青藏高原中部伦坡拉盆地新生代地层年代、古环境与古高度研究综述

Research progress on the age,paleoenvironment,and paleo-elevation of Cenozoic strata in the Lunpola Basin,Central Tibetan Plateau

新生代以来,地球上最为显著的地貌变化就是印度与亚洲板块碰撞导致的青藏高原隆升.长期以来,关于碰撞的起始时间、碰撞方式与过程以及古高度等均有较大的争议.本文主要针对近年在古高度重建方面最受关注的高原中部伦坡拉盆地的研究进行了综述,旨在总结年代学、古环境演变以及古高度重建方面的进展和分歧.首先,伦坡拉盆地的新生代地层年代近年获得了长足的进展,特别是在伦坡拉盆地的牛堡组和丁青组地层陆续发现了10层左右的凝灰岩,为限定地层时代提供了很好的绝对年龄约束,牛堡组的时代已经从此前的古新世-始新世调整为始新世-早渐新世,丁青组则为晚渐新世至晚中新世.其次,伦坡拉盆地牛堡组的植物化石揭示了早始新世晚期(47 Ma)为热带亚热带湿润的森林环境,始新世晚期(39 Ma)则为亚热带开阔森林环境.丁青组的植被类型以暖温带针阔叶混交林为主.丁青组中、下部地层记录了40万年准周期的干湿旋回,表明地球轨道的长周期偏心率波动驱动了该时期伦坡拉盆地的古降水变化;而丁青组上部的稳定同位素记录揭示了14 Ma以来高原中部发生的降温与干旱增强事件,而且该事件是对14 Ma以来东南极冰盖扩张、全球持续变冷事件的响应.此外,伦坡拉盆地牛堡组中段及丁青组含有丰富的动植物化石.近年,越来越多的生物学证据(包括植物大化石与孢粉)表明,伦坡拉盆地在晚始新世的古高度基本在2000~2700 m左右,晚渐新世-早中新世初期(26~20 Ma)的古高度为3000~3200 m.即便到晚中新世早期的12 Ma,水生浮叶植物的证据表明,伦坡拉盆地古湖面的最大高度为3400 m,由此表明伦坡拉盆地的海拔高度很可能在晚中新世以后仍有1200 m左右的上升.

The most significant topographic change on Earth is the uplifted Tibetan Plateau that was caused by the India-Eurasia collision.There has been a long-standing dispute about the time of collision and the uplifted paleo-elevation history,which can help to characterize not only the uplift process,but it also serves as a link between the Earth's deep lithosphere and the upper crustal-surface spheres.In this context,it is an important scientific issue to study the paleo-elevation of the plateau in different tectonic domains and in different time-slices.Only by accurately reconstructing the paleo-elevation can we better examine the uplift mechanism of the Tibetan Plateau and its impact on regional and global climate.During the past 20 years,great progress has been made in reconstructing the paleo-elevation of the Tibetan Plateau,but we still have a long way to go to solve such a complex and difficult scientific problem.At present,there are still controversies about the specific age of many Cenozoic strata and consequently uncertainties about different paleo-elevation reconstructions.This paper aims to review the most recent progress in the chronology,paleoclimatic change,and paleoelevation reconstructions of the Lunpola Basin in central Tibetan Plateau,where has received most comprehensive studies in recent years.First,the chronology of the Cenozoic strata in the Lunpola Basin has been well defined,particularly due to the discovery of up to ten tuff layers in the Niubao and Dingqing Formations,which have provided new absolute age constraints of their deposition.The age of the Niubao Formation has changed from Paleocene-Eocene to Eocene-Early Oligocene,and the age of the Dingqing Formation ranges from Late Oligocene to Late Miocene.Second,plant fossils from the Niubao Formation in the Lunpola Basin have revealed a tropical and subtropical humid forest in the late Early Eocene(47 Ma),and a subtropical open forest in the late Eocene(39 Ma).This ecological shift was related to global cooling after the early Eocene climatic optimum and to the reduction of water vapor transport caused by the retreat of the Neotethys Sea.The vegetation type of the Dingqing Formation is dominated by warm-temperate mixed coniferous and broad-leaved forests.The increase of coniferous forests and the existence of vertical vegetation gradient indicate that the orogenic belt surrounding the Lunpola Basin had reached a certain height at the end of the Oligocene.High-resolution paleoclimatic records indicate that the lower and middle Dingqing Formation recorded dry-wet alternations with a 400 ka quasi-cycle,implying that the eccentricity of the Earth's orbit drove the dry-wet oscillations in the Lunpola Basin between 25.5 and20 Ma;whereas the stable isotopic records of the upper Dingqing Formation reveal that an enhanced cooling and dry climate prevailed after 14 Ma,which was a response to the East Antarctic ice-sheet expansion and to global cooling after the Mid-Miocene Climatic Optimum.Finally,the middle Niubao Formation and the Dingqing Formation are rich in animal and plant fossils,and their abundant paleontological data suggest that the paleo-elevation of the Lunpola Basin was mostly at 2000-2700 m during the late Eocene,and at 3000-3200 m in the Late Oligocene-Early Miocene(26-20 Ma).During the early Late Miocene(~12 Ma),pollen evidence of aquatic floating-leaved plants suggests that the maximum paleoelevation of the ancient lake surface in the Lunpola Basin was at~3400 m,which means that the Lunpola Basin underwent a~1200 m rise in elevation after the Late Miocene.