Crustal thickness variations in the Zagros continental collision zone (Iran) from joint inversion of receiver functions and surface wave dispersion

Variations in crustal thickness in the Zagros determined by joint inversion of P wave receiver functions (RFs) and Rayleigh wave group and phase velocity dispersion. The time domain iterative deconvolution procedure was employed to compute RFs from teleseismic recordings at seven broadband stations of INSN network. Rayleigh wave phase velocity dispersion curves were estimated employing two-station method. Fundamental mode Rayleigh wave group velocities for each station is taken from a regional scale surface wave tomographic imaging. The main variations in crustal thickness that we observe are between stations located in the Zagros fold and thrust belt with those located in the Sanandaj–Sirjan zone (SSZ) and Urumieh–Dokhtar magmatic assemblage (UDMA). Our results indicate that the average crustal thickness beneath the Zagros Mountain Range varies from ～46 km in Western and Central Zagros beneath SHGR and GHIR up to ～50 km beneath BNDS located in easternmost of the Zagros. Toward NE, we observe an increase in Moho depth where it reaches ～58 km beneath SNGE located in the SSZ. Average crustal thickness also varies beneath the UDMA from ～50 km in western parts below ASAO to ～58 in central parts below NASN. The observed variation along the SSZ and UDMA may be associated to ongoing slab steepening or break off in the NW Zagros, comparing under thrusting of the Arabian plate beneath Central Zagros. The results show that in Central Iran, the crustal thickness decrease again to ～47 km below KRBR. There is not a significant crustal thickness difference along the Zagros fold and thrust belt. We found the same crystalline crust of ～34 km thick beneath the different parts of the Zagros fold and thrust belt. The similarity of crustal structure suggests that the crust of the Zagros fold and thrust belt was uniform before subsidence and deposition of the sediments. Our results confirm that the shortening of the western and eastern parts of the Zagros basement is small and has only started recently.     

Crustal thickness in Vrancea area,Romania from S to P converted waves

Crustal thickness (CT) in Vrancea region (Romania) and adjacent area is investigated using 1294 S to P converted waves from the Moho discontinuity. A total of 269 local earthquakes in the depth range 99.8 to 171.1 km and recorded by 76 permanent and 46 temporary stations of the Romanian Seismological Network are used. Time difference between the converted wave and the direct P phase is corrected to a first order for epicentral distance and for the errors in focal depth, being finally inverted to CT. Greatest values for the Moho depth are observed for stations located in the Carpathians molasse foredeep and smaller values are observed in the Southern part of the Moesian Platform, for stations in the eastern part of Moldavian (East-European) Platform and in Dobrogea area, close to the Black Sea shoreline. In Vrancea epicentral area, an important CT variation is observed, from 42 km at MLR and 41.8 km at SIR, stations placed in the south-western part of the epicentral area, to 30.9 km at VRI, located above north-eastern part of the seismogenic volume. Stations CVO and OZU, placed in Transylvanian Basin in the proximity of the epicentral area, have CT values of 32.1 and 24.1 km, respectively. The results seem to support that a mantle delamination process is responsible for Vrancea intermediate depth seismicity.     

Upper-mantle S-velocity structure across the Zagros collision zone resolved by nonlinear teleseismic tomography

Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies into the upper-mantle is suppressed by travel-time corrections calculated based on a 3-D crustal model for the study area. Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between 70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust.     

利用虚震源反射成像法研究江南造山带东段上地壳结构

江南造山带是华南地区扬子地块与华夏地块碰撞的产物,其地壳构造记录了两地块的碰撞过程,研究江南造山带的地壳构造有助于重建扬子地块与华夏地块的碰撞过程。本研究在江南造山带上布设了两条流动地震台阵,利用虚震源反射法提取其所记录的远震事件初至P波在地表的反射波（PPdp）波形,重构了研究区内两条测线下方的上地壳结构。结果显示:江绍断裂两侧上地壳沉积层的厚度变化明显,推断该断裂是扬子地块与华夏地块的东边界;相较华夏地块,江南造山带与扬子地块的层位连续性更强,符合江南造山带先与扬子地块合为整体后再与华夏地块碰撞的多期构造过程及其对应产生的亲扬子地块属性;江绍断裂西北侧的地层不整合以及赣东北断裂区域的断陷构造,可为了解古华夏洋向扬子陆块俯冲及碰撞和随后的构造运动过程提供参考依据。      

