藏南地壳和上地幔结构的多剖面、多震相联合层析成像研究

发布时间：2018-10-12 13:37

【摘要】：利用冈底斯中东部197个宽频带天然地震台站记录到的远震P波(直达P波、PKPab、PKiKP)和远震S(直达S波、sS、ScS和SKKS)震相走时数据和远震走时层析成像方法,获得了该区域的P波和S波速度扰动图像。层析成像结果显示研究区地壳和上地幔地震波速度结构存在着复杂的空间变化。首先,在藏南拆离系断层(STD)以北的特提斯喜马拉雅地壳中存在着较强的低速异常,但是该低速异常的北端在远离裂谷带的地方并没有明显越过雅鲁藏布江缝合线(YZS),这与前人的观测结果略有不同;在亚东-古露(YGR)和措美-桑日(CSR)裂谷带的下方存在低速异常,但异常强度和延伸深度都没有前者大;在两个裂谷带之间的拉萨地块中-南部,壳内存在一个强的高速异常体。这些结果表明,影响青藏高原地壳构造演化的"地壳通道流(Crustal Channel Flow)"在藏南主要分布在特提斯喜马拉雅下面,在雅鲁藏布江缝合线以北的冈底斯地区,可能主要局限于沿裂谷带分布。其次,被解释为印度岩石圈地幔的上地幔高速异常,在研究区西部,抵达了雅鲁藏布江缝合线以北100km或更远的地方,而在研究区东部,并没有越过雅鲁藏布江缝合线,而是停留在缝合线以南～100 km的高喜马拉雅下方,印证了前人给出的印度板块俯冲角度在研究区附近存在东西向变化的层析成像结果(如Li et al.,2008)。此外,我们的层析成像结果还印证了深部的低速异常根值于冈底斯东南侧的上地幔底部(如Ren et al.,2008),不过我们认为该现象可能与由于巽他块体的顺时针旋转引起向东俯冲的缅甸岩石圈板片向西后撤有关(参见Russo,2012)。为了更好地理解远震层析成像方法给出的藏南地震波速度结构的复杂性,我们同时还开展了 SKS/SKKS横波分裂测量研究,测量结果证实了藏南下面地壳和上地幔结构在91.5° E附近存在着巨大的东西向变化。在91.5° E以西地区,前人接收函数资料显示印度板块前沿或已经底垫到了嘉黎断裂附近(Shietal.,2016),在这些下面可能存在印度板块的地区未观测到明显的横波分裂现象(另见Huang et al.,2000;Chen WP et al.,2010),而在91.5° E以东地区,前人接收函数资料显示印度岩石圈在雅鲁藏布江缝合线以南就已经发生了陡深俯冲(Shi et al.,2016),在俯冲板块北侧的冈底斯下面存在着软流圈地幔楔(Shi et al.,2015),在软流圈可能发生垂直流动的地方,横波分裂现象不明显(另见Fu et al.,2008),而在软流圈可能发生水平流动的地方,观测到了明显的横波分裂现象,正是由于软流圈在地幔楔中流动方向的复杂改变,导致我们观测到各向异性方向出现的复杂变化以及远震层析成像结果所给出的复杂图像。
[Abstract]:Using the phase travel time data of P wave (direct P wave, PKPab,PKiKP) and S (direct S wave, sS,ScS and SKKS) recorded by 197 wide band natural seismostations in the central and eastern part of Gangdes, and using the tomography method of remote seismic travel time, The velocity perturbation images of P-wave and S-wave in this region are obtained. The seismic wave velocity structure of the crust and upper mantle in the studied area is characterized by complex spatial variations. First of all, there is a strong low velocity anomaly in the Tethys Himalayan crust north of the (STD) fault in the southern Tibetan detachment system. However, the northern end of the low-velocity anomaly did not obviously cross the (YZS), suture line of the Yarlung Zangbo River far from the rift zone, which is slightly different from the previous observation results, and there is a low-velocity anomaly under the Yadong-Gulu (YGR) and the (CSR) rift zone of Cuomei Sangri. However, the anomalous intensity and extension depth are not as great as the former, and there is a strong high velocity anomaly in the crust in the middle to the south of Lhasa block between the two rifts. These results indicate that the crustal channel flow (Crustal Channel Flow), which affects the crustal tectonic evolution of the Qinghai-Tibet Plateau, is mainly distributed under the Tethys Himalayas and north of the Yarlung Zangbo River suture in the Gangdis region in the south of Tibet. It may be mainly confined to the distribution along the rift zone. Second, the upper mantle velocity anomaly of the Indian lithospheric mantle was interpreted as reaching 100km or further north of the Brahmaputra suture line in the western part of the study area, while in the eastern part of the study area, it did not cross the Brahmaputra suture line. Instead, it stays below the high Himalayas from the south of the suture to 100 km, which confirms the previous computed tomography results (such as Li et al.,2008) showing that the Indian plate subduction angle varies from east to west near the study area. In addition, Our tomography results also confirm that the low velocity anomalies in the deep depth are located at the bottom of the upper mantle (such as Ren et al.,2008) on the southeast side of Gundis, but we believe that this phenomenon may be related to the eastward subduction due to the clockwise rotation of the Sunda block. Associated with the westward retreat of Myanmar lithospheric plates (see Russo,2012). In order to better understand the complexity of seismic velocity structure in south Tibet, we have also carried out the research of SKS/SKKS shear wave splitting measurement. The results show that the structure of the lower crust and upper mantle of southern Tibet has a great east-west change near 91.5 掳E. To the west of 91.5 掳E, Previous received function data show that the Indian plate front or has been underlaid near the Jiali fault (Shietal.,2016). No apparent S-wave splitting was observed in the areas below which there may be an Indian plate (see also Huang et al.,2000;Chen WP et al.,2010), but to the east of 91.5 掳E. Previous received function data show that the Indian lithosphere has already undergone a steep and deep subduction (Shi et al.,2016) south of the Yarlung Zangbo River suture line, and that there is a submersible mantle wedge (Shi et al.,2015) beneath the northern side of the subduction plate, and that there may be a asthenosphere in the asthenosphere. Where vertical flow occurs, The phenomenon of shear wave splitting is not obvious (see also Fu et al.,2008), but where horizontal flow may occur in the asthenosphere, the obvious shear wave splitting phenomenon is observed, which is due to the complex change of the direction of the flow of the asthenosphere in the mantle wedge. The complex changes in anisotropic direction and the complex images obtained from remote seismic tomography are observed.
【学位授予单位】：中国地质科学院
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P315.2

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