云南思茅—中甸地震剖面的地壳结构

Crustal structure beneath the wide-angle seismic profile between Simao and Zhongdian in Yunnan

云南思茅-中甸宽角反射／折射地震剖面切割松潘-甘孜、扬子和华南三个构造单元的部分区域．我们利用初至波和壳内反射波走时层析成像获得地壳纵波速度结构．在获得新的地壳速度结构模型基础上，利用地震散射成像思想和低叠加次数的叠前深度偏移方法重建了研究区的地壳、上地幔反射结构．综合分析研究区地壳P波速度模型和壳内地震反射剖面发现：沿测线从北至南地壳厚度从约50km减薄至35km左右，地壳厚度的减薄量主要体现在下地壳，剖面北段下地壳厚度约为30km，剖面南段下地壳厚度仅为15km左右；上地幔顶部局部位置P波速度值偏低，一般为7．6～7．8km／s，反映出云南地区是典型的构造活动区的特点．剖面沿线地壳内地震反射发育，其中莫霍强反射出现在景云桥下方；在景云桥弧形断裂带8～10km深处出现宽约50km的强反射带．

The wide-angle seismic reflection/refraction profile between Simao and Zhongdian in Yunnan crosses Songpan-Garze, Yangtze and Cathaysia blocks. By means of seismic traveltime tomography we used first arrivals and other reflections from interfaces in the crust to obtain crustal P-wave velocity structure, and then reconstructed crust-upper mantle seismic reflection structure by using the low-fold pre：stack migration in depth domain and seismic scattering scheme on the basis of the above-mentioned crustal P-wave velocity model. By analyzing crustal P-wave velocity model and crustal seismic reflection structure, we can conclude that crustal thickness thins from about 50 km under the north segment of the profile to about 35 km on the south of the profile, the thinning occurs mainly in the lower crust, where the lower crustal thickness is about 30 km in the north segment, and about 15 km in the south segment; along the profile P-wave velocity in the uppermost mantle is about 7.6 - 7.8 km/s, which is lower than average and indicates the characters of a typical tectonically active area. The reflection wave is developed within crust, a strong reflection zone of Moho exists beneath Jingyunqiao fault, and a strong reflection zone about 50 km wide is located at a depth of 8 - 10 km beneath Jingyunqiao fault.