阿坝—遂宁宽角地震剖面重建藏东缘龙门山地区地壳速度结构

龙门山断裂带位于青藏高原东缘,在中生代和晚新生代经历强烈的构造变形,急剧抬升,是研究青藏高原隆升和扩展动力学过程的重要窗口.本文利用起伏地形下的高精度成像方法,对"阿坝一龙门山一遂宁"宽角反射/折射地震数据重新处理,通过走时反演重建研究区地壳速度结构.剖面自西向东跨越松潘一甘孜块体、龙门山断裂带和四川盆地,不同块体速度结构表现了显著的差异.松潘甘孜块体地表复理石沉积层内有高速岩体侵入,低速层低界面起伏不平反映了该区的逆冲推覆构造.中下地壳速度横向上连续变化,平均速度较低(约6.26 km·s~(-1)).四川盆地沉积层西厚东薄,并在西侧出现与挤压和剥蚀作用相关的压扭形态.中下地壳西薄东厚,平均速度较高(约6.39 km.s~(-1)).龙门山断裂带是地壳速度和厚度的陡变带,Moho面自西向东抬升约13 km.在整个剖面上Moho面表现为韧性挠曲,中下地壳横向上连续变化,推测古扬子块体已到达松潘甘孜块体下方.松潘甘孜块体下方中下地壳韧性变形,并在底部拖曳着被断裂切割的脆性上地壳,应力在不同断裂上积累和释放,诱发大量地震.

Crustal structure beneath the Longmenshan area in eastern Tibet:new constrains from reprocessing wide-angle seismic data of the Aba-Longmenshan-Suining profile

The Longmenshan area in the eastern Tibetan Plateau experienced strong crustal deformation and faulting during Mesozoic and late Cenozoic epochs resulting in significant uplift of the Longmenshan Mountains, which serves as an important window to see the uplift and lateral expansion of the Tibetan Plateau. In this paper, the deep seismic sounding profile named the Aba-Longmenshan-Suning profile is reprocessed using a high-precision traveltime tomography for velocity structure with an irregular surface to reconstruct the crustal structure beneath this area. The obtained velocity structure along the profile could be divided into three major sections: the Songpan-Garzê block, Longmenshan fault zone and Sichuan basin. The sedimentary cover of the Songpan-Garzê block is about 10 km thick, with high-velocity bodies in 5~10 km depths, which correspond to the faults and likely the intrusion of Mesozoic granites. The undulating interface of the sediment may indicate the thrust-nappe tectonics. Velocity structures of the middle and lower crust change continuously with lower average velocity (about 6.26 km·s^-1). In the Sichuan basin, the sediment in the west is thicker than that in the west. The sediment in the west even exhibits compression and torsion forms, which may be caused by compression and erosion. The middle and lower crust in the west is thinner than that in the east with higher average velocity (about 6.29 km·s^-1). The Longmenshan fault zone is a transition zone of the crustal velocity and thickness, with the Moho uplifting about 13 km from the west to the east. Along the whole profile the Moho shows toughness winding, while the velocity of the middle and lower crust changes continuously in the lateral direction, which may indicate that the ancient Yangtze block has reached beneath the Songpan-Garzê block. In the Songpan-Garzê block, which is considered to be a passive margin, the middle and lower crust experienced ductile deformation under strong compression. The brittle upper crust with thrust-nappe belts is dragged by the middle and lower crust, while stress is accumulated and released on the faults, which may induce a large number of earthquakes.