秦岭─大别造山带及其南北缘地震层析成像

利用秦岭─大别造山带及其毗邻地区３１０个地震台站记录到的区域地震２３６００条Ｐ波到时数据，重建了该区地壳和上地幔三维速度图像。结果表明：１．秦岭─大别造山带及其毗邻地区地壳和上地幔存在显著的横向不均匀性，直至１１０ｋｍ深度处依然明显。２．地壳上部的速度图像与地表地质构造密切相关：造山带隆起区显著高速；盆地及坳陷区明显低速。由速度鲜明对比勾勒出的秦岭─大别造山带南界基本上位于扬子北缘主边断裂带上。３．中地壳的速度图像表明，造山带内部的一些低速区对应于一些大型推覆构造。４．４０＋０ｋｍ深度处的速度图像反映了该区莫霍界面深度的起伏。大致以１０７°Ｅ为界，以东地区地壳厚度小于４０ｋｍ，以西地区大于４０ｋｍ，且呈现出往西地壳逐渐加厚的趋势。５．位于滦川、商县、丹凤的北秦岭构造带，上地幔顶部出现低速异常，异常速度值约为７．３９—７．５５ｋｍ／ｓ。结合地球物理测深的结果，可能是由下地壳、上地幔顶部的热过程所致。     

地球物理综合观测揭示秦岭—桐柏—大别复合造山带地壳及上地幔结构

秦岭—桐柏—大别复合造山带(以下称为秦岭大别造山带)属于中国中央造山带的一部分,由华北克拉通与扬子克拉通汇聚形成.对于秦岭大别造山带及其周缘地区的研究,可以为这一大陆碰撞造山带的形成与演化过程提供重要信息.本文整合研究区域的接收函数与背景噪声数据,采用H-κ叠加分析、接收函数与背景噪声联合反演、克希霍夫偏移成像等方法,得到了沿秦岭东西方向具有高分辨率的地壳及上地幔结构.研究结果显示:(1)莫霍面深度由西向东逐步抬升,由剖面西侧最深约55 km上升至剖面东侧最浅约30 km;莫霍面于东西秦岭之间起伏明显;桐柏以及东大别下方莫霍面局部加深.(2)西秦岭中下地壳观测到的高速异常阻隔了青藏高原东北缘地壳低速异常的向东扩张,反映了青藏高原东北缘的中下地壳流没有通过西秦岭继续向东流动.(3)西秦岭岩石圈地幔顶部高速异常延伸至100 km深度(剖面底部),桐柏—西大别岩石圈地幔顶部高速延伸至70 km深度,东大别、东秦岭岩石圈地幔顶部未见较大深度范围的高速异常.     

大别造山带与郯庐断裂带壳、幔结构和陆内“俯冲”的耦合效应

地质学家基于岩石年代学、峰期变质的压力差和阿尔卑斯变质沉积岩中柯石英的发现，推断在大别造山带地区发现的柯石英和榴辉岩乃是地幔深处120km左右折返至地壳浅部的结果.为了提供大别造山带与郯庐断裂带地域的壳、幔深部结构与深层动力过程的定量判据，本文利用瑞利（Rayleigh）波的频散效应反演该区S波的三维群速度结构分布.研究结果表明：（1）该区存在一由东向西陆内俯冲的高速板舌，其下插深度仅达160 km.在秦岭—大别造山带地域上地幔70km深处存在一顶部宽约500km的低速地幔热点.（2）出露的含柯石英榴辉岩是大别造山带深部物质和能量的交换，郯庐断裂带强烈平移错动及板内俯冲效应共同作用导致的深部物质(上地幔)运移和动力学效应的综合产物.     

中国东部随县─启东地带上地幔结构研究

随县─启东人工地震探测剖面长７００ｋｍ，穿越郯城─庐江深大断裂带．提取了该区莫霍界面以下上地幔中的反射界面信息，获得了上地幔的分层结构．结果表明，该区上地幔中有两个深度分别为７６ｋｍ和１００ｋｍ的界面．７６ｋｍ深度以下为一低速层，１００ｋｍ深度以下则为速度更低的低速层．该区岩石层厚度为７６ｋｍ，与华北地区相似，具有薄岩石层性质．     

