Crustal Configuration of the Northwest Himalaya Based on Modeling of Gravity Data

The gravity response and crustal shortening in the Himalayan belt are modeled in detail for the first time in the NW Himalaya. The Bouguer gravity anomaly along a ~450-km-long (projected) transect from the Sub-Himalaya in the south to the Karakoram fault in the north across the Indus-Tsangpo Suture Zone is modeled using spectral analysis, wavelet transform and forward modeling. The spectral analysis suggests three-layer interfaces in the lithosphere at 68-, 34- and 11-km depths corresponding to the Moho, the Conrad discontinuity and the Himalayan decollement thrust, respectively. The coherence, admittance and cross spectra suggest crustal shortening because of convergence compensated by lithospheric folding at 536- and 178-km wavelength at the Moho and the upper-crustal level. An average effective elastic thickness of around 31 km is calculated using the coherence method. The gravity data are modeled to demarcate intracrustal to subcrustal regional thrust/fault zones. The geometrical constraints of these faults are obtained in the space scale domain using the wavelet transform, showing good correlation with the major tectonic boundaries. The crustal configuration along the transect shows how the Moho depth increases from 45 to 80 km towards the north with the locus of flexure of the Indian crust beneath the Higher Himalayan zone. The combination of forward modeling and wavelet analysis gives insight into the subsurface extent and geometry of regional structures across the NW Himalaya.     

长江中下游成矿带及邻区三维Moho面结构:来自人工源宽角地震资料的约束

为深入理解长江中下游地区在中生代成矿的深部动力学过程,对跨越宁芜矿集区地质廊带内的非纵剖面反射/折射地震数据进行动校正和时深转换处理,获得了非纵方向的Moho面深度;联合纵测线和非纵测线上Moho面深度数据,获得了长江中下游成矿带及邻区的三维Moho面深度结构.结果显示宁芜矿集区下方的Moho面整体较浅,约32~34km,华北块体合肥盆地内Moho面整体较深,约34~35km.Moho面深度和区域布格重力异常变化趋势对应良好.宁芜矿集区下方Moho面呈上隆特征,支持长江中下游地区成矿模式中增厚岩石圈发生拆沉、软流圈的上隆及底侵作用等动力学过程.Moho面平行于成矿带走向的变化趋势,预示长江中下游成矿带地壳和上地幔在板块边界发生了NE-SW向的切向流动变形.郯庐断裂带两侧,Moho面深度变化较大,表明地表近陡立的郯庐断裂为深大断裂,深部可能切穿Moho面并延伸至上地幔.     

用近垂直方法提取莫霍面——以六盘山深地震反射剖面为例

根据近垂直地震反射原理和Moho面上下深地震反射资料频率低的特征,运用深地震反射资料的近垂直反射数据,通过静校正、资料净化、共检波域叠加等叠前和叠后处理技术,对六盘山莫霍面结构进行成像.初步结果显示出横过六盘山莫霍面起伏、断错的结构特征.六盘山东侧的鄂尔多斯地块莫霍面东高西低、变化平缓、结构完整;靠近六盘山前缘莫霍面错断,可能与壳内走滑作用有关;西侧的青藏高原东北缘莫霍面西高东低,靠近六盘山呈现汇聚挤压、明显缩短的特征;六盘山复杂的莫霍面结构,揭示了六盘山地壳早期双向挤压、晚期山前侧向走滑的构造图像.     

可可西里岩石圈向北俯冲到柴达木地幔的地震学证据

利用中美德INDEPTH IV合作项目2007—2009年间布置于青藏高原中、北部140个宽频地震台站记录到的天然地震数据,经过接收函数成像处理,获得了3条穿过昆仑—阿尼玛卿缝合带清晰的壳幔结构图像.结果显示柴达木南缘莫霍面位于约50 km深度,羌塘地块、可可西里地块、东昆仑造山带莫霍面位于约65 km深度,昆仑—阿尼玛卿缝合带以北约50 km存在莫霍面深度突变.在可可西里和柴达木岩石圈地幔之间观测到北倾界面,这可能是可可西里岩石圈向北俯冲到柴达木地幔之下的证据.可可西里地块地壳内宽缓的负转换震相带是低速带的反映,其向北挤入到东昆仑山下发生挤压增厚,可能是东昆仑山隆升的原因;由于刚性柴达木岩石圈的阻挡,物质向东改向,则可能是该地区向东旋转的构造应力场产生的原因.本文研究结果不支持亚洲岩石圈地幔在东昆仑—柴达木交界处向南俯冲,据此,我们提出了新的东昆仑造山模式.     

