联合反演面波与体波研究青藏东北缘构造变形的地幔动力学机制

There is significant lithospheric reactivation, such as the deformation, rifting, and mountain building processes, occurred in the wide area of Northeast Tibet, including the Ordos block and Qinling Orogen, accompanied with the Indian lithosphere under-thrusting Asia northward during the Cenozoic. However, to date, there is no general agreement as to the actual causes and processes associated with the widespread tectonic reworking in Northeast Tibet. Here, we take advantage the recent development of seismic arrays with more than 1,000 portable and permanent seismic stations to obtain an unprecedented high resolution crustal and upper mantle model of Northeast Tibet. The proposed work will apply the newly developed joint inversion method using body wave travel time and surface wave from both ambient noise tomography and teleseismic surface wave tomography. Our proposal will focus on imaging the paths of eastward extruded mantle materials from the interior of Tibet and the interactions between the mantle flow and the overlying lithosphere beneath the Northeast Tibet area.

印度板块-亚洲板块的碰撞挤压不仅造成了青藏高原的大规模隆升，而且伴随着青藏东北缘包括了鄂尔多斯和秦岭造山带等广大地区强烈的、广泛的构造变形。本项目拟利用近些年在青藏东北缘布设的密集台阵大约1000个宽频带地震仪记录的数据，提取远震体波走时以及背景噪音面波和远震面波相速度，联合反演体波和面波获得青藏东北缘高分辨率的地壳和上地幔三维剪切波速度结构，重点研究（1）青藏软流圈热物质东向溢出的通道和深度范围以及（2）青藏高原上地幔物质的东向溢出与青藏东北缘大规模构造变形之间的关系，进而揭示青藏东北缘大范围构造变形的地幔动力学过程和机制。研究结果对于揭示造山带大规模构造变形的动力学过程和演化模式都有极为重要的意义。

印度板块和亚洲大陆的持续挤压碰撞造成了青藏高原岩石圈的缩短和地表的隆升，于此同时，中国东部岩石圈经历了大规模的变形和活化，但是其深部动力学机制存在较大争议。本项目发展了有限频远震体波和面波联合反演的成像方法，通过两种独立数据的约束，能够很好弥补单一数据反演出现的问题，提高反演结果的可靠性，构建高分辨率的三维地壳-上地幔速度结构。我们利用布设在青藏东北缘约1000个地震台阵的观测数据，构建了青藏东北缘广大区域的高分辨率三维壳-幔结构，揭示了1）鄂尔多斯块体南部和四川盆地北部上地幔下方存在较强、深度可以延伸至300~400km的高速异常体，表明这两个块体整体上保存了古老克拉通的岩石圈特征；2）秦岭-大别造山带北部和渭河盆地下方150km-300km深度是青藏高原上地幔物质向东逃逸的通道；3）向东逃逸的热物质在东向逃逸过程中，受到鄂尔多斯块体阻挡，在其东南角形成一个局部扰流并对其下方冷的岩石圈结构产生侵蚀作用，导致鄂尔多斯块体东部岩石圈的局部改造，并导致大同、鹤壁等陆内火山的活动。