桐柏造山带及其邻区地壳上地幔三维结构与地球动力学研究

Tongbai orogen is located in the joint zone between Qinlin and Dabie-Sulu orogenes. It was cohered of the Paleozoic arc-continent collision orogenic belt in the north part and the Mesozoic orogenic belt formed by uplift of the high press (HP) and ultrahigh press (UHP) metamorphic rocks in the south part. The final west-narrowing architecture of the Tongbai-Dabie HP and UHP metamorphic belt in Mesozoic orogenic belt attracted a lot of researchers. Now subduction /exhumation model of HP and UHP metamorphic rocks and mechanism of dynamics still is disputed. It may hold a key to understanding the tectonic evolution of different terranes in the Qinling-Dabie orogenic belt, the collision tectonics of Yangtze plate with North China plate and the differential subduction /exhumation model of different HP and UHP slices proposed for the Tongbai and western Dabie regions. Obviously, exploring the crust and upper mantle velocity structure of Tongbai orogen and its adjacent areas is the key of understanding above questions. Up to now little initiative geophysics exploration in research regions obstructed our comprehension about its deep structure. This project plans to dispose 40 temporary Broad-Band stations forming a crossed profile in Tongbai orogen and adjacent areas. The profile will traverse Tongbai orogen from north to south and bestride HP and UHP exhumation zone from east to west. We will use waveform data recorded by temporary Broad-Band stations in Tongbai orogen, supplying some fixed Broad-Band staions recorded data by China National Digital Seismograph Network, some provinces seismograph network including Henan, Hubei, Anhui, Jiangxi province and ISC(International Seismic Center) bulletin. 3-D seismic velocity structure of the crust and upper mantle in Tongbai orogen and its adjacent areas will be obtained by the TomoDD method developed by Prof. Haijiang Zhang and the joint inversion tomographic method of Tomo3D tomography program developed by Prof. Dapeng Zhao. The receiver function method will be used to inverse the discontinuities velocity boundary. This research can help us to find the image of Yangtze plate subduction. This work can help us to analyse and know this area tectonic characteristic and the mechanism of dynamics of UHP metamorphic rocks returned to the ground.

桐柏造山带位于秦岭-桐柏-大别-苏鲁造山带的中部，经历了北侧的古生代增生造山和南侧的中生代碰撞造山过程。中生代高压/超高压变质岩，呈现从东部大别的超高压变质向西部的桐柏地区的高压变质过渡的特征。因此，桐柏造山带是研究华北与扬子板块之间深俯冲及超高压变质带东宽西窄的差异性出露折返机制的理想场所。对其深部结构的研究，有助于全面认识陆陆碰撞俯冲及超高压变质岩折返之地球动力学机制。到目前为止该区没有开展任何地球物理主动观测研究，缺乏深部结构的资料。本申请拟布置40台宽频带仪器进行"十"字剖面观测，南北穿越桐柏山，东西横跨高压/超高压变质带。通过该剖面探测，补充省级固定台网和国家级台网的地震波形资料，利用地震波层析成像和接收函数技术获取研究区的地壳上地幔速度和泊松比结构特征和重要速度不连续界面的特征，揭示扬子板块俯冲图像，探讨华北板块与扬子板块碰撞俯冲/折返的深部地球动力学机制。

利用流动台站和固定台网记录的远震P波和近震P波到时数据对桐柏造山带地区深部结构进行了层析成像反演。流动台站的观测分2期完成。第一期工作，在2014年6月-2015年5月共布置了35套地震仪，在大别山南，向北进入大别腹地，再向西沿着西大别和桐柏山近东西走向延伸至湖北广水-河南信阳一线，基本上跨越江西、湖北、河南三省，经过东大别超高压变质带、桐柏高压变质带等关键地质构造带。第二期工作，在2017年6月-2018年4月用中国地质科学院地质所的仪器沿桐柏山向西直到南阳盆地边缘布置10台宽频带仪器。到现在为止，共采集了45个流动观测台站的147.8GB的原始波形数据。. 利用上述45个流动观测台站和54个中国地震台网固定台站记录的1643个地震远震事件，共39384条射线，进行了远震P波层析成像反演。同时，利用54个省级固定台站，即河南、湖北、安徽、江西等省份的地震台网中心的2001-2007年度的震相报告数据进行了近震P波层析成像反演。使用了535个地震近震事件，近震射线数为4584条。在远震和近震P波层析成像图像中，大别造山带的高速特征明显，桐柏山地区的25km以上也是以低速特征为主，42km后转为高速特征。因此，桐柏-大别造山带高压超高压变质岩分布东宽西窄的差异性出露的深部背景为东浅西深，即桐柏地区高压超高压岩石存在于深部，只是未出露。华北板块高速向深部延深不深，而且有向南俯冲的趋势。尤其在117°E的剖面图中显示了华北板块在100km之下向南俯冲拆沉。而从南北向剖面图中未见有扬子板块的向北俯冲图像。合肥盆地和鄂东南、九瑞等长江中下游成矿带的低速特征异常明显。