远震P波接收函数揭示的张家口(怀来)-中蒙边境(巴音温多尔)剖面地壳厚度与泊松比

本文使用时间域迭代反褶积算法,从张家口(怀来)-巴音温多尔一线布设的41个宽频地震台站、1年期间记录的连续三分量数据中提取到高质量的P波接收函数1844个.用H-κ扫描方法获得了测线下方Moho深度与波速比值(VP/VS)进而计算出泊松比,用共转换点(CCP)叠加方法获得了沿测线Moho面起伏图像.结果显示:(1)测线下方Moho深度平均40 km,仅各块体边界处出现Moho深度小尺度急剧变化.整体上,Moho面产状相对于索伦缝合带大致对称,在缝合带南侧的温都尔庙带和白乃庙带下方呈南倾趋势,在缝合带北侧的宝力道带、贺根山杂岩带下方呈北倾趋势.(2)华北克拉通北缘泊松比总体较高,兴蒙造山带整体较低;各次级块体内部泊松比分布相对稳定,块体分界带附近往往存在泊松比值的升降扰动.(3)整条测线地壳厚度和泊松比之间存在弱的负相关关系,表明存在构造作用的影响.(4)整条测线泊松比呈现以索伦缝合带南缘为对称轴的非线性分布.本文所获得的地壳上地幔结构以及泊松比分布特征,支持古亚洲(索伦)洋(南北)双向俯冲,最终沿林西断裂闭合的动力学模式.     

Heterogeneities in the mantle transition zone from observations of P-to-SV converted waves

The method of detection of P-to-SV converted waves from distant earthquakes (Vinnik, 1977) was applied to sets of long-period records from a few seismograph stations in Europe and the west of North America. The results obtained suggest that the converted phases related to the major boundaries in the mantle can be reliably detected and the depths of conversion evaluated with an accuracy of a few kilometres. The depth of the olivine-spinel transition is close to 400 km and no difference between the estimates for the north of Europe and the west of North America is found. The depth of the boundary separating the upper and lower mantle is close to 640 km, which is 30 km less than in the recent Earth-reference models. Fine S velocity stratification of this transition changes laterally from a high-gradient layer 50 km thick, terminated at the bottom by a sharp discontinuity, to a gradient layer 100 km or more thick without the discontinuity. A striking anomaly of the mantle transition zone is found in the Rio Grande rift area where a well pronounced boundary is found at 510 km depth.     

The effective elastic thickness of the lithosphere in the collision zone between Arabia and Eurasia in Iran

The effective elastic thickness, T_e, has been calculated in the collision zone between Arabia and Eurasia in Iran from the wavelet coherence. The wavelet coherence is calculated from Bouguer anomalies and topography data using the isotropic fan wavelet method, and gives T_e values between 14.2 and 62.2 km. The lower value is found in the Central Iranian Blocks and the East Iranian Belt which are bounded by several large strike-slip faults with lithospheric origin. The higher value occurs in the east of the South Caspian Sea Basin. The resulting T_e map shows positive and negative correlation with shear wave velocity and surface heat flow, respectively. A comparison between the seismogenic thickness (T_s) and T_e in Iran suggests that T_e > T_s. Results of the load ratio in Iran indicate that in most of the study area surface loads are much more prevalent than subsurface loads, except in the Central Iranian Blocks and NW of Iran. Intermediate to low T_e values in Iran were inherited from multiple rifting and orogenic activities from Late Precambrian (∼650 Ma) to present day which are not only reflected in thin and warm lithosphere but also an increasing seismicity rate.     

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.     

有限元数值模拟龙门山断裂带地震循环的地壳变形演化

现今地壳变形数据显示横跨龙门山断裂带的地壳缩短速率低于3 mm·a^-1,如此小的地壳缩短速率与龙门山断裂带附近的长期地质造山（平均高程约4.5 km）形成强烈对比.我们构建并使用了一个二维平面应变黏弹塑性有限元模型来模拟龙门山断裂带的地震循环位移变化,从而探讨了短期变形与长期变形之间的关系.模型模拟了地震循环的各个阶段（震间加载期、同震瞬间和震后黏性松弛调整期）以及多个地震循环（万年尺度）的地表变形,揭示了变形在地震循环中是如何累积、释放、调整以及最终形成永久变形导致了造山.模拟结果显示,岩石圈流变结构以及断层几何形态均对地震循环的地表位移变化有着显著的影响.经过多个地震循环,青藏高原东缘整体产生水平缩短与增厚抬升,而四川盆地基本保持稳定,区域的水平缩短主要由断层位错及青藏东缘的缩短抬升来调解,造成了青藏东部与川西盆地的差异抬升.研究结果将地震循环时间尺度的短期变形与长期地质造山联系起来,帮助我们理解青藏高原东部的隆升机制.

     

Crustal electrical conductivity of the Indian continental subduction zone: New data from the profile in the Garhwal Himalaya

We present the results of studying the geoelectrical structure of the zone of continental subduction of the Indian lithospheric plate within the Gahrwal Himalaya. In the framework of the Russian–Indian project, the data of the broadband magnetotelluric soundings conducted by the Indian Institute of Technology Roorkee on the regional profile across the structures of the orogen were expanded, processed, and interpreted by the new program tools adapted for the measurements in the mountain conditions and for the presence of industrial noise. The constructed model of the deep electrical conductivity cross section for Garhwal revealed its two-dimensional (2D) features and more accurately delineated the location of the midcrustal conductor associated with the ramp structure of the detachment plane. The correlations with the regional distribution of the earthquake hypocenters and the seismotomographic images suggest a common, fluid-related nature of the seismic and geoelectrical anomalies in the crust of the Garhwal Tectonic Corridor and enabled the identification of the seismogenerating zones. Among the data of the expanded profile set of magnetotelluric and magnetovariational transfer functions, the response of a poorly explored deep conductive body is revealed. This object is located east of the profile and is probably associated with the activation of the ancient trans-Himalayan cratonic structures which prepares the segmentation of the Himalayan arc.     

郯庐断裂带中南段地壳厚度与泊松比研究

郯庐断裂带中南段与西太平洋板块俯冲以及华北克拉通破坏密切相关,研究其深部结构对于了解郯庐断裂带构造演化过程、强震的孕震机制具有重要意义.本文利用密集台阵波形资料,采用接收函数H-κ叠加方法获得郯庐断裂带中南段的地壳厚度和泊松比.结果显示,郯庐断裂带中南段的地壳厚度分布以庐江和新沂为分段点呈现出与地质构造相关的南北向分段特征：南段的断裂带西侧的地壳厚度比东侧厚约5 km,中段的断裂带两侧厚度差异减小至约3 km,而北段则表现为断裂带下方隆起特征.郯庐断裂带沿线的地壳泊松比较高,推测是地幔物质的热侵蚀和化学侵蚀所致.根据艾里地壳均衡理论,南段地壳厚度与地表高程相关,基本符合艾里地壳均衡模型;而中段两侧地表高程基本相同,两侧地壳厚度差异表明不符合艾里地壳均衡模型,可能与断裂带西侧的华北克拉通岩石圈比东侧扬子克拉通岩石圈具有更高密度有关,高密度岩石圈产生向下拖拽力导致莫霍面相对理论值偏深;北段的莫霍面也偏深,与中段断裂带西侧情况类似,该区域基本位于华北克拉通块体.在郯城地震破裂范围内,存在莫霍面的不平滑过渡、活动块体的边界带以及断裂的分叉与交汇这三类结构特征.在上述结构特征的区域应力更集中,从而孕育产生了1668年郯城8.5级地震.     

青藏高原东北缘地壳S波速度结构及其动力学含义--远震接收函数提供的证据

利用青海和甘肃地震台网2007-2009年记录的远震波形资料,提取多频段P波接收函数,反演得到了青藏高原东北缘及相邻地块下方0~100 km深度的地壳和上地幔S波速度结构.结果表明:(1)青藏高原东北缘的上、下地壳之间普遍存在一个S波速度低速层,其深度由南端的约35 km 向北变浅约为20 km,推测该低速层为一壳内滑脱层,表明东北缘地区的上地壳变形与下地壳解耦,从滑脱层的深度分布可以认为青藏高原东北缘的地壳缩短自南向北进行,现阶段以上地壳增厚为主;(2)昆仑-西秦岭造山带的下地壳厚度较北侧的祁连地块薄,一种推测是西秦岭造山带的下地壳抗变形能力更强,也可能这种差异在块体拼合前已经存在;(3)青藏高原东北缘及鄂尔多斯和阿拉善地块的下地壳S波速度随深度的增加而增加,这种正梯度增加的S波速度结构反映较高黏滞性的下地壳,推测青藏高原东北缘的地壳结构不利于下地壳流的发育.     

