华北中部造山带南段岩石圈上地幔速度间断面结构及性质研究

The Trans-North China Orogen is a link between the east and west blocks of the North China Craton, and its lithospheric structure and properties are of great importance for studying the tectonism and evolution of the North China Craton. However, recent results from different geophysical methods are unable to reach an agreement on the lithospheric thickness in the southern Trans-North China Orogen, and some researchers even propose that there is a mid-lithospheric discontinuity with a velocity drop beneath the southern Trans-North China Orogen, just as what has been found in the cratonic Ordos Block. The S receiver function is of great advantages in detecting the velocity discontinuities in the sub-lithospheric mantle. On the basis of previous work, we intend to collect available seismic data from dense portable arrays and permanent networks in the southern Trans-North China Orogen and the adjacent regions, apply wave equation-based poststack migration for S receiver functions to image the lithospheric structure more comprehensively, and study the nature of the discontinuities by S receiver function synthetic modeling and frequency-dependency analysis. At last, combined with existing geological and geophysical observations, the results are expected to provide new insights into the origin and tectonic significance of the mid-lithospheric discontinuity, which may be helpful to study the lithospheric modification and destruction since the Mesozoic and the relevant dynamic processes in the North China Craton.

华北中部造山带作为联系华北克拉通东西块体的纽带，其岩石圈结构和性质对于研究华北克拉通的深部构造特征和动力学演化过程尤为重要。近些年来，不同的地球物理学方法在华北中部造山带南段得到的岩石圈厚度信息存在较大差异，同时陆续有学者提出华北中部造山带南段岩石圈内部可能存在与鄂尔多斯地区类似的波速随深度下降的间断面。S波接收函数因不受地壳多次波的影响，广泛应用于岩石圈上地幔尺度速度间断面的研究。本项目拟在前人已有工作的基础上，进一步收集华北中部造山带南段及其周边的密集流动台阵和国家固定台网远震数据，通过S波接收函数叠加偏移成像对其下方的速度间断面结构进行全面约束，并采用S波接收函数正演模拟和频率依赖性分析的方法对间断面的性质进行研究。最后将所得结果与收集到的其它观测资料相结合，探讨岩石圈内部间断面的形成机制和构造意义，为研究华北克拉通的中新生代改造和破坏，以及相关动力学过程提供参考。

本项目通过对华北克拉通中部造山带南段已有的230个地震台站的数据资料进行整合和处理，采用S波接收函数波动方程偏移成像方法，获得了研究区岩石圈底界面和岩石圈内部间断面结构模型，揭示了华北中部造山带是华北岩石圈改造和破坏的过渡区，区域小尺度结构变化剧烈，南段部分保留了原始的克拉通根（约110-170km），且内部存在MLD（约80-110km）；对不同区域叠加后的S波接收函数进行了正演拟合和频率依赖性分析，得到了速度间断面处梯度带厚度和速度变化信息，其中MLD处梯度带约厚8-10km，速度跳变约为7-12%，LAB处梯度带约厚8-15km，速度跳变约为4-8%，同一位置MLD相比下方LAB更为尖锐，东部块体的LAB比西部块体的LAB更为尖锐；综合成像、模拟结果和多学科观测资料，探讨了华北克拉通岩石圈构造演化特征，揭示了华北中部造山带经历了差异性减薄和改造，验证了华北克拉通受太平洋板片俯冲的影响，从内部薄弱带减薄和改造的模式；本项目的开展为基础地球科学研究提供了珍贵的数据资料，对理解华北克拉通的演化机制、陆块聚散过程和深部动力学背景等方面具有重要的科学意义。