塔里木地块北部横向构造及断条模式

提要:塔里木盆地北部与西南天山毗邻区域发育的浅部构造系统是喀什坳陷—柯坪塔格逆冲推覆构造带—库车前陆坳陷冲断带,它们在山前以1～3排方式平行造山带展布,是一套由底板滑脱断坪和断坡构成的逆冲推覆系统,具有明显的横向分段性;该区深部构造系统却是一组以北西向为主展布的断条构造,并可以划分出4个一级断条构造。笔者从构造层变形、重力、航磁异常场及天然地震平面分布密度等几个角度,对区内深部横向构造系统进行了研究,并利用天然地震深度/频次统计结果,识别出圈层拆离解耦面的深度分布,进而探讨了深部横向构造运动的岩石圈地壳圈层归属,对区域北西向横向构造系统做了详细的论述。本文选择喀拉玉尔滚断裂带和库尔勒断裂带进行重点解剖,研究了横向构造与浅部构造的转换关系,最终提出了塔里木地块北部断条构造几何学运动学模式,即塔里木地块深部向北西方向运动受到古西南天山的阻挡性约束而“被迫”俯冲,岩石圈地壳发生拆离解耦,原有的横向构造——北西向构造带(断裂带)被激活,使得俯冲系统以断条为单位进行俯冲;在俯冲过程中,岩石圈地壳的拆离及横向构造被激活的方向是从约束体(南天山)附近开始,向塔里木盆地(北部)的腹地方向推进,因此,越是接近造山带区域,断条俯冲状态的差异越明显,横向分段性越突出;而正因为塔里木地块以断条形式向南天山的俯冲行为,使得山前坳陷冲断带(及天山)发生分段。

Transverse structure and model of fault slivers in the northern part of the Tarim block

The shallow tectonic system in the northern Tarim basin adjacent to the southwestern Tianshan Mountains consists of the Kashi depression, Kalpin Tagh thrust nappe tectonic belt and Kuqa foreland depression thrust belt, which are distributed in the form of 1-3 rows parallel to the orogenic belt in the mountain front. It is also a thrust nappe system composed of the basal detachment fault flat and fault ramp, showing distinct E-W-directed transverse segmentation. The deep tectonic system is marked by a set of NW-trending faults, which divide the area into four fault slivers. The authors systematically study the deep transverse tectonic system in the area in the contexts of the structural layers, gravity and aeromagnetic anomalies and density of earthquake occurrences and recognize the depths of detachment surfaces of the lithospheric layers by using the statistical results of the depth and frequency of the earthquakes. On that basis, the authors discuss the regional NW-trending transverse tectonic system in detail. In the paper, the Karayulgun fault belt and Korla fault belt are selected for the study of the transformation relation between the transverse structure and shallow structure and finally a geometric and kinematic model of the fault slivers in the northern part of the Tarim block is proposed, i.e.:the Tarim block is "forced" to be subducted as the NW-directed movement of its deep part is hindered and constrained by the ancient southwestern Tianshan, thus resulting in decoupling of the lithospheric crust, activation of the original transverse tectonic system (NW-trending fault belt) and subduction of the subducting system in the form of fault slivers; in the process of subduction, the detachment of the lithospheric crust and activation of the transverse structure started near the "constraining body" (southwestern Tianshan) and progress toward the interior of the Tarim basin (northern part), and therefore the difference in subduction state between the fault slivers is more obvious and the transverse segmentation is more prominent toward the orogenic belt. It is just because the subduction of the Tarim block in the form of fault slivers that segmentation of the piedmont depression thrust belt (and Tianshan belt) takes place.