峨眉山大火成岩省地壳横波速度结构特征及其动力学意义

摘要

峨眉山大火成岩省是我国境内最早获得国际学术界广泛认可的大火成岩省,对于认识地幔柱形成与作用机理、生物与环境演化、资源富集与成矿机制等具有重要意义.本文利用峨眉山大火成岩省宽频带地震台阵(COMPASS-ELIP)以及云南、四川区域地震台网的部分台站资料,基于分格加权叠加策略实现接收函数和面波频散在信息来源和分辨尺度方面的协同;进而开展联合反演,重建了峨眉山大火成岩省关键剖面下方的地壳横波速度结构.研究结果显示:研究区地壳平均S波速度,沿剖面呈现自西向东先增大后减小的分带性,内带中、下地壳速度较高,尤其是下地壳存在明显的高速异常(Vs约3.8～4.2 km· s-1);丽江—小金河断裂带和水城—紫云断裂带的东西两侧,中上地壳存在低速层(Vs约3.3 km· s-1),尤其是水城—紫云断裂带东西两侧的中地壳低速层尤为明显.结合本文以及现有的系列研究结果,进一步确认内带中、下地壳高速对应二叠纪古地幔柱作用的遗迹,大规模岩浆的底侵和内侵,不仅改造了滇中块体的地壳结构和组分,而且也改变了地壳的流变强度,进而对现今青藏高原东南缘的深部过程产生了深远影响.%As the first large igneous province in China that is widely recognized by the international geosciences community,the Emeishan Large Igneous Province (ELIP) is of great significance for understanding the formation and mechanism of mantle plume,the co-evolution of biology and environments,and the resource enrichment and mineralization.Although the structural geometry and physical properties recorded in the crust and lithospheric mantle are well constrained by comprehensive geophysical investigation,the shear wave velocity structure in the ELIP remains poorly known.In this paper,based on the data of the COMPASS-ELIP experiment and the Regional Digital Seismic Networks in Yunnan and Sichuan Provinces,the crustal shear-wave velocity structure along the profile is reconstructed using a joint inversion of receiver functions and ambient noise surface waves.A weighted stacking strategy is used to match the information source and spatial resolution between receiver functions and surface waves.The results show that from west to east,the average crustal shear-wave velocity is featured by zonation along the profile,and the average crustal Vs within the inner zone is the relatively highest with a range of 3.6～3.8 km · s-1.A major contribution to this feature of the high velocity within the inner zone comes from the middle and lower crust,especially from the significant high-velocity anomaly (about 3.8～4.2 km · s-1) exiting at the base of the lower crust.The low-velocity zone in the upper and middle crust can be observed on both sides of the Lijiang-Xiaojinhe fault and Shuicheng-Ziyun fault,which is more prominent in the middle crust nearby the Shuicheng-Ziyun fault.Combined with other evidence from previous geophysical investigations conducted in the ELIP,the middle and lower crustal high-velocity anomalies we observed within the inner zone strongly further suggest they are the seismic signatures of the large-scale magmatic underplating and subsequent magmatic intraplating related to the late Permian mantle plume activities.Such ancient plume activities not only modified the crustal structure and composition within the central Yunnan block,but also changed the crustal strength there,and eventually affected the present deep process beneath the southeastern margin of the Tibetan Plateau.