青藏高原东缘地壳密度结构及其地球动力学意义

CRUSTAL DENSITY STRUCTURE OF THE EASTERN TIBETAN PLATEAU AND ITS GEODYNAMIC IMPLICATIONS

青藏高原东缘是高原物质向E及SE扩展的重要通道,掌握青藏高原东缘的地壳密度结构对研究青藏高原的隆升、变形机制具有重要意义。文中在前人研究成果的基础上,选取了地面实测的9条交叉的重力测线数据,以深地震反射剖面为约束,采用人机交互模式反演得到了青藏高原东缘地下的二维密度结构,并通过克里金插值法获取了三维密度结果。反演结果表明,青藏高原东缘地区具有巨厚的地壳,莫霍面埋深最深约为61km,而四川盆地的莫霍面埋深约为42km,以龙门山-安宁河-小金河断裂为界,两侧形成了莫霍面深度变化梯度带;从反演得到的沉积层厚度来看,沉积层在青藏高原东缘几个块体内呈现中心普遍厚度较大、边缘厚度较薄的特点。结合该地区的地震空间分布特征分析,青藏高原东缘的莫霍面和沉积层厚度分布与该地区的地震分布均具有很强的相关性,这对未来地震预测也具有重要的参考价值。

The continuous collision and convergence between the Indian and Eurasian plates have caused strong uplift and deformation within the Tibetan plateau and the surrounding areas.The eastern Tibetan plateau,as an important channel for the eastward and south-eastward expansion of plateau materials,is an critical area for understanding the interaction between the Tibetan plateau and the eastern tectonic blocks and for understanding the eastward escape of plateau deep materials,which is of great significance for studying the uplift and deformation mechanism of the Tibetan plateau.A large number of studies on the eastern Tibetan plateau have provided an important basis for revealing the uplift mechanism of this region.However,its complex geology makes it difficult in understanding the uplift mechanism from the single geophysical interpretation.The gravity field reflects the density properties of the subsurface material,and can be related to the wave velocity properties of the seismic data by certain translation relationships.In addition,gravity data can improve the crustal model of the area not adequately covered by seismic data,which can not only provide the three-dimensional crustal density structure of the area,but also reflect the relationship between the spatial distribution of earthquakes and the crustal structure from a gravity perspective.In this paper,based on the previous research results,we selected field survey gravity data of nine intersecting lines and used the deep seismic reflection as the constraint to invert the density interface depth distribution of each line by using human-computer interaction mode,and then used the kriging interpolation method to obtain the three-dimensional Moho depth and basement depth in the area,and then we obtained the sediment thickness by analyzing the difference between the topography and the basement depth.The inversion results show that the overall trend of Moho depth is deep in the west and shallow in the east,with the deepest depth in the west being 61km and the shallowest in the east being about 40km.There is a large difference between the two sides of the arc belt formed by the Longmenshan-Anninghe-Xiaojinhe fault,with the northwest side of the arc belt basically above 52km,among which the Moho depth is about 58km in the Bayankara block and the northern part of the Chuan-Dian rhombus block,and about 53km in the Chuan-Dian rhombus block and the southern part of the Indo-China block.The Moho depth is about 42km in the Sichuan Basin on the east side of the arc belt,which constitutes a gradient zone of Moho depth around the Tibetan plateau.There also exists a depressional zone of Moho in the Bayankara block,which may be related to the eastward flow of plateau material and the blockage of Sichuan Basin,so that part of the asthenosphere material accumulates and squeezes,thus forming a relatively thicker crust and the sinking of Moho.Part of the eastward overflowing asthenosphere material turns to the south and south-east direction,resulting in the thickness of the crust in the southwest of the Chuan-Dian rhombus block is greater than the east and west sides.At the same time,the late Paleozoic mantle column activity led to the uplift of the lithosphere and the intrusion of high-density material into the lithosphere,which also blocked the southward flow of material from part of the Tibetan plateau.From the inverted sediment thickness,the sediment on the eastern Tibetan plateau is relatively thicker in the center of several tectonic blocks,up to 7km thick,while the sediment at the edges of the blocks is relatively thinner,and even bedrock is exposed in some areas.Combined with the spatial distribution characteristics of earthquakes in this area,the Moho depth and sediment thickness distribution in the eastern Tibetan plateau are strongly correlated with the distribution of earthquakes in this area,which has important reference value for future earthquake prediction.