中生代羌塘前陆盆地充填序列及演化过程

中生代羌塘前陆盆地位于青藏高原巨型造山带内 ,夹于金沙江缝合带与班公湖—怒江缝合带之间 ,是一个与两侧缝合带逆冲作用相关的沉积盆地 ,由羌北盆地 (对应于金沙江缝合带 )、羌南盆地 (对应于班公湖—怒江缝合带 )和中央隆起带构成 ,其中中央隆起是北部前陆盆地和南部前陆盆地共有的前陆隆起 ,显示为对称型复合前陆盆地 ;该盆地形成于晚三叠世 ,并持续发育至早白垩世 ,盆地中充填了巨厚的同构造期的复理石和磨拉石 ,具有总体向上变粗变浅的充填序列 ,以不整合面可将其划分为 5个由顶底不整合面限制的构造层序 ,其中晚三叠世诺利期构造层序对应于金沙江缝合带主碰撞期 ,晚三叠世瑞替期构造层序对应于金沙江缝合带碰撞闭合后冲断抬升 ,早侏罗世构造层序对应于班公湖—怒江缝合带初始逆冲推覆 ,中侏罗世—早白垩世构造层序对应于班公湖—怒江缝合带主碰撞期 ,中白垩世构造层序为班公湖—怒江缝合带碰撞闭合后冲断抬升与金沙江缝合带冲断抬升的产物 ,为中生代羌塘盆地关闭后的磨拉石建造

FILLED SEQUENCE AND EVOLUTION OF THE MESOZOIC QIANGTANG COMPOSITE FORELAND BASIN IN THE QINGHAI-TIBET PLATEAU

Mesozoic Qiangtang Foreland Basin is located between the Jinshajiang and Bangong-Nujiang suture belts. It is an important petroliferous basin in the northern Qinghai-Tibet Plateau, and belongs to composite foreland basins according to Jordan's classification of the foreland basin(1988). The foreland basin is filled with 3000—6000m thickness of Late Triassic to Middle Cretaceous marine sediments. The filled strata in the basin indicate that the basin texture looks like a symmetric body, and can be subdivided into three tectonic geomorphic units from north to south, i.e. the northern basin, central uplift, and southern basin. The depth of the northern basin varies from 3000 to 6000m,the depth of the central uplift from 0 to 1000m, and the depth of the southern basin from 3000 to 5000m, showing that the the Mesozoic filled strata are thicker in the north and the south of the basin, but thinner in the central uplift of the basin. So there are two centers of subsidence of the basin, both of which are located in the south and north foredeep belts, lying in the front of the suture belt. The basin is a foreland basins between suture belts and belongs to typical symmetric foreland basin. This kind of basin geometry allows very thick synsedimentary flysch and molasse sediments to be preserved and is related to thrusts in suture belt. The provenance and constituent of the clasts in sandstone and conglomerate indicate that there are three provenances in the foreland basin, including Jinshajiang suture belt, Bangong-Nujiang suture belt and the central uplift which formed in the Late Triassic. There are three marginal facies belts developing in the basin. Two of them are distributed in the front of suture belt and one along the both sides of central uplift. The sedimentary facies can be symmetrically subdivided into four facies belts from central uplift to north or to south, namely central uplift, foreland slope, fore-deep, and the foothill of the thrust belt. The statistical measured data on cross-bedding, rippled mark and flute mold indicate that there are two palaeocurrent systems and drainage distribution patterns in the basin and the central uplift was a drainage divide. The filled sequence of the foreland basin of Late Triassic to Middle Cretaceous age can be divided into five tectonic sequences by unconformity. A tectonic sequence is a body of genetically related strata isolated by unconformity, deposited in a basining stage responding to a collision or thrusting episode of suture belt. The Norian tectonic sequence, which is characterized by flysch, marked the collision of the Jingshajiang suture belt. The Rhaetian tectonic sequence, which is characterized by molasse, marked thrusting of the Jingshajiang suture belt. The Early Jurassic tectonic sequence, which is characterized by flysch, marked thrusting of Bangong-Nujiang suture belt. The Middle Jurassic to Early Cretaceous tectonic sequence, which is characterized by molasses, marked the collision of Bangong—Nujiang suture belt. The Middle Cretaceous tectonic sequence is characterized by collisional successor molasses.