准噶尔盆地东北缘乌伦古坳陷中生代逆冲推覆构造

Mesozoic Thrust and Nappe Structures in the Wulungu Depression of the Northeastern Junggar Basin

乌伦古坳陷位于准噶尔盆地的东北缘,中生代以来逆冲断裂特别发育。本文通过对典型二维地震剖面进行构造解释,结合平衡剖面的正演方法,对乌伦古坳陷内逆冲推覆构造的分带性、分期性和拓展方式及动力过程进行研究。研究认为乌伦古坳陷内部逆冲推覆构造从北向南分为根部带的红岩断阶带、中部带的索索泉凹陷、前锋带的喀拉萨依断裂。剖面平衡结果显示,从晚三叠世到白垩纪,准噶尔盆地东北缘受到自北向南的挤压作用,逆冲断裂整体表现出基底卷入的前展式叠瓦逆冲的特点,具有从北向南拓展规律,即北部断裂活动较早,而南部断裂活动较晚。对比平衡剖面不同时代地层的收缩率变化可知,逆冲断裂活动总体可以划分为4个演化阶段:印支末期、燕山早期、燕山中期和燕山晚期。其中印支期是逆冲体系的启动期;燕山早期为主活动期,逆冲活动波及全区;燕山中期逆冲活动表现为震荡特征;燕山晚期逆冲活动微弱。中生代前期,乌伦古坳陷地区的逆冲体系形成的动力主要来自南阿尔泰造山带持续挤压,中生代后期叠加了北天山造山带自南向北的推覆挤压,使坳陷南部喀拉萨依断裂具有了走滑性质,该走滑活动与准噶尔内部微块体之间发生挤出-逃逸的调节过程有关。

Located in the east edge of the Junggar Basin, the Wulungu Depression has developed many reverse faults since Mesozoic. Based on the structural interpretation of the typical two-dimensional seismic sections, combined with the forward modeling method of balanced cross-sections, the zonation, staging, propagation mode and dynamic processes of thrusting of the Wulungu Depression were investigated. The thrust and nappe structures of the Wulungu Depression, from north to south, are divided into the Hongyan Imbricated Thrust Belt in the root, the Suosuoquan Sag in the central part, the Kalasayi Fault in the front. The balanced and restored cross-sections show that Late Triassic to Cretaceous thrust system, which suffered from the S-N directed compression in the east edge of the Junggar Basin, has the characteristics of the imbricate basement-involved thrusting. The thrust system developed forward, propagating from the north to the south. That is to say, the fault activities in the north are relatively earlier than those in the south. According to stratum shrinkage rates at different stages, the evolution of thrust system can be divided into four stages, i.e. the Late Indosinian, the Early, Middle and Late Yanshanian. Fault analysis may suggest that the thrust system started in the Indosinian, the Early Yanshanian is the major active period when thrusting covered all over the study region, while the thrusting of the Middle Yanshanian is various, whereas the fault activity is weak in the Late Yanshanian. At the early Mesozoic, the thrust system in the Wulungu Depression underwent continuous compression from the southern Altaid Orogenic Belt. Later, it is still influenced by northward indentation from the northern Tianshan Orogenic Belt, which makes the Kalasayi Fault at the south of the Wulungu Depression to be a strike-slip fault, resulting from the extrusion-escape processes of adjustment among the micro-blocks within the Junggar Basin.