A seismic array of 80 broadband seismic stations and 16 seismic network stations was deployed in the western Alxa of Inner Mongolia and northwest of Gansu Province from October 2013 to June 2015,granted by Project of China's Seismic Array Observation on the northern South-north Seismic Zone.By use of waveform correlation method,we pick out high-quality 49052 relative traveltime residuals from 787 earthquakes recorded by the array.Then,we use the fast marching tomography method to image the P-wave velocity structure beneath the eastern Altyn Tagh fault and its vicinity down to 400 km depth.The tomographic images reveal that the crustal structure beneath the Altyn Tagh fault,Qilian orogenic belt and the Beishan orogenic belt has obvious low velocities,showing clear features of orogenic structure.The high velocity anomalies of crustal structure beneath the Alxa terrane characterize continental tectonics.The Alxa terrane was subducted to south beneath the Qilianshan orogenic belt along the northern boundary thrust,and its lithosphere frontier collided with the northward-subducted Qaidam lithosphere face to face.The eastern Altyn Tagh fault extends not northward into the Alxa terrane,instead eastward along the Alxa south-edge fault as obstructed by the rigid Alxa lithosphere.%本文利用“中国地震科学台阵探测——南北地震带北段”项目在内蒙古阿拉善西部及甘肃西北部地区布设的80个流动宽频带地震仪及16个固定台站,于2013年10月 2015年6月所记录的787个远震事件,采用波形相关方法拾取了共49052个高质量的P波走时残差数据,并利用Fast-Marching远震走时层析成像方法,反演获取了研究区下方的三维P波速度结构.结果显示:阿尔金断裂带东段、祁连山、北山地区下方地壳结构表现为低速异常特征,具有明显的造山带构造特征;阿拉善地体下方地壳结构表现为高速异常特征,为典型的大陆地壳结构特征;阿拉善地块沿着青藏高原北边界逆冲断裂(NBT)南向俯冲,其在祁连山造山带下与北向俯冲的柴达木岩石圈形成了面对面的碰撞接触关系;阿尔金断裂带的末端并没有北东向延伸到阿拉善块体,而是受到刚性的阿拉善岩石圈阻挡沿着其南缘断裂带继续向东发展.
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