蒙古及周边地区重力异常和地壳不均匀体分布

Gravity anomalies and the distributions of inhomogeneous masses in the crust of Mongolia and its surrounding regions

基于全球EGM2008自由空气重力异常模型,本文计算了蒙古及周边地区的布格重力异常和AiryHeiskanen均衡重力异常.在此基础上,本文采用Crust 1.0地壳模型为参考,通过重力正演方法,对蒙古及周边地区不同深度地壳密度结构模型的重力异常进行了计算,并对得到的正演布格重力异常与实际重力异常进行了对比和分析.研究结果表明:蒙古西部杭爱山地区与阿尔泰山地区的构造变形差异性明显,现今均衡重力异常中杭爱山周边没有明显的均衡异常高值区,而阿尔泰山地区西南方向存在均衡重力异常高值分布,分析与新构造运动密切相关;Crust 1.0模型给出的壳幔横向密度不均匀体分布对于计算Moho面起伏引起的重力异常作用明显;Crust 1.0给出的地壳内界面变形可以反映深大活动断裂的深部构造变形.研究结果对于认识蒙古东西部构造特征差异,以及现今西部活动断裂的地球物理场特征具有参考意义,也可以为进一步应用Crust 1.0模型为参考开展三维密度结构反演提供一定帮助.

The Mongolia arc structural belt has a complicated formation and development history.It is an important boundary that affect the internal tectonic deformation and evolution of China mainland.From the gravity anomalies to study the crustal deformation patterns,to further reveal the continental crust and lithospheric mantle deformation characteristics in the northern part of China,has an important geological and geophysical significance.In order to study the density structures of sources at different depths,we need to separate the gravity anomalies and understand tectonic deformation patterns from the field sources of different depths.First,on the basis of the EGM2008 model,we have the free air gravity anomalies,using the gravity correction approach,calculated Bouguer and Airy gravity anomalies in the Mongolia and its surrounding areas.Then,introduce the Crust 1.0crustal model,and establish the reference density model from the surface to the 55 km depth in Mongolia and its surrounding areas.Finally,we extract 6profiles and determine the hierarchical gravity anomalies in each profile,and make the quantitative analysis on gravity anomaly features of the crustlithospheric mantle density field source bodies of in Mongolia and its surrounding areas.On the basis of above data and methods,our results include：（1）The gravity anomaly models in Mongolia and its surrounding areas indicate the obvious differences of tectonic deformation between the Hangai dome and Altay Mountains in the western region.There is no area with apparent high-value isostatic gravity anomalies in the Hangai dome.But the high-value regions are concentrated in the southwest of the Altay Mountains.We infer that this feature is related to the new tectonic movement.（2）Based on the reference crustal density model by Crust1.0,we have investigated the distributions of lateral inhomogeneous bodies in the crust and upper mantle,and find the lateral density structure is important to forward modeling of the gravityanomalies caused by the Moho relief.（3）We have the comprehensive profiles with density structure from the Crust 1.0 model.The results indicate that the density feature is consistent with deep crustal deformation and structures of large active faults.From the analysis of the hierarchical gravity anomalies and the reference density models,we suggest a dynamic model that accounts for the deep crustal movement from west to east in Mongolia,in which the force source is not from the squeeze process between the Siberian plate and Eurasian plate,rather related to the northeastward movement of the block in the southwestern Altay Mountains.The formation of strike-slip faults in both north and south of the Hangai Dome is also related to this movement.The deformed interface determined by the Crust1.0model is consistent with the distribution of surface ruptures.There is the correlation between the topography and deep deformation,which consists with the isostatic hypothesis.Therefore,it is a feasible approach to improve gravity inversion using Bouguer gravity anomalies based on the lateral inhomogeneous density model in regions with complex structures.Our results also indicate that the consistency between deep and shallow mass distribution and deformation in the arc-like tectonic deformation zone of Mongolia.