西藏日喀则白垩纪弧前盆地:沉积物和盆地演化

THE XIGAZE FOREARC BASIN (CRETACEOUS, TIBET): SEDIMENTS AND BASIN EVOLUTION

研究区位于拉萨地块及其深成岩类(冈底斯带)以南沿印度河-雅鲁藏布江缝合带的120km范围内。在圈捕洋壳或过渡地壳顶部的藏南日喀则弧前盆地的演化始于中白垩世。原先的被动陆缘沉积残余,特别是浅水碳酸盐,保存于强烈变形(缩短约65％)和部分侵蚀的盆地充填物的北缘。保存的弧前复理石沉积厚达6—8km,主要由与俯冲有关的火山弧(冈底斯带)排出的火山碎屑(安山岩质和安粗岩质)物质组成。除诸如陆棚碳酸盐之类的再搬运的盆内组分之外,较深侵蚀面处或较远源的深成岩和沉积岩均提供盆地充填物。可划分出5个主要的深海水道体系作为位置大体固定的点源。水道中的水流方向总是指向南面的生长加积楔或俯冲带,因此指示该盆地永久性地充填到外脊并逐渐变浅。弧前复理石至少可细分为三个巨层序,从宽阔而切割较深的粗粒水道充填物开始,而以半远洋泥灰岩(沉积于碳酸盐补偿深度CCD之上)和黑色页岩告终。水道的侧向迁移、水道舌状体的转换以及火山脉动产生了主要为向上变细的高频率旋回。弧前盆地内的海相沉积作用于麦斯特里希特期(Maastrichtian)或古新世结束,代之以富含源自深侵蚀的岩浆弧的粗碎屑的始新统一渐新统秋乌组(与凯拉斯和更西的印度河磨拉石等时)河流沉积。因为弧前复理石和磨拉石型秋乌组均在中新世(?)同期变形,我们认为秋乌组代表海相弧前盆地充填作用在大陆的继续,如加利福尼亚大峡谷弧前盆地中观察到的一样。

The evolution of the Xigaze forearc basin in southern Tibet, as investigated in a 120 km segment along the Indus—Yarlung suture zone, south of the Lhasa block and its plutonic rocks (Gangdise belt) commenced in the mid-Cretaceous on top of trapped oceanic or transitional crust. Relies of the former passive margin sediments, especially shallow-water carbonates, are preserved along the northern edge of the strongly deformed (approximately 65 shortening) and partially eroded basin fill. The preserved foreare flysch sediments reach a thickness of 6 to 8 km and consist to a large degree of volcaniclastic (andesitic to latitic) material shed from the subduction-related volcanic arc(Gangdise belt). Particularly in the west, plutonic and sedimentary rocks from deeper erosion level or more distal sources contribute to the basin fill, apart from reworked intra-basinal components such as shelf carbonates. Five major deep-sea channel systems could be identified, which acted as point sources of more or less constant position. Currents in the channels were always directed southward to the growing accretionary wedge of the subduction zone, thus indicating that the basin was permanently filled up to outer ridge level and therefore gradually shallowing. The forearc flysch may be subdivided into at least three mega-sequences, which begin with wide and relatively deeply cut, coarse-grained channel fills and end with hemipelagic marls (deposited above the CCD) and dark shales. Lateral channel migration, channel lobe switching, but also volcanic pulses generated a predominantly fining-upward high-frequency cyclicity. Marine sedimentation in the forearc basin terminated in the Maastrichtian or Paleocene. It was replaced by the fluvial deposits of the Eocene-Oligocene Qiuwu Formation (time-equivalent to the Kailas and Indus molasse farther west) rich in coarse gravel derived from the deeply eroded magmatie arc. Because both forearc flysch and molasse-type Qiuwu Formation were deformed simultaneously during the Miocene (?), we assume that the Qiuwu Formation represents the continental continuation of the marine forearc basin filling, such as also observed in the Great Valley forearc basin in California.