西藏南部晚侏罗世—白垩纪沉积与构造背景探讨

THE LATE JURASSIC-CRETACEOUS SEDIMENTARY- TECTONIC DEVELOPMENT OF THE SOUTHERN XIZANG

根据岩石组合特征、沉积构造和沉积相序的分析,结合岩石化学测试数据和粒度分析结果,将西藏南部的晚侏罗世一白垩纪地层划分为5个沉积区,从南往北依次为:喜马拉雅陆棚-外陆棚沉积区;拉轨岗日斜坡沉积区;雅鲁藏布深海盆地沉积区;日喀则弧前盆地沉积区;拉萨弧间盆地沉积区。进一步确定了西藏南部沉积与构造演化的时空关系。

In this paper, based on the field work in 1988, the authors suggested some ideas about late Jurassic to Cretaceous sedimentary-tectonic development of the southern Xizang as follows.The Upper Jurassic and Lower Cretaceous in southern Xizang could be divided into five tectonic-sedimentary area, they are: 1. north Himalaya area, which was the north continental margin of the Gandwanalandj 2. Larguigan-gri area, corresponding to the continental slope; 3. Yarlung Zangbo area, representing the oceanic basin of the 'Himalaya-Tethys'; 4. Xigaze area, which was the forearc basin in front of the Gangdesi magmatic arc; 5. Lhasa area, dominated by the late Jurassic to Cretaceous intraarc basin.During late Jurassic, with the spreading of 'Himalaya-Tethys and the thinning of the crust in this area, a deep-water oceanic basin was formed. In the both side of the oceanic basin, the different continental margins occurred. In Himalaya area of the south side, a series of stable-type clastic and carbonate sediments accumulated on the continental shelf, in which the carbon-ate-plateform facies and tidal-flat facies were dominant. Northward, in the Larguigangri area there were some deeper water sediments, represented by siliceous rocks, deep-water red mudstone and carbonate rocks, which implied the incompensated sedimentation on the continental slope. Meanwhile, some deep-water deposits, such as radiolaria- bearing abyssal mudstone and siliceous rocks accumulated in Yarlung Zangbo area, associated with a large number of ophiolites and olistostromes, marked the position of a oceanic basin. Above mentioned showed that the sea-water gradually deepened from south to north, and the tectonic-paleogeographical framework was of a transition from the shallow sea of continental shelf through deeper sea of continental slope to the abyssal oceanic basin, all of which indicated the future of a passive continental margin on the south side of 'Himalaya-Tethys' ocean. On the north side, across the Gangdise magmatic arc, some carbonate-plate-form type limestone of Duodigou Formation accumulated in the Lhasa intraarc basin.In early Cretaceous, the extension of 'Himalaya-Tethys' reached the maximum, which led to widespread deepen of sea water. In Himalaya area, for example at the Gucuo section, we could find the early Cretaceous thick black shale with a lot of large ferriconcretions, reminicent of the Oceanic Anoxic Event in Mid-Cretaceous of the world. At the same time, we might see some thick black shale, siliceous shale with large number of pyrite, yielding belemnite and ammonoides in Larguigangri area, in which there were many deepwater sedimentary structures,such as submarine slides and olistostromes, representing the deep-water deposition on the continental slope. To the north, lower Cretaceous was characterized by deep-water sediments of oceanic basin as upper Jurassic. But there were a series of onshore -nearshore ocal-bearing clastic and shallow sea shelf carbonate-clastic rocks of early Cretaceous in Lhasa intraarc basin, in which included also some delta and tidal-flat deposits.By late Cretaceous, while the'Himalaya-Tethys'ocean subducted northward, the southern Xizang remained the tectonic-sedimentary framework as previous, but the sea-water began to shallow, particularly in Himalaya area, that the deep-water black shale was replaced by the shallow sea shelf-facies carbonate-terristrial clastic rocks. In the same time, some flysch deposits, represented by the Ngamring Formation, accumulated in Xigaze forearc batin. It is worthy to point out that there were some distinctive sedimentary structures such as groove marks, load casts and graded sequenced in lower part of Ngamring Formation, indicating the deep-water turbidite flysch deposits. But in its upper part, the ripple marks, storm graded bedding and shallow water trace fossils were discovered, showing that sedimentary environment of upper part had been rather shallow sea than deep-water, and the basin had begun to uplift in middle late Cretaceous. It may be suggested that the collision of the Himal