南天山西段南缘断裂构造特征及对乌什凹陷发育的控制

FEATURES OF FAULT STRUCTURE IN SOUTHERN MARGIN OF WEST SEGMENT OF THE SOUTH TIANSHAN,AND ITS CONTROL TO EVOLUTION OF THE WUSHI SAG

乌什凹陷位于南天山西段以南,新生代受南天山崛起的陆内造山作用制约,与库车坳陷有相同的成盆动力学背景。因乌什凹陷南有温宿凸起阻挡了挤压应力的传播,东与库车坳陷间有北西向的构造变换带(西秋里塔格构造带)所隔,新生代变形与库车地区有不同的特征。以吉迪克组的泥岩层为滑脱面,乌什凹陷的中-新生界可分为上、下两个构造层,总体看上构造层以背冲构造为特征,下构造层则发育基底卷入的对冲构造。乌什凹陷的这一变形特征还与新生代挤压应力场自东向西迁移和走滑作用吸收了部分冲断变形有关,反映了南天山新生代构造对海西-印支期碰撞构造的继承性。文章简述了南天山造山带的构造演化,介绍了乌什凹陷周缘的主要断裂的特征,恢复了乌什凹陷中-新生代的演化并与库车坳陷进行了对比。在此基础上探讨了乌什凹陷的成盆动力学机制,分析了乌什凹陷与南天山造山带的耦合演化,指出该区的油气勘探应以三叠系为主要目的层。

The Wushi Sag situated to the south of the western segment of the Chinese South Tianshan, with the East Sub-Sag being its main body, and generally being regarded as westward extension of the Kuqa Depression （ a Cenozoic molasses basin of intracontinental orogeny in South Tianshan area）. Controlled by the Cenozoic intracontinental orogeny in area of the Tianshan Mountains, the Wushi Sag created with the same basin-forming dynamics of the Kuqa Depres- sion. In Wushi, the Cenozoic deformation had different features from the Kuqa Depression, because there was the Wensu Rise, to the south of the Wushi Sag, which resisted southward expanding of the compressional stress, and a NW-striking structural transfer belt （the West Qiultag structural belt） separated the Wushi Sag from the Kuqa Depression. The Meso-Cenozoic strata in the Wushi Sag could be divided, by the gypsum-sah bed in the Miocene Jidike Formation, into two structural layers. Generally speaking, the deformation in the upper structural layer was characterized with back-thrust, and a basement-evolved ramp thrust occurred in the lower structural layer. In addition, a thrust in the Pre-Mesozoic basement should be emphasized, since the Wensu Rise acted once a provenance to the Wushi Sag during Meso-Cenozoic. These deformation features in Wushi might be related to westward migration of the Cenozoic compressional stress, and some thrust deformation absorbed by the strike-slipping along the northern and southern boundaries of the Wushi Sag, which indicated an inheritance of Cenozoic structures to the Hercynian-Indosinian collisional tectonics in the Chinese South Tianshan. The paper expressed briefly tectonic evolution of the South Tianshan orogen, introduced main chatacterictics of boundary fractures of the Wushi Sag, reconstructed Meso-Cenozoic evolution of the Wushi Sag and compared with that of the Kuqa Depression. Based on what mentioned, the paper explored the basin-formation dynamics, analysed the coupled evolution between the Wushi Sag and the South Tianshan orogen, and pointed out that the main target for the oil-gas exploration in Wushi should be Triassic Huangshanjie Formation.