巴颜喀拉-川西边缘前陆盆地演化

EVOLUTION OF THE BAY AN HAR-WEST SICHUAN PERIPHERAL FORELAND BASIN IN SOUTHWESTERN CHINA

关键词:  巴颜喀拉-四川盆地  中晚拉丁期转折  边缘前陆盆地演化

Key words:  Bayan Har-Sichuan Basin  transition during the middle and late Ladinian  evolution of the peripheral foreland basin

摘要:

巴颜喀拉古特提斯洋的消亡过程反映在巴颜喀拉残留盆地到边缘前陆盆地转化的沉积记录中。鉴于这个前陆盆地与其向克拉通延伸的组成部分——四川盆地现为龙门山逆冲带所分隔,以致已往的沉积盆地研究多将其相割裂,本文将结合巴颜喀拉洋的消亡过程,把这两个盆地视为一个统一整体来加以分析,研究其演变历程。
晚二叠世,扬子板块向西楔入的同时,发生向北（昆仑-柴达木陆块）和向南（羌塘-昌都陆块）的双向俯冲消减。本文提出了巴颜喀拉洋的主体闭合,从而开始转化为边缘前陆盆地阶段的时间是在拉丁（T₂²）中晚期,而不是晚三叠世的见解。这点可由拉丁中晚期时,四川盆地川中广大地区形成与前陆挠曲沉降相对应的前陆隆起得以证明。此时期发生的前陆沉降,结束了被动边缘的饥饿（T₁—T₂¹）沉积盆地状态,充填了厚逾2,000—10,000m的类复理石沉积,并向扬子克拉通边缘超覆。随着逆冲带的由北向南推进,在诺利一瑞替期形成了滨海含煤磨拉石和陆相含煤磨拉石（逆冲褶皱带地区大多后期被剥蚀）。晚三叠世中晚期,逆冲带侵位推进到四川盆地西部边缘的龙门山地带,从而前陆盆地迁移入四川盆地内,进入陆内汇聚的后造山陆相磨拉石前陆盆地阶段。晚白垩世一早第三纪,因四川盆地晚期的抬升,这一前陆盆地便逐渐萎缩消亡。

Abstract:

The consumption of the Bayan Har Ocean in the Palaeo-Tethys was reflected in the depositional records of transition from remnant oceanic basin to peripheral foreland basin. Since the Bayan Har foreland basin was separated from its extended part toward Yangtze craton (Sichuan Basin) by the present-day Longmenshan overthrust zone,the previous study of each sedimentary basin was carried out independently.In this paper,an integrated study has been conducted so as to give a viable explanation for the evolution of the two basins.
While the Yangtze plate as a subduction one wedged westwards during the Late Permian,there occurred bidirectional subduction toward the Kunlun-Qaidam landmass in the north and toward the Qiangtang-Qamdo landmass in the south.The authors contend that the initiation of the transition from the remnant oceanic basin to peripheral foreland basin, when the bulk of the Bayan Har Ocean was closed,dates back to the middle-late Ladinian (T₂²)rather than the Late Triassicas is interpreted by most geologists at home.This new idea is supported by the formation of the foreland bulge in the vast area of central Sichuan basin in correspondence with the flexural subsidence of the foreland basin during the middle and late Ladinian.The subsidence of the foreland basin led to the termination of the starved sedimentary basin(T₁-T₂¹)on the passive margin,where more than 2000- to 10000 m thick flyschoid deposits were laid down and overlapped onto the Yangtze cratonic margin.The littoral coal-bearing molasse deposits and continental coal-bearing molasse deposits were accumulated in response to the progradation of the overthrust zone from north to south during the Norian and Rhaetian.The foreland basin was migrated into the area of western Sichuan Basin during the middle and late Late Triassic when the evolutionary stage of the postorogenic foreland basin with the continental molasse commenced under the influence of intracontinental convergence.Finally,the later uplifting of the Sichuan Basin permitted the termination of the foreland basin during the Paleogene.