青藏高原东北缘与阿拉善地块构造边界带深部结构研究

As a leading edge of the Tibetan plateau’s northeastern expansion, the northeastern Tibetan plateau (NE Tibet) has been vastly shortened during Cenozoic. The deep mechanism of plateau growth in this area is definitely recorded in the deep lithospheric structure beneath the tectonic boundary zone between the Qilian orogen and the Alxa block in the north against NE Tibet. Then it’s clear that investigating the deep lithospheric structure beneath this boundary holds the key to unravel the transition process from plateau to the surrounding blocks (here is the Alxa). This study plans to conduct research on the crustal and upper mantle structures on NE Tibet based on observations from two dense broadband seismograph station profiles that traverse the Qilian orgen in separately different Longitude zones. We shall do P-wave and S-wave receiver function imaging as well as joint inversion of P receiver functions with ambient noise dispersion data and crustal Vp model data from active-source wide-angle reflection/refraction (WAR/R) profiling. With these observations, we aim to obtain relatively high-resolution sections of crustal and upper mantle structures cutting through this tectonic boundary zone of Tibetan plateau, and the corresponding Vs and Vp/Vs structure sections. Finally, we will attempt to construct a comprehensive lithospheric structural model of NE Tibet that would help unravel the deep transition process from plateau to the Alxa block. Furthermore, based on these works, we can discuss the lithospheric deformation mechanism and geodynamic pattern accommodating plateau growth on this tectonic boundary.

作为青藏高原向东北方向扩展的一个前缘地带，青藏高原东北缘地区在新生代发生了大规模的地表缩短，祁连造山带与阿拉善地块接触的构造边界带的深部结构无疑记录了高原扩展的深部机制，探测查明边界带下方的岩石圈深部结构显然是揭示青藏高原如何向周缘块体过渡和扩展的关键所在。本研究拟基于两条在不同经度区域穿越祁连造山带布设的宽频地震密集台站剖面开展壳幔结构的研究工作，利用P波和S波接收函数成像以及P波接收函数、区域噪声和人工源宽角反射/折射的地壳Vp数据的联合反演工作，力求获得切过这一青藏高原构造边界带下方的较高分辨率的地壳和上地幔顶部岩石圈结构断面，以及相应的Vs和Vp/Vs结构剖面。从而构建青藏高原东北缘地区综合岩石圈结构模型，揭示从高原到阿拉善地块的深部过渡关系，探讨这一构造边界带的深部岩石圈变形机制和高原扩展的地球动力学模式。

作为青藏高原向东北方向扩展的一个前缘地带，青藏高原东北缘地区在新生代发生了大规模的地表缩短，祁连造山带与阿拉善地块接触的构造边界带的深部结构无疑记录了高原扩展的深部机制，探测查明边界带下方的岩石圈深部结构显然是揭示青藏高原如何向周缘块体过渡和扩展的关键所在。本研究基于三条穿越青藏高原东北缘与周缘块体不同边界区域的宽频地震密集台站剖面开展壳幔结构的研究工作，利用P波和S波接收函数成像以及P波接收函数、区域噪声和人工源宽角反射/折射的地壳Vp数据的联合反演工作，获得了青藏高原——秦岭构造转换带和青藏高原——阿拉善构造转换带这两个边界区域的地壳和上地幔顶部结构精细成像结果。我们的结果指示了，西秦岭北缘断裂带为分隔青藏高原岩石圈（较热/软弱）和亚洲岩石圈（较冷/刚性）的一条重要边界带，祁连造山带下方下伏了华北克拉通板块的冷的岩石圈，可能来自早古生代造山并在新生代发生活化后的地体楔入，这一过程促使了同构造期的地壳楔的形成，壳内滑脱层的存在有利于地壳楔的发育，区域各向异性体制特征很好地映证了我们的这一解释。我们的观测也指示了，受区域大型断裂带控制的中下地壳软弱物质挤出和由软流圈地幔流触发的岩石圈底部拆沉导致的重力均衡，共同作用于青藏高原东缘——秦岭/四川盆地构造转换带区域的高原隆升和扩展。青藏高原东缘物质挤出之深部过程与处于青藏高原北缘的青藏高原——阿拉善构造转换带区域的高原向北扩展之深部机制存在差异。