华北上地幔各向异性研究

对华北地震科学台阵的200个宽频带和甚宽带地震台站所记录的远震SKS(SKKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和叠加分析方法求得每一个台站的SKS(SKKS)快波偏振方向和快、慢波的时间延迟,并结合已发表的固定台站的结果,获得了华北上地幔各向异性图像.从得到结果看,华北东部各向异性快波方向基本为NWW-SEE方向,而西部的快波方向转到NW-SE或NNW-SSE.快、慢波时间延迟范围是0.50~1.47 s,华北西部的平均快、慢波时间延迟小于华北东部.在华北东部,快波方向与绝对板块运动(APM)方向基本一致,预示了NWW向的软流圈地幔流是引起该区域上地幔各向异性的主要原因,它使得上地幔橄榄岩等晶体的晶格优势取向沿地幔物质流动方向,从而导致了NWW趋向的快波方向.然而,在稳定的西部,快波方向既不与绝对板块运动方向一致,也不与构造走向一致,这种弱各向异性很可能是遗留在古老克拉通的厚的岩石圈内的"化石"各向异性.     

Intensity and direction of lattice-preferred orientation of olivine: are electrical and seismic anisotropies of the Australian mantle reconcilable?

It has been hypothesised that seismic and electrical anisotropy at the base of the lithosphere are caused by strain-induced lattice-preferred orientation (LPO) of olivine [100] axes parallel to present-day plate motion. This would imply that seismic and electrical anisotropy observations can provide geodynamicists with fundamental information for characterising mantle flow. The qualitative agreement between the fast direction of SV-waves and direction of maximum electrical conductance modelled deeper than 150 km below the North Central craton of Australia appear to support a common alignment mechanism, and the observed, anisotropic electrical conductances can be generated by hydrogen diffusivity in a water-poor (<1000 ppm H/Si) olivine mantle. A quantitative test is proposed for the hypothesis that electrical anisotropy is generated by anisotropic hydrogen diffusion rates (D) in olivine. Electrical anisotropy factors are computed using random resistor network models assuming that D[100]≈20×D[010]≈40×D[001]. Electrical and seismic anisotropies calculated from olivine LPO angular distribution functions modelled for a range of shear strains under a simple shear deformation demonstrate that the intensity of olivine [100] alignments (and associated shear strains) that would be required to explain the electrical anisotropy in the mantle below central Australia are significantly greater than predicted by Rayleigh wave anisotropies. The poor agreement between the observed electrical anisotropies and the electrical anisotropies that would be predicted from the Rayleigh wave anisotropies indicates that either (i) electrical anisotropy in the upper mantle below central Australia is not generated by hydrogen diffusivity alone or (ii) the seismic anisotropy is underestimated. The orientation of the olivine [100] axes maxima is inferred to be ∼30° rotated relative to the direction of present-day absolute plate motion (APM) that is determined relative to the hotspot reference frame (HS2-NUVEL1). Both the APM direction that is determined relative to a reference frame defined by requiring no-net rotation of the lithosphere (NNR-NUVEL1) and GPS-derived plate motion vectors fit the geophysical observations of upper mantle anisotropy better. This may support the contention that hotspots are not stationary relative to the deep mantle.     

板块运动与地震各向异性及应力场的相关性统计分析

利用收集到的各种来源共计7 959组的地震各向异性观测数据和21 750组应力场数据,结合板块绝对运动模型计算给出的各板块的运动规律,分别统计分析了板块运动与地震各向异性及应力场的相关性,并对板块运动对地震各向异性及应力场特征产生的影响进行了分析． 统计结果表明,阿拉伯、 加勒比、 胡安德富卡、 北美、 纳兹卡、 太平洋和南美板块上地震各向异性与板块运动均具有较好的相关性,而非洲、 南极洲、 澳大利亚、 欧亚、 印度和菲律宾板块上二者的相关性则相对较差. 讨论分析发现,板块运动拖动软流圈流动、 橄榄岩晶格优选方位、 化石各向异性和地幔流动或岩石圈流动等因素均在一定程度上控制并影响着地震各向异性与板块运动的一致性． 而板块基底拖曳力、 洋脊推力、 浮力作用和碰撞及俯冲作用等多种因素共同制约了板块运动与应力场的相关性,使得非洲、 可可斯、 欧亚、 胡安德富卡、 北美、 纳兹卡、 菲律宾和南美板块上二者的相关性较好,其它板块上其相关性则较差． 对于俯冲带地区,由于俯冲机制的复杂性和软流圈、 岩石圈地幔流动方向的不确定性,其板块运动与地震各向异性及应力场的相关性图像表现复杂,需要结合具体的俯冲带构造进行近一步研究．      