中国东部随县─启东地带上地幔结构研究

随县─启东人工地震探测剖面长７００ｋｍ，穿越郯城─庐江深大断裂带．提取了该区莫霍界面以下上地幔中的反射界面信息，获得了上地幔的分层结构．结果表明，该区上地幔中有两个深度分别为７６ｋｍ和１００ｋｍ的界面．７６ｋｍ深度以下为一低速层，１００ｋｍ深度以下则为速度更低的低速层．该区岩石层厚度为７６ｋｍ，与华北地区相似，具有薄岩石层性质．     

大别造山带壳幔界面的断错结构和壳内低速体

2001年4月~2002年3月, 北起河南兰考附近的崔林(34°40′N, 114°49′E), 南到江西大冶附近的大箕铺(30°20′N, 115°03′E), 横跨大别造山带, 布设了总长度约500 km, 由34台宽频带流动地震仪组成的二维地震台阵观测剖面. 台站采用不等间距布设. 在大别造山带范围内, 台站间距为3~8 km, 其他地区为15~20 km. 利用台阵记录的远震P波波形数据和接收函数方法, 获得了横穿大别造山带的接收函数剖面和各台站下方100 km深度范围内地壳上地幔的Ｓ波速度结构. 研究结果表明: 大别造山带地壳在垂直山体走向的方向上具有明显的非对称分块结构; 地壳最大厚度为42 km; 壳幔界面具有与地壳分块结构相应的断错结构, 最大断距达到8 km; 在造山带核部, 存在壳内横波低速体; 壳内低速体分为两部分, 分别对应南大别和北大别; 在垂向上两者可能曾有差异运动; 其下方地壳速度具有随深度增加的梯度结构, 上地幔顶部直到70 km深度范围内速度异常偏低, 而其下方70~100 km的深度上有高速异常体.     

大别造山带西段宽频带数字地震台阵观测与地壳上地幔结构

2001年5～11月,在大别造山带西段的新县、红安地区（31 °20～3°50′N,114°30′-115°E）架设了宽频带数字地震流动台阵．采用接收函数方法对台阵记录的高质量远震P波数据进行反演,获得了大别造山带西段的S波速度剖面和地壳上地幔精细结构．研究结果显示,研究区的地壳厚度整体上较薄,约为32～38km,莫霍面自南向北倾斜．在台阵北缘对应桐城一桐柏剪切带处,莫霍面发生错断,断距达到4-6km,显示桐城．桐柏剪切带为早中生代扬子板块与华北板块碰撞的古缝合带的南界．在上地幔顶部存在向北倾斜东西向延伸的s波低速带,显示出大别山造山带与毗邻华北块体之间的拼合关系．在台阵南部下地壳底部存在高速体,这可能和拆沉作用以后,发生大规模拉张作用相伴随的幔源基性岩浆在下地壳下部的底侵作用有关．     

华北及邻区地壳上地幔三维速度结构的地震走时层析成像

利用华北及邻区475个地震台站的区域地震走时资料,反演了该地区的地壳上地幔三维P波和S波速度结构。地震走时的计算用近似弯曲射线追踪方法,三维速度模型的反演用LSQR算法。用检测板方法对走时数据进行成像分辨率分析,结果表明反演模型在水平方向上以0.5°×0.5°的节点分布,垂直方向上以1km、10km、25km、42km、60km为节点作网格划分是合理的。研究区域内,秦岭—大别造山带两侧的华北块体与扬子块体有不同的速度异常特征：华北块体地壳速度结构复杂,而扬子块体则相对简单。华北块体地壳内存在较明显的低速异常,而扬子块体则正常或高速异常。自中新生代以来华北块体地壳经历挤压到伸展的强烈变形,而扬子块体相对稳定。华北块体的构造活动依然强烈,表现为频繁的地震活动。华北地块地壳速度结构的主要特征是：①主要构造带（如燕山构造带、太行山山前构造带、汾渭构造带、郯庐断裂带以及秦岭-大别构造带）位于地壳上地幔的低速或高低速过渡区内;②在唐山及附近地区25 km、42 km和60 km深处连续的低速异常,可能意味着上地幔热的物质上涌,到达上地壳的下部后停止上升过程。     

