One Year Renewal Request: Investigation of Syncollisional Extension and Basin Development in the High Himalaya

最初的印度-亚洲洲际碰撞被认为是大约5500万年前沿着印度河缝合带开始的。这项工作主要集中在沿缝合带堆积的厚实的新生代碎屑沉积序列，并记录了侧翼物源地体、沉积环境和区域古海拔的演化。凯拉斯组由厚厚的冲积扇-湖相沉积组成，主要来自缝合带北侧。来自互层凝灰岩的锆石U-Pb年龄表明，其年龄在2,600万至2,300万年前，大约比文献中记载的年轻2,000万至3,000万年。因此，凯拉斯组不再被解释为始新世大陆间碰撞的结果。此外，沉积学和古植物学数据表明，大型深水湖泊中存在沉积，保存了丰富的藻类和陆生植物材料以及脊椎动物化石。孢粉学数据显示了潮湿的热带花粉组合，并提出了凯拉斯湖形成于中低海拔地区的可能性，完全是在假定的碰撞开始后3000万年。今天，这些岩石出露在海拔6000米以上，表明西藏西南部地区达到高海拔的时间比之前设想的要晚得多。以前在西藏中部对与凯拉斯组相同年龄的岩石进行的研究表明，到晚渐新世，该地区的海拔很高。因此，当凯拉组沿着缝合带以明显低得多的海拔堆积时，西藏中部已经变得非常高。总之，这项研究提供了新的见解，但也提出了许多问题，如最初碰撞的时间和性质，青藏高原的发展机制，以及它对改变海洋化学和古气候动力学的影响。这个项目的重点是复杂地带的地质历史，它标志着印度和亚洲大陆之间的碰撞地点。这场碰撞可能早在5500万年前就开始了，并产生了地球上最大的山脉-喜马拉雅-西藏造山带。喜马拉雅山脉海拔8000米以上，青藏高原的平均海拔相当于美国西部三分之一的面积，高于毗邻美国的最高山。这个巨大的造山系统对全球气候动力学和海洋化学产生了深远的影响，但人们对该系统是如何以及何时发展的了解很少。这个项目旨在通过收集恰好沿着碰撞带或缝合带沉积的沉积岩的数据来阐明印度支那-亚洲碰撞的时间。这项研究的结果将挑战这样的观点，即碰撞早在5500万年前就开始了，因此，将对整个碰撞过程的机制产生重大影响，并影响对新生代海洋和气候演化的科学理解。这项工作是与丁林和张海军博士以及来自中国科学院青藏高原研究所的几名研究生合作进行的。

Initial Indo-Asian intercontinental collision is considered to have begun approximately 55 million years ago along the Indus suture zone. This work is focused on a thick succession of Cenozoic clastic sediments that accumulated along the suture zone and recorded the evolution of flanking source terranes, depositional environments, and regional paleoelevation. The Kailas Formation consists of thick alluvial fan to lacustrine deposits that were derived mainly from the north side of the suture zone. U-Pb zircon ages from interbedded tuffs indicate an age of 26 to 23 million years ago, roughly 20 to 30 miliion years younger than previously documented in the literature. As such, the Kailas Formation no longer can be interpreted as the result of intercontinental collision during Eocene time. Moreover, sedimentological and paleobotanical data demonstrate deposition in large deep lakes, which preserved abundant algal and terrestrial plant material and vertebrate fossils. Palynological data suggest humid tropical pollen assemblages, and raise the prospect that the Kailas lakes formed at moderate to low elevation, fully 30 million years after the putative onset of collision. Today these rocks crop out at elevations above 6,000 meters, demonstrating that attainment of high regional elevation in southwestern Tibet was much later than previously assumed. Previous research in central Tibet on rocks of identical age to the Kailas Formation showed high elevations in that region by late Oligocene. Therefore, central Tibet had already become very high by the time that the Kailas Formation was accumulating at apparently much lower elevation along the suture zone. Together this research provides new insights but also raises many questions about the timing and nature of the initial collision, the mechanics of development of the Tibetan Plateau, and its implications for changing ocean chemistry and paleoclimate dynamics. This project is focused on the geological history of the complex zone that marks the site of collision between the Indian and Asian continents. This collision began perhaps as early as 55 million years ago and has produced the largest mountain range on Earth - the Himalayan-Tibetan orogen. The Himalayan mountains are higher than 8,000 meters and the Tibetan Plateau has an average elevation over an area equivalent to the western third of the United States that is higher than the tallest mountain in the conterminous United States. This colossal orogenic system has a profound effect on global climate dynamics and ocean chemistry, but understanding of how and when the system developed is poor. This project is designed to shed light on the timing of the Indo-Asian collision by gathering data from sedimentary rocks that were deposited right along the collision zone, or suture zone. The results of this study would challenge the idea that the collision began as early as 55 million years ago and, thus, would have major implications for the mechanics of the entire collision process as well as impact scientific understanding of Cenozoic ocean and climate evolution. The work is being conducted in collaboration with Drs. Ding Lin and Zhang Haijun and several graduate students from the Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing.