Unraveling the Dynamic Processes beneath the Northern Tibetan Plateau: Paleozoic Collision and Cenozoic Destruction of Mantle Lithosphere and Plateau Uplift

我们提议在青藏高原北部开展中德合作地震实验，以解决大陆岩石圈的改变和破坏过程以及造山带下岩石圈动力学控制的基本问题。与西藏其他地区相比，藏北地区具有新生代火山活动广泛、锡波传播差、Pn波速度低、浅地幔地震速度低、地震各向异性强的特点。提出了两个端元模型来解释青藏高原北部的这些观测结果：(1)印度和欧亚地幔岩石圈的俯冲、堆积或纯剪切增厚；(2)欧亚地幔岩石圈的拆离或对流迁移。另一个问题涉及到古生代特提斯洋俯冲的极性重建，这可以通过研究古生代缝合线的深层结构来解决。西藏以往实验的分辨率不足以解决这些问题，估计的地震速度和岩石圈厚度仍然存在很大差异。在这里，我们计划在西藏东北部部署和分析两个线性无源地震台阵：(1)可可西里和羌塘北部的NW-SE宽带台阵，由50个台站组成，台距10公里；(2)跨越金沙江缝合带的密集的N-S台阵，由300个短周期台站组成，台距500m。宽带台阵将应用各种地震方法，包括接收函数、远程地震体波层析成像、利用环境噪声和地震的面波层析成像和横波分裂，以探测地壳和地幔岩石圈的地震结构，并确定岩石圈的厚度。我们将应用和开发利用短周期波形相干性的方法，例如，利用远震连续波场和不同转换相的叠加互相关获得金沙江缝合带的详细图像。

We propose a collaborative Sino-German seismic experiment in the northern Tibetan plateau in order to address fundamental questions on the processes that modify and destroy continental lithosphere and control lithospheric dynamics beneath orogenic belts. Northern Tibet is characterized, with respect to the rest of Tibet, by the widespread Cenozoic volcanism, poor Sn wave propagation and low Pn wave velocity, low seismic velocities in the shallow mantle and strong seismic anisotropy. Two end member models were proposed to explain the these observations beneath the northern Tibetan plateau involve (1) underthrusting, stacking or pure-shear thickening of Indian and Eurasian mantle lithosphere and (2) delamination or convective removal of Eurasian mantle lithosphere. A further question relates to the reconstruction of the polarity of Paleozoic Tethys ocean subduction, which can be resolved from examining the deep structure of Paleozoic sutures. The resolution of previous experiments in Tibet was not sufficient to resolve theses questions and large discrepancies remain in the estimated seismic velocities and thickness of the lithosphere. Here, we plan to deploy and analyse two linear passive-source seismic arrays in northeastern Tibet: (1) a NW-SE broadband array in Hoh Xil and northern Qiangtang, consisting of 50 stations with a station spacing of 10 km; and (2) a dense N-S array across the Jinsha River Suture, comprising 300 short-period stations with a station spacing of 500 m. A variety of seismic methods will be applied to the broadband array, including receiver functions, teleseismic body wave tomography, surface wave tomography using ambient noise and earthquakes and shear wave splitting to explore seismic structures in the crust and mantle lithosphere and to determine the thickness of the lithosphere. We will apply and develop methodologies that exploit the coherence of waveforms at short period, for example to obtain detailed images of the Jinsha River Suture using stacked cross-correlations of the continuous wavefield and distinct converted phases from teleseismic earthquakes.