中国大陆西北造山带及其毗邻盆地的地震层析成像

根据新疆、甘肃、青海和吉尔吉斯斯坦地震台网提供的地震数据,利用地震层析成像法重建了中国大陆西北造山带及其毗邻盆地的地壳上地幔三维速度图像．上地壳造山带大都为高速区,盆地和地陷区的低速显然与较厚的松散沉积层有关．地壳中部东、西天山之间存在低速边界,造山带及青藏高原北部的莫霍面深度较大,盆地和坳陷区的莫霍面相对较浅．上地幔软流层在青藏高原、阿尔泰山、祁连山等地较浅,在塔里木盆地和天山一带较深．地幔热物质有可能在板块碰撞中沿构造边界上升到造山带的底部,它们的动力学性质与中国大陆西北造山带的形成演化有着密切的联系。     

大别─苏鲁碰撞造山带的地震层析成像研究──岩石圈三维速度结构

对包含大别－苏鲁碰撞造山带在内的东经 １１２°－１２４°,北纬２８°－３９°区域进行地 震层析成像研究,重建其地壳及上部地幔的三维速度图像．结果表明：造山带岩石圈速度横 向不均匀性显著;大别造山带以商城－麻城断裂为界,东侧的大别地块与西侧红安地块在地 壳速度上是两个不同的速度块体;中地壳 １５-２５ｋｍ深度范围内存在低速带,与伸展滑脱构造 有关;南、北大别构造单元之下,莫霍面下凹,地壳内发育了速度为６．５－６．６ｋｍ／ｓ、向北倾斜的 相对高速体,与超高压变质岩体相对应;在大别－苏鲁造山带下方的上部地幔中存在向北倾 斜的板片状高速体,结合已有地质、地球化学证据推测,它是三叠纪俯冲的扬子地块的残留 体;俯冲板片在深部发生了断离．本文利用地震层析成像方法揭示的造山带岩石圈速度结构 细节,对研究与地表地质有关的地球动力学无疑是十分重要的．     

中国大陆及其邻近地区的地震层析成象

本文利用中国地震台网及ISC提供的区域地震和远震的P波走时数据,重建了中国大陆及其邻近地区的三维速度图象。 主要结果是:1.本文给出的速度图象揭示了中国大陆及其邻近地区的地壳和上地幔速度存在明显的横向不均匀性,这种不均匀性甚至在下地幔的1100km深度还依然存在。上地幔的速度图象同地表已知的地质构造特征的相关性可以追踪到110km,从220km以下很难找到它们之间的明显关系。2.45^-0km和45⁺⁰km深度处的速度图象明显地表示出中国大陆的地壳厚度可以102.5°E附近为界分为两部分:其东部地壳薄,厚度都小于45km;西部有一条自若尔盖-松潘（34°N,102.5°E附近）向西北沿38°N往西至塔里木盆地南缘的分界线;其南部除滇西南之外,整个青藏高原的地壳厚度都大于45km;其北部除天山山脉之外,地壳厚度一般不大于45km.3.110km深度处的速度图象表明,速度异常呈块状分布。同中国大地构造分区略图比较之后发现,其中,扬子准地台和塔里木地台对应于高速区,中朝准地台则大都表现为低速异常;华南褶皱系为低速区,青藏地块南缘喜马拉雅和冈底斯念青唐古拉褶皱系则表现为高速异常。4.220km深度处的速度图象表明,中国大陆相当多的地区软流层有明显的显示。5.同45_⁰km和45⁺⁰km的速度图象一样,400km和600km的速度图     

Seismic tomography beneath the orogenic belts and adjacent basins of northwestern China

Three-dimensional velocity images of the crust and upper mantle beneath orogenic belts and adjacent basins of the northwestern continent of China are reconstructed by seismic tomography, based on arrival data of P wave recorded in seismic networks in Xinjiang, Qinghai, Gansu of China and Kyrgyzstan. The velocity images of upper crust demonstrate the tectonic framework on the ground surface. High velocities are observed beneath orogenic belts, and low velocities are observed in the basins and depressions that are obviously related to unconsolidated sediments. The velocity image in mid-crust maintains the above features, and in addition low velocities appear in some earthquake regions and a low velocity boundary separates the western Tianshan Mts. from eastern Tianshan Mts. The orogenic belts and the northern Tibetan plateau have a Moho depth over 50 km, whereas the depths of the Moho in basins and depressions are smaller than 50 km. The velocity images of upper mantle clearly reveal the colliding relationship and location of deep boundaries of the continental blocks in northwestern China, indicating a weakness of the upper mantle structure of orogenic belts. The top depth of upper mantle asthenosphere varies from place to place. It seems shallower under the northern Tibetan plateau, Altay and Qilian Mts., and deeper under the Tarim and Tianshan regions. Hot mantle probably rose to the bottom of some orogenic belts along tectonic boundaries when continental blocks collided to each other. Therefore their dynamic features are closely correlated to the formation and evolution of orogenic belts in northwestern China.     

