冈底斯中东部底垫-俯冲转换带附近地壳上地幔结构远震P波层析成像研究

Crustal and Upper-mantle Structure of the Underplating-to-Subduction Transition Zone in Central-eastern Gangdese Revealed by Teleseismic P-wave Tomography

利用冈底斯中-东部197个宽频带天然地震台站记录到的数据和远震P波走时层析成像方法,获得了该区域的P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,但异常强度都没有前者大;在两个裂谷带之间的拉萨地块中-南部,地壳表现为强高速特征。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地区,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~100km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。此外,我们的层析成像结果还印证了冈底斯东南侧的上地幔低速异常根植于上地幔底部,我们认为该现象可能与巽他块体的顺时针旋转引起向东俯冲的缅甸弧向西后撤有关。

The P-wave velocity disturbance images beneath central-eastern Gangdese were obtained through teleseismic P-wave travel-time tomographic inversion for the data collected from 197portable broadband seismic stations.Our resultant images reveal a complicated seismic spatial pattern of the velocity structure occurring in the crust and upper mantle in the study region.Firstly,a strong low-velocity anomaly belt occurs beneath the Tethyan Himalaya north of the South Tibet Detachment（STD）.But the north of the strong low-velocity anomaly,far from the rifting zone,shows no obvious evidence to go through the Yarlung Zangbo Suture（YZS）.This conclusion is quite different from those by previous geologists.Lowvelocity anomaly occurs beneath the Yadong-Gulu（YGR）and the Comei-Sangri（CSR）rift systems,but intensities of the anomaly are lower than that beneath Tethyan Himalaya.Instead,strong high velocity anomaly was observed in the central-southern Lhasa Terrane bounded between the YGR and the CSR.The results show that the Crustal Channel Flow affecting the structural evolution of the crust in the QinghaiTibet Plateau is mainly distributed beneath the Tethyan Himalaya region,as well as in the Gangdese region to the north of the YZS,likely along the rifting belt.Secondly,the high-velocity anomaly of the upper mantle to west of the study area,interpreted as a manifest of the Indian lithospheric mantle,extends about 100km or more to the north of the YZS,while the anomaly to east of the study area does not go through the YZS but stagnates under High Himalaya,~100km south of the YZS.This is consistent with some previous results which show a systematic west-to-east change of the subduction angle of Indian lithospheric mantle near the study region.In addition,the tomographic result suggests that deep-seated low-velocity anomalies to southeast of Gangdese root into the base of upper mantle.This phenomenon may be related to the westward retreat of the eastward subducted Burmese arc during the clockwise rotation of the Sunda block.