六盘山断裂带及其邻区地壳结构

新生代期间,中国大陆西部受印度一欧亚板块碰撞和青藏高原隆升影响,以地壳缩短、增厚、陆内造山和强烈地震活动等为主要特征.在青藏高原东北边缘,高原物质侧向移动被鄂尔多斯地块所阻,在六盘山地区发育了一系列左旋斜冲断裂.断裂带周缘构造变形强烈,地震活动频繁,是研究青藏高原横向扩展控制大陆内部弥散变形的理想场所.本文对穿越青藏高原东北缘一六盘山断裂带一鄂尔多斯地块的宽角反射与折射地震资料使用层析成像和射线反演算法进行成像,获得了研究区地壳速度结构模型,其结果反映出六盘山断裂带两侧地壳结构、构造特征差异显著:1)上地壳层析成像结果显示鄂尔多斯盆地一侧地壳上部速度较低,等值线呈近水平状,具有典型的沉积盆地特征,而青藏高原东北缘一侧上地壳速度相对较高,横向变化剧烈,呈褶皱状,二者的分界为海原一六盘山逆冲走滑断裂;2)全地壳射线反演结果显示鄂尔多斯地块地壳速度梯度大,下地壳底部速度高由铁镁质物质组成,具有典型稳定古老克拉通的特征,青藏高原东北缘地壳速度总体较低,主要由长英质及长英-铁镁质过渡物质组成,具有典型造山带的特征,而六盘山断裂带下方地壳速度结构复杂,层面呈拱形,部分层出现速度逆转,为两个构造单元的接触过渡带;3)青藏高原东北缘一侧地壳厚度~50 km,鄂尔多斯地块地壳厚度~42 km,六盘山断裂带下方莫霍面发生叠置,揭示出青藏高原东北缘、鄂尔多斯地壳在六盘山下汇聚,较薄且刚性的鄂尔多斯地壳挤入较厚且塑性的青藏高原东北缘地壳中的构造模式.

Crustal structure beneath the Liupanshan fault zone and adjacent regions

Since Cenozoic, the western Chinese mainland has experienced widespread deformation characterized by crust shortening, thickening, intracontinental orogeny and strong earthquake activities because of the India-Eurasia collision and uplift and expansion of the Tibetan plateau. The transition between the northeastern margin of the Tibetan plateau and the Ordos block is a key to understand how the diffused deformation controls intracontinental tectonics. In 2012, a 410 km long seismic wide-angle reflection and refraction seismic profile was deployed, processed and interpreted. The 2D P-wave velocity model shows that: 1) the upper crust of the Ordos basin has relatively lower velocities with sub-horizontal velocity contours which represent stable sediments, while the upper crust of the northeastern margin of the Tibetan plateau has strong velocity variation. It should be a result of northeastward compression of the Tibetan plateau, implying that the Haiyuan-Liupanshan fault is the boundary between the plateau and the Ordos block; 2) The northeastern margin of the Tibetan plateau has relatively lower velocities with low gradients in the middle and lower crust, while the Ordos block has higher velocities in the lower crust, meaning it is a stable craton; 3) The crustal thickness varies from ~50 km in the west to ~42 km in the east, and a double Moho appears beneath the Liupanshan fault, which suggests that the thin and rigid Ordos crust was pushed into the thick and soft northeastern margin of the Tibetan plateau crust, which is probably the far-field effect of the collision between the Indian and the Eurasian plates.