中国大陆及邻区海域地壳上地幔各向异性研究

利用分别由Love波和Rayleigh波得到的S波速度结构的差值（VSH-VSV）对中国大陆及邻区海域（70°E～145°E，15°N～55°N）地壳上地幔中的偏振各向异性进行研究.初步研究结果表明，各向异性在空间分布上存在不均匀性：（1）在小于150 km的深度范围内，VSH>VSV的各向异性体占主导地位，反映出在地球的浅部岩石圈内的水平应力作用及软流圈顶部物质的水平向流动对各向异性的形成起主导作用.在大陆地区，各向异性的强度随深度有显著变化.上地壳和上地幔盖层中的各向异性普遍较弱，而在流变性较强的下地壳和软流圈存在较大范围的各向异性.这一现象说明下地壳在岩石圈变形中可能有解耦作用.（2）在大于200 km深度的软流圈下部主要表现为VSH<VSV的各向异性，说明地幔物质垂直运动相对占优势地位.（3）在中国大陆东部可以看出一个大致趋势：在构造比较稳定的地区，岩石圈中VSH>VSV的各向异性比较显著，而软流圈中VSH<VSV的各向异性较弱；在构造活动比较强烈的地区，软流圈中VSH<VSV的各向异性占主导地位.（4）印度板块低角度向青藏高原下俯冲影响了中国大陆西部地区各向异性的特征.印度板块向北运动水平挤压中国西部大陆，使得物质定向重结晶，从而在岩石圈下部产生显著的VSH>VSV各向异性.     

Seismic anisotropy beneath Tibet: evidence for eastward extrusion of the Tibetan lithosphere?

Strong seismic anisotropy beneath Tibet has recently been reported from the study of SKS shear wave splitting. The fast split waves are generally polarized in an easterly direction, close to the present day direction of motion of the Tibetan crust relative to stable Eurasia, as deduced from Holocene slip rates on the major active faults in and around Tibet. This correlation may be taken to suggest that the whole Tibetan lithosphere is being extruded in front of indenting India and that the anisotropic layer is the deforming asthenosphere, that accommodates the motion of the Tibetan lithosphere relative to the fixed mantle at depth. Uncertainties about this motion are at present too large to bring unambiguous support to that view. Assuming that this view is correct however, a simple forward model is used to compute theoretical delay times as a function of the thickness of the anisotropic layer. The observed delay times would require a 50–100 km thick anisotropic layer beneath south-central Tibet and an over 200 km thick layer beneath north-central Tibet, where particularly hot asthenosphere has been inferred. This study suggests that the asthenospheric anisotropy due to present absolute block motion might be dominant under actively deforming continents.     

用面波方法研究上扬子克拉通壳幔速度结构

本文研究采用单台法和双台法提取了穿越上扬子的基阶面波相速度和群速度频散;通过对提取的面波群速度和相速度频散进行联合反演,得到的1-D SV速度模型显示上扬子块体下地壳S波速度与典型克拉通区域相当,其上地幔顶部80~170 km深处存在高速的岩石圈盖层,较AK135模型要快2%~3%,其岩石圈厚度约为180 km.在上扬子地区,径向各向异性集中分布在300 km以浅的岩石圈与软流圈部分,其中岩石圈部分SH波比SV波波速要快2%~4%,软流圈部分SH波比SV波波速要快3%~5%;Rayleigh波相速度方位各向异性分析结果显示,上扬子块体周期为25~45 s（大致相当于30~70 km深度范围内）的Rayleigh波相速度存在1.8%~2.7%不等的方位各向异性,其快波方向介于147°~174°.我们认为上扬子块体径向各向异性集中分布在岩石圈、软流圈部分,且各向异性随深度变化, 其岩石圈部分各向异性为大陆克拉通化的遗迹,软流圈部分各向异性与现今板块运动相关.     