Lateral variation in Moho depth around the southern Tanlu fault zone and its adjacent area

We estimated Moho depth beneath the southern Tanlu fault zone and its adjacent area using common-conversion-point(CCP)stacking of receiver fun-ctions,which were computed from teleseismic records of the CEArray.Our estimated Moho depth matches well with 2-D profiles derived from active-source deep seismic reflection surveys,suggesting that the calculated the Moho depth map is likely accurate beyond the 2-D profiles.Overall,the estimated Moho depth map showed a high spatial correlation with tectonic provinces,i.e.,Moho topographic boundaries are in good agreement with geological boundaries.Beneath the Dabie orogenic belt and the mountainous areas in southern Anhui Province,the Moho lies relatively deep,and there is an obvious difference in Moho depth between the two sides of this segment of the Tanlu fault.We further selected four depth profiles with dense instrumentation to show Moho depth changes across different tectonic blocks in the study area.We saw two step-like changes in Moho depth beneath the Xiangfan-Guangji and Gushi-Feizhong,which run parallel along the WNW-ESE direction and delineate the southern and northern bounds of the northern Dabie orogenic belt,which is likely the suture zone between the North China Block and South China Block.Crust beneath the northeast corner of the study area is significantly thinner than other areas,which is consistent with the crustal detachment model proposed for suturing between the North and South China blocks in the region east to the Tanlu fault.     

Crust and upper mantle structures beneath Northeast China from receiver function studies

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.     

Topography of the Seismic Velocity Discontinuities beneath the Chugoku and Shikoku Districts,Southwest Japan

The first P-arrival-time data from 513 local earthquakes were analyzed to study lateral variation of the depth to the Conrad and Moho discontinuities beneath the Chugoku and Shikoku districts, southwest Japan, as well as to determine earthquake hypocenters and P-wave station corrections. The depth to the discontinuity was estimated by minimizing the travel-time residuals of more than 8700 first P arrivals observed at 55 seismic stations. The Conrad and Moho discontinuities are located within depth ranges of 15–25 km and 30–40 km, respectively. The Moho is deeper under the mountain area than under the Seto Inland Sea area, and especially deep under the Pacific Coast of the Shikoku district and the mountain area in the Chugoku district. The depth variation of the Moho is quite similar to the Bouguer gravity anomaly distribution and the lateral variations of the P-wave velocity. The deep Moho under the southern Shikoku is located at the portion in which the continental Moho under the island arc meets the oceanic Moho that is the boundary interface between the oceanic crust and the Philippine Sea (PHS) plate dipping toward the back arc. Although there are high mountains in the northern and middle Shikoku, the Moho is not so deep because subduction of the PHS plate prevents the Moho from getting deep, while the Moho is deep due to isostatic balance under the mountain area in the Chugoku district. In addition, we indicated the possibility that the upper boundary of the oceanic crust just above the high-velocity PHS plate is in contact with the deep Moho under the western Chugoku. The contact of the Moho with the oceanic crust can explain the markedly negative gravity anomaly observed in the western Chugoku and the later phase that appears just after the first P arrival from local earthquakes.     

Using large dynamite shots to image the structure of the Moho from deep seismic reflection experiment between the Sichuan basin and Qinling orogen

The Qinling orogen was formed as a result of the collision between the North and South China blocks. The Qinling orogen represents the location at which the southern and northern parts of the Chinese mainland collided, and it's also the intersection of the Central China orogen and the north-south tectonic belt. There is evidence of strong deformation in this orogen, and it has had a long and complex geological history. We investigated the structure of the Moho in the southern Qinling orogen using large dynamite shot imaging techniques. By integrating the analysis of the single-shot and the move-out corrections profile, we determined the structure of the Moho beneath the northern Dabashan thrust belt and the southern Qinling orogen, including the mantle suture beneath Fenghuang mountain. The Moho is divided into two parts by the mantle suture zone beneath Fenghuang mountain:(1) from Ziyang to Hanyin, the north-dipping Moho is at about45–55 km depth and the depth increases rapidly; and(2)from Hanyin to Ningshan, the south-dipping Moho is at about 40–45 km depth and shallows slowly. The mantle suture is located beneath Fenghuang mountain, and the Moho overlaps at this location: the shallower Moho is connected to the northern part of China, and the deeper Moho is connected to the southern part. This may indicate that the lithosphere in the Sichuan basin subducts to the Qinling block and that the subduction frontier reaches at least as far as Fenghuang mountain.     

