基于远震体波全波形反演的青藏高原-秦岭构造转换区岩石圈结构的高分辨率成像研究

Numerous geophysical studies have revealed pervasive low velocity zone with high electrical conductivities and anomalously low shear wave velocities in the crust-mantle of eastern margin of the Tibetan Plateau. Experimental petrology research has shown that these low velocity materials are mechanically weak and may cause the ductile flow. Because of the convergence between India and Eurasia and the strong crustal obstacle of rigid Sichuan Basin in the east, some geoscientists have speculated that these hot and soft crust-mantle materials may escape from beneath northeastern Tibet and be extruded toward the northeast through the Qinling Belt in a deep crustal flow or a mantle flow pattern. However, due to the limitations of data acquisition conditions in this area and shortcomings of the traditional imaging techniques, there are still significant differences between many inversion results in the Tibet-Qinling Transition Zone, which leads to the existence of channel for the extruded material beneath the Qinling Belt remaining controversial. At this point, the more deterministic geological interpretations of the Tibet-Qinling Transition Zone will require 3D images of density and velocity crust-mantle structures with a higher resolution than in current images. In order to obtain finely resolved tomographic images, we perform Full waveform inversion (FWI) on the broad-band teleseismic P waves records collected by dense temporary transects and adjacent seismological stations deployed across the Tibet-Qinling Transition Zone in northeastern Tibet. We use a hybrid method which couples a global wave propagation method in a 1D Earth model to a 3D spectral-element method in a regional domain to model broad-band teleseismic wavefields. By using this hybrid method, the sensitivity kernels of the least square waveform misfit function with respect to elastic and density perturbations in the regional domain are computed with the adjoint state method. In order to obtain the regional velocity and density structural model with high resolution, these waveform sensitivity kernels are used in an iterative full waveform inversion algorithm to invert broad-band waveform data recorded by stations in this region. The movement patterns of the materials in the crust and mantle and geological evolution model of the northeastern Tibetan Plateau are studied based on the obtained FWI structural model and previous studies.

大量地球物理研究表明青藏高原东北缘壳幔中存在一明显的低速异常体。岩石力学实验表明，该低速体的软弱物质可能会发生塑性流动。因为同时受到北部欧亚大陆的挤压和东部刚性四川盆地的阻挡，人们推测青藏高原东北缘下方软弱物质可能以地壳流或地幔流形式朝东北沿秦岭造山带挤出。由于数据覆盖和传统成像技术的限制，对该边界区域的诸多成像结果存在明显差异，导致秦岭造山带下方软弱物质挤出通道的存在性仍充满争议。更具确定性的地质解释需要更高分辨率的壳幔结构模型给予支持。因此本研究利用在青藏高原-秦岭构造转换区域部署的临时地震阵列及临近台站收集的宽带远震体波记录进行区域全波形反演成像。研究采用全局一维、局部三维的高精度混合方法来正演宽带远震波场，利用伴随方法计算局部区域内的参数敏感核，对宽带波形数据进行波形反演得到区域内介质结构的高分辨率模型。通过所得结构模型，综合前人研究来探讨该区域的壳幔物质运移模式和地质演化模型。

作为青藏高原的东北向扩展前缘，前人积累的地震学研究成果表明以地壳缩短增厚、陆内造山为主要特征的青藏高原东北缘壳幔中存在一明显的低速异常体。该低速软弱物质的侧向塑性移动受到北部欧亚大陆挤压和东部刚性四川盆地阻挡，人们推测其可能以地壳流或地幔流形式朝东北沿秦岭造山带挤出。但由于早先地震数据有限和分布不均，特别是经典的射线基成像方法的固有局限性和较低分辨率，对该边界区域的诸多成像结果间存在明显差异 ，导致秦岭造山带下方软弱物质挤出通道的存在性确定和精确地质解释仍存有大量争议。更具确定性的地质解释需更高分辨率的结构模型给予支持。因此，本研究采用先进的高分辨全波形反演方法结合近年来密集流动观测技术的进步，以获得青藏高原-秦岭构造转换区域岩石圈精细结构特征及深部构造信息，对人们发展青藏高原扩展动力学具有关键意义。本研究利用围绕秦岭造山带部署的高密度地震观测台阵收集的高品质宽频带远震波形记录，基于新发展的高精度混合方法和伴随梯度计算方法，最大化利用宽频带波场信息提升反演分辨率，在青藏高原东北缘区域首次开展了理论上具备最高分辨率的远震全波形反演研究以得到高分辨率岩石圈速度模型。基于反演所得结构模型来探讨该区域的壳幔物质运移模式和高原前缘扩展模型。