塔里木盆地南部上地幔各向异性研究

塔里木南缘位于塔里木块体同青藏高原碰撞的前缘,是认识青藏高原同周围块体相互作用的重要位置.横波分裂方法可以获得岩石圈及软流圈地幔的介质各向异性特征,进而探讨岩石圈变形和地幔流动.本文利用横波分裂方法对中国科学院地质与地球物理研究所、北京大学和南方科技大学联合布设在塔里木盆地南部的8个宽频带流动地震台站记录的SKS和SK...     

大别—苏鲁及邻区上地幔的各向异性

大别—苏鲁是扬子与华北的碰撞造山带,对该地区上地幔各向异性的研究有助于了解该区的地幔动力学机制.本文选用了中国数字化地震台网和区域数字地震台网(山东、安徽、江苏、河南、湖北)三分量宽频带的远震地震波形数据,分别采用最小能量法和旋转相关法,对大别—苏鲁及邻区进行剪切波偏振分析,计算了研究区台站下方介质的各向异性分裂参数:快波偏振方向(Φ)和快慢波延迟时间(δt).本文研究结果发现,研究区内快、慢波延迟时间0.5~1.63 s,推测各向异性层深度为57.5~187.6 km,由软流圈和岩石圈地幔的各向异性共同作用引起.快波偏振方向在4个不同构造区表现出不同的特点:华北板块快波偏振方向为近E-W向,根据地质资料,我们分析认为华北板块的各向异性受地幔软流圈流动的影响明显;大别造山带各向异性平行于大别主构造,反映造山过程中岩石圈物质沿大别造山轴部NW-SE向迁移的特点; 在大别南侧和东侧的扬子板块快波偏振方向分别表现为近垂直于造山带走向和NEE-SWW,苏鲁造山带各向异性结果为NEE-SWW,与地表构造有一定的夹角,同时与板块运动方向相差较大,分析认为扬子板块和苏鲁造山带各向异性是由地幔软流圈流动和印支—燕山期构造运动残留在岩石圈地幔的"化石各向异性"共同作用的结果.     

PRESENT-DAY KINEMATICS OF THE ORDOS BLOCK AND ITS SURROUNDING AREAS FROM GPS OBSERVATIONS

Located among the South China block, Tibetan plateau, Alxa block and Yinshan orogenic belt, the Ordos block is famous for its significant kinematic features with stable tectonics of its interior but frequent large earthquakes surrounding it. After the destruction of the North China Craton, the integrity, rotation movement and kinematic relations with its margins are hotly debated. With the accumulation of active tectonics data, and paleomagnetic and GPS observations, some kinematic models have emerged to describe rotation movement of the Ordos block since the 1970's, including clockwise rotation, anticlockwise rotation, clockwise-anticlockwise-alternate rotation, and sub-block rotation, etc. All of these models are not enough to reflect the whole movement of the Ordos block, because the data used are limited to local areas.
In this study, based on denser geophysical observations, such as GPS and SKS splitting data, we analyzed present-day crustal and mantle deformation characteristics in the Ordos block and its surrounding areas. GPS baselines, strain rates, and strain time series are calculated to describe the intrablock deformation and kinematic relationship between Ordos block and its margins. SKS observations are used to study the kinematic relationship between crust and deeper mantle and their dynamic mechanisms, combined with the absolute plate motion(APM)and kinematic vorticity parameters. Our results show that the Ordos block behaves rigidly and rotates anticlockwise relative to the stable Eurasia plate(Euler pole: (50.942±1.935)°N, (115.692±0.303)°E, (0.195±0.006)°/Ma). The block interior sees a weak deformation of~5 nano/a and a velocity difference of smaller than 2mm/a, which can be totally covered by the uncertainties of GPS data. Therefore, the Ordos block is moving as a whole without clear differential movement under the effective range of resolution of the available GPS datasets. Its western and eastern margins are characterized by two strong right-lateral shearing belts, where 0.2°~0.4°/Ma of rotation is measured by the GPS baseline pairs. However, its northern and southern margins are weakly deformed with left-lateral shearing, where only 0.1°/Ma of rotation is measured. Kinematics in the northeastern Tibetan plateau and western margin of the Ordos block can be described with vertical coherence model with strong coupling between the crust and deeper mantle induced by the strong extrusion of the Tibetan plateau. The consistency between SKS fast wave direction and absolute plate motion suggests the existence of mantle flow along the Qinling orogenic belt, which may extend to the interior of the Ordos block. SKS fast wave directions are consistent with the direction of the asthenosphere flow in Shanxi Rift and Taihang Mountains, indicating that the crustal deformation of these areas is controlled by subduction of the Pacific plate to North China. The week anisotropy on SKS in the interior of Ordos block is from fossil anisotropy in the craton interior. After comparing with the absolute plate motion direction and deformation model, we deem that anisotropy in the interior of Ordos block comes from anisotropy of fossils frozen in the lithosphere. In conclusion, the Ordos block is rotating anticlockwise relative to its margins, which may comes from positive movement of its margins driven by lithospheric extrusion or mantle flow beneath, and its self-rotation is slight. This study can provide useful information for discussion of kinematics between the Ordos block and its surrounding tectonic units.     

