【摘要】 利用连续地震背景噪声记录和互相关技术获得瑞利面波格林函数,进而反演获得了青藏高原东部和周边地区的地壳三维速度结构.地震数据源于北京大学宽频带流动观测地震台阵,国家数字测震台网数据备分中心提供的部分固定台站的连续记录及INDEPTH IV宽频带流动观测地震台阵.首先对观测数据进行处理和分析取得所有可能台站对的面波经验格林函数和瑞利波相速度频散曲线,反演得到了观测台阵下方周期从6~60s的瑞利波相速度异常分布图像.并且进一步反演获得研究区域三维剪切波速度结构和莫霍面深度分布.短周期(6~14s)相速度异常分布与地表地质构造特征吻合较好,在青藏高原和四川盆地之间存在一个明显的南北向转换带.而本文最重要的结果是周期大于25s的相速度异常分布图像显示,以昆仑断裂带为界,柴达木盆地和祁连山脉地区呈现与青藏高原截然不同的中地壳速度结构,反而与青藏高原东缘地区和川滇菱形块体速度结构相似.反演获得的剪切波速度在27.5~45km深度的切片也明显地揭示:青藏高原的松潘—甘孜地块和羌塘地块呈现均一的低速层;然而,柴达木盆地和祁连山脉地区则呈现较强的横向不均匀性,尤其是柴达木盆地的高速异常和四川盆地的高速异常相对应.这些结果为前人提出的青藏高原东北向台阶式增长模式提供了重要的地震学观测证据.与全球一维平均速度模型(AK135)相比较发现,本文测量和反演获得的研究区域内平均相速度和剪切波速度都比AK135模型慢很多,尤其是青藏高原的中地壳(25~40km)剪切波速度显著低于全球平均速度模型.进一步的层析成像反演证实松潘—甘孜和羌塘地块中地壳(27.5~45km)呈现大范围均一的低速层,为青藏高原可能存在大规模中下地壳"层流"提供地震学观测证据.在祁连山脉的27.5~45km深度观测到的明显低速异常体可能对应于该造山带下地幔岩浆活动导致的底侵作用,表明引起该地区地壳增厚的主要机制可能是来自地幔岩浆的底侵作用.更多还原

【Abstract】 The crust structure of the Tibet Plateau is still controversial.Whether there is a partialmelting layer in the anomalously thick crust is critical to understand the dynamics and evolution history of the formation and on going process.We address the issue with robustly inverted surface wave and shear wave velocity models from the west China dense array data.Ambient noise tomography was applied to obtain the crust and upper mantle velocity models of the Tibetan Plateau.The method is based on extracting Rayleigh wave phase and group velocity dispersion curve from continuous waveform cross-correlation.Our data comes from the Peking University seismic arrays,INDEPTH IV passive seismic array,and four Chinese Provincial seismic networks.The usable data was collected from 05,2007 to 12,2009.The long time span assures high signal to noise ratio when the cross-correlation is performed.We extract the empirical Green′s functions from the cross-correlation and perform a surface wave tomography inversion with the cross-correlation functions.With the phase and group velocity models from the previous step,a 3Dshear wave velocity model is obtained.Our surface phase/group velocity models and inverted shear velocity model show strong heterogeneity and complexity over the whole studied region.Surface geological features are accurately depicted by the short period surface waves(6sto 14 s,both phase and group velocity model).The Qaidam basin and Sichuan basin are well correlated to low velocity anomalies due to the thick sediments over these regions.Mountains,like Qilian,Kunlun,and Longmenshan are marked by high velocity anomalies.We also observe a continuous transition zone in between the Tibet plateau and Sichuan Basin at short period.For Rayleigh wave tomography models longer than 25 s,there are significant differences between Qaidam-Qilian region and the plateau south of Kunlun fault zone.The newly formed plateau area,i.e.Qaidam-Qilian region shares similar velocity structure signatures with the eastern boundary of the Tibetan Plateau and Sichuan basin.Both our phase velocity measurements and inverted shear wave velocity models show that the Tibetan plateau has a large-scale uniform low velocity layer in the middle crust(25~40km in depth).The velocity at this depth of the region is much slower than the global average velocity of AK135.It is also shows distinct deviation from the eastern boundary of the Tibetan Plateau.The low velocity layer supports the channel flow model that explains the eastward material transportation of Tibet.From the intermediate period velocity maps,we observe a low velocity zone at Qilian Mountain region and it might be related to the elevated temperature in the middle and lower crust of this region.The variation of the temperature at this depth might correlate to the underplating of magma from the local mantle upwelling.These observations suggest that the on-going crustal thickening at Qilian Mountain area is possiblly due to the underplating process.Our detailed structure of Tibetan Plateau crust is critical to understand the on-going plateau formation process.The Rayleigh wave group/phase velocity models and 3Dshear wave velocity model indicate there is a large-scale uniform low velocity within the plateau.The low velocity layer could explain the eastward material transportation of Tibet.The newly formed QaidamQilian region,as a new plateau shows distinct velocity signatures in our velocity models.And the low velocity zone below Qilian Mountain is possibly an indication of the ongoing magma underplating process in that region.更多还原