郯庐断裂带临沂-广济段岩石圈电性结构及其动力学意义研究

The Linyi-Guangji segment of Tancheng–Lujiang (Tan-Lu) Fault, which separates the Dabie orogeny belt on the western side from the Sulu orogeny belt on the eastern side, is the important tectonic boundary of the Yangtze Block and North China Block. The Tancheng Ms8.5 large earthquake occurred on this segment, so it is significant to determine its lithospheric structure. In this study, 12 existing magnetotelluric (MT) profiles data were collected, and 4 new MT profiles, 3 MT profiles extending the existing ones in Dabie orogeny belt, are arranged across the Linyi-Guangji segment of Tan-Lu Fault. Thus we have total 16 MT profiles across Tan-Lu Fault which are used to generate nearly a uniform profile spacing of MT data set along Dabie-Tanlu-Sulu tectonic zone.In order to obtain reliable lithospheric electrical structure about Dabie-Tanlu-Sulu zone and adjacent regions, multi-site, multi-frequency tensor decomposition and two-dimensional MT inversion, three-dimensional MT inversion are used. Based on these results, combining other geophysical and geological information, some fields, such as the deep structure beneath Linyi-Guangji segment of Tan-Lu Fault, the comparison of structure under Dabie, TanLu and Sulu zone, the spatial distribution of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic belt, the focal mechanism and seismogenic environment of the Linyi-Guangji segment of Tan-Lu Fault, et al, will be studied. Further more, some important continent dynamics topic, such as the formation and evolution of the Tan-Lu fault, the continental deep subduction and the crust-mantle interaction, will be discussed in-depth.

郯庐带临沂-广济段连接着苏鲁造山带和大别造山带，为华北古板块和扬子古板块的重要边界带，其间曾发生郯城8.5级大地震，故其深部构造具有重要研究意义。本项目在已收集到的该地区已有的12条大地电磁（MT）剖面数据的基础上，新布设4条MT剖面，并将大别地区原有的3条MT剖面往东南方向延长，构成苏鲁-郯庐-大别构造系上基本等间距的16条MT剖面网络；采用先进的多测点-多频点阻抗张量分解技术、二维反演技术和最新的三维反演技术，获得苏鲁-郯庐-大别及其邻近地区可靠的岩石圈电性结构；以此为基础，综合其他资料，研究郯庐带临沂-广济段的分段特征和深部延伸状况、苏鲁-郯庐-大别各构造单元的深部结构对比、高压-超高压变质岩体的空间分布情况、郯庐断裂带中南段的孕震环境与发震机制等，进而探讨郯庐断裂带的形成与演化、苏鲁-郯庐-大别构造系中各构造单元之间的耦合转换关式和大陆深俯冲及其折返机制等复杂的壳幔相互作用过程。

本项目通过新增、收集和融合郯庐带临沂-广济段附近的大地电磁数据，以三维带地形反演为主、二维反演为辅，结合阻抗张量成像技术，对整个苏鲁-郯庐-大别构造系的电性结构展开了系统的研究。发现苏鲁造山带、大别造山带、张八岭隆起均呈现为高阻特征。苏鲁高阻体最为显著，延伸深度超过80公里，张八岭的延伸深度次之，超过60公里，而大别高阻体在中、上地壳特别明显，但延伸深度较浅，在30公里以下就逐渐消失。张八岭隆起与大别的高阻体之间存在拖曳现象。依据这些结果和其他资料，本研究认为大别造山带脱离苏鲁造山带发生的大规模由北向南的运动最终形成了郯庐断裂带，其驱动力是来自北方的推挤，推测与燕山运动以及长江中下游剧烈岩浆活动的大型成矿是同一个力源。此外，电性主轴成像揭示下扬子区中上地壳与下伏岩体之间存在结构不整合现象，三维电性结构显示郯城大地震的震中可能位于临沂附近的高阻条带上、霍山地震区的地震活动可能是因为新断裂继承利用了老断裂的构造薄弱层、高压/超高压变质岩沿造山带的高导边界向上折返等。为使研究结果更为可靠，本项目还资助了多项大地电磁数据处理、反演算法和可视化集成软件的研究，包括大地电磁三维带地形反演云系统的后期研发与升级维护，提出并计算验证了大地电磁相间阵列观测技术等。