冈底斯弧弧后早白垩世裂谷作用的沉积学证据

Sedimentologic Evidence for Early Cretaceous Back-arc Rifting of the Gangdese Magmatic Arc, Tibet, Western China

冈底斯弧弧后地区早白垩世地层的一个显著特点是 ,由下而上普遍从陆相 -海陆交互相碎屑岩变化为海相碳酸盐岩。该地区在早白垩世中期开始了广泛的海侵 ,沉积范围由早期仅局限于班公湖 -怒江缝合带附近而扩展至羌塘地体南缘和拉萨地体 ,沉积了巨厚的台地相灰岩 ;与塔里木南部和思茅地区同期海平面变化非常不同 ,那里在晚白垩世才出现海侵。砂岩组分研究显示 ,早白垩世早期碎屑物源主要来自北侧的造山带 ,向上则逐步受到南侧火山弧的控制。在海侵层系的下部 ,发现了丰富的双峰型火山岩和双峰式火山岩碎屑。因而推断该区在早白垩世发生了强烈的裂谷沉降作用。与此同时的在印度和巴基斯坦境内的 L adakh- Kohistan弧后裂谷作用还形成了具有洋壳基底的Shyok边缘海。因此 ,在早中白垩世 ,欧亚大陆南缘为西太平洋型的活动大陆边缘 ,因强烈的弧后裂谷作用产生了一系列边缘海盆地 ;在包括青藏高原南部在内的欧亚大陆南缘 ,既没有构造动力、也没有古地理和古地形证据支持在早白垩世末 ( 99Ma± )即出现强烈的抬升。

Early Cretaceous sedimentary system in the back-arc area of the Gangdese magmatic arc, central Tibet, western China, is characterized by a distinct change from clastics to carbonates in an ascending order. An intensive marine transgression occurred in this area during middle Cretaceous time, which led to accumulation of a massive succession of platform limestone in the southern margin of the Qiangtang terrain and the Lhasa terrain. The trend of eustasy in this area is quite different from that in the Tarim and Simao basins, where a coeval marine regression was in progress. Sandstones (Q78F11L11) in early Early Cretaceous were mainly derived from northern orogen but, upwards, they were gradually dominated by a (southern) magmatic arc provenance. Both bimodal volcanic rocks and bimodal volcanic debris were sometimes found within the middle Cretaceous sedimentary sequences in the southern margin of the back-arc area. An early Cretaceous back-arc rifting event is thus proposed regarding the Gangdese magmatic arc. This back-arc rifting gave rise to the Shyok marginal sea that was floored by oceanic basement, in the back-arc area of the Ladakh-Kohistan arcs. We believe that, during (early-) middle Cretaceous time, the entiresouthern Eurasian margin could have been active, leading to a series of back-arc rift basins. Some basins may have ultimately been floored by oceanic crust while others remained underlain by continental or transitional basement, much like the present western Pacific margin. There is no mechanism and no paleogeographic and topographic evidence to support the view that the Tibetan plateau had intensely been elevated by the end of Early Cretaceous time.