川西高原重磁异常特征与构造背景分析

川西高原位于青藏高原东缘,是我国大陆地壳构造变形及地震活动最强烈的区域.利用最新重力、航磁资料,通过异常分析和反演计算,研究了该区鲜水河断裂、理塘断裂、金沙江断裂的重磁异常特征、莫霍面特征、居里面特征,分析得出了这些断裂的深部地质结构与构造背景.计算表明:川西高原莫霍面东南浅、西北深,地壳厚度在43~63km之间.居里面特征表现为条带状,深度在17~23km之间.其中,鲜水河断裂带对应莫霍面深度梯度带,居里面为高低起伏圈闭.理塘断裂带北段莫霍面局部隆坳相间,南段莫霍面逐渐抬升,居里面呈现由西向东加深的梯度带.金沙江断裂带,居里面形成局部抬升,深部可能存在高温地热异常源.综合分析认为,川西高原地壳结构主要特点为:增厚的下地壳,热-塑性变形的中地壳,脆性变形的上地壳.

The study of gravity-magnetic anomaly and tectonic background in Sichuan west region

The Western Sichuan Plateau, the eastern margin of the Tibetan Plateau, is the important part of the Sichuan-Yunnan rhombus block and the most intense tectonic deformation of the continental crust and high-seismicity region. The Batang earthquake zone, the Litang earthquake zone and the Kangding earthquake zone are related to the Jinsha River fault, the Litang fault and the Xianshui River fault respectively. Most of focal depth distributed in the upper crust of the Western Sichuan Plateau. The middle and deep crustal structure obviously related to the thermal abnormal process which revealed from the Curie isothermal surface undulation. The characteristics of gravity and magnetic anomalies, the Moho depth and the Curie isothermal depth in the Xianshui River fault, the Litang fault and the Jinsha River fault were studied. The analytical and inversion methods were used to the latest gravity and aeromagnetic data in the paper. The characteristic of Bouguer gravity anomalies of the Western Sichuan Plateau are higher in the southeast than in the northwest, showing a beads shaped. The band shaped magnetic field can be divided into three districts from west to east with Ganzi-Litang boundary:the strong changing district in Baiyu-Deqin, the quiet district in Ya River-Jiulong, the strong district in Luding-Mianning. The Moho depth is shallower in the southeast and characterized with chair-step shape from the gravity inversion results. The crust thickness in the Western Sichuan Plateau varies from 43 to 63 km and the average thickness is 55 km. The Curie isothermal surface depth ranges from 17 km to 23 km and with an average of 20 km. Xianshui River fault corresponding to the Moho surface gradient zone and the amplitude of the Curie isothermal surface trap. The Moho surface characterized with uplift and depression in the northern of Litang River fault, and uplift gradually in the southern. The Curie isothermal surface deepens from east to west. Along Jinsha River fault, the Curie isothermal surface shows local uplift, where the heat geothermal anomalies source maybe existing under this area. The two-dimensional inversion results of the Batang-Luding profile indicate the vertical crustal structure variant among different tectonic units. The Moho surface shows uplift and the minimum crust thickness is only 50 km in the 50 km to the east of Batang. The Moho depth below Litang is 60 km with a concave trend. The Curie isothermal surface shows weak variation and uplift gradually from west to east between 22 km under Batang and 17 km under Luding. In general, the Curie isothermal surface in the Western Sichuan Plateau is in zoning distribution from north to south, which in accordance with the fault strike. The Curie isothermal surface shows ups and downs on either side of the fracture. The Moho depth gradually deepens from west to east, but the Curie isothermal surface above the Moho surface shows weak variation. We concluded that the Western Sichuan Plateau can be characterized with lower crust thickening, mid-crust thermal deforming and upper crust brittle deforming from comprehensive analytical approach.