楔形构造在山前冲断构造位移量消减中的作用——以准噶尔盆地南缘为例

The Effect of Wedge Structure on Displacement Subduction of Piedmont Thrust Structure:A Case Study of the Southern Margin of Junggar Basin

介于复活的天山造山带与稳定的准噶尔克拉通之间的准噶尔盆地南缘前陆冲断带,是印度板块与欧亚大陆碰撞的远距离效应产物,也是新近纪以来青藏高原隆升并向北推挤的直接结果。前陆冲断带吸收了来自造山带的水平缩短构造位移量后,克拉通一侧构造趋于稳定。准噶尔盆地南缘与世界上多数前陆冲断带构造地质特征相似,通过区域地震剖面的精细构造几何学和运动学解析,发现其中的楔形构造非常典型,是前陆冲断带内部冲断构造位移量消减的主要方式之一,控制着前陆冲断带分布范围和变形方式。准噶尔盆地南缘构造变形主要由南侧的天山造山带向北逆掩冲断,但是大部分冲断构造位移量是通过楔形构造反向传递后消减。紧邻天山北麓的齐古-喀拉扎-昌吉等构造带,山前深部的楔形体沿侏罗系西山窑组煤层向北扩展过程中,部分位移量沿构造楔顶部的反冲断层向南消减,并切割上覆地层形成第一排背斜带,另一部分位移量则继续向北传递,在断坡位置引发褶皱变形,形成霍-玛-吐第二排构造带和安集海-呼图壁第三排背斜带。准噶尔盆地南缘第二、三排构造带中-新生界内部发育多个小型的构造楔型体,这些互相叠置的楔型构造横向延伸不大,加大了构造变形的复杂性和构造圈闭识别的难度。

The foreland thrusting belts in the southern margin of Junggar Basin, lying between the stable Junggar craton and rejuvenated Tianshan orogen, is the product of long distance effect of collision between India plate and Eurasia plate in the Cenozoic, and also the directive result from the Tibet plateau uplifting and pushing toward north. The structure on one side of the craton tended to be stable after the foreland fold and thrust belts absorbed the thrust structural displacement resulted from orogen. The southern margin of Junggar basin shares similar geological characteristic as many other foreland thrusting belts in the world. Through analyses of fine geometry and kinematics of structural geology in south Junggar basin, it is found that the unique wedge structure is one of major approaches to absorb thrust displacement within the foreland thrust faulting zone and controls the scope of thrust distribution and deformation style. The structural deformation in the basin is dominated by northward overthrusting of the Tianshan orogenic belt in south, but most displacement was consumed by backward movement of the wedge structure. During northward expansion of the wedges along the Jurassic Xishanyao Formation coal seam in the deep foreland close to the Qigu-Kalazha-Jichang structural zone in north Tianshan Mts, part of displacement was absorbed southward along overthrusting layer at top of structural wedge and cut overlying strata to form the first row of anticline zone. And part of displacement propagated northward and deformed, triggering folding deformation at faulted slope and forming the Huo-Ma-Tu structural zone （the second row） and Anjihai-Hutubi the anticline zone （the third row）. Within the Cenozoic of the second and third structural zones of the southern margin of Junggar Basin developed many small structural wedges. These superimposed wedges have limited horizontal extension, and this adds much difficulty in complexity of structural deformation and identification of structural traps.