中国南北带地壳和上地幔的三维速度图象

本文采用作者提出的地震层析成象法得到了中国南北带地壳和上地幔的三维速度图象。通过误差和分辨分析,以及同爆炸地震测深剖面的比较,证明了成象结果的可靠性。 成象结果表明:1.南北带的地壳和上地幔存在显著的横向不均匀性,深达450km这种不均匀性还依然存在;2.地壳上部的速度图象与地表的已知地质特征明显相关:四川盆地显著低速,康滇地轴显著高速;3.中地壳在很大范围内存在低速层,其最低速度值达5.60km/s;4.在25°N-38°N和100.0°E-103.2°E的长条带内,上地幔顶部出现低速异常,异常速度值约为7.49km/s。 成象结果还清晰地勾划了各块体间的焊接边界。120km深度的速度图象表明,扬子准地台自秦岭以南以龙门-大巴和盐源-丽江台缘褶带为其西部边界;西南以哀劳山褶皱带为界;东南则以右江褶皱带内的南盘江为界。 统计表明,地震活动与南北带的速度结构相关:从20km以上的速度图象发现,大地震大都发生在高速和低速间的过渡条带上。     

JOINT INVERSION OF SURFACE WAVE DISPERSION AND RECEIVER FUNCTIONS FOR CRUSTAL AND UPPERMOST MANTLE STRUCTURE BENEATH CHINESE TIENSHAN AND ITS ADJACENT AREAS

The Tienshan orogenic belt is one of the most active intracontinental orogenic belts in the world. Studying the deep crust-mantle structure in this area is of great significance for understanding the deep dynamics of the Tienshan orogen. The distribution of fixed seismic stations in the Tianshan orogenic belt is sparse. The low resolution of the existing tomographic results in the Tienshan orogenic belt has affected the in-depth understanding of the deep dynamics of the Tienshan orogenic belt. In this paper, the observation data of 52 mobile seismic stations in the Xinjiang Seismic Network and the 11 new seismic stations in the Tienshan area for one-year observations are used. The seismic ambient noise tomography method is used to obtain the Rayleigh surface wave velocity distribution image in the range of 10~50s beneath the Chinese Tienshan and its adjacent areas (41°~48° N, 79°~91° E). The joint inversion of surface wave and receiver function reveals the S-wave velocity structure of the crust and uppermost mantle and the crustal thickness below the station beneath the Chinese Tienshan area(41°~46° N, 79°~91° E). The use of observation data from mobile stations and new fixed seismic stations has improved the resolution of surface wave phase velocity imaging and S-wave velocity structure models in the study area.
The results show that there are many obvious low-velocity layers in the crust near the basin-bearing zone in the northern Tienshan Mountains and the southern Tienshan Mountains. There are significant differences in the structural characteristics and distribution range of the low-velocity zone in the northern margin and the southern margin. Combining previous research results on artificial seismic profiles, receiver function profiles, teleseismic tomography, and continental subduction simulation experiments, it is speculated that the subduction of the Tarim Basin and the Junggar Basin to the Tienshan orogenic belt mainly occurs in the middle of the Chinese Tienshan orogenic belt, and the subduction of the southern margin of the Tienshan Mountains is larger than that of the northern margin, and the subduction of the eastern crust is not obvious or in the early subduction stage. There are many low-velocity layers in the inner crust of the Tienshan orogenic belt, and most of them correspond to the strong uplifting areas that are currently occurring. The thickness of the crust below the Tienshan orogenic belt is between 55km and 63km. The thickness of the crust(about 63km)is the largest near the BLT seismic station in the Bazhou region of Xinjiang. The average crustal thickness of the Tarim Basin is about 45km, and that of the Junggar Basin is 47km. The S-wave velocity structure obtained in this study can provide a new deep basis for the study of the segmentation of the Tienshan orogenic belt and the difference of the basin-mountain coupling type.     

