the symmetry axis oriented along N15°E±10° fit well the observed data for events the focal depths of which are less than 80 km. For the greater depths, the orthorhombic symmetry of medium is not excluded. The anisotropy coefficient increases generally with depth from 1–2% in the crust to 4–7.5% in the subducting plate. Variations in time delays show a general increase up to 10–15 ms/km during 1996–1997 before the large crustal earthquake series (M≈5.5–7) in the Avacha Bay and before the Kronotsky Earthquake. Analysis of fast S-wave azimuths of mantle events reveals a temporal cyclic variation. The most regular variations are observed for fast azimuths of deep events with a period of about 172 days over the 3-year period. The fast polarizations of crustal events behave comparatively stable. It is assumed that the major instabilities in stress state are localized in the descending slab and influenced the upper mantle and comparatively stable crust. 相似文献
19.
大陆深俯冲后效作用的地球化学记录——华北中生代岩石圈地幔源区特征变异的讨论 总被引：8，自引：3，他引：8
早中生代扬子板块与华北板块之间的俯冲及其后两陆块间的碰撞为大陆深俯冲后效作用研究提供了一个少见的实例。本文在系统论述华北中生代深部年代学记录、深源岩类源区地球化学特征的时空变化规律,普遍的 Sr 同位素高度富集现象及其空间变化指向、高场强元素强烈分馏以及时间制约分析基础上,提出了导致这些源区特征变化最可能的机制为扬子板片深俯冲中析出的熔体/流体交代并改造了华北大陆岩石圈地幔。而其后中生代全球事件引发的中国东部广泛而强烈白垩纪构造-岩浆幕则提供了侵位于地壳层位深源岩类的成岩条件。本文强调指出对华北岩石圈地幔这种大尺度实体上的改造,应被视为扬子板片深俯冲后效作用中最具深远意义的一种表现,并对中生代华北岩石圈整体转型产生了重要影响。 相似文献
20.
Recent interpretations of upper continental mantle seismic anisotropy observations have often relied on fabric measurements and calculated anisotropies of upper mantle xenoliths. Seismic ray paths of P and S waves, which provide information on azimuthal compressional wave anisotropy and shear wave splitting, are tens to hundreds of kilometers, whereas, xenoliths are usually only a few centimeters in diameter. To place better constraints on field-based anisotropy observations and to evaluate anisotropy information provided by xenoliths, it is important to examine anisotropy in large ultramafic massifs which have originated in the upper mantle. One such massif is the Twin Sisters Range located in the western portion of the North Cascades of Washington State, USA. The Twin Sisters massif, a slab of unaltered dunite, is 16 km in length, 6 km in width and 3 km thick. Exposed along its south and west sides are mafic granulite facies rocks, which likely represent lower continental crustal fragments. The ultramafic rocks are porphyroclastic in texture, consisting of strained, flattened porphyroclasts of olivine and enstatite and strain-free olivine mosaics. Olivine fabrics are typical of those formed at high temperatures and low strain rates. Petrofabrics and calculated anisotropies of individual samples vary throughout the massif, however, overall anisotropy of the body is significant, with maximum P and S waves anisotropies of 5.4% and 3.9%, respectively. The maximum delay time for split shear waves traveling through a 100-km-thick slab is 0.8 s and two directions of shear wave singularity are observed. The directions of maximum shear wave splitting and shear wave singularities do not coincide with the directions of maximum and minimum compressional wave velocity. In general, individual hand samples show significantly higher anisotropy than the overall anisotropy of the massif. It is concluded that simple averages of xenolith anisotropies are unreliable for use in the interpretation of field anisotropy observations. 相似文献