西藏西南部札达盆地新近纪沉积序列研究

札达盆地为一个藏南晚新生代断陷盆地，基于岩性岩相、古流向和物源分析，认为札达盆地主要经历了裂陷充填期(9.2~7.8 Ma)，稳定发展期(7.8~2.6 Ma)和裂后消亡期(2.6~1.7 Ma)。沉积相主要有辫状河相、淡水湖泊相和冲积扇相，以湖相为主；古流向由南西向转变为盆地周缘指向湖盆中心，发展到最后为南东向；物源主要来自北侧的阿伊拉日居山地区，古地势由北东高南西低，经差异抬升变为北西高南东低，沉积中心位于湖盆南缘。至约1.7 Ma之后，贡巴砾岩的出现代表了札达盆地的消亡。札达盆地的演化表明藏南裂陷盆地经历了拉张形成、湖盆最大化而后快速消亡的过程，揭示了藏南在获得最大高度后进一步构造伸展垮塌的演变历程。
 

Neogene Sedimentary Succession in the Zanda Basin,Southwest Tibet

The Zanda Basin is a Neogene rifted basin in South Tibet.On the basis of previous vertebrate palaeontology,palynology,magnetostratigraphy,and ESR dating research,the age interval can be bracketed between ~9.2 and 1 Ma through strata correlation.Based on lithology,paleo-current measurements and provenance analysis,this paper studies sedimentological succession of the Neogene Basin,and three sedimentological phases are identified: initial rift filling phase(9.2~7.8 Ma),stable subsidence phase(7.8~2.6 Ma) and subduction phase(2.6~1.7 Ma).During 9.2~7.8 Ma,the braided river lithofacies with an SWS paleo-current predominates in the Lower Tuolin Formation,and the sediment sources are mainly in the north-esatern side of the basin,the sedimentary centrer is located at the southside of the basin.Then,the lacustrine lithofacies dominates in the basin,accompanied by lake-delta lithofacies,with a basin-centric paleo-current in the Upper Tuolin Formation from 7.8 Ma to 2.6 Ma,the lake area reach to maximum,and the source of the sedimentary basin comes from mountains surrounding the basin margin.During 2.6~1.7 Ma,the alluvial-fan lithofacie dominates,with a SES paleo-current in Xiangz Formation,implying the the source material are from southwestern side of the basin,and the facies change from fine lacustrine to coarse alluvial fan rapidly.The Pleistocene Gongba Formation is in angular unconformity with the underlying Xiangz Formation,implying end of the sedimentary basin.In summary,the lithofacies compose of braided river,lacustrine and alluvial-fan,dominated by lacustrine lithofacies.Paleo-current directions change from SW(lower part) to SE(upper part),with a basin-centric direction in the middle part.Furthermore,regional geography changes from northeast high,southwest low to northwest high,southeast low,and the sedimentary central is located at the south margin of the basin.After 1.7 Ma,the Pleistocene Gongba Formation appears and implies disappearance of the basin.It indicates that the rift-sag basin in southern Tibet rises firstly,forming a paleo-lake,and then falls abruptly,dating east-west extensional tectonic evolvement when the plateau attains its maximum elevation.