准噶尔盆地南缘山前带阿什里背斜精细构造建模

准噶尔盆地南缘山前冲断带经历了多期叠加构造活动,构造变形特征复杂,对研究陆内造山变形机制具有重要意义。阿什里背斜处于北天山后方前陆部位,构造样式为分层滑脱变形体系控制的复式叠加背斜,垂向上包括浅层薄皮推覆构造系统和中深层复合构造楔系统。钻井和地震反射信息揭示,阿什里地区主要滑脱层为基底滑脱层,石炭系、二叠系泥岩层,中下侏罗统八道湾组、西山窑组煤层。阿什里背斜侏罗系底部不整合面受基底发育的叠加构造楔(由2~3个冲断席构成)控制,反冲断层之上石炭系-三叠系构成不对称背斜。阿什里西南大型石炭系推覆体之下发育泥盆系-石炭系组成的冲断席,构成(楔端点向上方突破的)构造楔。阿什里背斜北侧以一向斜与喀拉扎背斜过渡,指示冲断位移沿浅部滑脱层向北继续传播。阿什里及邻区发育的石炭系与三叠系-中下侏罗统不整合、二叠系内部不整合、二叠系与三叠系削截不整合、三叠系与侏罗系不整合、新近系与第四系不整合揭示了中-晚二叠世以来多期构造活动。其中,阿1井核部二叠系梧桐沟组之下钻遇的凝灰岩锆石SHRIMP U-Pb同位素分析结果显示其年龄为289.1±7Ma(95%置信度),指示了晚海西期的构造活动。根据阿什里地区地震剖面的精细构造几何学、运动学解析,结合关键不整合面,划分了5个关键构造演化期次:中二叠世阿什里西南逆冲推覆形成古隆起;晚二叠世-晚三叠世阿什里地区存在两期小规模冲断活动;侏罗纪整体稳定沉降或弱坳陷;白垩纪-古近纪多幕隆升构造活动使阿什里地区沿基底发育叠加构造楔;中新世北天山剧烈造山活动中阿什里基底构造楔向北突破形成阿克屯-喀拉扎背斜。

Structural modeling of the Ashili anticline in the piedmont thrust belt of the southern margin of Junggar Basin

The southern Junggar fold and thrust belt experienced multiple stages of superposing structural activities which resulted in the complex nature of tectonic deformation.It is of great significance to study the mechanism of intracontinental orogenic deformation.The Ashili anticline is located in North Tianshan rear-foreland position,and its structural style is a duplex-superimposed anticline which formed multiple detachment level system,including a thin-skinned nappe structure in the shallow part and a superposing structural wedge in the middle-deep part.It develops main detachment layers in the basement,the Carboniferous and Middle Permian mudstone layers and Lower-Middle Jurassic(Badaowan and Xishanyao formations)coal layers as shown in drilling and seismic reflections.It is worth noting that the deformation of the bottom Jurassic unconformity is controlled by a huge superposed structural wedge which consists of 2~3 thrust sheets.The Carboniferous-Triassic strata above this tectonic wedge’s roof is shown as an asymmetrical fold.To the southwest of Ashili anticline,a great Carboniferous nappe which is composed of a tectonic wedge(the wedge endpoint broken upward)and its interior stratum is supposed to be the Devonian-Carboniferous strata.To the north,a syncline between Ashili and Kalazha anticline which indicates the displacement propagated northward by shallow slip horizon.The evidence of multiple tectonic deformations from Middle-Late Permian lies in the unconformities between Carboniferous and Jurassic(Middle-Lower)strata,internal Permian strata,Upper Permian and Triassic strata,Upper Triassic and Jurassic strata,Neogene and Quaternary strata.The zircon SHRIMP U-Pb isotope analysis of the tuff zircon drilled under the Wutonggou Formation of the Permian in Well A1 shows that the age of 289.1±7Ma(95%confidence)which indicates the Late Hercynian tectonic activity.Through analyses of fine geometry and kinematics of structural geology in Ashili and depending on the critical unconformity,it is suggested that from Late Permian to present,five stages of tectonic activities can be subdivided.The first stage occurred during the Mid-Late Permian,and the paleo-uplift was formed in the south-west of Ashili anticline by a series of intense thrusting nappe activities.What happened in the second stage is that two phases of small scale thrusting in Late Permian to Late Triassic.After that,this zone experienced a stable subsidence or weak extension stage from Early-Middle Jurassic.During Late Cretaceous to Paleogene,this zone underwent poly-phase orogenies which brought about the formation of a superposed wedge along the basement.The last stage started from Neocene to present,as the North Tianshan rose,the Ashili anticline basement tectonic wedge broke through to the north which formed the Kalazha-Aketun anticline.