三江地区北段江达弧间盆地的沉积演化

Development of Jomda Interarc Basin in the North of Eastern Tibet

江达弧间盆地位于西藏东部的三江地区北段 ,现今被厚约 10 0 0 0m的三叠系所覆盖 ,三叠纪该区以海相沉积为主 ,并以巨厚浊积岩与频繁火山活动形成的钙碱性系列弧火山岩为特征。本文进行的盆地分析基于两个方面 :一是运用层序地层学 ,通过精细沉积相与沉积相模式研究 ,在露头剖面上共识别出 10个三级层序界面和 10个三级层序 ,建立了三叠纪层序地层年代格架 ;二是沉降史分析 ,重建的构造沉降曲线上反映出两次抬升 (分别在 2 5 0Ma和2 35Ma)与二次明显的沉降 ,估算的沉积速率介于 87～ 5 37m/Ma。研究表明江达盆地演化可划分为 6个沉积充填序列 。

Jomda interarc basin is located in the north part of Three-River area, eastern Tibet, and today covered by Triassic with about 10 000 m in thickness. Marine sediments predominated, and were characterized by the largely thick turbidity rocks and arc volcanic rocks of calc alkalic suites that resulted from frequent volcanic events . The basin analyses were on the bases of two sides: (1) utility of sequence stratigraphy, based on outcrop profiles,10 three order boundaries and sequences were recognized by research of fine sediment facies and its models. Isochronous grillwork for Triassic sequence stratigraphy were established. The recognized marks of boundaries were built by the distinguished key sequence boundaries and geometric or superposed patterns of system traces, and compared isochron of sedimentary and volcanic events.10 sequence stratigraphic boundaries were discriminated with marks of tectonic and strata, including 6 Ⅰ type and 4 Ⅱ type boundaries. 10 sequences were divided, including 6 Ⅰ type and 4 Ⅱ type sequences. These sequences developed during 37 Ma, the estimated average time of a sequence is 3～7Ma, with generally 1～2 Ma, the longest about 10Ma. (2) Analyses of subsidence, the rebuilt of tectonic subsidence curves suggested that there were clearly two uplifts (at 250 Ma and 235 Ma) and two sinks. The estimated sedimentary rates were among 87～537 m/Ma. It is inferred that relations among tectonic subsidence, water depth and high frequency sedimentary velocities represented characteristics of depositional process in the arc area. It is concluded that evolution of Jomda basin were classificated into 6 depositional accumulation stages. The 6 stages were respectably: ①sedimentation of first shallow shelf; ②formation of carbonate platform; ③development of the volcanic terrigeous aubmarine fan; ④ advancement of the volcanic terrigeous shelf; ⑤ evolution of carbonate ramp and ⑥ advancement of the terrigeous shelf to delta. It is pointed out that interarc basin of Jomda experienced processes of both early extensive deepness and late compressed shallowness.