青藏高原东北缘重力场深部结构及其动力学特征

本文采用欧拉反褶积、场源参数成像(SPI)、场源边界提取(SED)、莫霍面反演、地壳三维可视化等多源方法,对青藏高原东北缘地区的布格重力场进行反演与分析,深入研究该地区的深部结构与变形特征,探讨区域深部孕震环境及动力学机制.研究表明,青藏高原东北缘的布格重力场整体呈负异常值,具有明显的分区性,表现出鄂尔多斯盆地异常值相对偏高、阿拉善块体次之、青藏高原块体极低的特点,其中海源断裂系形成了一条宽缓的弧形重力梯度条带,梯度值达1.2 mGal·km^-1.欧拉结果显示,鄂尔多斯盆地相比于青藏高原块体而言,场源点具有较强的均一性,场源强度值高(密度值高)且深度稳定在25~32 km范围内,而高原块体的中下地壳尺度广泛分布着低密度异常体.SPI图可知,海源弧形断裂系位于“浅源异常”弧形区,反映其地壳较为活跃,易发生中强地震.SED图揭示青藏高原地壳向东北扩展,经过几大断裂系的调节后运动矢量向东或东南转化,SED与GPS、SKS运动特征大致相同,说明地表-地壳-地幔的运动特征有着较强的一致性.青藏高原东北缘地区壳幔变形是连贯的,加之莫霍面由北向南、由东向西是逐渐加深的,因此属于垂向连贯变形机制,不符合下地壳管道流动力学模式.区域形成了似三联点构造格局,其中海源弧形断裂系的深部地壳结构复杂,高低密度异常体复杂交汇,是青藏高原、阿拉善、鄂尔多斯三大块体相互作用的重要枢纽,其运动学特征总体为中段走滑尾端逆冲,而断裂系正处于大型的弧形莫霍面斜坡带之上,具备强震的深部孕震环境,因此大尺度的运动调节与深部孕震条件共同促使了该地区中强震的多发.

Deep structure of the gravity field and dynamic characteristics of the northeastern margin of the Tibetan Plateau

This paper studies the Bouguer gravity field of the northeastern margin of the Tibetan Plateau by data inversion. Several methods are employed including Euler deconvolution, Source Parameter Imaging (SPI), Source Edge Detection (SED), Moho inversion and Three-Dimensional (3-D) crustal visualization. The purpose is to reveal the deep structure and deformation characteristics, and discuss the deep seismogenic environment and dynamic mechanism in this region. The research shows that the Bouguer gravity field of the northeastern Tibet Plateau has obvious regionalization with negative gravity anomalies decreasing gradually from the Ordos Basin, through the Alxa Block to the Tibetan Plateau. The Haiyuan fault system displays a wide arc gravity gradient belt with a gradient value of 1.2 mGal·km^-1. The Euler result indicates that compared with the Tibetan Plateau, the Ordos Basin has stronger homogeneity of field source points and higher strength (higher density) of field source with depth of 25~32 km. Low density anomalies are widely distributed in the middle and lower crust of the Tibetan plateau block. The SPI map shows that the Haiyuan fault system is located in the arc area of "shallow source anomalies", which indicates that the crust is more active, so that it is easy to generate moderate and strong earthquakes. The SED map reveals that the crust of the Tibetan Plateau extends to the northeast, and the motion vectors transform eastward or southeastward by the adjustment of several major fault systems. The movement characteristics of the SED are similar to the GPS and the SKS, indicating that the movement characteristics of the surface, crust, and mantle are consistent. The crust-mantle deformation of the northeastern Tibetan Plateau is coherent with the Moho gradually thickening from north to south and from east to west, which indicates that it can be explained by vertical coherent deformation mechanism rather than the lower crustal channel flow model. This study region forms a quasi-triple point structure. The high and low density anomaly bodies intersect complexly in deep crust of the Haiyuan fault system, which is an important hub for the interaction of the Tibetan Plateau, Alxa and Ordos. The Haiyuan fault system is characterized by the strike-slip movement of the middle segment and thrust movement of the tail. And it is located in the large arc Moho slope zone, indicating the deep seismogenic environment of strong earthquakes. Therefore, large-scale motion regulation and deep seismogenic conditions jointly lead to occurrence of moderate and major earthquakes in this area.