中国东北地区高分辨率地壳结构:远震接收函数

利用分布在东北地区的国家地震局台网、NECESSArray台网、吉林大学在长白山及其周边地区布设的26个临时台站总计259个台站接收到的16,070条高质量的P波接收函数,采用H-k和CCP(Common Conversion Point,共转换点)叠加成像方法,获得该区高分辨率的地壳结构.观测结果显示,东北地区莫霍界面深度和地表高程总体呈镜像关系;西部大兴安岭—太行山重力梯级带附近存在莫霍界面深度陡变带;中部的松辽盆地地区受晚中生代的地壳拉伸作用影响,地壳厚度较薄,北部的小兴安岭地区和南部的华北北缘造山带可能同样受拉伸运动影响,具有较小的地壳厚度;松辽盆地莫霍界面深度由西向东逐渐减小,推测这与太平洋板块俯冲作用有关;东部地区莫霍界面结构比较复杂,依兰—伊通断裂与敦化—密山断裂之间出现复杂震相,可能与该区存在地幔物质的底侵作用有关;长白山火山地区地壳厚度较大,对应较高的波速比,推测在该区地壳内存在岩浆囊.     

Crustal structure of Northwest Zagros (Kermanshah) and Central Iran (Yazd and Isfahan) using teleseismic Ps converted phases

Receiver functions are widely employed to detect P-to-S converted waves and are especially useful to image seismic discontinuities in the crust. In this study we used the P receiver function technique to investigate the velocity structure of the crust beneath the Northwest Zagros and Central Iran and map out the lateral variation of the Moho boundary within this area. Our dataset includes teleseismic data (M _b ≥ 5.5, epicentral distance from 30° to 95°) recorded at 12 three-component short-period stations of Kermanshah, Isfahan and Yazd telemetry seismic networks. Our results obtained from P receiver functions indicate clear Ps conversions at the Moho boundary. The Moho depths were firstly estimated from the delay time of the Moho converted phase relative to the direct P wave beneath each network. Then, we used the P receiver function inversion to find the properties of the Moho discontinuity such as depth and velocity contrast. Our results obtained from PRF are in good agreement with those obtained from the P receiver function modeling. We found an average Moho depth of about 42 km beneath the Northwest Zagros increasing toward the Sanandaj-Sirjan Metamorphic Zone and reaches 51 km, where two crusts (Zagros and Central Iran) are assumed to be superposed. The Moho depth decreases toward the Urmieh-Dokhtar Cenozoic volcanic belt and reaches 43 km beneath this area. We found a relatively flat Moho beneath the Central Iran where, the average crustal thickness is about 42 km. Our P receiver function modeling revealed a shear wave velocity of 3.6 km/s in the crust of Northwest Zagros and Central Iran increasing to 4.5 km/s beneath the Moho boundary. The average shear wave velocity in the crust of UDMA as SSZ is 3.6 km/s, which reaches to 4.0 km/s while in SSZ increases to 4.3 km/s beneath the Moho.     

2010年玉树7.1级地震震源区P和S波接收函数成像

利用玉树固定地震台站和7个流动宽频带地震台站远震P波和S波接收函数,对2010年4月14日发生M_S7.1级地震的玉树地震震源区下方地壳及上地幔顶部速度结构进行了成像.结果表明:研究区下方存在明显的双地壳结构;主震所在的金沙江缝合带可能错断了地壳和岩石圈;错断的地壳在印度板块向北的推挤下,发生了叠置,导致金沙江缝合带下方呈现双地壳结构;根据发生在金沙江缝合带上的历史地震进一步推断,印度板块对金沙江缝合带这种深大走滑断裂的向北推挤,为断裂带上大地震的发生提供了闭锁条件,这可能是巴颜喀拉地块周边地震孕育的一种典型的深部地球动力学模式;在玉树地震主震区所在深度附近,发现局部存在高速夹层,这种局部高速结构,可能是积累引力,孕育地震的一个重要原因.     

四川芦山7.0地震和汶川8.0地震震源区地壳岩石圈变形特征分析

地壳和岩石圈变形特征研究对于深入了解中强地震的深部孕震环境具有重要科学意义.本文联合P和S波远震接收函数偏移成像结果,对发生过芦山7.0地震和汶川8.0地震的龙门山断裂带及附近区域地壳和岩石圈结构进行分析.结果揭示出在青藏高原向四川盆地过渡的龙门山断裂带,Moho面和岩石圈底界面(LAB)呈现出强烈变形,特别是芦山地震和汶川地震震源区下方地壳出现了错断、下凹,岩石圈也呈现下凹变形特征.这种地壳及岩石圈变形所代表的高应力的积累可能是汶川和芦山地震发生的重要深部地球动力学背景.     