Deformational behaviour of continental lithosphere deduced from block rotations across the North Anatolian fault zone in Turkey

Theoretical considerations of lithosphere deformation across transform plate boundaries predict an expression in terms of 3istributed deformation. The magnitude of rotation is expected to diminish away from the fault zone in a way which depends on the length of the fault, the amount of displacement, and the ductility of the lithosphere. Palaeomagnetic studies across the North Anatolian transform fault zone, which separates the Eurasian Plate and Anatolian Block in northern Turkey, show that clockwise rotations predicted from the sense of dextral motion are indeed present and have attained finite rotations of up to 270° during the 5 Ma history of Neotectonic deformation. Such rotations are, however, confined to narrow ( 10 km wide) zones between system-bounding faults and appear to have resulted from rotation in ball-bearing fashion of equidimensional blocks a few kilometres in size. Outside of this zone only anticlockwise rotations are observed; these are unrelated to deformation across the fault zone and record regional anticlockwise rotation of Turkey which is complementing clockwise rotation of Greece and accompanying Neogene opening of the Aegean Sea. The observed behaviour of continental lithosphere satisfies no plausible value of power law behaviour. We therefore conclude that relative motion across this transform boundary occurs as a discrete zone of intense deformation within a brittle layer comprising the seismogenic upper crust. This is presumed to be detached from a continuum deformation response to shearing in the lower crust and mantle beneath.     

Detection of waves converted from P to SV in the mantle

A method is described for the detection of P to SV converted waves in the long-period P coda. The procedure involves axis rotation, transformation of records to a standard form and stacking of processed records from events of various epicentral distances. When applied to NORSAR records, the procedure detected converted phases corresponding to the boundaries in the 410–440- and 640–690-km depth ranges.     

Crustal structure along the coast of California from Rayleigh waves

The dispersion curve of Rayleigh waves is obtained for the path San Fernando Valley-Berkeley, using the single-station surface wave method. The crustal model obtained from dispersion data is in excellent agreement with seismic refraction results. Dispersion data enable the rejection of one of the models previously proposed on the basis of refraction data.     

Source characteristics of the 1992 Nicaragua tsunami earthquake inferred from teleseismic body waves

We analyzed the broadband body waves of the 1992 Nicaragua earthquake to determine the nature of rupture. The rupture propagation was represented by the distribution of point sources with moment-rate functions at 9 grid points with uniform spacing of 20 km along the fault strike. The moment-rate functions were then parameterized, and the parameters were determined with the least squares method with some constraints. The centroid times of the individual moment-rate functions indicate slow and smooth rupture propagation at a velocity of 1.5 km/s toward NW and 1.0 km/s toward SE. Including a small initial break which precedes the main rupture by about 10 s, we obtained a total source duration of 110 s. The total seismic moment isM _o =3.4×10²⁰ Nm, which is consistent with the value determined from long-period surface waves,M _o =3.7×10²⁰ Nm. The average rise time of dislocation is determined to be 10 s. The major moment release occurred along a fault length of 160 km. With the assumption of a fault widthW=50 km, we obtained the dislocationD=1.3 m. From andD the dislocation velocity isD=D/0.1 m/s, significantly smaller than the typical value for ordinary earthquakes. The stress drop =1.1 MPa is also less than the typical value for subduction zone earthquakes by a factor of 2–3. On the other hand, the apparent stress defined by 2E _s /M _o , where andE _s are respectively the rigidity and the seismic wave energy, is 0.037 MPa, more than an order of magnitude smaller than . The Nicaragua tsunami earthquake is characterized by the following three properties: 1) slow rupture propagation; 2) smooth rupture; 3) slow dislocation motion.     

深反射大炮揭示的青藏高原侧向碰撞带地壳骨架结构

青藏高原东南缘处于印度板块与欧亚板块碰撞的侧翼,揭示该地区的岩石圈结构有助于完整理解青藏高原碰撞造山的动力学过程,对构建大陆碰撞成矿理论框架至为关键.本研究对横过青藏高原侧向碰撞带的一条深反射地震剖面的15个大炮资料,进行了针对性静校正、去噪等处理和单次叠加成像,结果剖面显示了侧向碰撞带岩石圈结构的骨架特征：（1）双程走时（TWT）8~10 s的强反射（Tc）将地壳分为上、下两层;Tc可能是大型滑脱构造的拆离面,其存在使上地壳的变形与下地壳解耦;（2）Moho间断面反射（Tm）为3~4个同相轴的窄带反射波组,横向不连续,与深大断裂交汇处被错断,但断距不大;（3）在兰坪—思茅地块下方TWT21 s和扬子克拉通西缘下方TWT22~24 s存在相向倾斜的反射波组（TL）;以Tc、Tm和TL构成的骨架结构,定性地描绘出剖面下方岩石圈地幔以汇聚为主、地壳块体以侧向滑移为主和上地壳为薄皮逆冲或滑脱的分层动力学模式.该岩石圈变形样式明显不同于以正向碰撞挤压、地壳缩短垂向增厚为主的"冈底斯模式".     