JOINT INVERSION OF AMBIENT NOISE AND SURFACE WAVE FOR S-WAVE VELOCITY OF THE CRUST AND UPPERMOST MANTLE BENEATH WEIHE BASIN AND ITS ADJACENT AREA

The Weihe Basin is the main component of the extrusion and escape shear zone between the ancient North China craton block in Ordos and the ancient Yangtze platform in Sichuan Basin, and carries the dynamic transmission from the main power source of the Qinghai-Tibet Block in the west to the North China and South China regions in the east. The basin itself plays multi roles in the east-west and north-south tectonic movement, and is an excellent site for studying the structural interlacing, dynamic transformation and transmission. At the same time, Weihe Basin is also a famous strong earthquake zone in China. Historically, there was a strong earthquake of magnitude 8 1/4 occurring in Huaxian County in 1556, causing huge casualties and property losses. In view of the special geological structures and the characteristics of modern seismicity activities in the Weihe fault-depression zone, it is necessary to carry out fine three-dimensional velocity structure detection in the deep part of Weihe Basin and its adjacent areas, so as to study the relationship between velocity structure and geological structural units and their evolution process, as well as the deep medium environment where earth￣quakes develop and occur. We investigate the S-wave velocity structure beneath Weihe Basin and its adjacent regions based on continuous background noise data and teleseismic data recorded by 257 broadband stations in Shaanxi Province and its adjacent regions and China Seismological Science Array Exploration Project, and by adopting seismic surface wave inter-station method and background noise cross-correlation method, a total of 10 049 fundamental-mode Rayleigh surface wave phase velocity dispersion curves in the periods of 5~70s are obtained. Firstly, using the average dispersion curve in this study area, we obtain the one-dimensional average S-wave velocity structure model of the study area, and then we apply the ray-tracing surface-wave-dispersion direct inversion method to obtain the S-wave velocity structure of the crust and uppermost mantle (3~80km) beneath Weihe Basin and its adjacent regions. The test results of a 1°×1° grid checker board show that the recovery is good, except for the areas east of 111° and south of 32° of the study area, where there is almost no resolution. The imaging results show that the velocity structure beneath each tectonic unit in the study area has a certain distribution rule, and there is a good correlation between surface geological structure and deep velocity structure. Based on the analysis of velocity slices at different depths and S-wave velocity structures of three profiles, and combined with existing geological structures, geophysics and other deep exploration research results, we obtain the following knowledge and conclusions:1)The thick sedimentary layer covering the top of Weihe Basin is the cause of low velocity anomaly in its shallow crust, the middle and upper crust of the basin are of low velocity structure, and the low-velocity zone extends about 25km, the Moho interface uplifts abruptly relative to both the Ordos Block and the Qinling orogenic belt on opposite sides, and high-speed materials from the upper mantle intrude into the lower crust, which may be related to the underplating of mafic-ultramafic materials from the upper mantle in Mesozoic-Cenozoic period; 2)The south Ordos Block is not a homogeneous whole, the low-velocity structure of the shallow crust in southern Ordos Block is thin in east and thick in west, which may be related to the overall tilting of the Ordos Basin since the Phanerozoic, as well as the differential uplift and strong and uneven denudation of the Ordos Block since the Late Cretaceous. The crustal structure of the south Ordos Block is relatively simple and homogeneous. There is no significant low-velocity structure in the curst of the block, which shows that the low-velocity structure in the crust does not penetrate the whole Ordos block. We speculate that the southern Ordos Block still maintains the stable craton property, and has not been reformed significantly so far; 3)The variation characteristics of deep structure of the Qinling orogenic belt reflect the deep crustal structure and tectonic deformation characteristics of the orogenic belt which are strongly reformed by land-land collision and suture between North China plate and Yangtze plate, intracontinental orogeny, uplift of Qinghai-Tibet Plateau and its northeastern expansion since the Late Hercynian-Indosinian period. The deep structure beneath the eastern and western Qinling orogenic belt is different and has the characteristics of segmentation. The low-velocity anomaly at the bottom of the lower crust of the orogenic belt may be affected by tectonic activities such as uplift and outward extension of the NE Tibetan plateau, and the analysis considers that there is little possibility of the existence of lower crustal circulation channel for the eastward flowing of Tibetan plateau materials in the Qinling orogenic belt. However, since the maximum depth from the inversion of this paper is 80km, which is located at the top of the upper mantle, our results cannot prove that there exists a mantle flow channel for the eastward flow of Tibetan plateau material beneath the Qinling orogenic belt.     

