【摘要】 地形地貌与区域地质构造分析揭示,龙门山一带上地壳的汇聚作用表现为松潘-甘孜块体东缘的褶皱拱曲与扬子克拉通西缘的高角度叠瓦状冲断,近地表形成了一个巨大的北东向展布的拱曲冲断构造带。松潘-甘孜块体东缘的拱起预示着可能有来自深部的顶托。人工地震测深得到的地壳P波速度结构剖面也揭示,松潘-甘孜块体东缘上地壳普遍有较大范围的拱起,上地壳底部的低速层也同步抬升,其隆起范围与近地表的拱曲冲断带比较一致,说明很可能存在来自中下地壳的上拱作用。面波层析成像揭示松潘-甘孜块体与扬子克拉通具有截然不同的壳幔结构,扬子克拉通速度较高,且不存在低速夹层;而松潘-甘孜块体速度偏低,地壳明显增厚,且下地壳及地幔上部存在S波低速层,地幔低速层上涌至壳幔过渡带,甚至侵入四川盆地之下,且上涌的范围与地表拱曲冲断带恰巧吻合,推测地幔盖层与下地壳塑性软弱物质的局部上涌促成了上地壳的拱曲。2008年汶川8.0级地震发生在拱曲冲断带中段,最大同震位移场位于龙门山拱曲冲断带及四川盆地西缘,揭示松潘-甘孜块体东缘的拱曲与扬子克拉通西缘的冲断共同受制于两大块体最新的汇聚作用。认为龙门山拱曲冲断构造带是陆内汇聚与壳幔通道流上涌联合作用的结果。爆破地震测深与地震层析成像不显示陆内俯冲图像,两大地块之间中地壳以下似为一近直立的汇聚带,这一构造格架将物质迁移主要限于垂向上,有利于龙门山保持大地形高差,而四川盆地一侧则因难以形成大的构造负载,前陆凹陷作用不显著。更多还原

【Abstract】 The Longmenshan(Longmen Mountain)is located in the middle part of the east margin of Tibetan Plateau, which is thought to be an active convergent mountain between Songpan-Ganzi Block and Yangtze Craton in Cenozoic time. In order to better understand the uplifting of Longmenshan margin in Cenozoic times, tectonic landform surveying was executed to constrain the total deformations of the upper crust and surface-wave tomography was implemented to probe the deeper structure of the lithosphere. Our field investigations and structural analyses revealed that the convergence of upper crust along Longmenshan is expressed as upward bending of the east margin of Songpan-Ganzi Block and high-angle imbricate thrusting of the west margin of Yangtze Craton,which resulted in a gigantic NE-trending bending and thrusting tectonic belt. The scope of uprising accords well with the locally up-warping of the upper crust revealed by seismic soundings across Longmenshan,which indicates that upward forces from the middle and lower crust may exist. Our surface wave tomography showed that the Songpan-Ganzi Block and the Yangtze Craton have distinctively different crust-upper mantle structures. The velocity is high and there is no low velocity layer in the Yangtze Craton; whereas in the Songpan-Ganzi Block the velocity is low, the crust is obviously thickened, and low S-wave velocity layers exist in the lower crust and upper mantle. The low velocity zone in upper mantle rises to the crust- mantle transition zone,even into the Sichuan Basin, and the scope of uprising accords well with the bending and thrusting belt in the upper crust. The tomographic images do not show intra-plate subduction; instead there is a nearly upright convergence zone below the middle crust between these two tectonic blocks. These findings are agreed with seismic soundings and receiver function inversions cross Longmenshan. A 150-km-wide bending of the upper crust along the east margin of Songpan-Ganzi Block favor the hypothesis that a eastward flow of weak and ductile material in the lower crust and upper mantle is blocked by the rigid Yangtze Block and turns upward, thus produces an upward force onto the capable upper crust, making it bend upwards locally. Furthermore,the uprising also exerts an upward shear force on the west side of the convergence zone,which facilitates high angle thrusting in the locked upper crust under horizontal compression. The Wenchuan Ms8.0 Earthquake of 2008 took place in the middle section of the bend-thrust belt. The main shock has a high- angle-thrusting feature and the maximum coseismic displacement field concords the bend-thrust belt and the west margin of Sichuan Basin. These indicate that the bending in the east margin of Songpan-Ganzi Block and the thrusting in the west margin of Yangtze Craton are all controlled by recent convergence of these two blocks.更多还原