The Landform Development And Environment Effects Of Three Parallel Rivers In The North Of Longitudinal Range-Gorge Region (LRGR)

The area of Three Parallel Rivers through which Nujiang River, Lancang River and Jingsha River flow parallel from north to south, is regarded as unique Longitudinal Range-Gorge Region (LRGR) in Southwest China. This area includes south-eastern part and fringe of Tibet Plateau. It is a key area not only to study natural environment evolution of southeastern Tibet Plateau and northwestern Yungui Plateau but also to unveil the response of environmental variations of Yungui Plateau to uplift of Tibet Plateau.Fluvial landforms in the north part of LRGR are characteristic with: (1)unique marvelous three parallel river spectacle consisting of outstanding longitudinal rang-gorges;(2) intense relief and deep incision of river;(3)tectonics (especially fault) control on evolution of gorge landforms;(4)diversities of landforms and their assemblements;(5)ecological vulnerability of geological and geomorpholgocial environment systems;(6)uneven tributary distribution of both sides of rivers deriving from plateau lakes, and so on. It is a typical representative area characteristic by high mountains, deep incision gorges, as well as assemblment of mountains and gorges in Hengduan Mountains even all river valleys in China. Based on investigations carried out in the whole area and focus on ages and causes of Hutiaoxia Gorge in Jingsha River, the results conclude that Yulong-Haba snow mountain is an almost integrated block and Hutiaoxia gorge reach of Jinsha River does not develop along the faults controlled by tectonics, headward erosion is not also responsible for this spectacle. The continuous extension of gorge layered landform surfaces from upper reach to low reach indicates that the identical layered landforms of upper reach and low reach are synchronous. The first bend of Yangtze River is not a bend due to river capturing but the junction of paleo-Shuilou River, paleo-Chong River and paleo-Jinsha River. The Hutiaoxia Gorge is shaped by incising downward antecedent stream bedrock under the background of tectonic uplift. Based on the ages of Yulong snow mountain glaciers, lacustrine sediments in Lijiang Basin, Pleistocene sediments in Diancangshan, mammalian fossils on terraces or dale surfaces, we primarily infer that the landforms in the Hutiaoxia Gorge formed by river incision since Middle-Pleistocene. Kunlun-huanghe Movement is a transformational event to trigger the evolvement of landforms and drainages.Impacted by uplift of Tibet Plateau, as well as active neotectonics, the planation surface in north Longitudinal Range-gorge Region was displaced and broken down into several pieces with different levels by local tectonic movement and Tibet Plateau uplift. Even in the same region, the heights of the surfaces at both sides of a fault are not identical. As a whole, the planation surface descends towards southeast due to uplift of Tibet Plateau, but its distribution is also complex. According to distribution of planation surface and existence of early Tertiary weathering crust or not, we conclude that there have been two extensive planation surfaces (Summit Surface and Main Surface), which are consistent with those on Tibet Plateau. Comparing the relevant sediments inthis area to neighboring ones, we argued that the formation age of Summit Surface is 23Ma B.P. and Main Surface was formed in about early days of Pliocene and broken up caused by Hengduan Movement at the age of 3.4Ma BP. In a word, all the evidences, existent planation surface, relevant basin sediments, together with evolvement of terraces in the rivers watershed, suggests that landforms evolution in north LRGR and the area of Three Parallel River results as multi-stage intermittent mountains uplift.Caused by river capture or not? The genesis of the First Bend of Yangtze River has been controversial for a long time. In fact, the genesis of the First Bend of Yangtze River is closely relative to evolutions of north LRGR and drainages in Yunnan. The north of LRGR and drainages in Yunnan have undergone several stages of regional geomorphic evolvementment, which are exterior drainages before the end of Pliocene, inland drainages from the end of later Pliocene to early Pleistocene, exterior and modern drainages since early Pleistocene. In later part of middle Pleistocene, The direction of ground stress in the region translation from NW to nearly SNE resulted in 8 nearly EW uplifted ranges served as watersheds of rivers and also dammed the Jinsha River runoff, forced it