西南天山前缘乌拉根背斜南翼逆断层的发现及其地质意义

THE DISCORVERY AND GEOLOGIC SIGNIFICANCE OF THE THRUST AT SOUTH LIMB OF THE WULAGEN ANTICLINE,SOUTH MARGIN OF THE SOUTHWESTERN TIANSHAN

塔里木盆地新疆喀什以西部分是西南天山和帕米尔两大对冲构造系统的会聚带,关于两者变形前缘和分界的确切位置存在不同认识。在乌恰县以南的玛依卡克盆地南缘,清晰可见属于帕米尔构造带、向N或NNE逆冲的帕米尔前缘逆冲推覆体(PFT)。最近野外调查在盆地北部发现了西南天山前缘的最新变形带:向南逆冲的乌拉根背斜南翼断层。断层总体近EW走向,在克孜勒苏河南岸和别尔托阔依河两岸河流阶地上形成了长约12km、坡向南的逆断坎,近地表断层产状为6°∠15°。由于乌拉根背斜及其南翼断层的活动,断层上盘阶地面上形成了多条弯滑断层,并发生了明显的反向掀斜。高阶地形成以来(区域对比表明,该阶地与木什背斜北翼T2大致为同一期阶地,其光释光年龄约为16ka),西南天山南缘吸收的总缩短量约71.4km,平均缩短速率约4.5mm/a。

The western Tarim Basin is a convergent zone of the Southwestern Tianshan and the Pamir,and there have been big debates about its exact boundary.However,in the Mayikake Basin,the boundaries of two tectonic systems are very clear：the north-vergent Pamir Front Thrust is the leading edge of the Pamir,and south-vergent thrust at south limb of the Wulagen anticline,which was discovered in recent field study,is the south margin of the Southwestern Tianshan.The thrust created 7.5～17.6m high scarps on the Tk3（the high terrace of the Kezilesu river）and Tb3（the high terrace of the Bieertuokuoyi river）,with an occurrence of 6°∠15°.To the west,the thrust cuts all terraces of the Bieertuokuoyi river and the underlying youngest alluvial fans ultimately.The total length of thrust trace is about 12km.As activity of the thrust,lots of subparallel flexural-slip scarps are formed on terrace surfaces,which make terrace surfaces obviously differential back-tilted（tilted to south）,and the locations of tilted degrees changed are corresponding to locations of flexural-slip faults.Shortening at south limb of the Wulagen anticline is absorbed by the thrust and flexural-slip faults,which is about ～71.4m since abandonment of the high terrace.Regional correlation indicates the high terrace is the same surface as the T2 located at north limb of the Mushi anticline with the age of～16ka,which indicates the average shortening rate of south limb of the anticline in late Quaternary is～4.5mm/a.