川滇地区莫霍面深度和地壳厚度

THE DEPTH OF MOHO INTERFACE AND CRUSTAL THICKNESS IN SICHUAN-YUNNAN REGION,CHINA

利用滑动平均法从川滇地区布格重力异常场分离出主要反映莫霍面起伏的低频布格重力异常场,将地震模型转换、并经过高程改正的密度模型作为初始模型,根据不同构造单元之间莫霍面上、下密度差不同,采用Parker法对低频布格重力异常场进行分区反演,得到了该区莫霍面深度和地壳厚度分布。结果显示:研究区西北部青藏块体从边缘向内部莫霍面深度和地壳厚度加深和增厚,分别为52～62km和54～66km;四川盆地莫霍面深度为38～42km;从南东向北西川滇菱形块体莫霍面深度为42～62km;红河断裂带以西的滇西地块,从南向北莫霍面深度为34～52km;龙门山、红河断裂带是莫霍面深度变化的梯度带;红河断裂带两侧自北向南莫霍面深度差异逐渐增加;攀枝花裂谷下方的莫霍面未发现明显隆起。川滇地区莫霍面深度分布明显受印度板块与欧亚板块碰撞及其向中南半岛侧向俯冲的制约。文中采用分区反演得到的莫霍面深度模型与已有的人工地震探测结果之间的均方差<1. 7km,同时通过与整体反演结果对比,表明在不同构造单元的地壳速度、密度结构存在明显差异的区域,以高分辨地震探测结果为约束,开展分区密度界面反演工作,可有效提高反演结果的可靠性。

By using moving average method to separate Bouguer gravity anomaly field in Sichuan-Yunnan region,we got the low-frequency Bouguer gravity anomaly field which reflects the undulating of Moho interface.The initial model is obtained after seismic model transformation and elevation correction.Then,we used Parker method to invert the low-frequency Bouguer gravity anomaly field to obtain the depth of Moho interface and crustal thickness in the area.The results show that the Qinghai-Tibet block in the northwest of the study area deepens and thickens from the edge to the interior,with the depth of Moho interface and the crust thickness of about 52~62km and 54~66km,respectively.The depth of Moho interface in Sichuan Basin is about 38~42km.In Sichuan-Yunnan block,the depth of Moho interface is about 42~62km from southeast to northwest.Beneath the West Yunnan block,west of the Red River fault zone,the Moho depth is about 34~52km from south to north.The Longmen Mountains and Red River fault zone are the gradient zone of the Moho depth change.Along the Red River fault zone,the depth difference of Moho interface is increasing gradually from north to south.No obvious uplift is found on the Moho interface of Panzhihua rift valley.The depth of Moho interface distribution in Sichuan and Yunnan is obviously restricted by the collision between the Indian plate and the Eurasian plate and the lateral subduction of the Indo-China peninsula.The mean square error of the depth of Moho interface is less than 1.7km between the result of divisional density interface inversion and artificial seismic exploration.At the same time,we compared the integral with divisional inversion result.It shows that:in areas where there is obvious difference between the crust velocity and density structure in different tectonic blocks,the use of high resolution seismic exploration data as the constraints to the divisional density interface inversion can effectively improve the reliability of inversion results.