深反射大炮揭示的青藏高原侧向碰撞带地壳骨架结构

Crustal skeleton structure of the lateral collision zone of the Qinghai-Tibet Plateau revealed by large-shot set of deep-reflecting profiling

青藏高原东南缘处于印度板块与欧亚板块碰撞的侧翼,揭示该地区的岩石圈结构有助于完整理解青藏高原碰撞造山的动力学过程,对构建大陆碰撞成矿理论框架至为关键.本研究对横过青藏高原侧向碰撞带的一条深反射地震剖面的15个大炮资料,进行了针对性静校正、去噪等处理和单次叠加成像,结果剖面显示了侧向碰撞带岩石圈结构的骨架特征:(1)双程走时(TWT)8~10 s的强反射(Tc)将地壳分为上、下两层;Tc可能是大型滑脱构造的拆离面,其存在使上地壳的变形与下地壳解耦;(2)Moho间断面反射(Tm)为3~4个同相轴的窄带反射波组,横向不连续,与深大断裂交汇处被错断,但断距不大;(3)在兰坪—思茅地块下方TWT21 s和扬子克拉通西缘下方TWT22~24 s存在相向倾斜的反射波组(TL);以Tc、Tm和TL构成的骨架结构,定性地描绘出剖面下方岩石圈地幔以汇聚为主、地壳块体以侧向滑移为主和上地壳为薄皮逆冲或滑脱的分层动力学模式.该岩石圈变形样式明显不同于以正向碰撞挤压、地壳缩短垂向增厚为主的“冈底斯模式”.

The southeastern margin of the Qinghai-Tibet Plateau is located on the flank of the collision between India and Eurasian plate.Revealing the lithospheric structure in this area is helpful to understand the deep dynamic process of the collision orogeny and crucial to construct the theoretical framework of continental collision metallogenesis.In this study,15 large-shots data of a deep reflection seismic profile across the lateral collision zone of the Qinghai-Tibet Plateau were processed by static correction,denoising and single coverage stack imaging.The results show that the skeleton characteristics of the lithospheric structure in the lateral collision zone are:(1)Strong reflection(Tc)appearing between TWT 8 to 10 s divides the crust into upper and lower layers;Tc may be a large scale detachment surface in the crust,which decouples the deformation of the upper crust from the lower crust;(2)Moho discontinuity reflection(Tm)is a narrow-band reflection wave with 3~4 coaxial axes,discontinuous lateral traceability,and recognizable offset at the position of regional faults across;(3)There are two reflection wave groups(TL)dipping opposite each other beneath the Lanping-Simao massif at TWT21 s and TWT22~24 s beneath the western margin of Yangtze craton.The crustal structure skeleton composed of Tc,Tm and TL outlines briefly the hierarchical dynamic model of lithospheric mantle convergence,crustal block lateral slip and upper crust thin-skinned thrust or slip under the section.This deformation pattern of the lithosphere is obviously different from the“Gangdise model”,which is dominated by forward collision,crustal shortening and vertical thickening.