南向俯冲在藏北高原下的亚洲岩石圈地幔前缘结构与陆陆碰撞效应

Rapid uplifting of the Tibetan plateau was resulted from collision between the Indian plate and the Asian plate since the Cenozoic. It is in doubt where the frontier of the Asian mantle southward subducted beneath in Northern Tibet located, as where widely emerged so plentiful geological marks, like mantle-derived ultrapotassic magmatic volcanoes regionally erupted since ~10 Ma. Strong interaction between the lower crust and upper mantle beneath the region was difficult to understand due to the absence of deep structures of the crust and upper mantle. And, broadband seismic observation is admitted as a valid tool to obtain deep structures. However, the region is devoid of any broadband seismic observing, mainly because of some limiting factors, such as particular topography with high elevation over 4500m, abominable climate with thinner air,etc..Following the seismic profile of Hi-Climb project with dense broadband stations to northward extend, a fire-new profile of broadband seismic observation with 40 broadband stations of 18-months working duration will be carried out in a rang between N34 to N38 , and E84 to E85 ，from south to north, which acrosses northern Qiangtang, the western part of the Jinshajiang Suture, the Kok hil terrane, huge-scale strike-slip faults of the Altyn tagh, ends at Qiemo in the south margin of Tarim basin. Tomography and receive functions will be used to image 3D velocity structure and 2D discontinuous boundaries within the crust and the mantle beneath the profile, in order to deldelineate the lithospheric mantle of the Asian plate southward subducted beneath Northern Tibet. In terms of geochemistry analysis to some hand samples of potassic magmatic rocks erupted in the region, material physical parameters can be calculated by a method of Hacker et al.(2000) with geophysical and geochemical data. Furthermore, under the same frame can be discussed the machanism of rapaidly uplifting of Northern Tibet and its effects in continent-continental collision..Thereby, the proposal will supply a gap with broadband seismic observations in the key region. Also, it belongs to independent basic research.

新生代以来青藏高原快速隆升是由印度板块与欧亚板块陆陆碰撞直接作用的结果。受藏北高原独特地貌与恶劣的自然环境等限制，使得宽频带地震观测在该区域还是空白，以及藏北高原特有的地质现象（如5Ma以来的钾质和超钾质火山岩广泛分布）导致了欧亚岩石圈地幔南向俯冲至藏北高原之下存在诸多争议。在前人研究工作基础上，本项目计划在该区域（E84 ～E85 ，N34 ～N38 ）布置40台宽频带流动台站，向北跨越阿尔金断裂带，北抵塔里木盆地南缘。利用地震波层析成像技术和接受函数技术获取该区域下的速度结构和重要不连续界面特征，来追踪南向俯冲的欧亚板块岩石圈地幔前缘结构变化特征。利用地球化学分析获取喷发时的环境参数，进而探讨与之有关的陆陆碰撞效应及藏北高原隆升机制。

利用宽频带地震观测来追踪藏北高原下的亚洲岩石圈地幔前缘是当前青藏高原研究的热点问题之一。原计划通过布置约40台宽频带地震台网并结合已有的HiClimb台阵，并利用宽频带地震学技术手段捕捉俯冲在藏北高原下的亚洲岩石圈地幔前缘及变形特征，进而研究藏北高原隆升动力学过程。在项目执行周期内，主要进展在6个方面：.（1）2015-2016年度布置了16个观测点，并采集了25个新生代以来的火山岩石手标本。回收了约100GB的野外原始观测数据和分析了48个岩石薄片。收集了青藏高原及其周缘的流动台站数据，挑取了高质量的26多万个远震P波初至及4万多个远震P波接收函数。（2）研发了基于S变换方法的震相识别与拾取软件及相位滤波方法技术。（3）利用接收函数方法（H-k，CCP叠加及一维速度结构反演）再研究羌塘中央隆起带构造成因，获得的深部结构特征显示羌塘中央隆起带为一缝合带。（4）通过区域重力均衡异常及二维布格重力异常剖面拟合研究显示，广泛分布在可可西里区内的约10Ma以来钾质-超钾质火山岩可能与南向俯冲在藏北高原下的亚洲岩石圈地幔前缘断离作用有关。（5）利用基于S波变换的震相识别与波形相关方法挑取了祁连山北缘的流动台站及固定台站波形数据，利用Fast-Marching 远震走时层析成像方法获得的三维P波速度异常结构显示沿着青藏高原北边界逆冲断裂南向俯冲的阿拉善地块岩石圈在祁连造山带下与北向俯冲的柴达木岩石圈形成了面对面的碰撞接触关系。（6）利用2009－2010两年的全国固定台站记录到的远震事件波形数据，通过接收函数方法并精细化处理，获得了高质量的中国大陆下的Moho深度和地壳内的平均Vp/Vs比值。.综合分析认为，在印度岩石圈地幔持续的北向俯冲作用下,南向俯冲在藏北高原下的亚洲岩石圈地幔大约新近纪时期发生了断离作用，导致了藏北高原钾质火山大面积喷发。已出版6篇文章，待刊3篇，培养3名硕士和1名博士。