利用接收函数研究太行山前断裂南部地区地壳厚度

利用太行山前断裂南部地区宽频带数字化地震台站的远震体波记录,采用频率域反褶积方法提取接收函数,由H—Kappa叠加方法反演得到各台站下方地壳厚度和泊松比。结果显示:1该区域地壳厚度分布为西部较厚,东部较薄,莫霍面呈小角度向西倾斜。西部长治盆地地壳厚度40.2 km,东部平原地区地壳较薄,厚度31.8 km,地壳变化落差约9 km;2在太行山与平原交接地带,特别是沿太行山弧形分布的主峰外缘,即一级夷平面和二级夷平面的分界线附近地壳厚度变化较快;3接收函数反演的地壳厚度与人工地震测深结果吻合,太行山隆起区地壳变厚,汤阴地堑地壳变薄,内黄隆起地壳变厚,表明深部莫霍面或上地幔的隆起、凹陷与地面起伏呈镜像对称;4区域内泊松比值变化不大,约0.23—0.27,表明该地区地下介质主要由中酸性岩组成,与较大范围的花岗岩分布有关,石英、长石含量高。     

南海中北部陆缘横波速度结构及其构造意义

纵横波联合勘探可以得到更多关于岩石圈层岩性、物性等介质属性方面的信息,有效提高地壳物质组成的约束性.在纵波速度结构模型的基础上,通过射线追踪和走时拟合对OBS2006-3地震剖面径向分量的转换震相进行了横波速度结构模拟.结果表明:沉积层1、沉积层2的横波速度分别为0.7~0.9 km/s和1.6~1.7 km/s,波速比由2.64~3.16变化到2.41左右,较高的波速比表明沉积层固结程度不高;中生代沉积层横波速度由浅部的2.6 km/s增大到3.0 km/s,波速比为1.74~1.80;地壳中波速比为1.74~1.82,表明该剖面地壳为陆壳性质;下地壳高速层的横波速度为4.0~4.2 km/s,纵横波速比为1.73~1.78,揭示高速层的物质组成为铁镁质的基性岩,推断其与海底扩张停止之后的岩浆底侵作用有关.     

利用共转换点叠加方法研究华北地区地壳结构

利用华北地震台阵L测线的35个台站记录的895个远震数据进行了接收函数的计算,并利用H-κ叠加方法得到华北克拉通西部陆块东侧和中部陆块内基岩台站下方的地壳结构.利用得到的基岩台站下方的地壳结构和通过波形模拟方法得到的渤海湾盆地的沉积层结构作为背景模型对测线进行共转换点(CCP)叠加成像.在渤海湾盆地,通过增大CCP叠加方法中的共转换点单元,使得由于沉积层基底反射信号过强而难以识别的Ps转换波信号得以增强,获得渤海湾盆地下方的地壳结构.由CCP叠加的结果可以看出:华北克拉通地壳西厚东薄.西部陆块东侧的吕梁山下方地壳出现了地壳的突然减薄,减薄幅度约3 km.在减薄处的西侧,地壳逐渐加深,由44 km左右加深至46 km左右,之后在减薄处突然减薄至43 km左右,推测该减薄处可能是西部和中部陆块在深部的分界点;中部造山带地区的重力梯度带的西侧,地壳较为平缓,厚约42 km左右,山西断陷盆地下的地壳出现了略微的上升,幅度为2 km左右.大同盆地东侧下方的台站出现了Moho面的不连续,可能是地幔物质上涌的通道.在太行山前缘的重力梯度带内地壳迅速变浅至33 km左右,渤海湾盆地内地壳厚约32 km左右,冀中坳陷带下方地壳变浅,最浅可达29 km.沉积层基底的深度与地壳厚度呈负相关关系,可能与渤海湾盆地受到的拉张剪切作用力相关.