The Structural Features And Evolution Of Coqen Basin In Tibet

Coqen basin is located in the hinterland of the Qinghai-Tibet Plateau, from north to south, it's approximately 130 kilometers wide, and approximately 700 kilometers long from east to west, nearly east-west distribution. Its north takes Bangong-Nujiang suture zone as boundary line, the south takes Gangdese magmatic arc as boundary line, the east connect with Biru basin, the west extension out of the country. The basin is in the nearly east-west strip and between BanGong-nujiang suture zone and Brahmaputra suture zone. Its formation and evolution closely related to the two suture zone.In this paper, analyse Coqen basin's sedimentary characteristics and structural characteristics at first, through the field outcrop of the inspection, the measured profile and the histogram of the data to analyze the Sedimentary rock's lithologic characteristic, sedimentary facies and the space distribution. Through tectonic position, tectonic setting, basin fold and fault structure as a comprehensive analysis of inversion Coqen basin evolution of the structure.Since the Triassic until the Eocene, the Coqen areas performance characteristics of the basin, Its filling stratigraphy including late Triassic, Middle-Late Jurassic, Cretaceous and Paleogene, Paleocene and Eocene; In the Late Paleozoic Period, Coqen region is made up of continental margin and littoral sediment mainly, Coqen Basin at this time has not yet formed, the main exposed strata are late Paleozoic Devonian, Carboniferous and Permian, which formed the fold basement of Coqen basin; since the Oligocene, The basin disappeared gradually, then Coqen areas deposited the Neogene and the quaternary. Coqen basin in a general lack of early-middle Triassic and early Jurassic.On the whole, the concave and convex strips in East-west direction of Coqen basin are often alternate with arrangement. From north to south, it can be broadly classified into three sag zone and three uplifted uone, the internal structure of the sag zone and uplifted zone also arranged nearly east-west strip distribution.Therefore, the basin were be divided into six unit in this article, from north to south there were the Dongcuo-Asu sag zone, Laguocuo-Dangxiong uplifted zone,Chuanba-Taricuo sag zone, Xiadong-Yanong uplifted zone, Coqen-SeLongla sag zone and Longgeer- Jiangrang fault-uplift zone. According to the contact relationship of strata and the tectonic setting and the deformation characteristics , the basin can be broadly divided into three tectonic epochs , Including Hercynian tectonic period, Yanshan tectonic period and the Himalayas tectonic period.Fold characteristics of Coqen basin: the folds growth in the Early Cretaceous strata, They were most concentrate in the northern part of Chuanba- Taricuo sag zone and Taruocuo-Coqen sag zone, the axial trace north-west-west mainly,and parallel arrangement constitute to combination of linear fold. Vertical view, the fundus of basin most are closeness folds, closed fold covered the cap rock mainly, up is the broad fold, The stratum from the old to the new, and deformation diminuendo.The formation mechanism of the folds main longitudinal bend fold, the main stress is the level of squeezing pressure on nearly north-south.The fault characteristics of Cuoqin basin : the largest number of the fault trend in the north-west-west to south-east-east direction, while a small number of the fault in north-west or north-east and north-south direction. The fault in north-west-west to south-east-east direction have high dip angle in north or south tendency, it reflect the main north-south stress state, and such a fault with characteristics of multiple activities. General, the fault in north-west or north-east direction were strike-slip faults, came into being pull-apart basins. Usually the fault in north-south direction were normal fault, came into being grabens or horsts. Faults have the high number and high density in the north of basin, these faults cut jurassic and cretaceous Duoni, Langshan formation, make up of fault zone in north-west-west direction; Less faults were distrbuted Dawacuo-Angzhicuo sag zone in the south of basin.Paleozoic era, Gangdese Terrane of the basin area was located in the north verge of the Gondwanaland. Qiangnan Terrane, Gangdese Terrane, Himalaya Terrane and India Terrane apart from Gondwanaland subsequently, piece together the southof the Eurasia in turn. In these period, the basin area come through Varisian movement , Indosinian movement and Himalaya movement. The evolvement of the Coqen basin was restricted by the evolution of the Tethys. The evolvement of the Coqen basin was divide into three moment in this article, the form of basin fundus;the evolvement of basin; back-turning and destruction of basin.The form of basin fundus: late Paleozoic era, Gangdese Terrane and Qiangnan Terrane located the south of the ancient Tethys, The evolvement is restricted by the evolution of the ancient Tethys, from open to close of the ancient Tethys, the backdrop of Coqen basin from fault depression to extrusion. The basin area fold and uplift, so came into being fundus of Paleozoic. The evolvement of basin: Meso-Tethys opend at late Triassic and southward subduction at mid-late Jurassic. The backdrop of Coqen basin from fault depression to extrusion again. At this point, basin was deposited Jienu Group. Early Cretaceous, Neo-Tethys northward subduction, the evolution of basin area was restricted by the two subduction zone. At this period, Coqen basin was deposited Zenong Group, Duoni Formation and Langshan Formation. Late Cretaceous, Meso-Tethys closed and continent-continent collisional, whole basin area uplift. It was deposited molasses of the Jingzhushan Formation.The history that large area deposition at Mesozoic was over.During Paleocene and Eocene, Neo-Tethys closed and continent-continent collisional, basin area volcanic and magmatic activity strongly, developed Linzizong Group lava. Back-turning and destruction of basin: after Oligocene, basin wither gradually. Miocene period, basin area was leveled, later it uplift along with Tibetan Plateau. Differences in the course of the uplift, it developed a series of rift basins and pull-apart basins. Pleistocene to Holocene period, in the transmeridional tensile stress, basin area developed a series of grabens (rift basins) and horsts. From Paleozoic to this day, Coqen basin area after several Separation and combination, several ups and downs, finally evolved into what we see now.