to flow counter to its original direction along a reach which was abandoned by paleo-Shuilouhe River due to Jinsha River capturing in three river junction. The First Bend of Yangtze River is a junction of paleo-Shuilou River, paleo-Chong River and paleo- Jinsha River. The Cenozoic landforms of north LRGR and Three Parallel River have gone through such process as compression ramping and basin inversion from late Eocene to the end of Oligocene, planation from the end of Oligocene to Pliocene, intense uplift from end of Pliocene to early Pleistocene, different uplift and development of longitudinal range-gorge landforms from early Pleistocene to present. The climatic environmental changes since early Tertiary in this area can be divided into several stages, i.e. subtropical trade wind climate before the early Tertiary, paleo-monsoon climate during late Tertiary and modern monsoon climate since Quaternary.The evolvement of tectonic landforms, climate and drainage leads to a unique lithosphere-atmosphere-hydrosphere-biosphere coupling system. So the landscapes of early Tertiary subtropical dry-hot vegetation, later Tertiary humid and semi-humid forest, Quaternary subtropical forest and natural vertical zones emerged in turn. Because of its "longitudinal range-gorge corridor barrier", high gradient effect and complicacies of local settings, this area is famous for it's the richest biodiversity in China even the whole world, also a center of species differentiation and species origin in Asia. In the light of global changes, the trend of ecological and environmental evolution of the Three Parallel Rivers in the north of LRGR would bring out both favorableness and harm in development of local ecology, societies and economies. Therefore we must harmonize the relation of human and nature with reason, adjust industrial structure and attach importance to ecological construction.The evolution of Southwest Monsoon which is regarded as an important member of Asian Monsoon system has been affected by uplift of Tibet Plateau and framework variation of sea and continent due to the collision between Eurasia and Indian sub-continent. Till about 8.5Ma B.P., translation from ancient ocean-continent monsoon to Southwest Monsoon had been finished. The Southwest Monsoon plays an important role on environmental evolution of the Three Parallel Rivers. The latitude, solar radiation, oceanic advance and regression, evolution of Tibet Plateau, tropic ocean and so on are mainly responsible for evolvement of Southwest Monsoon while "glacial cycles" can hardly affect it. The dominant periods are 41ka and 23ka. The Southwest Monsoon transports humidity latitudinally and its dominant wind is southwesterly wind. The areacontrolled by the Southwest Monsoon is characteristic with obvious rainy and dry seasons. In early Pleistocene, the Plateau was too low to bring out the barrier function, so the Southwest Monsoon dominate a larger area including Southwest China, North China and Central China. During the Middle Pleistocene, the barrier function was strengthened by abrupt uplift of the Plateau and the transfer event that affected Quaternary environment in China with Southwest Monsoon and Southeast Monsoon took place. The area controlled by Southwest Monsoon had shrunk to the southwest of Yangtze River upper reach and the southeast of Tibet Plateau.The special geomorphic spatial layout of LRGR results in obvious corridor-barrier, Rang-gorge high gradient effects (consisting of energy, precipitation, climate, climatic landform, natural zone pedigree, resource distribution and using, society and economy, resident condition gradient effects and local lithosphere- atmosphere-hydrosphere-biosphere-human coupling effects) and is also responsible for dry-hot earth and ecological landscape in this area. The dry-hot valley is characteristic with ecological environment vulnerability, active earth surface process, vegetation degeneration, dry-hot valley and wet-cold summit and single dry-fast vegetation. The formation of dry-hot valley is not only due to foehn effect, mountain valley circulation and human actions but due to many factors. In these factors evolution of geologic environment is dominant, atmosphere circulation is exterior control and human actions deteriorate environment. The dry-hot valley is a syntheses caused by tectonic movement- landform-paleoecology effect, circulation -monsoon "chimney" effect, mountain wave- precipitation-local circulation-foehn effect and vegetation -land use-human activity. It is characteristic with a crude earth and ecological landscape in principle.