甘肃武威盆地南缘断裂带晚第四纪活动特征及变形分析

Activities and geomorphic deformation of the fault belt on the southern margin of Wuwei Basin,Gansu Province during the Late Quaternary

武威盆地南缘断裂带位于青藏高原东北缘的前锋地带,关于该构造带的空间展布和晚第四纪活动习性等科学问题尚不清楚。此外,1927年8.0级古浪地震也造成断裂带上的一些地表破裂。运用构造地质学与地貌学原理及年代学测试方法,展开了相关调查和研究。结果表明,①武威盆地南缘断裂带为NWW—SEE走向、呈右阶斜列式展布。②自西向东,断裂带的晚第四纪活动方式整体表现为以逆冲为主,逐步过渡为逆冲兼走滑运动或走滑活动为主;垂向上的活动强度整体上西弱东强,且中间增加的幅度最大,而断裂带的水平左旋滑移分量中-东段较大。③断裂活动具有明显的分段性,即西段的最新活动一般早于区域性T1阶地的形成时间,中-东段切割了T1阶地面,反映了断裂最新活动具有东向迁移的变化趋势。④结合露头揭示的1927年8.0级古浪地震地表破裂遗迹,该断裂中-东段的最新活动发生于全新世末期。⑤依据武威盆地南缘断裂带的晚第四纪活动方式、断面擦痕、晚新生代地层内发育的剪切节理运动学特征及共轭张性节理等,该区域的第四纪晚期最大主压应力水平方向表现为NNE—SSW向,且自西向东应力场方位略有变化。上述认识,对探索武威盆地三维变形的晚新生代构造演化、青藏高原东北缘地层生长和构造地貌变形过程,以及古浪地震的破裂机制与气候环境变迁等,提供了重要依据。

The fault belt on the southern margin of Wuwei Basin is located in the front of the northeastern margin of the Qinghai-Tibet Plateau,and its spacial distribution and activity in the Late Quaternary are still not very clear.In additon,the magnitude 8.0 Gulang eurthquake in 1927 also caused some surface reptures on the fault zone.Based on the principles of tectonic geology and geomorphology and the method of chronological test,the relevant investigation and research were carried out.The results show that the fault belt on the southern margin of the Wuwei Basin extends strikes NWW to SEE in a right-step oblique array.From west to east,the overall behavior of the fault belt during the Late Quaternary was mainly thrusting and gradually transformed to thrusting and strike-slip or strike-slipping.The vertical activity intensity was generally weak in the west and strong in the east.Moreover,middle part exhibited strongest activity,and the horizontal left-lateral slip component was great.Its activity has obvious segmentation,that is,the latest activity of the western section is generally earlier than the regional T1 terrace,and the middle-eastern section cuts the ground of the T1 terrace,which reflects the trend of eastward migration of its latest activity.Combined with the surface rupture relics caused by the 1927 M 8.0 Gulang earthquake,the latest activity in its middle-eastern section occurred at the end of the Holocene.According to the Late Quaternary activity pattern of the fault belt,face scratches,the kinematic characteristics of shear joints developed in the Late Cenozoic strata,and conjugate tension joints,the horizontal direction of the maximum principal compressive stress field during the late Quaternary was the NNE-SSW,and the orientation of the stress field changed slightly from west to east.These insights provide important basis for exploring the Late Cenozoic tectonic evolution of the three-dimensional deformation of Wuwei Basin,the strata growth and tectonic geomorphic deformation process in the northeastern margin of the Qinghai-Tibet Plateau,the rupture mechanism of the Gulang earthquake,and climate and environment changes.