龙门山构造带壳幔电性结构特征及其与汶川、芦山强震关系

青藏高原东缘龙门山构造带是研究青藏高原地壳物质向东侧向挤出的焦点地区.为探索龙门山构造带活动构造特征及其与发震构造的关系，本文通过布置垂直龙门山构造带南段芦山地震震源区的大地电磁测深剖面，运用多种数据处理手段，得到研究区可靠的电性结构，并通过与已有龙门山中段和北段剖面进行对比分析.研究表明：（1）青藏高原东缘岩石圈存在明显的低阻异常带--松潘岩石圈低阻带，该低阻异常带沿龙日坝断裂-岷山断裂-龙门山后山断裂分布，形成松潘-甘孜地块向扬子地块俯冲的深部动力学模式，通过统计研究区的历史强震，发现震源主要沿低阻异常带东侧分布，同时，低阻异常带也是低速度、低密度异常带，松潘岩石圈低阻带可能是扬子地块的西缘边界；（2）青藏高原物质东移过程中，受到克拉通型四川盆地的强烈阻挡，龙门山构造带表层岩块和物质发生仰冲推覆，表现为逆冲推覆特征的薄皮构造，中下地壳和上地幔顶部物质向龙门山构造带岩石圈深部俯冲，印支运动晚期，扬子古板块持续向华北板块俯冲，在上述构造运动作用下，呈现出刚性的上扬子地块西缘高阻楔形体向西插入柔性青藏块体的楔状构造；（3）根据电性结构推断，芦山地震受到深部上里隐伏壳幔韧性剪切带向上扩展的影响，构成芦山地震的深部主要动力来源；汶川地震的发生，在龙门山南段形成应力加载区，是触发或加快芦山地震孕育发生的另一个动力来源.

Crust and upper mantle electrical resistivity structure in the Longmenshan tectonic belt and its relationship with Wenchuan and Lushan earthquakes

Longmenshan tectonic belt, located at the eastern border of Qinghai-Tibet Plateau, is an important area for studying the eastward lateral extrusion of crustal matter in Qinghai-Tibet Plateau. In order to explore the active tectonic features of Longmenshan tectonic belt and its relation with seismogenic structure, we arrange a magnetotelluric sounding profile at the focal area of Lushan Earthquake in the southern section of the vertical Longmenshan tectonic belt, gains a reliable electric structure of the research area by using various data processing methods and makes a comparison of the sounding profile of the middle section and north section of Longmenshan. The research shows: (1) A clear low-resistivity abnormal zone exists in the eastern Qinghai-Tibet Plateau, which distribute along Longriba fault-Minshan fault-back range faults at longmenshan fault. This low-resistivity abnormal zone forms a deep dynamics mode which features the subduction of Songpan-Garzê block to Yangtze block. Through statistics analysis of strong earthquakes in the research area in history, it is found that the earthquake focus is mainly distributed along eastern of the low-resistivity zone. Meanwhile, low-resistivity abnormal zone is also a low-seismic velocity and low-density abnormal zone. The lithosphere at the western border of Yangtze Block is bounded by the low-resistivity abnormal zone; (2) During the eastward movement of substances in the Qinghai-Tibet Plateau, the mantle rock and substances, blocked by the Sichuan Basin, get obducted and thrusted. The thin-skinned structure, featuring over-thrust, and substances from the middle-lower crust and the top of the upper mantle subduction deeply to the lithosphere of the Longmenshan tectonic belt. At the later stage of the Indosinian movement, Yangtze ancient block continues to subduct to the Northern China block, and, under the impact of the above-mentioned structure movement, forms wedge structure brought about by the westward breaking of high-resistivity wedge in the western border of rigid upper Yangtze block into the soft Qinghai-Tibet Plateau block; (3) According to the electrical resistivity structure, the affect of expansion of concealed crust-mantle ductile shear zone in the deep part may be one of the major power sources in the deep part of Lushan earthquake. The occurrence of Wenchuan earthquake, caused a stress loading area in south of Longmenshan structure zone, is another power source to trigger or accelerate the Lushan earthquake gestation and occurrence after a certain time of energy accumulation and stress transfer.