采用剪切波分裂方向约束的板块绝对运动模型研究

板块绝对运动(即岩石圈板块相对于深部地幔的运动)导致软流圈深度存在较强的剪切作用,为软流圈呈现较强的地震波速各向异性提供了一种物理解释.相应地,软流圈地震各向异性的实测数据为反演板块绝对运动提供了一种定量的约束.本文利用前人发表的由474个剪切波分裂数据组成的全球软流圈地震各向异性方向数据集,结合板块相对运动模型MORVEL,通过加权最小二乘法反演板块绝对运动.计算结果表明,由实测数据约束的板块绝对运动模型不能将观测数据拟合到原始数据测量误差要求的统计水平上,反映出以地震各向异性方向指示板块绝对运动方向存在原始数据测量误差之外的系统性误差.该误差或可归因于板块绝对运动控制软流圈地震各向异性的物理机制复杂性以及小尺度地幔流动的区域复杂性.因此,地震各向异性方向只能从统计平均的意义上约束板块绝对运动方向,两者间的差别除了测量误差外还应包含复杂物理成因的模型误差.在原始数据测量误差的基础上增加20°的模型误差并且剔除一个离群数据后,本文得到了最优拟合剪切波分裂数据集的SKS473模型,并与由热点数据反演得到的板块绝对运动模型进行对比.由于约束板块绝对运动的地震各向异性数据和热点数据都存在误差较大、地理分布不均的局限性,结合两类数据的联合反演或可成为未来建立更高精度板块绝对运动模型的有效途径.     

华北中西部和青藏高原东北缘上地幔各向异性变形特征

基于华北中西部和青藏高原东北缘3个流动台阵共480个台站新得到的远震XKS(SKS、SKKS和PKS)波分裂结果,并结合研究区已得到的987个台站的分裂结果,获得了高分辨率的上地幔各向异性图像.分析表明,鄂尔多斯块体的时间延迟较小,反映了其稳定性和弱的各向异性变形特征,可能保留了古老克拉通根的"化石"各向异性,但其靠近...     

Depth constraints on azimuthal anisotropy in the Great Basin from Rayleigh-wave phase velocity maps