Subsidence analysis on the Sardinian margin and the central Tyrrhenian Basin: Thermal modelling and heat flow control; deep structure implications

The subsidence of the Tyrrhenian Basin is analysed based on numerous MCS reflection data and related seismic velocities, and calibrated using subsidence curves versus time at ODP Leg 107 sites 654 and 652 on the Sardinian margin. Two main phases of rifting are clearly defined for the margin-basin evolution: the first, beginning at 7 Ma, split the Sardinian-Calabrian margins, the second, beginning at 5 Ma formed the Central oceanic subbasins.Thermal models, based on thinned crust geometry from reflection-refraction data, are used which consider both vertical and horizontal conduction over time along a NW-SE transect across the entire Tyrrhenian Basin from Sardinia to Calabria. In general, there is a close agrrement between the predicted and the observed surface heat flow and the rate of subsidence over time. However, within the oceanic domain an initial subsidence must be chosen about 1000 m greater than observed on typical spreading centers, i.e., at about 3500 m as postulated in numerous back-arc basins.Thermal models, constrained by subsidence analysis and gravity observations, imply a large difference for the Moho depth beneath the Sardinian margin as determined after refraction seismic experiments. The upper mantle, hot but with high density, would be situated much higher than is postulated by seismic data and probably includes 7.3–7.4 km/s velocity layers previously interpreted as deep crustal layers.     

基于三重震相的青藏高原东缘岩石圈地幔波速结构

本文利用中国数字地震台网记录到的中国青海和缅甸弧发生的两次浅源地震的区域波形资料,在以Crust2.0改进AK135模型所构建的参考模型C2AK的基础上,通过三重震相波形拟合的方法,获得了青藏高原东部下方从莫霍面至上地幔顶部180 km深度范围内的P波和S波最佳拟合模型。最佳模型显示:松潘—甘孜地块（A和B剖面）下方的P波速度比C2AK模型高5%,而川滇地块（C剖面）下方上地幔顶部的P波速度要比参考模型低5%,且随深度逐渐增加,直至120 km处与C2AK模型值相同;松潘—甘孜地块下方的S波速度较C2AK模型要高3%。上述区域性速度结构差异表明,相对于松潘—甘孜地块,川滇地区的岩石圈地幔存在着更明显的挤出效应。      

藏北羌塘盆地中部莫霍面形态及其动力学成因

本文通过对羌塘盆地内49个临时宽频带地震观测台阵数据的接收函数分析,采用H-κ叠加和CCP 叠加成像两种方法,获得到了藏北羌塘中部莫霍面深度以及泊松比分布.作为羌塘盆地构造单元的南缘边界,班公湖-怒江缝合带下的Moho存在一个南深北浅、断距约10 km的台阶;把羌塘盆地分为两部分的羌塘中央隆起带下存在一个3 km的Moho台阶;北羌塘盆地下的Moho 平均深度约为60 km,而南羌塘约为63 km.羌塘高原下的近水平Moho结构可能是受到印度大陆北向俯冲作用下的青藏高原隆升过程中Moho再均衡所致或者与其构造演化有关.泊松比值具有明显的构造分区特征,如南羌塘下的泊松比平均为0.31,双湖缝合带下的泊松比接近正常值,为0.265,而北羌塘的泊松比平均为0.285.     

New constraints on lithospheric thickness of the Iranian plateau using converted waves

The study of mantle lithosphere plays a key role to reveal predominant tectonic setting process of a region. The current geological and tectonic setting of Iran is due to the ongoing continental–continental collision of the Arabian and Eurasian plates. We applied a combined P and S receiver function analysis to the teleseismic data of nine permanent broadband seismic stations of the International Institute of Earthquake Engineering and Seismology located in different tectonic zones of Iranian plateau. More than 4 years of data were used to estimate the thickness of the crust and mantle lithosphere. According to our results, the crust is 50 km thick beneath the Zagros fold and thrust belt (ZFTB). We found the maximum Moho depth of approximately 70 km under the Sanandaj-Sirjan zone (SSZ) indicating the overthrusting of the crust of Central Iran onto the Zagros crust along the main Zagros thrust (MZT). Below the northeasternmost part of the Urumieh–Dokhtar Magmatic Arc (UDMA) and Central Iran, the Moho becomes shallower and lies at 40 km depth. Towards northeast, beneath the Alborz zone, the crust is 55 km thick. Based on S receiver functions, we provided new insights into the thickness of the Arabian and Eurasian lithospheres. The location of the boundary between these plates was estimated to be beneath the SSZ, which is slightly shifted northeastward relative to the surficial expression of the MZT. Furthermore, the Arabian plate is characterized by the relatively thick lithosphere of about 130 km beneath the ZFTB reaching 150 km beneath the SSZ, where the thickest crust was also observed. This may imply that the shortening across the Zagros is accommodated by lithospheric thickening. In contrast, UDMA and Central Iran are recognized by the thin lithosphere of about 80–85 km. This thin lithosphere may be associated with the asthenospheric upwelling caused by either lithospheric delamination or Neo-Tethys slab detachment beneath the Zagros collision zone.     

