贺兰山西麓断裂右旋走滑的地质地貌证据及其构造意义

摘要

贺兰山西麓断裂的水平运动性质对厘定阿拉善地块与华北地块的现今界线,探讨青藏高原向NE扩展的影响范围均具有重要意义.通过野外地质地貌调查发现:贺兰山西麓断裂切割了新近系背斜的西翼,干河沟组和清水营组之间的地质界线被右旋错动,位移＞ 800m;在断裂附近的第四纪洪积高台地上,多处发育了与主断裂相交的次级张性节理(裂隙),其锐角指示主断裂具有右旋走滑性质;贺兰山西麓断裂南端发育的与主断裂斜交的正断层,表明断裂西盘向N运动并在端部形成拉张调整区,反映了主断裂水平运动为右旋走滑;形成于不同时期不同规模的冲沟跨断裂发生了明显的右旋扭动.因此,贺兰山西麓断裂的水平运动是右旋走滑,而非前人认为的左旋走滑.从断裂活动和新生代地层变形的相互关系分析,认为晚新生代以来,在贺兰山西麓断裂附近存在2个阶段的构造变形:即早期褶皱变形,后期断裂活动.这2次构造变形是青藏高原对阿拉善地块的持续推挤,导致其向NE侧向挤出的结果.青藏高原扩展的影响范围在上新世末已抵达贺兰山西麓地区,并导致贺兰山西麓断裂的右旋走滑运动,形成了阿拉善地块和华北地块的现今边界,也是青藏高原扩展的最新前缘.%The horizontal movement of the Helan Shan west-piedmont fault is important to determination of the present-day boundary between the Alashan and North China blocks as well as to the exploration of the extent of the northeastward expansion of the Tibetan plateau.Field geological surveys found that this fault cuts the west wing of the Neogene anticline,which right-laterally offset the geological boundary between Ganhegou and Qingshuiying Formations with displacement over 800m.The secondary tensional joints (fissures) intersected with the main faults developed on the Quaternary flood high platform near the fault,of which the acute angles indicate its dextral strike slip.The normal faults developed at the southern end of the Helan Shan west-piedmont fault show that the west wall of this fault moves northward,and the tensional adjustment zone formed at the end of the strike slip fault,which reflects that the horizontal movement of the main fault is dextral strike slip.The dextral dislocation occurred in the gully across the fault during different periods.Therefore,the Helan Shan west-piedmont fault is a dextral strike slip fault rather than a sinistral strike slip fault as previous work suggested.The relationship between the faulting and deformation of Cenozoic strata demonstrates that there were two stages of tectonic deformation near the Helan Shan west-piedmont fault since the late Cenozoic,namely early folding and late faulting.These two tectonic deformations are the result of the northeastward thrust on the Alashan block by the Tibet Plateau.The influence range of Tibetan plateau expansion has arrived in the Helan Shan west-piedmont area in the late Pliocene leading to the dextral strike slip of this fault as well as formation of the current boundary between the Alashan and North China blocks,which is also the youngest front of the Tibetan plateau.