We present fundamental-mode Rayleigh-wave azimuthally anisotropic phase velocity maps obtained for the Great Basin region at periods between 16 s and 102 s. These maps offer the first depth constraints on the origin of the semi-circular shear-wave splitting pattern observed in central Nevada, around a weak azimuthal anisotropy zone. A variety of explanations have been proposed to explain this signal, including an upwelling, toroidal mantle flow around a slab, lithospheric drip, and a megadetachment, but no consensus has been reached. Our phase velocity study helps constrain the three-dimensional anisotropic structure of the upper mantle in this region and contributes to a better understanding of the deformation mechanisms taking place beneath the western United States. The dispersion measurements were made using data from the USArray Transportable Array. At periods of 16 s and 18 s, which mostly sample the crust, we find a region of low anisotropy in central Nevada coinciding with locally reduced phase velocities, and surrounded by a semi-circular pattern of fast seismic directions. Away from central Nevada the fast directions are ～ N–S in the eastern Great Basin, NW–SE in the Walker Lane region, and they transition from E–W to N–S in the northwestern Great Basin. Our short-period phase velocity maps, combined with recent crustal receiver function results, are consistent with the presence of a semi-circular anisotropy signal in the lithosphere in the vicinity of a locally thick crust. At longer periods (28–102 s), which sample the uppermost mantle, isotropic phase velocities are significantly reduced across the study region, and fast directions are more uniform with an ～ E–W fast axis. The transition in phase velocities and anisotropy can be attributed to the lithosphere–asthenosphere boundary at depths of ～ 60 km. We interpret the fast seismic directions observed at longer periods in terms of present-day asthenospheric flow-driven deformation, possibly related to a combination of Juan de Fuca slab rollback and eastward-driven mantle flow from the Pacific asthenosphere. Our results also provide context to regional SKS splitting observations. We find that our short-period phase velocity anisotropy can only explain ～ 30% of the SKS splitting times, despite similar patterns in fast directions. This implies that the origin of the regional shear-wave splitting signal is complex and must also have a significant sublithospheric component.     

1976年唐山地震震时和震后变形的模拟

本文采用三维粘弹性有限元方法拟合唐山地区１９７６-１９８５年观测到的地震震时和震后的水平与垂直地形变，反演华北板块下方深部物质的流变学性质．模型采用多层弹性覆盖层与线性粘弹性层的有限块体，发展断层面上存在着位错运动，并用正交设计法拟合观测数据．模拟计算表明，华北板块下方软流层粘度为７．１×１０^１８Ｐａ·ｓ；上地幔粘度为２．１×１０^１９Ｐａ·ｓ．     

南海东北部及其邻近地区的Pn波速度结构与各向异性

利用中国地震台网和ISC台站1980～2004年的地震数据,反演了南海东北部及其邻近地区的Pn波速度结构和各向异性.上地幔顶部的速度变化揭示出区域地质构造的深部特征：华南地区速度较高并且变化平缓,具有构造稳定地区的岩石层地幔特征;华南沿海尤其是滨海断裂带附近出现低速异常,表明该断裂可能穿过壳幔边界深达上地幔顶部.南海北部至台湾海峡较高的速度与华南地区类似,反映出大陆边缘和陆架地区的岩石层地幔性质;西沙海槽附近较高的速度不仅反映了华南大陆向南的延伸,而且与海槽裂谷拉张引起的地幔上拱有关,整个南海北部没有发现大规模地幔热流的活动痕迹.相比之下,南海东部次海盆的上地幔顶部存在明显的低速异常,对应于海底扩张中心的地幔上涌区,表明岩石层地幔强烈减薄甚至缺失;台湾东部－吕宋－菲律宾北部的低速异常与地震、火山活动以及岩浆作用紧密相关,揭示了西太平洋岛弧俯冲带的活动特征;南海东北部的洋－陆边界清晰,南海东部和菲律宾海西部较高的速度代表了海洋岩石层地幔的性质.Pn波各向异性反映出区域性构造应力状态及岩石层地幔的变形痕迹：华南地区的各向异性较小,说明这一构造稳定地区的岩石层地幔变形程度较弱;南海北部的快波方向与地壳浅表层构造的伸展方向一致,主要反映了中、新生代以来的大陆边缘张裂和剪切作用对岩石层地幔结构的影响;琉球－台湾－吕宋岛弧两侧各向异性十分强烈,平行于海沟的快波方向表明菲律宾海板块和欧亚大陆的相互作用导致俯冲板块前缘的岩石层地幔强烈变形;台湾东南海域快波方向的变化可能与欧亚大陆和菲律宾海板块俯冲机制的转换以及岩石层被撕裂有关.     

