青藏高原东北缘祁连山西段与东段岩石圈结构差异研究

The lithospheric structure differences between the western and eastern Qilian in the northeastern Tibetan Plateau

作为青藏高原东北缘的祁连山,其东段与西段的地貌以及新生代地表变形样式差异显著.在新生代青藏高原向北东扩展这一共同构造背景之下,引起祁连山东段和西段这些差异的深部原因尚不清晰.本文基于一条宽频密集台站观测剖面,利用P波速度结构约束下接收函数和面波频散的联合反演方法给出祁连山东段的壳内及岩石圈地幔结构.结合前人在青藏高原东北缘的研究成果,对比祁连山东段和西段的岩石圈结构,探讨东、西段壳内变形方式的差异,从而揭示引起祁连山东西段这些差异背后的可能原因.东西段岩石圈结构的差异主要有:1)壳内低速层差异.在20~40 km的深度处,祁连西段(测线BB′)中地壳存在壳内低速物质(3.2~3.4 km·s^(-1)),而祁连东段(测线AA′)的中地壳并不存在低速层;2)岩石圈地幔差异.祁连西段的岩石圈地幔低速异常表现为分块的特征,而祁连东段的岩石圈地幔表现为比较统一的高速体.通过对比研究发现祁连西段壳内强度相对软弱,地壳的缩短变形程度更大,表现为高海拔的山脉以及更厚的地壳,而祁连东段壳内变形及缩短程度相对较弱,表现为地势相对较平坦的临夏盆地和陇中盆地以及较薄的地壳厚度.东、西段壳内强度以及变形方式的差异可能与深部岩石圈地幔的结构相关.

As the northeastern margin of the Tibetan Plateau,the Qilian Orogen has significant differences in geomorphology and Cenozoic surface deformation patterns between the eastern and western segments.Under the common tectonic background of the Tibetan Plateau extending to the northeast in the Cenozoic,the causes of these differences between the eastern Qilian and western Qilian are still unclear.Based on the observation data from a broadband dense station array,the intracrustal and uppermost mantle structures in the eastern Qilian are derived by using the joint inversion of receiver functions and surface wave dispersions with P-wave velocity constraints.Combined with the previous research results in the northeastern Tibetan Plateau,this paper compares the lithospheric structures of the eastern Qilian and western Qilian,and discusses the differences in the crustal deformations of the eastern Qilian and western Qilian,thus revealing the possible causes of these differences.The crustal structure differences in the eastern Qilian and western Qilian are mainly as follows:(1)Differences in the crustal low-velocity zone:At a depth of 20~40 km,there is a low-velocity zone(3.2~3.4 km·s^(-1))in the middle crust of the western Qilian(profile BB′),while the middle crust of the eastern Qilian(profile AA′)does not exist the low-velocity material.(2)Differences in the lithospheric mantle:The lithospheric mantle in the western Qilian is characterized by blocky low-velocity anomalies,while the lithospheric mantle in the eastern Qilian is characterized by relatively uniform high-velocity material.The crustal shortening and deformation of the western Qilian are greater and the crustal strength is relatively weaker,which is manifested as high-altitude mountains and relatively thicker crust,while the crustal shortening and deformation of the eastern Qilian are weaker,showing the relatively flat Linxia and Longzhong basins and thinner crust thickness.The differences in crustal strength and deformation between the eastern and western Qilian may be related to the structure of the deep lithospheric mantle.