鄂尔多斯西缘北段的地壳结构和块体间变形关系

本文对喜马拉雅计划二期部分台站的远震波形数据进行接收函数提取,利用接收函数共转换点叠加方法获得阿拉善地块、鄂尔多斯地块以及银川—河套盆地下方0~80 km深度的速度间断面结构.结果表明:鄂尔多斯地块成层性好,地壳厚度为38~42 km,康拉德界面为18~22 km,阿拉善地区的Moho面深度为38~45 km.河套盆地地壳厚度约52 km,银川断陷盆地和贺兰山下方的Moho面最深为~55 km.鄂尔多斯西缘构造边界下方Moho面变化明显,且黄河断裂为深大断裂直接切割莫霍界面.根据本文的间断面成像结果我们进一步确定阿拉善地块与鄂尔多斯地块分属不同的大地构造单元.与此同时,我们推测贺兰山以西70~80 km范围内和鄂尔多斯地块西缘北段存在地壳增厚变形的可能.     

Crustal structure beneath Qiangtang and Lhasa terrane from receiver function

To determine the crustal structure in central Tibet, we used teleseismic waveform data recorded by 18 stations in the INDEPTH-Ⅲ seismic array across the central Tibet from the central Lhasa terrane to the central Qiangtang terrane. The S-wave velocity structures beneath stations are determined by inverting the stacked radial receiver function using the GA method. The first order features in the receiver function are modeled. Our results show that the Moho in Qiangtang is about 8 km shallower than that in Lhasa terrane along the INDEPTH-Ⅲ profile. It maybe suggests the northward subduction of the Lhasa mantle lid beneath the Qiangtang terrane is affected by the India-Asia collision. We conclude that there exist low velocity zone in the middle crust across the northern Lhasa and Qiangtang terrane, which can be related to the high temperature upper mantle beneath that.     

利用接收函数频率特征研究莫霍面形态及应用

基于不同莫霍面模型的全波形理论地震图,计算了不同频率的接收函数,分析对比了不同形态莫霍面在不同频率上的接收函数变化特征。数值试验结果显示,当莫霍面的形态复杂时,高频接收函数上P?S转换波和多次波会出现多峰值特征。之后对不同形态莫霍面的模型在不同频率的接收函数进行了分类总结,据此判别实际观测资料所表征的莫霍面性质。以位于青藏高原东北缘的高台（GTA）地震台为例,分析了该台站不同频率的接收函数。结果表明,该台站下方莫霍面总体为遵循同一变化规律的速度过渡带,但在沿龙首山断裂方向附近速度变化不同于主要变化方式。基于此,通过对观测结构进行拟合构建了该台站下方地壳及莫霍面模型,并结合地质学和岩石学等方面的结果对这种莫霍面形成的原因进行了探讨,进而推断此种莫霍面是由于多种构造因素以及上地幔热物质上涌引起地区壳幔物质的分异与交换所导致。      

长白山-镜泊湖火山区地壳结构接收函数研究

利用71个远震的波形资料,用接收函数方法提取了布设在长白山—镜泊湖火山区的34个宽频带流动数字地震台站的接收函数,通过对接收函数反演,获得了台站下方的S波速度结构.研究结果表明,沈阳—敦化一线莫霍面深度32～33km,向西地壳厚度加厚,到长春附近地壳厚度约为36km.在天池火山口莫霍面深度为达38km,而镜泊湖火山口森林的莫霍面深度约为39km.总体看研究区的地壳厚度是南浅北深.长白山天池火山口附近地下10km左右有一明显的低速层存在;镜泊湖火山口森林附近30km也可能有低速体存在;研究发现莫霍面上S波速度梯度在火山口附近和远离火山口有明显区别.在火山口附近其莫霍面的S波速度梯度比非火山口地区的S波速度梯度明显小,说明火山口下与一般的地壳莫霍面结构有差别.研究发现沈阳—敦化一线两侧的莫霍面深度有较大变化,其位置与地表的敦化—密山断裂基本一致,说明敦化—密山断裂是研究区的一条非常重要的地质构造带.