南北构造带北段上地幔各向异性特征

对布设在南北构造带北段的中国地震科学探测台阵项目二期674个宽频带流动台站和鄂尔多斯台阵21个宽频带流动台站记录的远震XKS(SKS、SKKS和PKS)波形资料作偏振分析,采用最小切向能量的网格搜索法和"叠加"分析方法求得每一个台站的XKS波的快波偏振方向和快、慢波的时间延迟,并结合该区域出版的122个固定台站的分裂结果,获得了南北构造带北段上地幔各向异性图像.快波方向分布显示青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘的快波方向主要表现为NW—SE方向,秦岭造山带的快波方向为近E—W方向,鄂尔多斯块体内部的快波方向在北部为近N—S方向,南部表现为近E—W方向.时间延迟分布来看,鄂尔多斯块体的时间延迟不仅明显小于其周缘地区,而且小于其他构造单元,特别是在高原东北缘、阿拉善块体和鄂尔多斯块体的交汇地区的时间延迟很大,反映了构造稳定单元的时间延迟小于构造活跃单元.通过比较快波方向的横波分裂测量值与地表变形场模拟的预测值,并结合研究区地质构造和岩石圈结构特征分析表明,在青藏高原东北缘、阿拉善块体和鄂尔多斯块体西缘各向异性主要由岩石圈变形引起,地表变形与地幔变形一致,地壳耦合于地幔,是一种垂直连贯变形模式;秦岭造山带的各向异性不仅来自于岩石圈,而且其岩石圈板块驱动的软流圈地幔流作用不可忽视;鄂尔多斯块体内部深浅变形不一致,具有弱的各向异性、厚的岩石圈和构造稳定的特征,我们认为其各向异性可能保留了古老克拉通的"化石"各向异性.     

Correlated Crust and Mantle Strain Field in China

INTRODUCTION Moststudiesonactiveblockshavebeenfocusedonidentificationofblockboundariesandtheiractivity;inotherwords,mostoftheworkwaslimitedtothehorizontalmovementoftheblocks.Inreality,theblocksarenotonlysurroundedbyactivefaultsofhorizontalmotion,butalsoco…     

呼包盆地周缘壳、幔结构研究

对呼包盆地周边七个台站的远震接收函数研究表明：研究区地壳厚度为43～46 km,地壳速度比和S波速度结构均无异常;阴山造山带岩石圈埋深为65～85 km,且上地幔S波平均速度偏低,是典型的异常地幔区.根据S波速度结构和地壳厚度随地形高程反比的变化关系,以及地表广泛出露的幔源玄武岩分布,推测该区地幔深部热物质上涌是阴山造山带隆升的原因之一.上升的软流层物质与地幔发生交代、侵蚀作用导致岩石圈减薄,S波速度降低.呼包盆地的形成可能与深部物质上涌造成的拉张效应有关.     

平均热点参考基准的建立和岩石圈西向漂移的研究

对于大地测量应用来说,目前IERS机构在定义地球参考系时推荐采用岩石圈无整体旋转(No-Net-Rotation-NNR)约束条件,然而对于地球物理应用来说,相对于NNR参考基准的绝对板块运动数据可能会对地幔对流等研究结果产生误导.考虑到热点的运动,提出建立平均热点(MHS-Medial HotSpot)参考基准的方法,给出建立该基准的约束准则,分别以地学模型NNR-NUVEL1A和实测模型ITRF2005VEL为基础,建立了平均热点参考基准MHS-NUVEL1A和MHS-ITRF2005,并与其它基于热点的绝对板块运动模型进行了比较和分析;讨论了岩石圈的西向漂移,给出了岩石圈相对于下地幔整体旋转的更精确的定量估计,即基于实测的热点参考架MHS-ITRF2005和地学模型NNR-NUVEL1A之间的整体旋转为0.26°/Ma,旋转极在(50°S, 62°E),这与由板块的受力模型给出的岩石圈的整体旋转的旋转极很接近,旋转速率大致快了10%.     

