Differential patterns of SH and P wave velocity structures in the transition zone beneath northwestern Tibet
Differential patterns of SH and P wave velocity structures in the transition zone beneath northwestern Tibet
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藏西北过渡带SH、P波速度结构的差异模式
DOI：
10.1007/s11430-011-4228-8
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发表时间：
2011-07
影响因子：
5.7
通讯作者：
Zeng RongSheng
中科院分区：
文献类型：
--
作者：
Zhang RuiQing;Wu QingJu;Li YongHua;Zeng RongSheng
利用三重波形模拟方法研究了西藏西北部上地幔过渡带的SH(VS)和P(Vp)波速结构。提出了震源深度移出叠加，以增强三重相的识别，并可用于测试我们数据的一致性。我们的结果表明，VS和Vp结构是去相关的，并且在660公里的不连续面上发生了大的VS跳跃，在其上方有一个小的VS梯度。相反，Vp模型的特征是在不连续面上具有相对较小的对比度，并伴随着TZ中的高Vp梯度。在两个模型中，660公里长的不连续面似乎都没有明显的深度异常。西藏西北部TZ的地震构造与最近研究的羌塘中部和拉萨西部地体下的地震构造相似。以印度下TZ构造为参照物，Vs正常，Vp略高，表明西藏西北部的Vp/Vs值较高。结合实验结果和矿物研究的资料，我们认为Vp和Vs的差异异常可以归因于化学不均一性，如较低的TZ中Al含量的增加。考虑到西藏的构造演化，这种化学不均质性可能与特提斯洋板的俯冲或拆离有关。
Triplicate waveform modeling is used to resolve SH (Vs) and P (Vp) wave velocity structures in the upper mantle transition zone (TZ) beneath northwestern (NW) Tibet. Focal depth move out stacking is proposed to enhance the identification of triplicate phases, and can be used to test consistency of our data. Our results show that theVs andVp structures are decorrelated, and that a largeVs jump occurred across the 660-km discontinuity, with a smallVs gradient above it. Conversely, theVp model is characterized by a relatively small contrast across the discontinuity, accompanied by a highVp gradient in the TZ. There seem no significant depth anomalies of the 660-km discontinuity in both models. The seismic structures in TZ beneath NW Tibet are similar to recent studies beneath the central Qiangtang and western Lhasa terrains. Taking the lower TZ structures under India as references,Vs is normal butVp appears slightly high, and thus a high ratio ofVp/Vs was indicated beneath NW Tibet. Combined results with experiment information from mineral studies, we suggest that the differential anomalies ofVp andVs can be attributed to a chemical heterogeneity, such as increased Al content in the lower TZ. Considering the tectonic evolution of Tibet, the chemical heterogeneity may be associated with subduction or detachment of the Tethys oceanic slab.
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