Rayleigh wave phase velocities of South China block and its adjacent areas

Using records of continuous seismic waveforms from 609 broadband seismic stations in the South China Block and its adjacent areas in 2010–2012, empirical Green's functions of surface waves were obtained from cross-correlation functions of ambient noise data between these stations. High quality phase velocity dispersion curves of Rayleigh waves were obtained using time-frequency analysis. These interstation dispersion curves were then inverted to build Rayleigh wave phase velocity maps at periods of 6–50 s. The results of phase velocity maps indicate that phase velocities at 6–10 s periods are correlated with the geological features in the upper crust. Major basins and small-scale grabens and basins display slow velocity anomalies; while most of the orogenic belts and the fold belts display high velocity anomalies. With the gravity gradient zone along Taihang Mountain to Wuling Mountain as the boundary for the phase velocity maps at period of 20–30 s, the western area mainly displays low velocity anomalies, while the eastern side shows high velocity anomalies. Phase velocities in the eastern South China Block south to the Qinling-Dabie orogenic belt is higher than that in the eastern North China Block to the north, which is possibly due to the differences of tectonic mechanisms between the North China Craton and the South China Block. The phase velocities at periods of40–50 s are possibly related to the lateral variations of the velocity structure in the lower crust and upper mantle: The low-velocity anomalies in the eastern part of the Tibetan Plateau are caused by the thick crust; while the Sichuan Basin and the southern part of the Ordos Basin display distinct high-velocity anomalies, reflecting the stable features of the lithosphere in these blocks. The lateral variation pattern of phase velocities in the southern part of the South China Block is not consistent with the surface trace of the block boundary in the eastern Yunnan Province and its vicinities. The phase velocities in the Sichuan Basin are overall slow at short periods and gradually increase with period from the central part to the edge of the basin, indicating the features of shallower basement in the center and overall stable lithospheric mantle of the basin. The middle and upper crust of the southern Ordos Basin in the North China Block is heterogeneous, while in lower crust and the uppermost mantle the phase velocities mainly exhibit high anomalies. High-velocity anomalies are widespread at the middle of the Qinling-Dabie orogenic belt, as well as the areas in southeastern Guangxi with Caledonian granite explosion, but its detailed mechanism is still unclear.     

西昆仑造山带下岩石圈地幔速度结构

在已完成的新疆地学断面研究计划实施中曾在西昆仑山前布置了14个宽频带地震台站.利用记录到的远震P波初至和层析成像方法，研究了西昆仑造山带下的岩石圈地幔结构特征.在已有地震学证据基础上，层析成像结果显示，西昆仑造山带下的高速岩石圈地幔可能是印度岩石圈地幔的俯冲前缘.沿东经80°深度剖面图像显示，在西昆仑造山带下的150～300km处，高速异常的岩石圈地幔前锋与低速异常的塔里木块体岩石圈地幔发生了面对面碰撞.     

大别造山带地壳结构反演及其动力学意义

大别造山带是全球最大的碰撞造山带之一,三叠纪时期,扬子板块深俯冲至地幔的200km处,经历了超高压变质作用。白垩纪早期,该造山带发生了强烈的伸展和垮塌,以及大规模的后造山地幔源岩浆侵入和火山活动。本研究收集了大别造山带及其邻区(29°~34°N、114°~119°E)的震相资料,采用双差层析成像技术,对大别造山带地壳结构进行反演,研究地壳结构与后造山地幔源岩浆侵入和火山活动之间的关系。结果显示,大别造山带中上地壳存在低速结构,该低速结构可能是熔融的幔源侵入物质,由于俯冲板片断裂,或下地壳/岩石圈发生拆沉,导致软流圈物质上涌至地壳底部、侵入地壳中,形成大别造山带地壳中的低速结构;同时,合肥盆地显示为低速区,可能是受浅部沉积层影响。研究中横切大别山的4条剖面显示,该地区下方存在北向倾斜高速结构,该高速结构可能是襄樊-广济断层,或者是扬子板块向华北板块下方俯冲的遗迹。