中国东北地区北部上地幔各向异性及其动力学意义

中国东北地区广泛发育新生代板内火山,晚中生代以来岩石圈遭受过多期拉张作用.作为中国唯一的深震孕育区,中国东北地区受到太平洋板块的西向俯冲,使得其成为研究岩石圈变形、板块俯冲和板内火山成因及其相互作用关系的天然实验室.通过分析架设在中国东北地区北部的147个流动和固定台站的SKS波形数据,共计得到了377对各向异性参数和251个无效分裂结果.结果表明,中国东北地区东西两侧具有不同的各向异性分布:西部地区各向异性方向变化范围为N143-199°E,平均N169°E,与晚中生代岩石圈伸展方向一致;其各向异性延迟时间平均值约为0.8s,说明来自地幔的各向异性比较微弱,主要由残留在岩石圈中的古老变形所引起.同时,在松辽盆地和佳木斯地块部分区域,观测到延迟时间较小的各向异性(~0.4s),可能是由于岩石圈的拆沉和热地幔物质的上涌侵蚀了保留在岩石圈的古老形变所致.在研究区东部,NNW-SSE朝向的各向异性被观测到,并伴随较大的延迟时间(大于1.0s),可能与太平洋板块撕裂回撤而产生的地幔流动有关.此外,近W-E方向的各向异性只在佳木斯地块被观测到,而太平洋板块在地幔过渡带中的俯冲可能是其产生的主要成因.     

CRYSTALLOGRAPHIC PREFERRED ORIENTATION(CPO) OF ANISOTROPIC MINERALS IN THE LITHOSPHERE AND ITS SIGNIFICANCE TO THE STUDY OFLITHOSPHERE DYNAMICS

Seismic anisotropy has been widely used to constrain deformation and mantle flow within the upper mantle of the Earth's interior, and is mainly affected by crystallographic preferred orientation(CPO)of anisotropic mineral in lithosphere. Anisotropy of peridotites caused by deformation is the main source of seismic anisotropy in the upper mantle. Olivine is the most abundant and easily deformed mineral to form CPO in peridotite, thus the CPO of olivine controls seismic anisotropy in the upper mantle. Based on simple shear experiments and studies of natural peridotites deformation, several CPO types of olivine have been identified, including A, B, C, D, E and AG-type. Studies on the deformation of olivine have shown that the CPO of olivine is mainly related to stress, water content, temperature, pressure, partial melting and melt/fluid percolation. Most of the seismic anisotropy has been explained by the A-type olivine CPO in the upper mantle, which is commonly found in upper-mantle peridotites and produced by the simple shear in dry conditions. Previous studies showed that anisotropy was attributed to the CPO of mica and amphibole in the middle-lower crust. The comparison between mantle anisotropy calculated from mineral CPO and regional anisotropy deduced from geophysical methods is therefore particularly useful for interpreting the deformation mechanisms and geodynamic processes which affect the upper mantle in different tectonic units such as subduction system, continental rift and continental collision zone in the world. The paper summarizes the characteristics of CPO and anisotropy of major anisotropic minerals in the upper mantle. Taking the lithosphere mantle xenoliths in the southeastern Tibetan plateau as an example, we perform detailed studies on the microstructures and seismic anisotropy to better understand the deformation mechanisms and upper mantle anisotropy in this region. Results show that the CPO of olivine in peridotite xenoliths in southeastern Tibetan plateau are A-type and AG-type. The mechanisms proposed for the formation of AG-type are different from that for the A-type. Therefore, the occurrence of AG-type olivine CPO pattern suggests that this CPO may record a change in deformation mechanism and tectonic environment of the lithosphere in southeastern Tibetan plateau. Provided that the strong SKS(shear wave splitting)observed in southeastern Tibetan plateau results from lithosphere mantle, the lithosphere mantle in this region is expected to be at least 130km thick and characterized by vertical foliation. Considering that the thickness of lithosphere in southeastern Tibetan plateau is much less than 130km and the lithosphere mantle cannot explain the anisotropy measured by SKS, other anisotropy sources should be considered, such as anisotropy in the asthenosphere and the oriented melt pockets(MPO)in the upper mantle. Therefore, detailed study of CPO of anisotropic mineral is essential for constraining geophysical measurements and analyzing the dynamic process of the lithosphere reasonably.